THE HEALTH OF OUR AIR

Toward sustainable agriculture in Canada

Compiled and edited by

H.H. JanzenR.L. DesjardinsJ.M.R. Asselinand B. Grace

Research BranchAgriculture and Agri-Food Canada

©Minister of Public Works and Government Services Canada 1998

Available fromDonna DewanPublishing OfÞcer, Strategic PromotionResearch Branch, Agriculture and Agri-Food CanadaSir John Carling Building, Room 777Ottawa, OntarioK1A 0C5Tel. (613) 759-7787Fax (613) 759-7768e-mail dewandm@em.agr.ca

Electronic version available in May 1999 atwww.agr.ca/research/branch

Publication 1981/ECatalog No. A53-1981/1998EISBN 0-662-27170-XPrinted 1999 3M:02/99

Cette publication est disponible en fran�ais sous le titreLa sant� de lÕair que nous respirons: vers une agriculture durable au Canada

Staff Designer

Johanne Sylvestre-DrouinStrategic PromotionAgriculture and Agri-Food Canada

Production Editors

Sharon RudnitskiStrategic PromotionAgriculture and Agri-Food Canada

Jane T. BuckleyGilpen Editing Service

ii

iii

Contents

Foreword v

Preface vi

1. Introduction 1

Importance of atmospheric health 1

Our changing atmosphere 1

Objectives of this report 2

Scope of this report 2

Reading this report 3

2. Canadian agriculture and greenhouse gases 5

A glance at Canadian agriculture 5

The greenhouse effect 8

Commitments to reduce emissions 9

Estimates of emission 10

Carbon dioxide 10

Methane 25

Nitrous oxide 30

Combined effect of the three greenhouse gases 43

Uncertainties in current estimates 44

Techniques to minimize emission of greenhouse gases 45

Reducing carbon dioxide emissions 46

Reducing methane emissions 53

Reducing nitrous oxide emissions 57

Putting it all together 59

Other effects of practices that reduce greenhouse gas emissions 60

iv

3. Ozone 61

Source of ground-level ozone 61

Effect of ozone on plants 62

Ozone exposure and absorption by crops 63

Measuring plant response to ozone 65

Examples of crop response to ozone 66

Environmental interactions 69

4. Other links between agriculture and the atmosphere 71

Ammonia 71

Background 71

Agricultural sources of ammonia 72

Reducing ammonia emissions 74

Other odors 76

Nitrogen oxides 77

Aerosols 77

Ultraviolet radiation 79

Background 79

Effect of ultraviolet radiation on crops 81

Pesticides 83

5. Conclusions 85

Current status 85

Opportunities to reduce emissions 85

Future challenges 86

Remaining questions 88

6. Bibliography and selected reading 89

Acknowledgments 93

Appendix I 95

v

It is only in the past four or Þve decadesthat scientists have increasinglyunderstood many of the interactionsbetween lands and oceans and theatmosphereÑand how human actionsmay be modifying these exchanges. Andit is only in the past two decades that thepublic and governments have beenconfronted with clear evidence of thechanging composition of the globalatmosphere and its likely effects. In theshort space of a century, the burning oflarge quantities of fossil fuels stored inthe ground over millions of years hashad the largest impact. But the way inwhich we manage the land and producefood and Þber is by no means negligible.Pollution of the atmosphere affectsdirectly all land creatures and plants, aswell as the climate that governsproductivity, human activities, andoccurrence of extreme events such asdrought, ßoods, and storms.

This book comprehensively addressesthose interactions between land andatmosphere that arise because ofagricultural practices in Canada. Some ofthe atmospheric changes may be benignor even beneÞcial to humans and plants.But there is much evidence to indicatethat adverse effects are occurring. Thesenegative effects will continue to increaseunless changes occur in how we manageour energy, food, and Þber economies.

The authors wisely point out that the netrelease of greenhouse gases fromagriculture "is usually a symptom ofinefÞcient use of resources." This is alsotrue of other economic sectorscontributing to atmospheric changes. It isnot just the case for greenhouse gases,but also for problems such as emissionsof precursors to low-level ozone and acidrains.

Various means of increasing theefÞciency with which we use ourresources in agriculture are outlined.Also examined is the signiÞcant potentialfor restoring organic carbon in our soilthrough conservation tillage and othermeans, thus reducing atmospheric carbondioxide. There are many "winÐwin"opportunities demonstrated, whereincreased soil and agriculturalproductivity go hand-in-hand withreducing pollution of the atmosphere.This book provides much of the scientiÞcinformation needed to develop aneffective strategy for CanadaÕsagricultural sector.

Let us hope that this cooperative way ofapproaching the problems confrontingagriculture and the global atmospherebecomes an inspiration for other sectors.As in agriculture, there are many cost-effective "win-win" opportunities forachieving energy efÞciency to be foundin transportation and forestry that canimprove CanadaÕs economic situationand simultaneously help protect the life-giving atmosphere of our small planet.

James P. Bruce, O.C., FRSCCanadian Climate Program Board

Foreword

This book has its roots in twointernational reports:

¥ the World Commission onEnvironment and Development, OurCommon Future, better known asthe ÒBrundtlandÓ report

¥ the 1990 report of theIntergovernmental Panel on ClimateChange, IPCC.

In 1987, Our Common Future broughtworld attention to problems such asglobal warming, ozone depletion,desertiÞcation, reduced biodiversity,burgeoning demands of a growingpopulation, and the need for a globalagenda for change that would makedevelopment sustainable. In short, theCommission sought ways to meetpresent needs without compromising theability of future generations to meettheirs.

The IPCC, set up by the WorldMeteorological Organization and theUnited Nations Environment Program,produced its Þrst scientiÞc assessment in1990. Several hundred scientists from 25countries helped prepare and review thescientiÞc data. The IPCC concluded thatemissions from human activities areincreasing the concentration ofgreenhouse gases. It warned that thiscould lead to a warming of the EarthÕssurface.

These two reports brought climatechange to the forefront of the worldenvironmental agenda. In 1992 nationsmet in Rio de Janeiro to sign the ClimateChange Convention, an agreement toreduce greenhouse gas emissions. TheIPCC released a second scientiÞc

assessment in 1995 stating that Òthebalance of evidence suggests adiscernible human inßuence on globalclimate.Ó A further internationalagreement, signed by 174 countries inKyoto, Japan, in 1997, agreed to setspeciÞc targets for greenhouse gasemissions.

Scientists in Canada, as in othercountries, now focused more attention onissues associated with atmosphericpollution, greenhouse gases, and climate.Within the Canadian federal government,the Atmospheric Environment Service(Environment Canada) and programssuch as the Green Plan provided furthersupport. Indeed, the four federal,science-based, natural resourcesdepartments of Environment, Fisheriesand Oceans, Natural Resources, andAgriculture and Agri-Food joined forcesand signed a Memorandum ofUnderstanding for Science andTechnology for Sustainable Developmentin 1995. The goal was to enhancecooperative research in areas of mutualconcern such as climate change.

The Research Branch of Agriculture andAgri-Food Canada initiated a researchprogram in greenhouse gases andground-level ozone in 1992 in support ofsustainable development. The programinvolved scientists working for thefederal government, universities,provincial agencies, and the privatesector. After 6 years, we now report ourÞndings to the Canadian public.

The Health of Our Air joins The Healthof Our Soils and the forthcoming TheHealth of Our Water as a series ofscientiÞc assessments evaluating thenatural resources upon which Canadianagriculture depends.

Preface

vi

vii

This book contains our most recentresearch Þndings. As the researchcontinues, better estimates of greenhousegas emissions will emerge. New, moreefÞcient technologies will be developed,and we will learn more about therelationship between agriculture and thehealth of air. The Research Branch ofAgriculture and Agri-Food Canada iscommitted to research in support ofsustainable development of the Canadianagriculture sector.

J.B. MorrisseyAssistant Deputy MinisterResearch BranchAgriculture and Agri-Food Canada

R. SlaterSenior Assistant Deputy MinisterEnvironment Canada

1

Agriculture is tied tightly to theenvironment. Future food productiondepends on preserving soil, air, andwater quality; in turn, the way we farminßuences these resources. As a result,we need to assess, from time to time, thechanges taking place in the environment,both to ensure that farming can besustained and to measure its effect onother ecosystems. An earlier reportconsidered The Health of Our Soils; inthis companion, we focus on The Healthof Our Air.

Unlike soil, which is Þxed in place, airmoves and mixes freely around theglobe. Air at the earthÕs surface canexchange with air many kilometres upwithin hours, and air currents can movearound the world in a matter of days.Some of the carbon dioxide released byÞres in Asia may be absorbed byorchards in Ontario, and some of thatreleased from decomposing straw inSaskatchewan is taken up by the junglesof South America. As a result, we haveto view the health of our air from aglobal perspective.

Importance ofatmospheric healthOur atmosphere has many essentialfunctions. It is a reservoir of gases uponwhich life depends: carbon dioxide andnitrogen for plant growth, oxygen for usto breathe, water vapor for rain thatrefreshes the land. It insulates the planetagainst temperature extremes and Þltersout harmful radiation from the sun. Iteven helps detoxify harmful substancesreleased into it, either hastening theirbreakdown or diluting and dispersingthem. Because living things depend so

completely and in so many ways on theatmosphere, any change in its makeupshould concern us.

Our changingatmosphereThe air is always being recycled byexchange of gases and particles withland, water, and living things. The ratesof gases entering the atmosphere areusually balanced by rates of gases lost,so that the composition of theatmosphere has remained nearly constantfor many centuries. This balance hasbeen disrupted, however, by increasingemissions from human activity, so thatsome gases are now accumulating,altering the composition of the air.

One of the most noticeable changes, inrecent years, has been an increase inconcentrations of some ÒgreenhousegasesÓ: carbon dioxide (CO2), methane(CH4), nitrous oxide (N2O), andchloroßuorocarbons (CFCs). These gasesabsorb some of the energy radiating fromthe earth, thereby warming theatmosphere. A build-up of greenhousegases, therefore, is expected to increaseglobal temperature over time.

Greenhouse gases are not the onlyconstituents of air whose concentrationis changing. The levels of otherconstituents such as nitrogen and sulfurgases, ozone (O3), gaseous organiccompounds, and suspended particles arealso increasing as a result of humanactivity, including agriculture. Thesechanges, like those in greenhouse gases,may affect climate and the health of theenvironment.

1. Introduction

2

Growing concern over the changingatmosphere has prompted global effortsto reduce emissions. These includerecent international agreements to limitemissions of O3-depleting chemicals(Montreal Protocol) and a CanadaÐUSagreement on air-pollution. Perhaps mostnoteworthy is a treaty signed in Kyoto,Japan in December 1997 where 174countries, including Canada, agreed tocurb emissions of greenhouse gases.

Objectives of thisreportAgriculture occupies a larger portion ofglobal land area (about 35%) than anyother human activity. Because of itsscale and intensity, agriculture emits alot of gases into the atmosphere. Forexample, agriculture is a main source ofgreenhouse gases, accounting for about25% of the CO2, 50% of the CH4, and70% of the N2O released via humanactivity globally. As well, agricultureaccounts for more than 50% of ammoniareleased into the air.

But because farmlands are managed sointensively, farmers can control, at leastpartly, the amounts of gases released.Various ways of farming producedifferent emissions; and by choosingnew practices, it may be possible toreduce emissions. For some gases,farmlands may, in fact, even be made toabsorb more than they emit, therebyhelping to restore air quality.

In this report, we focus on the effect ofCanadian agriculture on the atmosphere.SpeciÞcally, we try to answer threequestions:

¥ How do farming practices affect thecomposition of the atmosphere?

¥ What is the amount of agriculture'semissions to the air?

¥ How can we reduce theseemissions?

Scope of this reportThe most pronounced change in theatmosphere, and the one with greatestpotential consequence, is the build-up ofgreenhouse gases. Hence, this reportaddresses in detail the amounts ofgreenhouse gas emission and possibleways of reducing them. We limit ourdiscussions mainly to agriculturalproduction itself and, except for ethanol,do not consider the fate of agriculturalproducts once they leave the farm. Manyof the Þndings presented were obtainedfrom a national research programinitiated by Agriculture and Agri-FoodCanada in 1992.

Besides focusing on greenhouse gases,we also consider several other currentatmospheric issues, though in less detail:ground-level O3, ammonia, ultraviolet(UV) radiation from the sun, aerosols,nitrogen oxides, pesticides, and farm-related odors. Wherever possible, ourdiscussion is based on Þndings fromCanadian studies but, where Canadianresults are only just emerging, we havedrawn on results from elsewhere.

Principal atmospheric constituents

Elements and compounds Symbol Atomic or molecular weight

Elements

Hydrogen H 1

Carbon C 12

Nitrogen N 14

Oxygen O 16

Gases

Methane1 CH4 16

Ammonia NH3 17

Nitrogen N2 28

Nitric oxide NO 30

Oxygen O2 32

Carbon dioxide2 CO2 44

Nitrous oxide3 N2O 44

Nitrogen dioxide NO2 46

Ozone O3 48

1 1 g of CH4 contains 0.75 g of C.2 1 g of CO2 contains 0.27 g of C. 3 1 g of N2O contains 0.64 g of N.

3

Changes to the global environment mayhave pronounced effects on Canadianagriculture in the future: changingconcentrations of CO2 may affect plantgrowth; increasing temperature mayallow greater diversity of crops but favorcrop pests; changing patterns ofprecipitation may favor some areas butinduce drought in others. Such changesremain hard to predict. Because of thisuncertainty and the constraints of spacein this report, how agriculture will adaptto future changes is only referred toindirectly; we await results of ongoingresearch to clarify this issue further.

Reading this reportThis report is written for students ofenvironmental issues, farmers,agricultural professionals, decision-makers, and others who want tounderstand the links between agricultureand air quality. The reader does not needa scientiÞc background to read thereport, though a rudimentary knowledgeof chemical notation will be helpful. Forexample, we refer to various compoundscontaining nitrogen (N), carbon (C),oxygen (O), and hydrogen (H), elementsthat are the main constituents ofgreenhouse gases and organic matter. Forthe inquisitive reader, we have listedsome important compounds in the boxon this page.

The information presented is gleanedfrom numerous sources, both Canadianand international. To allow for easierreading, we have not quoted individualsources but have provided a generalbibliography at the end. More detailedinformation as well as some of theoriginal sources may be found there.

5

Canadian agriculture is diverse, with avariety of crops and livestock in a rangeof climates and soils. Emissions ofgreenhouse gases are also highlyvariable, changing with type of farmingoperation and even within individualfarms. To estimate the emission ofgreenhouse gases from Canadian farms,therefore, we have to Þrst considerbrießy the nature of farming in Canada.

A glance atCanadianagricultureThe Canadian landmass has beenclassiÞed into 15 ecological zones(ecozones) based on soil and climate.Most of CanadaÕs land is forested, andonly about 5% is suitable for farming,mainly in two ecozonesÑthe Prairiesand the Mixed Wood Plains of the St.Lawrence River (Fig. 1). The Prairiesalone account for about 80% of CanadaÕs68 million hectares of farmland. Two-thirds of all farmland is used for cropsand ÒimprovedÓ pastures (those that areseeded, drained, fertilized, or weeded);

Figure 1Farmland as a proportion of land area in various parts of Canada in 1996. (F. Wang and D.B. Gleig, AAFC)

2. Canadian agriculture and greenhouse gases

Farmland %

5.0Ð10.010.1Ð30.030.1Ð60.0>60.0Not rated

6

the rest is occupied by ÒunimprovedÓpastures (largely native grasslands),buildings, barnyards, bush, sloughs, andmarshes. The various types of pasturetogether account for about 30% offarmland (Fig. 2).

The relative areas devoted to annualcrops and animal husbandry vary widelyacross the country. For example, largeareas of the Prairies are used almostexclusively for cropland (Fig. 3),whereas small pockets of concentratedlivestock production exist in areas ofBritish Columbia and the southernregions of Alberta, Ontario, and Quebec(Fig. 4).

Although all ecosystems share commonnutrient pathways, agriculture has uniquefeatures when compared to other landuses such as forestry. Farmlands,particularly those devoted to annual

crops, are intensively managed.Moreover, the time cycle for agriculturalcrops is short, often annual. As a result,agriculture can respond quickly toclimatic, economic, and policy events bychanging land use and cropping systems,and there can be large shifts in just a fewyears (Table 1). Finally, agriculturalecosystems are quite Òopen,Ó involvingcontinual transfer of material in (e.g.,fuel, fertilizers, and pesticides) andmaterial out (e.g., crop yields and animalproducts). Unlike forests, whichgradually increase their store of wood,farmlands rarely accumulate vegetationover the long term. Because of theseunique features, studying and estimatinggreenhouse gas emissions from farmsdiffers from that in other ecosystems.

Figure 2Pasture (improved and unimproved) as a proportion of farmland in 1996. (F. Wang and D.B. Gleig, AAFC)

Pasture %

²5.05.1Ð15.015.1Ð40.040.1Ð60.0>60.0Excluded from analysis

7

Figure 4Distribution of livestock in Canada in 1996. One animal unit is the quantity of livestock that produces manure containing 170 kg of N peryear. For example, approximately 2 dairy cows are equivalent to 1 animal unit. (F. Wang and D.B. Gleig, AAFC)

Figure 3Annual crops as a proportion of farmland in 1996. (F. Wang and D.B. Gleig, AAFC)

Annual crops %

²20.020.1Ð40.040.1Ð70.0>70.0Excluded from analysis

Units per ha

²0.050.06Ð0.150.16Ð0.300.31Ð0.60>0.60Excluded from analysis

8

The greenhouseeffectThe earth is warmed by the sunÕsradiation (including visible light) thatstrikes it. The earth, in turn, radiatesenergy back into outer space, but thisoutgoing radiation differs from that ofthe sun: it has a longer wavelength and isinvisible to the human eye. Furthermore,some of this outgoing, long-waveradiation is absorbed by various gases inthe air, thereby warming the atmosphere.This warming is referred to as theÒgreenhouse effectÓ (though the warmingeffect inside glasshouses is really quitedifferent!). The warming from thegreenhouse effect is highly beneÞcial;without it, the average temperature onour planet would be about 33oC colder,making the earth inhospitable.

The gases causing the warming of theatmosphere are known as Ògreenhousegases.Ó The most important are watervapor, CO2, CH4, N2O, and CFCs.Foremost among these is water vaporbecause it is a powerful absorber oflong-wave radiation and is present inrelatively high concentration. This gas,however, is already present in highenough concentration in the loweratmosphere that further increases in itsconcentration would have minimal effecton temperature.

Much of the current concern aboutgreenhouse gases has arisen from therecent recognition that the concentrationof other greenhouse gasesÑCO2, CH4,N2O, and CFCsÑhas been increasingsteadily since the industrial revolution,almost certainly because of humanactivity. By 1992, CO2 had increased by30%, CH4 by 145%, and N2O by 15%.Current rates of increase are 0.5% peryear for CO2, 0.6% for CH4, and 0.3%for N2O. The CFCs were not even

The Ògreenhouse effectÓ

Short-wavelength radiation emitted from the sun is absorbed by the earth and re-radiatedat longer wavelengths. Carbon dioxide, CH4, and N2O account for 90% of thisÒgreenhouse effect.Ó In the long term, incoming radiation is balanced by outgoingradiation. Because of the greenhouse effect, the average surface temperature of the Earthis about 15¼C, instead of Ð18¼C.

Incomingradiationenergy

Reflected energy Outgoingradiationenergy

Energy trappedby greenhouse gases

Table 1 The area of farmland in Canada occupied by annual crops (million ha)

1981 1986 1991 1996

Total farmland 65.9 67.9 67.7 68.0

Croplands 31.0 33.2 33.5 35.0

Summer fallow 9.7 8.5 7.9 6.3

Improved pasture 4.4 3.6 4.1 4.4

Nonimproved pasture 20.8 22.6 22.2 22.3

9

The Intergovernmental Panel on Climate Change

In 1988, the World Meteorological Organisation and the United Nations EnvironmentProgram created the Intergovernmental Panel on Climate Change (IPCC). The IPCCevaluates research and policy options and publishes reports on climate change and therisk of global warming.

The latest synthesis report, based on 1995 science, includes the following conclusions:

¥ The balance of evidence from observed changes suggests a discernible human inßuence on global climate

¥ Human-induced climate change represents an important additional stress, particularly to the many ecological and socioeconomic systems already affected by pollution, and nonsustainable management practices

¥ SigniÞcant reduction in net greenhouse gas emissions are technically possible and can be economically feasible . . . in all sectors including . . . agriculture

The assessment report now being planned will provide the major science input to thefuture evolution of the UN Framework Convention on Climate Change and the Kyotoprotocol.

present in the atmosphere until a fewdecades ago. If the current rates ofincrease continue, many scientists expectsigniÞcant impact on the worldÕs climate.For example, the IntergovernmentalPanel on Climate Change predicts thatthe doubling of the CO2 concentration,likely to happen in the 21st century,would increase average globaltemperatures by 1 to 3oCÑa rate ofwarming unprecedented in the last10 000 years. As well, the enhancedgreenhouse effect could amplify climatevariability.

In short, greenhouse gases have adesirable effect, as they warm theatmosphere and create favorableconditions for biological activity. Furtherincreases in these gases, however, maylead to an Òenhanced greenhouse effectÓwith uncertain, possibly disruptive,consequences.

Commitments toreduce emissionsConcern about the enhanced greenhouseeffect has prompted international actionto reduce emissions. A Þrst agreement,intended to stabilize emissions at 1990levels by 2000, was signed in 1992 at theEarth Summit in Rio de Janeiro. A morebinding agreement was reached atKyoto, Japan in 1997. This protocol wasaimed at reducing emissions fromparticipating countries to at least 5%below 1990 levels, by 2008 to 2012.This treaty will come into effect,however, only when ratiÞed by at least55 countries representing 55% of totalgreenhouse gas emissions fromdeveloped countries.

The Kyoto protocol

At Kyoto, developed countries agreed to reduce their combined emissions of greenhousegases by 5.2% from 1990 levels. This target will be realized through national reductionsof 8% by Switzerland, many Central and East European states, and the European Union;reductions of 7% by the United States; and reductions of 6% by Canada, Hungary, Japan,and Poland. Russia, New Zealand, and Ukraine are to stabilize their emissions, whileNorway may increase emissions by 1%, Australia by up to 8%, and Iceland by 10%.

The protocol aims to lower overall emissions from a group of six greenhouse gases by2008Ð2012, calculated as an average over these 5 years. Cuts in the three most importantgasesÑCO2, CH4, and N2OÐwill be measured against a base year of 1990. Cuts in thethree long-lived industrial gasesÑhydroßuorocarbon, perßuorocarbon, and sulfurhexaßuorideÑwill be measured against either a 1990 or a 1995 base year, depending onwhat year is most beneÞcial.

10

In the Kyoto protocol, Canada agreed toreduce its emissions to 94% of 1990levels by 2008 to 2012. But CanadaÕsemissions are already well above 1990levels. Based on increases from 1990 to1997 and assuming a Òbusiness as usualÓscenario thereafter, one estimate suggeststhat Canada will need to reduce itsemissions by about 21%. Consequently,a widespread effort involving all sectorsof our economy will be required to meetCanadaÕs commitments.

In 1992, Agriculture and Agri-FoodCanada initiated a research program toestimate emissions of greenhouse gasesfrom Canadian agriculture and to devise

Greenhouse gas research methodology

The sources and patterns of emission of carbon dioxide, methane, and nitrous oxide arecomplex. Laboratory and experimental plot studies are needed to improve ourunderstanding of biological processes. Then the emissions must be assessed over wholeÞelds and groups of Þelds, to account for soil, landscape, and management variations.Finally, interactions between these three greenhouse gases must be considered, regionaland climatic variations taken into account, and the global effect integrated over completeecosystems and all of Canada.

N2O

N2O

N2O

CO2

CO2

CO2

CH4

CH4

CH4

Integration • temporal/spatial scaling up• model validation• interaction of gases

Level Objectives

Process • identification of sources/sinks• characterization of

Ecosystem • net balances for greenhouse gases• feasibility of practices for reducing emissions

rate-determiningfactors

1) net contributions of greenhouse gas2) options for reducing emissions

OutputResearch Approach

ways of reducing these emissions.Findings from this effort, some of whichare summarized in this report, may helpCanada meets its reduction target.

Estimates ofemission

Carbon dioxide

The global carbon cycle

There are about 40 000 petagrams (Pg)of C in global circulation (Fig. 5). MostC is in the oceans but large pools alsooccur in soils, vegetation, and theatmosphere. Of these three pools, theatmosphere is the smallest but mostactive. The CO2 in the air is continuallybeing removed by plants throughphotosynthesis and being absorbed intothe oceans. At the same time, however,CO2 in the air is being replenished byrelease from plants, soils, and oceans.Thus, though C is always cycling, theconcentration of atmospheric CO2 hasremained constant from year to year.Analysis of air bubbles trapped in oldglaciers and shells buried in oceansediments reveals that the atmosphericconcentration of CO2 had stayed at about270 parts per million by volume (ppmv)for about 10 000 years.

Soil carbon map of Canada

The Canadian Soil Organic Carbon Database, consisting of over 15 000 soil landscape polygons, contains information describing the soillandscape and carbon content of each polygon. The total carbon in the Þrst metre of all Canadian soils is 260 Pg (billion tonnes), whichrepresents 13% of the worldÕs total organic carbon. However, most of the carbon is in the northern wetlands and permafrost. Only about 10Pg (billion tonnes) or 4% of the carbon is contained in the soil of agricultural ecosystems.

(C. Tarnocai, AAFC)

11

12

Figure 5

A simpliÞed view of the global carbon cycle.

That changed with the advent of theIndustrial Revolution. Since then, thedemand for energy has resulted in ever-increasing amounts of fossil fuels beingextracted from deep reserves andconverted to atmospheric CO2. Thisprocess, in effect, withdraws C from aninactive pool and emits it into theatmosphere as CO2. Other activities havealso favored increases in atmosphericCO2: removal of forests has resulted invegetative C being converted to CO2,and the cultivation of previouslyundisturbed soils has resulted in soil Cbeing converted to CO2. Because ofthese processes, the emissions of CO2

into the atmosphere now exceed thewithdrawals, resulting in the gradualbuildup of CO2 (Fig. 6)

Figure 6Long-term atmospheric CO2concentrations as determined from ice coredata (before 1950) and atmosphericmeasurements (after 1950).

380

340

300

260

01600 1700 1800 1900 2000

Year

CO

2 (p

pmv)

760CO2

610

39 × 103

(Pool size in Pg)

Soil1580

In 1995, fossil fuel combustion alonereleased 23.5 Pg (billion tonnes) of CO2

into the atmosphere. The natural C cyclecan absorb some of this increased CO2

emission: some is absorbed by oceans,some by increased photosynthesis inplants. Nevertheless, the total amount ofCO2 in the atmosphere is still increasingby about 11.7 Pg (billion tonnes) of CO2

every year. These increases are readilyapparent in weekly measurements ofatmospheric CO2 at Alert, NWT, which,despite seasonal variations reßectingplant growth, show a clear, undeniableupward trend (Fig. 7).

Figure 7Seasonal variations of CO2 concentrationsmeasured at Alert, NWT. Most land and,therefore, vegetation on the earth is in theNorthern Hemisphere. This vegetationdraws heavily on the atmospheric CO2pool in summer but returns the CO2 as thevegetation dies in winter.

Carbon cycles inagricultural ecosystems

The carbon cycle in cropped land is quitesimple, at least in principle (Fig. 8). Carbon dioxide is absorbedfrom the atmosphere by plant leaves andis transformed, via photosynthesis, intoC-containing compounds such as sugars,carbohydrates, cellulose, and lignin.

Annual averageSeasonal variations370

365

360

355

350

345

340

0

CO

2 (p

pmv)

Year97969594939291908988

13

Figure 9Conceptual C cycle of a livestock-basedcropping system.

In systems that have remained largelyunchanged for several decades, theamount of C entering the soil as plantresidues is usually balanced by theamount of C converted to CO2 bymicrobial activity. Consequently, thoughC is continually added to the soil, theamount of C stored in the soil may notchange measurably. For example, in thecorn system illustrated (see Fig. 8),residue inputs of 4.5 Mg (tonne) C perhectare are exactly balanced bymicrobial production of CO2 from thesoil, so that there is no change in theamount of C stored in the soil.

Management effects oncarbon cycle

A change in the way land is managedcan disrupt the C cycle, affecting theamount of C stored. Perhaps the mostdrastic example was the initialcultivation of soils for farming. Thisevent, which happened on manyCanadian farmlands more than a centuryago, resulted in high losses of soil C:many soils lost about 25% of the Coriginally present in the C-rich surfacelayer, releasing a lot of CO2 into the

Soil organic matter

CO2

Product

Some of this material is used by theplant for its own energy and convertedback to CO2. Of the C remaining in theplant, a portion is removed duringharvest (e.g., in grain) and the rest isreturned to the soil. This residue,including roots, becomes part of the soilorganic matter. Microorganisms in thesoil, in turn, decompose the soil organicmatter, releasing CO2 back into theatmosphere and closing the loop. Thiscycle is essentially the same in allcropping systems, but rates varydepending on climate, soil, and crop type.

Figure 8Conceptual C cycle for corn (values areestimates of annual ßows of C in Mg/ha).

Where present, livestock add anothercomponent to the carbon cycle (Fig. 9).Instead of being exported, much of theharvested plant material is fed to animalsor used as bedding. Some of this C isreleased by the animals to the atmosphereas CO2, some is removed as animalproducts, but much is returned to the soilas manure. Consequently, livestock-basedsystems often retain higher proportions ofC on the farms. In many ways, this cycledoes not differ from that in crops grownfor human food. But the CO2 and wastesfrom human consumption of crops areoften released far from the farm andtherefore are not usually thought of as partof the agricultural C cycle.

2.5

10 3

4.5

4.5

4.5

CO2

Soil organicmatter

Product

14

atmosphere. There are several reasonsfor this loss. First, farming involves theharvest of C from the Þelds, and theremoval of this C means less input ofnew C. As well, cultivation and growingannual crops often speed up theconversion of soil C to CO2 by soilmicrobes. After soils have beencultivated for a few decades, however,losses of C usually slow down or ceaseentirely, and the level of soil C is againstable (Fig. 10).

Figure 10Theoretical changes in soil C as inßuencedby management.

The effect of the initial cultivation on theC cycle is largely past. Today we areinterested more in how current practicesor future modiÞcations might affect theC cycle. By choosing their crops, tillagepractices, fertilizer treatments, and otheroptions, farmers can alter the C cycle,thereby changing the amount of C storedin the system.

Soil

carb

on

agriculture

∆C<0 ∆C=0 ∆C>0

Initialcultivation

Managementchange

Effect onatmosphericCO2

~

Measuring managementeffects on carbon cycle

How do we determine the inßuences offarming practices on the C cycle? Oneway is to measure all the ßows in the Ccycle in a farm Þeld (see Fig. 8). Bysubtracting the amounts of C leaving theÞeld from the amounts entering, we cancalculate the net change in C. Suchmeasurements are useful in describinghow management affects the C cycle, butthey are time-consuming and are usedonly at selected research sites.

Another way is to measure the netexchange of CO2 between vegetationand the atmosphere above it. Usingsensors placed on towers, researcherscan measure CO2 transfer above the cropcontinuously for months or even years,allowing them to calculate CO2

exchange over an entire Þeld. Thisapproach, using towers, aircraft, andother variations, provides an average ofnet CO2 emissions from larger areas,thereby overcoming the naturalvariations that occur across a Þeld. Themain disadvantage of this method is costand the difÞculty of integrating over longperiods.

A third method, and that most widelyused, is to measure the change in theamount of stored C after a number ofyears. In farm Þelds (as opposed toforests), virtually all the C is stored inthe soil organic matter. By measuring theamount of soil C once and then againseveral years later, scientists can tellwhether the Þeld has gained or lost Cunder certain practices (Fig. 11).

A common variation on this approach isto measure the change under onetreatment relative to another. Forexample, if we are interested in the effectof tillage on C storage, we can maintain

15

Tower-based ßux measurements

Tower-based long-term measurements of CO2, water vapor, and energy exchange from manyecosystems are now available for North America and Europe. Scientists make thesemeasurements to

¥ collect critical new information to help deÞne the global CO2 budget

¥ improve predictions of future concentrations of atmospheric CO2

¥ enhance understanding of CO2exchange between atmosphere and biosphere

¥ determine response of CO2 ßuxes to changes in environment and climate

¥ provide information on processes controlling CO2 ßux and net ecosystem productivity

¥ help calibrate and verify data for CO2ßux models.

(S. McGinn and E. Pattey, AAFC)

Twin Otter aircraft

The Twin Otter aircraft (see photo), operated by the Flight Research Laboratory of theNational Research Council, provides an excellent platform for investigating gas exchangenear the surface. It is equipped with sophisticated turbulence and trace gas sensors. At aßying speed of 60 m/s and at altitudes of 30Ð100 m, the instruments record atmosphericdata every 2 m. In ßight, the measurednet ßuxes of a particular gas, such asCO2, water vapor, O3, CH4, and N2O,can be determined as the averageproduct of vertical wind and the actualconcentration of the gas. The ßux valuecan be positive (indicating that more gasis released by the surface than is beingabsorbed), zero, or negative (meaningthat more gas is being absorbed than isbeing released).

(J.I. MacPherson, NRC, and

R.L. Desjardins, AAFC)

two systems side by sideÑone tilled, theother notÑand then measure the increasein stored C in the untilled plot bycomparing it to that in the tilled plot. Butmeasuring changes in soil C is not easy.Any increase may be small, say 3 tonnesC per hectare, compared to the amountinitially there, say 60 tonnes C perhectare. This problem is furthercomplicated by the natural variability ofC in the Þeld, which is often muchgreater than the difference we hope tomeasure. Accurate measurement of soilC change, therefore, requires carefulsampling and analysis. Some researchershave focused on speciÞc forms of soil Cor on atomic markers (isotopes) tomeasure soil C changes more precisely.

Figure 11Estimating soil C gain after adoption ofno-till.

To estimate the effects of managementon the C cycle over large regions, wehave to rely on models. These modelsmay be simple equations or highlycomplex computer programs that takeinto account many variables such asweather, soil type, and farming practicesto predict C processes on the farm.Whatever their complexity, these modelsneed to be checked against actualmeasurements to ensure that they arereliable. By using measurements fromspeciÞc locations, researchers can verify

Org

anic

C (M

g/ha

) 60

40

20

100

0

Absolutegain

Netgain

Time after adoption of no-till (years)

Conventional tillage No-till

Tower-based system for measuring gasexchanges.

16

only increased crop yield but alsoprovided direct addition of C.

Figure 12Change in organic C in two croppingsystems at Breton, Alta., as affected bynutrient application. (R.C. Izaurralde,University of Alberta)

Fertilizer application to corn

Application of fertilizer can increase soilC. At a long-term research site inOntario, soil under fertilized corn hadhigher soil C than that under unfertilizedcorn after 32 years (Fig. 13). Using Cisotopes to distinguish between C fromcorn and that from previous organicmatter, the researchers also showed thatthe increase came entirely from the cornresidueÑfertilization had no effect onthe organic matter that was there beforecorn was Þrst planted. Adding fertilizerto this soil increased yields, therebyincreasing the amount of residuesreturned to the soil. Where there is noyield response to fertilizer, there may beno increase in soil C.

Org

anic

C (g

/kg

soil)

0

5

10

15

25

20

20

15

10

5

01930 1940 1950 1960 1970 1980 1990

Cereal–forage rotation

Fallow_wheat

ManureFertilizerNo nutrients

the models and present their predictionsfor large areas with some conÞdence.

Examples of managementeffects on carbon cycle

Scientists have measured the effect ofmanagement on the C cycle at numeroussites across Canada. Rather than attemptto summarize all these, we offer a few asexamples of recent Þndings.

Crop rotation in forest soil

The Breton plots near Edmonton, Alta., areamong the longest-running research sitesin Canada. This experiment shows that anappropriate crop rotation, includinglegumes and cereal crops, can result inlarge increases in the C content of this soil,originally cleared from forest (Fig. 12). Bycomparison, soil under fallowÐwheatshowed no appreciable gains of C. Withineach crop rotation, soil receiving fertilizerhad higher gains of C than unfertilizedsoils, probably because of higher residueinputs with fertilization. Manureapplication increased soil C even morethan fertilizer, because the manure not

Get a feel for magnitudes

Multiplier Name Other name Abbreviation

100 gram g

103 grams kilogram kg

106 grams megagram tonne Mg

109 grams gigagram thousand tonnes Gg

1012 grams teragram million tonnes Tg

1015 grams petagram gigatonne or billion tonnes Pg

100 m ´ 100 m = 1 hectare (ha).1 ha = 2.5 acres.

17

Figure 13Soil C after 32 years of growing cornshowing the proportion derived from cornand that remaining from previous organicmatter. (E. Gregorich, AAFC)

Tillage

Historically, tillage was one of the main

tools available to farmers for controlling

weeds and preparing land for seeding.

But with new herbicides and seeding

equipment, intensive tillage is no longer

always necessary. Some farmers have

opted to eliminate tillage entirely, a

practice referred to as Òno-tillÓ farming

or Òdirect seeding.Ó This practice can

lead to substantial increases in soil C. A

partial survey of studies across Canada

shows no-till can increase soil C by as

much as 10 Mg (tonne) per hectare,

when compared with tilled soil (Table 2).

But such gains are not automatic. In

some cases, researchers were not able to

detect any effect of tillage on soil C. The

inconsistency of the results is not

surprising, because the response of soil

C is affected by climate, soil properties,

length of time under no-till, crop

rotation, and many other factors. Some

of the variability may simply reßect the

90

80

70

0Fertilized Unfertilized

Soil

C (M

g /h

a)

Original C

C derivedfrom corn

Soil management

Some of the many techniques used by farmers include¥ Conventional tillage: soils are routinely cultivated to eliminate weeds and prepare

soil for seeding¥ Reduced, minimum, or conservation tillage: tillage is reduced to keep residues on

the surface¥ No-till: seeds are planted directly without any prior tillage; weeds are controlled

by chemicals¥ Summer fallow: no seeding for one season; weeds are controlled by cultivation or

by chemicals.

DeÞnitions of tillage practices differ from region to region.

No-till can have several advantages. It requires less time and machinery. The organicresidues left on top of the soil help to preserve moisture and protect it against erosion.

Use of no-till* in Canada

Area under no-till(%)

1991 1996

Atlantic 2 2

Quebec 3 4

Ontario 4 15

Manitoba 5 8

Saskatchewan 10 20

Alberta 3 9

British Columbia 5 9

Canada 7 14

* No-till includes direct seeding into stubble or sod, or tillage of only the ridge of rows.

Table 2 Examples of the effects of no-till on soil C in selected long-term studies in Canada

Location Duration Cropping Soil C

(years) system gain/loss*

(Mg/ha)

Ontario 11 Corn -0.9

Ontario 18 CornÐsoybean 11.5

Saskatchewan 11 Wheat 1.8

Saskatchewan 11 FallowÐwheat 0.6

*C in no-till - C in tilled.Tilled treatments and depth of analysis vary among sites.(C. Campbell, AAFC; C. Drury, AAFC; T. Vyn, University of Guelph)

18

Summer fallow in Canada

The major development that allowed agriculture in the climatically restricted prairiesoccurred by accident. In the spring of 1885, the farm horses of Indian Head, Sask., wereconscripted for the army that was suppressing the Rebellion. By the time the horses werereleased, it was too late to plant. However, the land was worked during the summer andproduced an excellent crop the next year, while drought caused an almost complete cropfailure everywhere else. Experiments at the Dominion Experimental Station at IndianHead, established soon after, led to the system of summer fallowing being developed thatturned PalliserÕs Triangle into the bread-basket of Canada. (PalliserÕs Triangle is the drysouthwestern area of Alberta and Saskatchewan, named after this early explorer.) Withmodern methods of weed control, fertilization, and planting, summer fallow is no longeras essential as it once was.

Fields not cropped for a year stillrequire weed control, either mechanicalor chemical. The bare soil is directlyexposed to wind and sun, enhancingerosion and organic matterdecomposition. Without a crop, littleorganic residue is returned to the soil.Use of summer fallow depends on soilmoisture and expected crop income. It isexpected that summer fallowing willcontinue to decrease and stabilize atabout 4.5 million hectares by about 2050.

difÞculty of measuring soil C change

precisely.

Summer fallow

Summer fallow, the practice of leavingland unplanted for a whole year, wasonce widely practiced in western Canadabecause it helped control weeds,replenish soil moisture, and increaseavailable nutrients in the soil. The areaof fallow has declined recently but stilloccupies about 6 million hectares everyyear. Soils that are frequently undersummer fallow usually have lower Ccontent than those that are croppedannually. For example, long-term studiesin Saskatchewan show that, after severaldecades, soil cropped to wheat everyyear have C contents several tonnes perhectare higher than those that arefallowed every second year (Fig. 14).Fallow has two negative effects on soilC: it hastens decomposition of soil C,and it reduces C inputs into the soilduring the year when there is no crop.

Mill

ion

ha

Year

12

10

8

6

4

2

019761971 1981 1986 19961991

Area of summer fallowin Canada

19

Figure 14Organic C in surface soil of fertilizedfallowÐwheat (FW) and continuous wheat(W) in long-term sites in Saskatchewan. (C. Campbell, AAFC)

Grass on previously cultivatedland

One of the fastest ways to increase soil Cis to return cultivated land to vegetationlike that under ÒnativeÓ conditions. Astudy at Lethbridge, Alta., compared theC cycle in four treatments: nativegrasses, crested wheat grass (a common,introduced grass), continuous wheat(wheat planted annually), andfallowÐwheat (wheat planted only everysecond year). These plots were started onland that had been under fallowÐwheatfor many decades. Using the C budgetmethod described earlier, researchersshowed that the grass plots were gaininglarge amounts of C (Table 3). ThefallowÐwheat plots, on the other hand,were losing C whereas the continuouswheat plots were neither losing norgaining C.

Manure application to silage corn

Animal manure is widely used as anutrient source for crops. In a study atSt-Lambert, Que., regular manureapplication increased the amount of Cstored in the soil after 10 years

Org

anic

C (M

g /h

a)

8070

60

50

40

30

20

10

0Swift

Current(30 years)

IndianHead

(40 years)

Melfort

(30 years)

FW

W

(Table 4). Part of this increase camefrom the direct addition of C in themanure. This C represents a recycling ofthe C from plant materials used to feedand bed the animals. But the manure, byproviding plant nutrients and improvingsoil aggregation and porosity, alsoincreased crop growth and the amount ofC returned to the soil as residues. Thus,using manure not only results in efÞcientrecycling of plant C but also promotessoil C gains by increasing plantphotosynthesis.

These few examples, along withnumerous similar studies across Canada,show clearly that the choice of farmingpractice can affect the C cycle andinßuence the net exchange of CO2 fromfarms.

Table 3 Carbon balance on plots seeded to grass or wheat in Lethbridge

Crested Native Continuous WheatÐ

wheatgrass grasses wheat fallow

g/m2

Net primaryproduction 423 315 291 215

Harvested matter -101 -66 -76 -58

Carbon input inthe soils 322 249 215 157

Carbon loss from the soils (organic matter decay) -191 -196 -207 -178

Net soil carbon gain(loss) 131 53 8 -21

(B.H. Ellert, AAFC)

20

atmosphere. We must consider this CO2,which is part of the C cycle on farms, ifwe want to look at the overall effect ofagriculture on the atmosphere.

The main use of fuel on Canadian farmsis to power the machinery for tillage,planting, harvesting, and other Þeldoperations. Additional amounts are alsoused for transportation, irrigation, dryingof crops, heating of buildings, andequipment used in livestock operations.

Aside from that used directly on thefarms, agriculture also depends onenergy for the manufacture and transportof inputs. For example, manufacturingpesticides, buildings, and farmmachinery uses energy. But the largestoff-farm use of energy is for making andtransporting fertilizer, notably thatcontaining nitrogen. The resultingrelease of CO2 varies depending onfertilizer form (Table 5). But, onaverage, producing and transporting 1 kgof fertilizer N releases about 1 kg of C(or 3.7 kg CO2) into the atmosphere. In

Table 4 Carbon inputs, soil carbon storage, and soil physical properties of a silty clay loam in Quebec following 10 years of biennial applications of solid dairy cattle manure

Manure application C added by C added by Soil C storage Aggregate Porosity

rate manure crop size

(t/ha/2 y) (kg/ha/y) (kg/ha/y) (kg /ha) (mm) (%)

0 0 350 4969 1.3 51

20 870 380 6078 1.6 52

40 1740 430 6459 1.5 54

60 2610 480 7080 1.7 55

80 3480 530 7505 1.7 56

100 4350 600 7708 1.8 56

(A. NÕDayegamyie, MAPAQ, Qc and D. Angers, AAFC)

Table 5 C released as CO2 from manufacturing andtransporting fertilizers

Fertilizer kg C per kg ofnutrient (N,P,K)

Anhydrous ammonia 0.8

Urea 1.2

Ammonium nitrate 1.1

Ammonium sulfate 1.0

UreaÐammonium nitrate 1.1

Monoammonium phosphate (N + P) 1.2

Potassium (K2O) 0.2

(E. Coxworth, Saskatoon, Sask.)

Energy use

Most cropping systems depend onexternal energy sources. Much of thisenergy comes from the burning of fossilfuels, which releases CO2 into the

21

national estimates of emissions,researchers usually assign these indirectuses of energy to other sectors (e.g.,manufacturing). But they still relate tofarming and offer a means of reducingCO2 emissions from farms.

The rate of CO2 emission from energyuse on Canadian farmland varies widely,depending on how intensive the farmingoperation is. For example, farmsproducing livestock on grassland mayrequire relatively little external energy.By comparison, farms with high inputsof fertilizer, intensive tillage, andirrigation may generate high amounts ofCO2 from using energy.

A typical farming system on croplandmay release C from energy use at a rateof roughly 100 kg C per hectare per year.For example, an analysis of farmingsystems at Indian Head, Sask., showedthat the total C emission from direct andindirect use of energy ranged from about100 to 115 kg C per hectare per year,depending on tillage intensity (Fig. 15).The largest sources of this CO2 were themanufacture and transport of fertilizerand the on-farm use of fuel.

The net effect of a farming system onatmospheric CO2 is the increase in soil Cminus the amount of C released fromenergy use. Thus, a farm that emits CO2

from fuel use at a rate of 100 kg C perhectare per year will have a net beneÞton atmospheric CO2 only if the rate ofsoil C gain exceeds 100 kg C per hectareper year. For example, suppose a Þeldgains 4 Mg (tonnes) C per hectare overseveral decades in response to bettermanagement and that soil C thenstabilizes at that new, higher level. Thenet beneÞt to the atmosphere will be thedifference between the soil C gain andcumulative CO2 release from energy use(Fig. 16). If CO2 from energy use is 100

kg C per hectare per year, then the CO2

release would be equal to the soil C gainafter about 40 years. Thereafter, the Þeldwould again be a net emitter of CO2,unless some further soil C gains aremade.

Figure 15Sources of CO2 from spring wheat atIndian Head, Sask., as affected by tillage.(E. Coxworth, Saskatoon, Sask.)

Figure 16Conceptual illustration of soil C gain andcumulative CO2 from fossil fuels in anagroecosystem.

Soil

C

increase

0 10 20 30 40 50 60 700

1

2

3

4

5

Year

C (M

g/ha

)

CumulativeCO2 from fossil fuel

140

120

100

80

60

40

200

CO

2 re

leas

e (k

g C

/ha)

Conv. till Minimum till Zero till

Fuel

Machinery

P fertilizer

Herbicide

N fertilizer

22

Modeling soil carbon content

The site-speciÞc model Century makes use of simpliÞed relationships of thesoilÐplantÐclimate interactions to describe the dynamics of soil carbon and nitrogen ingrasslands, crops, forests, and savannas. It accounts for several agricultural managementpractices including planting, applying fertilizer, tilling, grazing, and adding organicmatter. It simulates above- and below-ground plant production as a function of soiltemperature and availability of water and nutrients. Century predictions of the change insoil carbon in Saskatchewan are shown for two cases: A) two soil types and a change from wheatÐfallow rotation to continuous barley

in 1930B) one soil type, Dark Brown Chernozem clay loam, but two different rotations

after 1930.

Century predictions for different soils and crop rotations

1910 1930 1950 1970

11000

10000

9000

8000

7000

6000

Soil

orga

nic

C (g

/m2 )

to

30 c

m

A

continuous barleywheat–fallow

Dark Brown Chernozem, ClayDark Gray Chernozem, Sandy Loam

Year

1990 1910 1930 1950 1970

Wheat_wheat_fallowWheat_fallow

Year

1990

B

(W. Smith, Ottawa, Ont.)

because soil properties and managementpractices vary across the country.Measuring the change directly wouldrequire enormous effort, so our estimatesrely on mathematical models.

In a recent study, a detailed model(ÒCenturyÓ) was used to predict changesin C content of Canadian agriculturalsoils, based on climate and soils datafrom across Canada. Information aboutfarming practices was taken from recentStatistics Canada data. The studyconsidered the predominant agriculturalsystems in Canada but did not include allpossible variations. Some of the factorsnot included were a) biomass burning, apractice no longer widely used; b) soilerosion, which moves C around thelandscape; c) manure addition; d) minorcrops such as potatoes and annuallegumes; and d) minimum tillage, whichis intermediate between ÒconventionalÓand no-till. Future analyses may includesome of these factors.

The model predictions agree withhistorical observations: soil C declinesrapidly after initial cultivation, but therate of decline diminishes gradually overtime as soils approach a new Òsteady-stateÓ at which they no longer lose C(Fig. 17). According to the model,current rates of C loss are negligible.The model predicts, further, thatagricultural soils will begin regainingsome of the lost C in the future, asfarmers adopt improved practices such asno-till and reduced summer fallow(Table 6). According to the model, theagricultural soils were losing C at a rateof about 3 Tg (million tonnes) of C peryear in 1970 but could be gaining C at arate of 0.4 Tg of C per year by 2010.Predicted rates of soil C change differamong regions, reßecting variableadoption of improved practices anddifferences in soil properties. For

Estimates of carbondioxide emissions inCanada

Scientists calculate the net emissions ofCO2 from Canadian agriculture byestimating the annual change in stored Cand adding CO2 release from fossil fuel(see Fig. 8). Most of the C stored inagroecosystems occurs in soil, so they canestimate the change in storage from thegain or loss of soil C.

Estimate of soil C change

Estimating soil C change for all theagricultural area of Canada is difÞcult,

23

example, the model suggests that rates ofgain are highest in Saskatchewan andlowest in Alberta.

Figure 17Long-term predictions of soil C changebased on the Century model, assumingonly a gradual adoption of no-till. (W. Smith, Ottawa, Ont.)

All these predicted rates of change arelow compared to the total amount ofstored C. For example, a C gain of 0.4Tg/y amounts to a rate of <0.01 Mg(tonnes) of C per hectare per year, whenaveraged across all cultivated soils inCanada. This value is very smallcompared to the total C content of soils,which is commonly about 60Ð100 Mg(tonnes) C per hectare.

The Century model predictions representour current best estimates of soil Cchange across the country. But theseestimates rely on several simplifyingassumptions and have not yet been fullytested for all conditions across Canada.For example, compared to some actualdata on the change in soil carbon underno-till, the predicted changes appear to

5000

4500

4000

3500

3000

2500

2000

1500

1000

500

1910 1930 1950 1970 1990 2010

Soil

orga

nic

C (T

g) t

o 30

cm

Year

0

Rate of change of carbon in Canadian agricultural soils, 1990

AgricultureÕs largest store of carbon is in its soils, where dead plants have accumulatedover the centuries. Cultivating the soil, however, has greatly affected this store of carbon,reducing it by about 15Ð35%. Agriculture and Agri-Food CanadaÕs research programconÞrmed that, in many cases, farmers have been able to reduce or even reverse the Closs with good management.

The Century model (a site-speciÞc computer simulation of the dynamics of soil organicmatter) was used to estimate the rate of change of carbon in Canadian soils for the year1990. Soil, crop coverage, tillage, and crop rotation data were obtained for 1229 soillandscape of Canada polygons. Century runs were carried out on 15% of the polygons.For each sampled polygon Century was run for one to Þve types of crop rotations underconventional tillage. It was also run for no-till practices for polygons for which no-tillrepresented 5% or more of the agricultural area.The map shows carbon change in agricultural soils on the Prairies during 1990. Theestimated average carbon loss corresponds to about 40 kg/ha/y, which is much smallerthan the amount that can be measured.

Rate of change in soil carbon in 1990 (t/ha)

LETHBRIDGEREGINA

WINNIPEG

SASKATOON

CALGARY

S A S K A T C H E W A NA L B E R T A M A N I T O B A

Lake Winnipeg

0.040 0.000 -0.040 -0.080 >-0.160-0.160-0.120

Black

Brown

Dark Brown

Dark Brown

(W. Smith, Ottawa, Ont.)

24

be low by as much as 50%. With furtherresearch and as the reliability of themodels improves, the estimates may beadjusted.

Emissions from the use of fossil fuel

The other major source of CO2 inagriculture, aside from the biological Ccycle, is burning of fossil fuel. Directfuel use on Canadian farms releasesabout 10 Tg (million tonnes) of CO2

annually (Table 7). Indirect sources,associated with the production ortransport of inputs, emit additional CO2.Of these, manufacture and transport offertilizer is the most important.Emissions from this source haveincreased steadily because of increasedrates of fertilizer application. Themanufacture of farm machinery,construction of buildings, and generationof electricity also emit large amounts ofCO2. Altogether, CO2 emissions fromindirect sources amounted to about 16Tg (million tonnes) of CO2 in 1996.

Direct and indirect use of fossil fuels onCanadian farms, therefore, amounted toabout 26 Tg (million tonnes) of CO2 (7Tg C) in 1996. In calculating nationalinventories, however, researchers countonly the CO2 produced from stationarycombustion (about 3 Tg CO2 in 1996) inestimates for agriculture; the remainderthey include in emissions frommanufacturing, construction, andtransportation sectors.

Total emissions

Total emissions of CO2 from Canadianagricultural activity are the sum of netsoil C loss, emissions from direct use offossil fuel, and emissions from indirectuses of fossil fuel (Table 8). Theseestimates suggest that, in 1996,agricultural activity released about 28 Tgof CO2 into the atmosphere, slightly less

Table 6 Soil organic C change in Canadian crop lands1 as estimated using the Century model

1970 1981 1986 1991 1996

Average C change (kg/ha/y) -67 -51 -48 -35 -11

Total C change (Tg/y) -2.7 -2.1 -2.0 -1.4 -0.5

1 Pastures are not included.2 Since 1910, there has been a 24% reduction of soil organic C (1053 Tg C)

in cultivated soils. Total C in Þrst metre of agricultural soils in Canada is 10 000 Tg.(W. Smith, Ottawa, Ont.)

Table 7 Estimated CO2 emissions from fossil fuel use in Canadian agriculture

1981 1986 1991 1996

(Tg CO2)

Direct useFuel used on farm 9.5 7.7 8.1 9.5

Indirect uses

Fertilizer manufacture,transport & application 4.4 5.5 5.1 6.6

Machinery manufacture& repair 4.8 4.8 4.8 4.8

Building construction (steel& cement manufacture) 2.5 2.2 2.3 2.2

Pesticide manufacture 0.2 0.3 0.3 0.3

Electricity generation 1.8 1.9 2.1 2.4

Total indirect fossil CO2 13.7 14.7 14.6 16.3

(R.L. Desjardins, AAFC; E. Coxworth, Saskatoon, Sask.)

25

than in 1981. Projections to the year2010 suggest that total emissions willnot change appreciably from those in1996. Scientists predict that emissionsfrom soils are likely to decline andbecome negative (that is, soils will gainC) but, at the same time, emissions fromindirect sources may increase, offsettingthese beneÞts. These estimates, however,assume a Òbusiness-as-usualÓ scenarioand do not yet take into account anybeneÞts that might occur from concertedefforts to reduce emissions.

MethaneMethane is perhaps most familiar to usas the main component of natural gas.Though present in the atmosphere atvery low concentrations (about 2 ppmv),it is a comparatively powerfulgreenhouse gas: one kilogram of CH4

has 21 times the warming effect of thesame amount of CO2, when calculatedover a 100-year period. This effect arisesnot only from the CH4 itself but alsofrom other indirect effects, including theCO2 to which it eventually converts.

The concentration of CH4 in theatmosphere, which had been increasingat a rate of 1.1%, is now increasing atabout 0.6% per year. Globally,agriculture is a prominent source of CH4,accounting for about two-thirds ofhuman-induced emissions.

Most of the CH4 emitted fromagriculture is produced by the microbialbreakdown of plant material. Normally,when oxygen supply is adequate, most ofthe C in decomposing plant materialconverts to CO2. But, in the absence ofoxygen, decomposition is incompleteand C is released as CH4 instead. Inagricultural systems, such conditionsoccur in the digestive system of ruminant

livestock (e.g., cattle) and in water-logged soils (e.g., rice paddies).Incomplete burning of fuel or organicwastes also produces small amounts ofCH4. Methane and CO2, therefore, aresomewhat complementary: C notconverted to CH4 is largely released as CO2.

The CH4 emitted into the atmosphere hasa lifetime of, on average, about 12 years.Chemical reactions in the atmosphereconvert most CH4 to CO2.Microorganisms living in the soil convertprobably less than 10% of CH4 releasedinto the atmosphere to CO2.

Table 8 Estimated CO2 emissions from Canadian agriculture from direct and indirect sources

1981 1986 1991 1996

(Tg CO2)

Direct emissions

Soils 7.7 7.3 5.1 1.8

Fuel used on farm 9.5 7.7 8.1 9.5

Total direct emissions 17.2 15.0 13.2 11.3

Indirect emissions 13.7 14.7 14.6 16.3

Total emissions attributableto agriculture 30.9 29.7 27.8 27.6

(R.L. Desjardins, AAFC)

26

Methane emission bylivestock

All animals produce CH4 when theydigest feed. But emission is especiallyhigh from cattle, sheep, goats, and otherruminants. These animals have a rumen,or Òfore-stomach,Ó where microbialfermentation partially digests feed.Because of this process, ruminants canefÞciently digest Þbrous feeds. But, sincethe fermentation occurs under restrictedoxygen supply, some C in the feed, oftenabout 5Ð10%, is released as CH4 ratherthan as CO2 (Fig. 18). Nonruminantanimals, such as pigs and poultry, alsoemit some CH4 during digestion, but theamounts released are almost negligibleby comparison (Table 9).

Figure 18CO2 and CH4 ßow in a livestock-basedagroecosystem.

Measuring methane emission

We can measure the amount of CH4

emitted by livestock in a number ofways. One method is to place the animalin an enclosed chamber and measureCH4 accumulating in the airspace. Thisapproach permits accurate analysis, butestimates may be distorted because theanimal is removed from its normalenvironment. Recently, therefore,

CH4

CO2

Soil organic mattermicrobes

Table 9 Estimated CH4 emissions from livestock and manure in 1991

Number of Mass of Methane Methane from Total animals manure from manure livestock methane

(Millions) (Tg) (Gg) (Gg) (Gg)

Dairy cattle 2 17 70 190 260

Beef cattle 11 98 10 558 568

Pigs 10 19 102 15 117

Poultry 103 3 8 N/A 8

Sheep/lambs 1 0.4 0.2 8 8

Total livestock 127 137 190 771 961

(R.L. Desjardins, AAFC)

27

Emissions from dairy cows

A complete barn, with a handling system for liquid manure, was instrumented to monitorCO2 and CH4 emissions from 118 dairy cows and their manure. Experiments such as thishelp to determine the amount of greenhouse gases emitted from cattle. The rate at whichfeed energy is converted to CH4 is based on the quantity and quality of feed. Dairy cowsemit much more CH4 per year than other cattle.

researchers have measured CH4 emissionfrom cattle in their natural setting. Theymeasured the CH4 concentration in airemitted from vents in a dairy barn andcalculated the emission from all cows inthe barn, including the manure theyproduced. Using this approach, theywere able to estimate not only theaverage rate of CH4 production peranimal (about 550 litres per cow per day)but also the daily and seasonalßuctuations in emission rates (Figs. 19and 20). For example, highest emissionsusually occurred immediately after eachfeeding.

Figure 19Diurnal pattern of CO2 and CH4 emittedby dairy cows. (H. Jackson and R.Kinsman, AAFC)

Figure 20Average monthly emission of CH4 from adairy barn in Ottawa. (H. Jackson and R.Kinsman, AAFC)

Lit

res/

cow

/day

1000

800

600

400

200

01993 1994 1995

680

580

480

6800

5800

4800

0:00 06:00 12:00 18:00 24:00Time of day

CH

4 (l

itre

s/co

w/d

ay)

CO

2 (l

itre

s/co

w/j

day)

CO2

CH4

Cows unknowingly participating in an experiment to measure greenhouse gas at the Central Experimental Farm in Ottawa, Ont.

(H. Jackson and R. Kinsman, AAFC)

Measuring CH4 from cattle on pasturesposes more difÞcult problems. Butresearchers now have a new technique,based on the use of a chemical marker,to measure directly CH4 emission fromgrazing animals. This method, used in agrazing study in Manitoba, showed thatemission rates were about 0.7 litre perkilogram body weight per day (0.5 gCH4 per kilogram body weight per day).

Factors affecting methaneemission

Many factors inßuence the rate of CH4

emission from ruminants. They arereasonably well known because CH4 lossrepresents incomplete use of feed energy.

28

forage plant, degree of chopping orgrinding, the amount of grain in the diet,and the addition of oils. For example,CH4 emission may be lower fromlegume rather than grass forage, fromensiled rather than dried feeds, and fromhighly concentrated rather than high-roughage diets.

Another important factor is the amount offeed intake. When intake of feed isincreased above maintenance levels, theamount of CH4 emitted per animalincreases, but the efÞciency of feed usagealso increases. Consequently, CH4

emission per unit of product (e.g., milk orbeef) is usually reduced at higher levelsof feed intake. For this reason, it is oftenbetter to assess CH4 emission per unit ofproduct rather than per animal or unit offeed.

For animals on pasture, the CH4

production may be affected by thegrazing regime. In a Manitoba study,halving the number of beef cattle perhectare increased CH4 emission peranimal but reduced the emission perhectare. Overall, CH4 emission perkilogram of weight gain (about 150 gCH4 per kilogram of gain) wasunaffected by grazing practice.

The animal itselfÑits breed, weight, rateof growth, and whether it is producingmilkÑaffects CH4 emission. Theenvironment may also affect CH4

emission. For example, some researchsuggests that emissions may increase atlower temperatures. Because of the largenumber of factors that inßuence CH4

release from livestock, it may be possibleto reduce emissions by changingmanagement practices.

As much as 15% of the gross energy infeed may be lost through CH4 emission.As a result, researchers studied thefactors affecting CH4 emission longbefore the environmental concerns aboutCH4 became prominent.

One important factor affecting the rate ofCH4 emission is the quality of the feed.In general, diets that increase the rate ofdigestion reduce CH4 emissions, becausethe feed does not stay in the rumen aslong. Thus, several characteristics of thefeed can affect CH4 emission: theamount of roughage in the diet,preservation method, growth stage of

Measuring methane emissions from grazing animals

Scientists can measure CH4 produced from grazing cattle by using sulfur hexaßuoride(SF6) as a tracer gas. Capsules that gradually release SF6 at a constant rate are placed inthe animals rumen. Then, by comparing the ratio of the concentrations of CH4 and SF6expired by the animal, the researchers can calculate the CH4 produced.

Steer equipped to measure CH4 production using a tracer gas.

(P. McCaughey, AAFC)

29

Estimates of methane emissionfrom livestock

Direct emission of CH4 from Canadianfarm animals can be estimated bymultiplying the number of animals by anaverage emission rate per animal. In1991, direct emission of CH4 fromCanadian farm animals was about 771Gg (thousand tonnes) (see Table 9). Ofthis, beef cattle accounted for 72% anddairy cattle for 25%. By comparison,direct emissions from other livestockwere almost negligible.

Emission of methane frommanure

Methane is emitted not only from theanimals themselves but also from the Cthey excrete (see Fig. 18). Manure, likeother organic materials, is decomposedby microorganisms. If the decompositionoccurs under well-aerated conditions,most of the C is released as CO2. Whenoxygen is deÞcient, however, a lot ofCH4 may be produced instead.

The ratio of CO2 to CH4 produceddepends on how the manure is managed.Much of the CH4 from manure isproduced during storage. When manureis stockpiled, inadequate aeration insidethe pile may lead to CH4 production.Even higher amounts of CH4 may bereleased from manure stored in liquidform because of limited aeration. Thuspig manure, commonly stored as a slurry,may emit high amounts of CH4. Oncemanure is applied to the land, it produceslittle additional CH4 because of adequateexposure to air.

Using estimates of manure produced andCH4 emission rates, it is possible toestimate the amount of CH4 emittedfrom manure in Canada (see Table 9). According to this

calculation, emission from manureaccounts for about 20% of the total CH4

emitted by livestock (manure + directemission). In particular, these estimatespoint to pig manure as an importantsource of CH4, both because of largenumbers of animals and because of theway the manure is stored.

Methane emission andabsorption by soils

Soils can either release CH4 or absorb it,depending largely on moisture content.When organic materials decompose insubmerged or water-laden soils, thewater reduces the oxygen supply causingthe release of large amounts of CH4.Globally, for example, rice paddies arean important source of atmospheric CH4.In the agricultural soils of Canada,however, CH4 emission is probablyconÞned to localized wetland areas andperhaps to brief periods when low-lyingsoils are submerged during snowmelt orafter high precipitation. Most soils haveenough aeration that they do not produceCH4; in fact, microorganisms in the soilsconvert CH4 to CO2 so that the soilsÒabsorbÓ CH4. The amount absorbeddepends to some extent on managementpractices. For example, CH4 absorptionis usually higher under grassland than intilled soils and is suppressed by applyingN fertilizers.

Although CH4 absorption by soils is animportant mechanism in the global CH4

cycle, the amounts absorbed byCanadian agricultural soils are probablysmall compared to total emissions fromfarms (Table 10). Researchers estimatenet absorption of CH4 by agriculturalsoils in Canada to be about 12 Gg(thousand tonnes) per year. Even largeincreases in amount of CH4 absorptionby soils would offset only a small

30

CH4 was emitted from Canadian farmsin 1996. Of this amount, about 80%came directly from livestock, theremainder from livestock manure.

Changes in emissions from year to yearreßect differences in livestock numbers,which ßuctuate depending on costs offeeds, market prices for the products, andexport markets. If livestock numbersincrease as expected, CH4 emissionsmay further increase unless farmersadopt new methods that reduceemissions per animal.

Nitrous oxideNitrous oxide is familiar to us as ananesthetic. It occurs naturally in theatmosphere at very low concentrations(about 0.3 ppmv), but the concentrationis now increasing at a rate of about 0.3%per year. Much of this increase comesfrom agriculture, which accounts for upto 70% of the N2O emissions fromhuman activity.

The increase poses two potential threats.First, N2O is a potent greenhouse gaswith a long lifetime in the atmosphere(about 120 years). Its warming potentialis about 310 times that of CO2 over 100years. Second, N2O released iseventually converted in the upperatmosphere to nitric oxide (NO), a gasthat breaks down O3. Ozone in the upperatmosphere Þlters out UV radiation fromthe sun, so its depletion results in higherdoses of harmful UV radiation reachingthe earthÕs surface. Higher N2O levels,therefore, not only contribute to thegreenhouse effect but may also increaseindirectly the intensity of UV radiation.

Most N2O from agriculture is producedin the soil. To understand the origins ofthe N2O and the factors that affect itsemission, it is helpful to review theoverall N cycle on farms.

proportion of current emissions fromlivestock and manure.

Other sources of methane

Fossil fuels used in agriculture releasesmall amounts of CH4 by volatilizationand combustion. This emission amountsto about 1 Gg (thousand tonnes) of CH4

per year (see Table 10). Some CH4 isemitted from the burning of cropresidues, but amounts are small and willdiminish further because this practice isbecoming obsolete.

Estimates of net emissionfrom all sources

Virtually all the CH4 emission onCanadian farms is from livestock (seeTable 10). According to currentestimates, about 1 Tg (million tonnes) of

Table 10 Estimated total CH4 emissions

1981 1986 1991 1996

Livestock 849 748 771 879

Manure 208 192 190 208

Soils -12 -12 -12 -12

Fuels 1 1 1 1

Total (Gg CH4) 1046 929 951 1076

Total (Tg CO2 equivalents) 22 20 20 23

(R.L. Desjardins, AAFC)

31

Nitrogen cycle

In terrestrial ecosystems, there are threemain pools of NÑsoil, plants, andatmosphere (Fig. 21). The largest ofthese is the atmosphere; in the column ofair above a hectare of land there areabout 76 million kg of N, roughly amillion times the amount that plants onthat hectare use in a year. Virtually allthis N, however, occurs as N2, a gas thatis almost inert and not directly availableto plants.

Figure 21Conceptual N cycle in an agroecosystem.

Despite living in a sea of gaseous N,plants obtain most of the N they needthrough their roots, by absorbing nitrate(NO3

-) and ammonium (NH4+)

dissolved in soil water. When the plantslater die, the N in the plant litter isreturned to the soil, where it becomespart of the soil organic matter. Soilmicroorganisms, in turn, graduallydecompose this organic matter, releasingNH4

+, which may be further convertedto NO3

-. These forms are then availableagain for plant uptake, completing thecycle. In ÒnaturalÓ systems, this cyclebetween soil and plants can continuealmost indeÞnitely, with only very smallinputs of N from the air via lightning orspecialized soil bacteria.

Manure

Legumes

Fertilizer N2N2O

NO

NH3

NO3-

NH4+

In farmlands the N cycle is morecomplicated, as grain and other productsremove large amounts of N from theÞeld. In fact, cropping systems are oftendesigned speciÞcally to maximize theamount of N (as protein) in the plantparts that farmers harvest and remove. Inhigh-yielding wheat, for example,harvesting the grain removes more than100 kg N per hectare from the Þeldevery year. Consequently, to continue thecycle and to maintain crop growth,inputs from outside must replace the lostN.

The main source of new N is the air.There are two ways of converting theotherwise inert N2 into a form availableto plants. One is the industrial approach,which uses energy from fossil fuel toconvert N2 into ÒchemicalÓ fertilizer.The other is a biological approach,which uses legumes such as alfalfa,clover, beans, and peas to ÒÞxÓ N2.These crops have nodules on their roots,containing bacteria that convert N2 intoplant-available form. The plants absorbthis N and, when they die anddecompose, release it back into the soilas NH4

+.

The N from fertilizers and legumes hasallowed large increases in foodproduction, but, if they are to feed thegrowing population, producers will needeven larger amounts of N. Already, theglobal additions of N from these sourcesexceed inputs from ÒnaturalÓ sources(mainly Þxation by lightning andbacteria not associated with agriculturalcrops). Although this injection of Nsustains food production, it exertspressure on the N cycle and often resultsin losses or ÒleaksÓ of N into theenvironment (see Fig. 21). Largeportions of applied NÑas much as 50%in extreme casesÑmay leach into thegroundwater. As well, N enters the air in

32

Similarly, most of the N fertilizers usedin Canada contain N as NH4

+, or in aform (like urea), which converts toNH4

+ soon after application. Most of theN applied to soil, therefore, passesthrough the nitriÞcation process.

During nitriÞcation, most of the N isreleased as nitrate (NO3

-), but a smallproportion of the N (usually less than1%) may be emitted as N2O (Table 11):

In general, the more NH4+ applied, the

more nitriÞcation occurs, and the greateris the potential for N2O release. But theproportion of N released as N2O is notÞxed; under conditions of good aerationand high NH4

+, for example, less of theN will appear as N2O than when oxygenor NH4

+ concentrations are low. As aresult, the amount of N2O released fromnitriÞcation may not correspond directlyto the amount of N entering the process.

DenitriÞcation

When movement of oxygen into soil isrestricted, nitrate (NO3

-) can beconverted into nitrogen gas (N2) in theprocess called denitriÞcation. Deprivedof oxygen in air, some bacteria use NO3

-

instead, thereby releasing N2. As fornitriÞcation, however, a small proportionof the denitriÞed NO3

- may be releasedas N2O:

NO3– N2

N2O

Organic N

Fertilizer NH4+ NO3

-

N2O

various gaseous forms: ammonia (NH3),nitric oxide (NO), N2, and N2O. Most ofthese ÒleaksÓ occur from the pool ofplant-available N (NH4

+, NO3-).

Consequently, losses are highest whenproducers add these forms in amountsgreater than the plants can use or at atime when plants are not growing.

Nitrous oxide formation

Nitrous oxide can originate from twoplaces in the N cycle: during nitriÞcation(converting NH4

+ to NO3-), and during

denitriÞcation (converting NO3- to

gaseous N2). Both processes are carriedout by bacteria living in the soil.

NitriÞcation

Most N enters the soil either as NH4+ or

in a form that converts to NH4+. For

example, the N in crop residues occurslargely in organic forms (like protein)which, when decomposed, release NH4

+.

Table 11 Estimates of proportion of N released as N2O from various fertilizers as estimated in laboratory studies

Synthetic fertilizer Amount of N fertilizerevolved as N2O

(%)

Urea 0.3

Ammonium sulfate ((NH4)2SO4) 0.1

Ammonium nitrate (NH4NO3) 0.3

Anhydrous ammonia 1.6

Nitrogen solution 0.3

Calcium nitrate (Ca(NO3)2) 0.2

(Adapted from a review by E.G. Beauchamp and G.W. Thurtell, University of Guelph)

33

Three main factors control the rate ofdenitriÞcation: the supply of oxygen, theconcentration of NO3

-, and the amountof available C (used by bacteria as anenergy source). Highest rates ofdenitriÞcation occur when all threefactors are present: low oxygen, highNO3

-, and high available C. The absenceof any one of these three may reducedenitriÞcation to negligible rates.Because it occurs only in the absence ofoxygen, denitriÞcation is most intense inwater-logged soils. Some denitriÞcationmay also occur inside the root nodules oflegumes.

The amount of N2O release, however,depends not only on the rate ofdenitriÞcation but also on the ratio ofN2O to N2 produced. This ratio is highlyvariable and tends to be lower underconditions favoring high rates ofdenitriÞcation.

Often, we think only of thedenitriÞcation that occurs on farm Þelds.But N that is lost from the soil may alsoconvert to N2 or N2O. For example, theNO3

- that leaches from the soileventually Þnds its way into thegroundwater or into sediments ofstreams and lakes. Once there it canundergo denitriÞcation. Consequently,the amount of N2O produced from farmpractices may be much higher than thatwhich is emitted directly from the soil.

Of the two processes, denitriÞcation isprobably more important thannitriÞcation as a source of N2O inCanadian farms. Emissions of N2O fromdenitriÞcation may be several timeshigher than those from nitriÞcation, butit is difÞcult to distinguish between thetwo sources, and their relativeimportance varies widely from place toplace.

Management practicesaffecting nitrous oxideemission

Because of larger N inputs and disruptedN cycling, agricultural soils often havehigher rates of N2O emission thancomparable soils under ÒnaturalÓvegetation. For example, a fertilizedbarley Þeld near Quebec City had N2Oemissions as high as 7 kg N per hectareper year, compared to negligible amounts(0.04 kg N per hectare) in a nearbyforest soil. But the rate of N2O emissionis highly sensitive to conditions in thesoil; under many conditions there may beno emission; in others there may be largebursts of N2O. By their effects on soilconditions, therefore, farming practicescan greatly affect N2O emission.

34

Fertilizer consumption in Canada

From 1930 to 1960, the world production of nitrogen, phosphate, and potash was aboutequal. Since 1960, the use of all three nutrients has greatly increased but that of nitrogenfertilizers has increased faster than that of phosphate and potash. Canada uses about 2%of the world fertilizers.

Much like the global trend, there has been a large increase in fertilizer use in Canadasince the 1960s. Most of this increase is in nitrogen fertilizer and occurred in the Prairies.In eastern Canada, fertilizer usage has stabilized or even decreased in the last decade.Compared to other developed countries, Canada has a low rate of fertilizer use perhectare.

Fertilizer consumption in Canada from 1966 to 1996

Ter

agra

ms

(mill

ion

tonn

es)

2.5

2.0

3.0

1.5

1.0

0.5

0.070 75 80 85 90 95

K P N

Form of fertilizer applied

In Canada, producers use a variety ofcommercial fertilizers to supplement soilN (see Table 11). Of these, urea andanhydrous ammonia (pressurizedammonia gas) are the most common,together accounting for almost 75% ofthe N applied. Most forms include Neither as NH4

+ or in a form that quicklychanges to NH4

+ after application. Forexample, anhydrous ammonia becomesNH4

+ immediately upon reacting withwater in the soil, and urea is convertedby soil enzymes to NH4

+ and CO2

within days of being applied. As a result,most of the N in fertilizers passesthrough the nitriÞcation process(conversion to NO3

-) with the potentialfor some to be lost as N2O.

During their initial reactions, fertilizersmay affect pH, soluble C content, andother properties of soil in theirimmediate vicinity. These effects varywith fertilizer form so that N2Oformation during nitriÞcation may varyamong fertilizers. Indeed, some researchsuggests that there may be largedifferences in N2O emission amongfertilizer forms. Highest emissions mayoccur from anhydrous ammonia, andlowest from calcium nitrate, presumablybecause the N in the latter does notundergo nitriÞcation.

Nitrous oxide emissions from variousfertilizer formulations were compared ina study at Elora, Ont. Equivalentamounts of N were applied to turfgrassin one of several forms: ammoniumnitrate (NH4NO3), urea (CO(NH2)2), andslow-release urea. There was little N2Oemission from the slow-release urea,probably because its gradual N releasecoincided with plant N uptake,preventing the accumulation of NH4

+ orNO3

-. The other two sources showedsigniÞcant N2O emission, with slightlyhigher values from ammonium nitratethan from urea.

The physical form and placement offertilizers may also inßuence N2Oemissions. For example, results of alaboratory study suggest that emissionsmay be higher from large granules thanfrom Þne particles mixed into the soil.The Þner fertilizer is more widelydispersed in the soil and, presumably,has less effect on the pH immediatelynext to individual particles. Bandingfertilizer, similarly, concentrates the N in

35

Selecting the fertilizer

Selecting a fertilizer is a question of convenience and cost.Convenience factors include the following: ¥ concentration of nutrient¥ machinery, training, and maintenance requirements¥ safety¥ ease of transportation and application¥ secondary effect on soil acidity¥ possibility of combining with other operations (irrigation,

spraying, seeding).

Economic factors include the following:¥ cost relative to other formulations¥ value of the crop¥ efÞciency of use by crop.

Nitrogen requirements of crops

Nitrogen is the nutrient needed most to ensure growth of nonleguminous crops, such as corn or wheat. Although leguminous crops, such asalfalfa and soybeans, derive some nitrogen from the soil, most comes from biological Þxation. Other sources of nitrogen include syntheticfertilizers and manure. Residues of alfalfa following ploughing or chemical burndown may also supply the succeeding crop with signiÞcantquantities of nitrogen.

The optimum rate of application for fertilizer or manure depends on the cropÕs need for added nitrogen, the anticipated yield, and theavailability of nitrogen from previous manureapplication or leguminous crop residue. Soilsdiffer signiÞcantly in their ability to furnishnitrogen to crops. Although data on historicalresponse to nitrogen are generally used topredict the amount of nitrogen required, soiltests can also be used.

Yields of nonleguminous crops may beincreased by as much as 50% by addingmanure or nitrogen fertilizer. But the amountshould not exceed that which will return themost proÞt. Maximum proÞt usually occurs atabout 95% of maximum yield. When applied atthe rate for maximum proÞt, the nitrogen willbe used efÞciently, yet as economically aspossible.

(E. Beauchamp, University of Guelph,

Guelph, Ont.)

Nitrous oxide emissions measured at Elora (using a tower-based ßux-measuring system) from a corn Þeld. Bursts of N2O emissions occur just

after spring thaw and following fertilizer application.

(C. Wagner-Riddle and G. Thurtell, University of Guelph, Guelph, Ont.)

36

localized areas and may therefore alsoaffect N2O emission.

Although these and other data suggestthat how a fertilizer is formulated andwhere it is placed may affect N2Oemission, this effect has not yet been fullydeÞned. Because N2O emissions alsodepend on other factors such as rate ofapplication, soil properties, timing ofprecipitation, and crop rotation, the effectof fertilizer formulation may not alwaysbe the same.

Manure management

Of the N consumed by livestock in feed,as much as 78% is excreted in urine andfeces. In 1 year, for example, a dairy cow

may excrete as much as 100 kg N ormore. Consequently, animal manurecontains large amounts of N; in Canada,the N excreted each year by livestockmay approach the amount of N applied asfertilizer.

Some N in manures is lost to theatmosphere as NH3, either immediately orduring storage, but most is returned to theland. The N content of manures variesdepending on animal, rations, andbedding material but is typically about2% of dry weight. This N occurs largelyin two forms: NH4

+ and organic N. Theformer is immediately available to plantsand behaves in the soil like NH4

+ fromfertilizer. The organic N, however, actsmore like a slow-release form, graduallybeing converted to NH4

+ by the action ofsoil microorganisms.

The N applied in manure is susceptible toloss as N2O. Because a large part of the Noccurs as NH4

+, some N2O may beformed during nitriÞcation to NO3

-.DenitriÞcation may produce much higheramounts, because manure is a source notonly of N but also of available C.Applying high concentrations of N andavailable C together favors denitriÞcation.In extreme cases, where soils havereceived excessive rates of manure formany years in succession, N2O emissionsmay be as high as 50 kg N per hectare peryear, though emissions are usually muchlower.

The amount of N2O emitted frommanured soils depends on method andrate of application, type of manure, andsoil properties. One study suggests thatliquid manure applied in bands mayproduce more N2O than manure applieduniformly on the soil surface. Placing themanure in bands concentrates the N andC, creating conditions more favorable fordenitriÞcation.

Flux

of N

2O (n

g/m

2 /s)

200

100

-100

0

0 60 90 120 150 18030

Thaw

Calendar day

37

Manure management may also haveindirect effects on N2O emission. A largeportion of N excreted from livestock, asmuch as 50%, may be released into theatmosphere as ammonia (NH3) gas. ThisNH3 is eventually deposited onto soil orwater, where it reverts to NH4

+ and canbe lost as N2O like N applied directly.

Crop residue input and soilmanagement

Crop residues (e.g., straw, roots) andother plant materials return much Nannually to the soil. In many cases, thisN is merely a recycling of N absorbedearlier from the soil. But legumes, whichcan capture N2 from the air, can actuallyadd new N to the soil. Sometimes cropsgrown solely for the purpose ofcapturing N are ploughed back into thesoil as Ògreen manures.Ó

The amount of N2O produced fromadded plant materials depends on the rateof N release. Some residues, such aswheat straw and corn stover, have a lowN concentration, commonly less than0.5%. When these materials decompose,they release little N; in fact, sometimesthey even result in the withdrawal ofNH4

+ or NO3- from the soil because the

microbes need extra N to decompose theresidue. In contrast, N-rich materialssuch as legume residues or greenmanures can quickly release largeamounts of NH4

+ (later converted toNO3

-) during decomposition. Likeanimal manure, these materials alsoprovide a ready source of available C,favoring the release of N2O fromdenitriÞcation. For example, alfalfaresidues may release 2Ð4 kg N2O-N perhectare and soybean residues 0.3Ð2 kgN2O-N per hectare per year.

The way in which farmers manage cropresidues may also inßuence N2O

Injecting liquid manure

Injecting liquid manure into the soil prevents rapid loss of nitrogen compounds into theair and minimizes release of unpleasant odors. If the soil is loosened-up at the same time,deep soil Þssures will be broken, and the liquid will not drain directly into the drainagetiles. Because manure tankers are very heavy and will compact moist soil, such as occursin early spring, it is often difÞcult to Þnd appropriate times to apply liquid manure.

emission. Tillage may be the mostimportant tool for managing residues.Normally, tillage mixes crop residuesinto the soil, but in no-till or otherÒminimum tillageÓ systems the residuesremain on the soil, alteringdecomposition patterns. Some studiessuggest that no-till techniques mayincrease N2O emission; others concludethat no-till can reduce emissions (Table12). How tillage affects N2O emission, itseems, depends on soil, cropping system,climate, and other factors. Aside fromtheir effect on residue placement, tillagepractices also inßuence soil moisture,temperature, and aeration, all of whichaffect N2O production.

Soils, even without recent additions of

38

will be minimal. Such ideal synchrony,however, rarely occurs. Often NH4

+, andparticularly NO3

-, accumulate in excessof the plantsÕ capacity to absorb them,resulting in high potential for N loss vialeaching or denitriÞcation. This situationis especially true if the NO3

-

accumulates after harvest, because thenit is vulnerable over the fall, winter, and,especially, the following spring, whendenitriÞcation is particularly intense.Consequently, matching the amount andtime of N application with plant Nuptake pattern is an importantmanagement tool to minimize N2Oemissions.

Nature of nitrous oxideemission

Nitrous oxide emissions are usuallysporadic. Unlike CO2, which is releasedfrom soil almost continuously, N2O isoften emitted in bursts or Òßushes.ÓUnder Canadian conditions, the mostimportant of these ßushes may occur inearly spring, as the snow melts. At a sitein central Alberta, for example, most ofthe N2O emitted in the entire yearoccurred during 10 days at the end ofMarch (Fig. 22). These bursts of N2Oemission at snowmelt may reßectfavorable conditions for denitriÞcationand N2O formation: high moisturecontent (oxygen deÞciency), adequateNO3

- and available C, and favorabletemperature. Or the N2O ßush mayreßect the abrupt release of N2O thatwas previously trapped under a layer offrozen soil or ice. Although the springßush is often the largest, additionalbursts of N2O follow heavy rains thatresult in water-logging of soils,especially in low-lying areas. As well,N2O may erupt immediately afterfertilizer is applied because of thesudden availability of N.

residues or other N, can emit N2O fromtheir decomposing organic matter.Organic soils, because of their richorganic N reserves, may releaseparticularly high amounts of N2OÑabout 5 kg N per hectare per year.Similarly, soils that are left unplanted fora year (a practice known as summerfallow) may emit signiÞcant amounts ofN2O. Soil microbes gradually breakdown the organic N in these soils intoNH4

+ and NO3-, and because there are

no growing plants to remove this N, itaccumulates and is highly susceptible toloss via denitriÞcation.

Amount and timing of nitrogenapplication

Often, N2O emission is assumed to bedirectly proportional to the amount of Napplied. But a better measure may be theamount unused by the crop. Matchingthe NH4

+ or NO3- released into the soil

precisely to their uptake by plantsprevents these N forms fromaccumulating in the soil, and N2O losses

Table 12 Comparison of N2O emissions in central Alberta as affected by tillage

1993Ð94 1994Ð95

N (kg/ha)

Till / with fertilizer 1.7 2.5

Till / no fertilizer 0.6 2.4

No-till / with fertilizer 1.7 0.9

No-till / no fertilizer 0.6 0.4

(R. Lemke, University of Alberta)

39

Emission of N2O is sporadic not onlyover time but also across space. Thisvariability stems, in part, from thedifferences in N and moisture (henceoxygen) content across the landscape. Atany time, there may be minimal releaseof N2O from most areas in a Þeld, buthigh emissions from small Òhot spotsÓwhere conditions are ideal for N2Oproduction.

Figure 22Seasonal pattern of precipitation and N2Oemissions from a fertilized wheat Þeld atEllerslie, Alta., 1993. (R. Lemke, Universityof Alberta)

A further complication is that much of theN2O is often produced in deeper soillayers. The release of this N depends onits rate of diffusion to the soil surface,which is controlled by soil porosity andthe presence of ice or water at the surface.The trapped N2O may also be dissolved insoil water or be further converted to N2 orto NO3

- by microbes, so that the N2Oformed at depth is not all released to theatmosphere. Consequently, N2O emissionfrom soils depends not only on how fast itforms but also on how fast it diffuses orconverts to other N forms.

Calendar day

N2O

em

issi

on(µ

g N

/m2 /

h)P

rec.

(mm

)

82 126 146 166 186 206 226

300200100

0

200

Nitrous oxide emissions in the winter

Eastern Canada, where soils can be covered with snow for up to 5 months, has arelatively short growing season. We once thought that N2O emissions during winter wereminor and of little importance in the annual N-budget. But we now know that signiÞcantlosses of nitrogen as N2O occur from under the snow cover. In certain cases, soils releasesubstantial amounts of N2O during the winter. FreezeÐthaw cycles also affect the N2Oemissions from soils. These cycles induce physical and biological changes to the soil;they disrupt soil structure and stimulate denitriÞcation leading to more N2O production.

(E. van Bochove, AAFC)

Until recently, we thought little N2Owould form over winter because of lowsoil temperatures. But this idea may nothold true where snow insulates the soil.In parts of eastern Canada, for example,snow blankets the soil thickly for up to 5months per year, keeping soiltemperature above or near freezing. As aresult, N2O can be produced all winterand be released through the porous snow.At a site near Quebec City, a fertilizedbarley Þeld, ploughed the previous fall,released up to 5 kg N per hectare duringthe winter and spring, equivalent to5Ð10% of the fertilizer N applied. The

40

direct emissions from livestockproduction, and indirect emissions fromfarms.

Direct emissions from soils include N2Oderived from fertilizer, land-appliedmanure, legumes, and crop residues.Researchers calculated emissions from thetotal N content of these sources, based onnational statistics, assuming that aspeciÞed proportion of the N was releasedas N2O (about 1%, depending on source).They also included estimates of N2Orelease from organic soils, though theseamounts are small. Based on thiscalculation, they estimated directemissions of N2O from agricultural soilsin Canada in 1996 to be 70 Gg (thousandtonnes) of N2O (Table 13). Whenaveraged over the area of cultivated landin Canada, this amount equates to about 1kg N per hectare per year. The estimatedemission rates, however, vary widelyamong regions (Fig. 23 a,b).

The scientists calculated direct emissionsfrom livestock by estimating the amountof N in manure and assuming that aspeciÞed portion of that N was emitted asN2O. They assumed the fraction of Nconverted to N2O to be 2% for grazedanimals and 0.1Ð2% for other livestock,depending on waste management. Usingthis approach, they estimated directemissions from livestock to be 24.5 Gg(thousand tonnes) of N2O in 1996 (see Table 13).

They also calculated indirect emissionsfrom estimates of atmospheric N (e.g.,NH3) deposited on the soil, N leachedfrom farm Þelds, and N produced fromhuman sewage. According to thesecalculations, leached N is the mostimportant, accounting for more than 80%of the roughly 38 Gg (thousand tonnes) ofN2O released from indirect sources in1996 (see Table 13). This estimate

same Þeld released only 2 kg N duringthe growing season.

Because of the sporadic andunpredictable pattern of N2O release,estimating amounts of emission isdifÞcult. Hence, current estimates ofN2O emission are probably less reliablethan those for the other greenhousegases.

Estimates of nationalnitrous oxide emission

Given our limited understanding of N2Oformation and release, we can estimateonly tentatively N2O emissions fromCanadian farms. Current estimates rely onsimple equations, developed by theInternational Panel on Climate Change(IPCC), that calculate N2O release fromthree sources: direct emissions from soils,

Table 13 Estimates of direct and indirect sources of N2O emissions from Canadian agriculture in 1996

Province Direct Direct Indirect Total N2O

emissions emissions emissions emissions

from soils from manure

(Gg N2O)

Atlantic 0.9 0.5 1.0 2.4

Quebec 5.7 2.4 4.0 12.1

Ontario 13.1 3.7 6.0 22.8

Manitoba 10.6 2.3 5.7 18.6

Sask. 19.1 4.5 9.2 32.8

Alberta 18.4 9.6 10.8 38.8

B.C. 1.9 1.5 1.5 4.9

Canada 69.7 24.5 38.2 132

(R.L. Desjardins, AAFC)

41

assumed that 30% of the N applied asfertilizer or manure leached into thegroundwater.

Based on the IPCC approach, totalemissions of N2O from agriculture inCanada in 1996 were about 132 Gg(thousand tonnes) of N2O (see Table 13).Of this, direct emissions from soilsaccounted for about half.

The trend in N2O emissions over timemay be as important as the total amount.Current estimates suggest that N2Oemissions have increased steadily since1981, increasing by 20% from 1991 to1996 alone. Much of the increaseresulted from higher N inputs asfertilizers and animal manure. Withincreases in livestock numbers andhigher crop yields expected in the future,N2O emissions may climb still furtherunless producers make improvements inN management.

Figure 23aEstimated direct N2O emissions from agricultural sources in western Canada for 1991.

Kg N2O per square kilometre

42

Figure 23bEstimated direct N2O emissions from agricultural sources in eastern Canada in 1991.

Combined effect ofthe three greenhousegasesThe three gases (CO2, CH4, and N2O)differ in their warming effects. Tocompare their relative effects, therefore,their emissions are usually expressed asÒCO2 equivalents.Ó One kilogram ofN2O has the warming effect of about 310kg of CO2 (when considered over 100years), so it represents 310 CO2

equivalents. Similarly, 1 kg of CH4

represents 21 CO2 equivalents.

According to best estimates, using theapproaches described for each gas,Canadian agriculture had emissions of67 Tg (million tonnes) of CO2

equivalents in 1996 (Table 14). Of this

Kg N2O per square kilometre

43

Global warming potential

Global warming potentials (GWPs) are a simple way to compare the potency of variousgreenhouse gases. They help policy makers compare the effects of reducing CO2emissions relative to another greenhouse gas for a speciÞc time horizon. For example, asmall reduction in N2O can be just as if not more effective than a larger reduction in CO2emissions.

The heat-trapping potential of a gas depends not only on its capacity to absorb and re-emit radiation but also on how long the effect lasts. Gas molecules gradually dissociate orreact with other atmospheric compounds to form new molecules, with different radiativeproperties. For example, CH4 has an average residence time of about 12 years, N2O 120years, and CO2 200 years. Over a 20-year period, CH4 has 56 times the radiative impactof CO2. However, as time proceeds some of the CH4 molecules are broken down intoCO2 and H2O. Therefore, over a 100-year period, CH4 has a global warming potential of21 times that of CO2.

Global warming potentials are presented for 20-, 100-, and 500-year time horizons. In TheHealth of Our Air, we use the 100-year GWPs recommended by IPCC. Calculations ofwarming potential are continually reÞned, so these numbers are subject to revision asunderstanding improves.

amount, about two-thirds was as N2Oand about one-third as CH4. Bycomparison, net emissions as CO2 werealmost negligible.

The estimates of CO2 emission,however, exclude most of the CO2 fromfossil fuels used to produce inputs,power farm machinery, and transportproducts. These sources, which areincluded in inventories for transport andmanufacturing sectors, emitted about 25Tg (million tonnes) of CO2 in 1996.

The emission of greenhouse gases fromCanadian agriculture are increasing,according to current estimates (see Table14). By 2010, emissions may be about9% higher than those in 1996, unlessproducers adopt better managementpractices. These projected increases stemlargely from predicted increases inlivestock numbers and N inputs asfertilizer and manure. Emissions of CO2

are expected to decline, but not nearlyfast enough to compensate for predictedincreases in the other gases.

Future emissions will depend on changesin farming practices that are hard topredict. Livestock numbers, crops thatare grown, fertilization patterns, andmanure management techniques can allchange quickly, throwing off our currentbest projections.

Uncertainties incurrent estimates

Current estimates of greenhouse gasemission are not without uncertainty. Weface many possible pitfalls in calculatingemissions for ecosystems as extensiveand diverse as CanadaÕs farmlands. Westill do not even understand all theprocesses that affect emissions. And sowe admit that each estimate is subject topotential error. Of the three gases, N2Ohas the highest degree of uncertainty

Relative global warming potential(CO2 equivalents per unit mass of gas)

Time horizon

Gas 20 y 100 y 500 y

CO2 1 1 1

CH4 56 21 6.5

N2O 280 310 170

(Fig. 24). Estimates for this gas could beoff by 50% or more. Despite theiruncertainty, these values are the Þrstcomprehensive estimates of greenhousegas emission from Canadian agricultureand provide a reference point forshowing trends.

Though valuable as a Þrst approximation,the estimates will likely change as welearn more. Ongoing research will teachus more about the processes leading to

44

emission and allow us to build bettermodels. As well, new techniques thatsimultaneously measure all three gasesover large areas will allow us to evaluatebetter the modelsÕ reliability. We cantherefore expect more deÞnitive estimatesin the future, but we need not wait fortheir arrival before trying to reduce actualemissions.

Figure 24Estimates of CO2, CH4, and N2O emissionsin CO2 equivalents from Canadianagriculture, showing relative uncertaintyfor each gas.

CO2 CH4 N2O

Em

issi

ons

(Tg

CO

2)

40

20

0

-20

60

Agri-environmental indicators

An agri-environmental indicator is a measure of change, or the risk of change, inenvironmental resources used or affected by agriculture. Although the indicators arenational in scope, regional variations are taken into account. Six indicators are beingdeveloped, each of which has several components as follows:

(T. McRae, AAFC)

Farm resource management:tracks farmersÕ use of environmentallysustainable management practices, bymeasuring soil residue cover andmanagement of agricultural land,fertilizers, pesticides, and manure.

Soil degradation risk:measures progress in reducing thevulnerability of agricultural soils todegradation and identiÞes soils still athigh risk of erosion, salinization,compaction, or loss of organic matter.

Water contamination risk:assesses progress in reducing the riskof water contamination by nutrientsused in agriculture and identiÞes theareas at risk of contamination.

Agroecosystem greenhouse gasbalance:estimates trends in the net emission ofCO2, N2O, and CH4.

Agroecosystem biodiversity change:monitors biodiversity in agriculturalecosystems by measuring changes inhabitat availability.

Input use efÞciency:measures the efÞciency of fertilizers,energy, and irrigation water used byfarmers to track possible effects on theenvironment.

Table 14 Estimates of total greenhouse gas emissions from CanadaÕs agroecosystems

1981 1986 1991 1996 2000* 2005* 2010*

(Tg CO2 equivalents)

CO2 9 7 5 3 1 0 0

CH4 22 20 20 23 23 24 25

N2O 32 33 34 41 43 45 48

Total 63 60 59 67 67 69 73

* Predicted using a scenario of medium growth from Canadian Regional Agricultural Model (CRAM) to 2007. All 2010 data follow a best-Þt trend using datafrom 1993 to 2007 from the CRAM. All fertilizer data were predicted using a best-Þt trend from 1981, 1986, 1991, and 1996 Census data. All sheep,chicken, and turkey populations were predicted using a best-Þt trend from Census data.

(R.L. Desjardins, AAFC)

45

Techniques tominimize emission ofgreenhouse gasesAgriculture is a net emitter ofgreenhouse gases. Furthermore, currentpredictions point to increased emissionsunless some changes are made tofarming practices. Fortunately, farmerscan adopt several measures to reduceemissions. Some of these would beexpensive, but some can be used withlittle cost or even at higher proÞt.Widespread use of such practices couldreduce emissions of all three greenhousegases and, for CO2, even make farms netabsorbers.

Reducing carbondioxide emissionsFarming means managing carbon. Onevery hectare of farmland, many tonnes(Mg) of C are removed from the airevery year and changed to organicmaterials through photosynthesis (seeFigs. 8, 9). At the same time,decomposing organic matter and theburning of fossil fuels releases roughlyequivalent amounts of CO2 back into theair. By their choice of farming practices,farmers can manage this cycle, altering itto reduce net emissions of CO2.

There are two main ways of reducingemissions: one is to increase the amountof C stored in soil; the other is to burnless fuel. Several practices are alreadyavailable to achieve each of these.

Greenhouse gas emissions from agriculture

Agriculture contributed about 10% ofCanadian anthropogenic greenhouse gasemissions in 1996. Using the globalwarming potentials, the major sources ofemissions of all the gases were convertedinto CO2 equivalents. From agriculture,the major sources are manure, entericfermentation, crops, and fertilizers.

(R.L. Desjardins, AAFC)

Population of hogs in Canada

Future greenhouse gas emissions depend on economic trend. For example, the CanadianRegional Agricultural Model (CRAM) estimates that, by 2010, the hog population inCanada will be about 14 million. This population is an increase of 36% over that recordedin 1990. This increase may cause N2O and CH4 emissions from manure to greatlyincrease. Hogs produce the second highest amount of manure per 1000 kg of live animalper day, which is equivalent to 10 kg CH4 per animal per year. Therefore, the CH4emissions from hog manure, in 2010, is expected to be about 143 Gg (thousand tonnes) ofCH4, which is 25 Gg higher than 1996 emissions from hogs.

Tg

CO

2 eq

uiva

lent

s

20

10

15

0

5

Entericfermentation

Other

Manure

Fertilizers

Crops

46

Conservation tillage

Conservation tillage prior to potato planting in Prince Edward Island. Minimum tillage incorn in Ontario and wheat in Saskatchewan.

Measuring greenhouse gases over farms

Scientists are now looking at ways ofmeasuring greenhouse gas emissions fromentire farms. One way is to use instrumentsmounted on a tethered balloon, Þlled withhelium, to measure changes in greenhousegases over time at various heights above thefarm.

(E. Pattey, AAFC)

Increasing soil carbon

In soils that have been managed in thesame way for many years, the C contentis reasonably constant. A change inmanagement, however, can result inlosses or gains of C. To increase soil C,we can do one of two things: increasethe amount of C added to the soil, orslow the rate at which soil C isdecomposed (decayed) back to CO2 (seeFigs. 8, 9).

Adding organic matter

Atmospheric CO2 enters the soil by wayof photosynthesis. This process trapsCO2 in organic forms, a portion of whichis added to the soil as residues (includingroots). The only direct way to increase Cadditions, therefore, is to use practicesthat favor higher photosynthesis; in otherwords, practices that increase plant yield.Farmers can achieve such increases byusing higher yielding crops and varieties,by improving crop nutrition (usingfertilizers and manures), or by reducingwater stress (by irrigation, waterconservation, or drainage). Actions thatimprove soil quality also promote higheryields. Perhaps most important is to usecropping systems that keep activelygrowing (and photosynthesizing) plantson the land as long as possible.

But increased photosynthesis helps buildsoil C only if at least some of theadditional trapped C is returned to thesoil. The more of the plant removedfrom the Þeld as grain or other products,the less the increase in soil C. Thus,using cropping practices that keep allresidues in the Þeld and planting crops(like forage grasses) that store much oftheir C in roots can achieve soil C gains.Often, animals help recycle the C backinto soil. In many livestock-basedsystems, a large part of the plant yield isreturned to the soil as manure, and only

47

a small portion is actually exported fromthe Þeld or pasture.

Reducing decay rate

The other way to build soil C is to slowthe rate of organic matter decay in thesoil. One method of doing that is tomake conditions less favorable for soilmicrobes. For example, residues on thesoil surface keep soils cooler, slowingdecay. Similarly, maintaining growingplants on the surface as long as possibleslows decay, because plants dry out thesoil and cool it by shading.

Decay rate can also be slowed byshielding the organic matter from soilmicrobes. Soils are usually granulated,with organic materials protected insidethe granules (or aggregates). Breakingthese aggregates open by intensivetillage exposes that organic matter to soilmicrobes. As a result, practices thatminimize soil disturbance tend topreserve soil C.

Another way to shield organic materialsis to place them where conditions are notfavorable for decay. For example, theycan be kept on the surface where theytend to stay dry, or placed deep in theproÞle, where soil is cool (although thisapproach would require intensivetillage).

Practices that increase soilcarbon

Much can now be done to promote soilC gain, either by adding more C orslowing decay (or both). The followingmethods are often effective, though theamount of C gain depends on climateand soil type:

Reduce tillage : Tillage was oncenecessary to control weeds and preparesoil for planting. But now weeds can becontrolled with herbicides, and new

seeding equipment can place seedsdirectly into untilled soil. As a result,intensive tillage is no longer necessary,and a growing number of farmers haveeliminated tillage entirely, using no-tillor Òdirect-seedingÓ practices. Thesepractices protect C inside aggregates andkeep residues on the surface where theydecay more slowly and cool the soilbeneath them. No-till and other ÒreducedtillageÓ practices also prevent erosion,thereby preserving soil quality andmaintaining future photosynthesis. No-till, already used on about 14% ofcropland in 1996, could be adopted on alarge proportion of CanadaÕs cropland.

Apply more nutrients : Where soils donot have enough nutrients, addingfertilizers, animal manure, or greenmanure increases yields, leading tohigher inputs of C. Manures may alsoimprove the physical condition or ÒtilthÓof the soil, further increasing yields andresidue additions.

Grow more perennial forage crops :Perennial crops can trap more CO2 thanannual crops because they continuegrowing for more months of the year.Because they dry out the soil more andthere is no tillage, decay rates may alsobe slower. Many perennial crops, likegrasses, have extensive root systems thatplace much C below-ground.

Remove land permanently fromcultivation : Probably the most effectiveway of increasing soil C is to allow theland to revert to its original vegetation,whether grasses or trees. Because thereis little or no removal of C in products,virtually all the C trapped byphotosynthesis is returned to the soil orstored in the wood. In theory, such Òset-asideÓ lands would eventually regain allthe C lost since cultivation began.However, this option means a loss in

48

Trees on agricultural land as a carbon reservoir

Farmers have long planted trees as shelterbelts and for other environmental reasons. SinceÒafforestationÓÑthe practice of planting trees on previously untreed landÑis explicitlyrecognized as a legitimate carbon offset under the Kyoto protocol, we need to know howmuch C can be stored in such trees and at what rate.

Prairie Farm Rehabilitation Administration (PFRA) Shelterbelt Centre at Indian Head,Sask., has determined the quantities and rates of C stored in prairie shelterbelts. Typicalshelterbelt trees contained from 162 to 544 kg C, with poplar trees having the most. Shrubshelterbelts contained as much as 52 tonnes C per kilometre.

(J. Kort, AAFC)

productivity so it is probably onlyfeasible on marginal lands. The practicemay also be applicable in small areas ofcultivated land planted to shelterbelts orgrassed waterways for control of windand water erosion.

Eliminate summer fallow : Leaving landunplanted for a growing season (summerfallow) helps control weeds andreplenish soil moisture. But it results insoil C loss because, during the fallowyear, no new residue is added and thesoil remains warm and moist, whichhastens decay. A shift to continuous

cropping (growing a crop every year)therefore favors increases in soil C. Thearea of summer fallow has declined inrecent years, but it still occupies about 6million hectares every year. Eliminatingsummer fallow may not be practical invery dry regions, such as parts of thesouthern prairies.

Use cover crops : Where the growingseason is long enough, a winter covercrop can be sown after the main crop hasbeen harvested. This practice can addmore residues to the soil and preventerosion.

Avoid burning of residues : Whenresidues are burned, almost all their C isreturned to the atmosphere as CO2,which reduces the amount of C added tothe soil.

Use higher yielding crops or varieties :Crops or crop varieties that have moreefÞcient photosynthesis often producemore residues, which increases soil C.But because plant breeders choosevarieties based on marketable yield (e.g.,grain yield), residue and root yields ofnew varieties may not increase as muchas the yield of harvested product.

Improve water management : Water isoften the limiting factor to crop growth.In the dry southern prairies, yields canbe increased by irrigating or by trappingand storing water more effectively (e.g.,using crop residue or windbreaks to trapsnow). In parts of central and easternCanada, conversley, crop growth may belimited by excess water in poorly drainedsoils. In these conditions crop growthand C additions to soil can be increasedby drainage.

Restore wetlands : Some low-lying areasin farmlands have been drained to allowcrops to grow. Re-submerging these soilswould cut off oxygen supply, preventing

49

decay. These restored wetlands orÒsloughsÓ could gain a lot of C quickly,though the small area of potentialwetlands would limit CO2 removal.

Integrate livestock into cropping systems :Feeding crops to livestock results ineffective recycling of C if the manure ismanaged well. Thus, although largeamounts of C may be removed from theÞeld as forage or silage, much of this Ccan be eventually returned as manure.The manure also promotes crop growthand photosynthesis, favoring further soilC inputs.

Improve grazing management : The waya grassland is grazed can affect the Ccycle in several ways. It inßuences theproportion of the plant ÒharvestedÓ bythe animal, the redistribution of C inmanure, the condition of the soil (viahoof action), and the speciescomposition. Because of these manyeffects, the relationship between soil Cand grazing regime is still unclear.Overgrazing, however, can result in largelosses of C via erosion. Reducing thenumber of animals per hectare on suchlands will likely increase the amount ofC stored.

Many studies across Canada have shownthat these practices can increase soil C.The amount of potential gain, however,is still unclear and will vary dependingon the initial soil C content, soilproperties, climate, and other factors.The extent to which farmers adopt thesepractices also inßuences the amount of Cgain. That will depend on crop prices,costs of production, and other factorsthat ßuctuate from year to year.

Despite the uncertainty, some estimatessuggest that agricultural soils in Canadacould gain as much as several million

tonnes of C per year if these C-conserving practices were widelyadopted. Such a gain would result in anet removal of CO2 from the atmosphere.With time, however, the rate of C gainwould decline because it becomes harderto add additional C as the C content ofsoil goes up.

50

Storing carbon in strawboard

Strawboard made from cropresidues can store C and may helpmitigate greenhouse gasemissions. At the end of theirlifetime, the boards could beburned in power plants, replacingfossil fuel, resulting in a truereduction in CO2 emissions.

One example is the industrialIsobord plant in Elie, Man. Itexpects to use 180 000 tonnes ofwheat straw per year, which isequivalent to sequestering 267 000 tonnes of CO2 per year.The plant at Elie has already sold80% of its Þrst 5 yearsÕproduction.

Storing carbon in plantmaterial

Although the soil is the main storehouseof C in farm ecosystems, plant materialcan store additional C. One way to storemore plant C is to grow trees onfarmland, either as shelterbelts (whichalso control erosion) or as woodlotsalongside farmsteads. The net beneÞt ofthis practice for atmospheric CO2

depends on the area of land devoted totrees, their rate of growth, and the fate ofthe wood. If the wood is burned, forexample, there is little long-term beneÞtunless its use reduces dependence onother fuels.

Another way of storing plant C is toconvert crop residues into products witha long lifetime. One approach is toconstruct Þberboard (wood-like panels)from cereal straws. These materials are

used for construction and, whereas muchof the C in straw returned to soil wouldnormally decay back to CO2, the C inthese construction materials remainstrapped for a long time.

Reducing fossil fuel use

Farms rely on energy from fossil fuels topower machinery, heat buildings, dryharvested crops, and transport goods.Energy is also used to supply materialsemployed on the farm, such as fertilizers,pesticides, machinery, and buildings.Most of these emissions are notattributed to agriculture in the nationalinventory of greenhouse gases. Even so,using less fuel on farms would reduceCanadaÕs total CO2 emissions.

The amount of fuel used on the farm andin the supply of farm inputs can bereduced in the following ways:

Reduce tillage : It takes a lot of energyto lift and turn soil during tillage.Reducing or stopping tillage can,therefore, save on fossil fuel use. OneOntario study showed diesel fuel usereduced from 30 litres per hectare forconventional tillage to only 4 litres perhectare in a modiÞed no-till system. Astudy on the Prairies, which consideredboth direct and indirect use of fuel,showed that reducing tillage decreasedemissions from direct fuel use by about40% (see Fig. 15). Emissions forpesticide inputs were slightly higherunder reduced tillage and emissions fromfertilizer were unchanged. When all thedirect and indirect factors were counted,emissions from no-till were 92% of thosein conventional tillage, and emissionsfrom minimum tillage were intermediate.

51

Use fertilizer more efÞciently : Makingand transporting fertilizer is energy-intensive. For each kilogram of fertilizerN used, about 1 kg of C is released intothe atmosphere as CO2. Consequently,methods of applying fertilizer thatproduce high yields from less fertilizercan reduce CO2 emissions. Possibleapproaches include placing fertilizer moreeffectively (e.g., banding); applying onlyas much as is needed, based on soil tests;and using variable rates of application ona Þeld to reßect differences in soil fertility(Òprecision farmingÓ).

Grow legumes : Legumes can often getmuch of the N they need from the air.When they die and decompose, they alsorelease N into the soil. Careful use oflegumes in cropping systems, therefore,can reduce the amount of N fertilizerneeded, and thereby lower CO2

emissions. For example, in a study atMelfort, Sask., introducing pea into thecrop rotation reduced CO2 emissionsfrom fossil fuel by about 28% (Table 15).

Use manure more efÞciently : Animalmanure contains many nutrients. Thesenutrients, however, are not always usedefÞciently, in part because of the highcost of transporting the heavy, bulkymanures. Avoiding excessive applicationrates of manure in localized areas wouldnot only prevent harmful loss ofnutrients to the environment but alsosave on fertilizer use, thereby reducingCO2 emissions.

Increase energy use efÞciency :Additional opportunities for reducingenergy use include drying crops in theÞeld wherever possible, using moreefÞcient irrigation systems, andinsulating farm buildings. As well, manyof the energy conservation measuresadvocated for urban areas also apply tothe farm.

An entirely different way of reducingemissions from fossil fuels is to growcrops that provide an alternate energysource. This ÒbiofuelÓ can displace fossiluse, thereby indirectly reducing CO2

emission. Instead of extracting C fromdeep within the earth and burning it toCO2, biofuel production simply recyclesthe C originally removed from theatmosphere by photosynthesis.

The most efÞcient way of using cropmaterials for fuel is to burn themdirectly. Although this approach is usedin some parts of the world, it is notalways practical in Canada, where thefuel may have to be transported greatdistances.

An alternative is to ferment crops,producing ethanol and mixing it, atproportions of about 10%, with gasoline.This mixture can be used in mostgasoline engines and reduces the amountof CO2 produced from fossil fuel. Thenet savings in fossil fuel use, however,depend on the amount of fuel used togrow the crop in the Þrst place.

The materials most easily converted intoethanol are those with high starchcontent. Thus cereal grains, such as cornand wheat, are preferred for ethanolproduction. One study suggests that, ifthe CO2 emitted in crop production are

Table 15 Impact of planting a legume on C emissions in aSaskatchewan cropping system

Rotation CO2 emissions

(legume or no legume) (kg C/ha/y)

PeaÐBarleyÐWheat 82

BarleyÐBarleyÐWheat 114

(E. Coxworth, Saskatoon, Sask.)

52

taken into account, use of corn-ethanolreduces CO2 emissions by about 40%,relative to the emissions from thegasoline it replaces. If the emissions ofother greenhouse gases are also takeninto account, then use of ethanol fromcorn or wheat reduces the globalwarming potential by 25Ð30%. InCanada, about 30 million litres ofethanol are currently produced annuallyfrom wheat and corn, reducing CO2

emissions by about 33 Gg (thousand

tonnes) per year. If Canadian ethanolproduction reaches the expected 265million litres by the end of 1999,reductions in net CO2 emission will beincreased by the same proportion.

Though ethanol is most easily madefrom high-starch materials, new methodsnow also make it possible to makeethanol from Þbrous matter, such as cropresidues, forages, and crop wastes. Anexcess of about 2 Tg (million tonnes) ofstraw and chaff may be produced everyyear, beyond the amount needed foranimal bedding and preventing soilerosion. If all this amount were used, itwould produce about 500 million litresof ethanol and replace about 0.5 Tg(million tonnes) of fossil fuel CO2

(equivalent to 2% of the emissions fromfossil fuel used in agriculture). Theprocess could also be used to produceethanol from perennial grasses grown on marginal lands.

Still another way to reduce reliance onfossil fuel is to produce fuel for dieselengines (ÒbiodieselÓ) from oilseed cropssuch as canola, ßax, soybean, andsunßower. Although technically feasible,producing biodiesel is still much moreexpensive than producing fossil fuel.

Ethanol as a fuel

In 1997, Canadians used about 40 million litres of ethanol and 34 billion litres ofgasoline, so ethanol represents about 0.1% of total gasoline sales in Canada. Ethanol has alower energy content than gasoline. But when carefully blended at less than 10%, mileageis not affected.

Ethanol is a liquid alcohol produced by fermenting either starch materials (corn, wheat,barley) or cellulosics (agricultural residues, wood, wood wastes). Much of the CO2released when biomass is converted to ethanol and burned in car engines is recapturedwhen new vegetation is grown, thus offsetting the greenhouse gas effect. Net lifecycleCO2 emissions from burning 10% ethanol-blended gasoline have shown about 3%reduction when compared to regular unleaded gasoline. Recent developments in theethanol industry are expected to increase Canadian production from wheat and corn toabout 350 million litres by 2000.

(M. Stumborg, AAFC)

53

Current status of methodsto reduce carbon dioxideemissions

The C cycle is central to farmingsystems. Methods to reduce CO2

emission rely mainly on managing thatcycle more efÞciently: recycling as muchorganic C as possible, minimizingdisruption of soil, optimizing use of thesunÕs energy (via photosynthesis), andrelying less on off-farm energy.

Because they promote efÞciency, manyof these methods also help sustain landresources and may even be proÞtable. Asa result, they are being adopted forreasons quite apart from their beneÞts toatmospheric CO2. For example, mostfarms in Canada now use less tillage thana generation ago, and an increasingproportion now use no-till practices.Similarly, the area of land devoted tosummer fallow has fallen from about 11million hectares in 1971 to about 6million hectares in 1996. The use of theseand other C-conserving approaches will likelycontinue to increase in coming decades.

The two general approachesÑstoringmore C and relying less on fossil fuelÑreduce CO2 emissions over somewhatdifferent periods. Storing C in soils hashighest beneÞts early, in the Þrst fewyears or decades, but net removal of CO2

declines with time because it gets harderand harder to add additional C as soil Caccumulates. Carbon dioxide savingsfrom reduced fossil fuel, on the otherhand, may seem rather small in the shortterm but can be signiÞcant when viewedover many decades. The net removal ofatmospheric CO2 from soil C gains isÞnite; that from reduced fossil fuel cancontinue indeÞnitely.

Reducing methaneemissionsMethane, like CO2, is part of the C cyclein farm ecosystems. It is released duringdecay of organic material when ashortage of oxygen prevents organic Cfrom being completely converted to CO2.Although both CH4 and CO2 aregreenhouse gases, CH4 has a muchhigher warming potential, so release of Cas CO2 is preferred.

Most CH4 from CanadaÕs farms comesfrom the livestock industry, eitherdirectly from the animals or from themanure they produce. A number ofmethods have been proposed to reduceemissions from these sources, some ofwhich are already in use.

Reducing methaneemissions from animals

Much of the CH4 produced on farms isfrom ruminantsÑlivestock such as cattleand sheep that have a rumen fordigestion of feed. SpeciÞc practices thatcan reduce emissions from these animalsinclude the following:

Change rations to reduce digestion time:Most CH4 is released from the rumen,where feed is fermented in the absenceof oxygen. The longer the feed remainsin the rumen, the more C is converted toCH4. As a result, any practice that speedsthe passage of feed through the rumenwill reduce CH4 production. One studywith steers showed that, when scientistsincreased the passage rate of matterthrough the rumen by 63%, CH4

emission fell by 29%. The passage offeed through the rumen can be hastenedby

¥ using easily digestible feeds grains,legumes, and silage

54

Effect of feed additives on methane emissions from dairy cows

Scientists at AAFC measured CH4 emissions from dairy cows in a barn over 3 years withan automatic gas sampling system. In one trial, 95 to 100 cows were fed a total mixedration (TMR) consisting of concentrate and ensiled forage (35:65 on a dry matter basis)and produced an average of 26.8 kg of milk per cow per day. After a control period,monensin was added to the ration. There was an immediate decrease in CH4 emissions. Atthe same time milk production increased and daily feed consumption decreased,indicating an increased efÞciency in feed usage. The effects of feeding monensin lasted 2months after it was removed from the ration. There was, however, indication that rumenbacteria became resistant to monensin when a second feeding trial was conducted 5months later. The use of monensin in dairy feeds is under consideration by regulatoryauthorities but has not yet been approved. This feed additive has been used in beef cattlesince 1975. These results show that feeding additives can signiÞcantly decrease CH4emissions by dairy cows. However, further work is needed to resolve rumen microbialresistance and to develop a rotational system of feed additives to overcome thispossibility.

(H. Jackson and F. Sauer, AAFC)

1000

900

800

700

600

500

400

300 0

10

20

30

40

50

Met

hane

em

issi

ons

(L/c

ow/d

)

0 10 20 30 40 50 60 70

Day of experiment

Milk

pro

duct

ion

and

feed

con

sum

ptio

n (k

g/co

w/d

)Endtreatment

Begintreatment

Milk

Feed

Methane

¥ harvesting forages at an earlier,more succulent growth stage

¥ chopping the feed to increasesurface area

¥ minimizing use of Þbrous grassesand hays

¥ feeding concentrated supplements asrequired.

Add edible oils : Adding canola,coconut, or other oils to the diet mayreduce CH4 production by inhibiting theactivity of CH4-producing bacteria.Though quite effective, this practice maynot always be economical.

Use ionophores : Ionophores are feedadditives that inhibit the formation ofCH4 by rumen bacteria. Already widelyused in beef production, they can reduceCH4 emission. However, some evidencesuggests that rumen microbes can adaptto a given ionophore, lessening its effectover time. For long-term effectiveness, itmay be necessary to use a rotation ofdifferent ionophores.

Alter the type of bacteria in the rumen :In the future it may be possible tointroduce into the rumen geneticallymodiÞed bacteria that produce less CH4.Though research efforts are promising,such inoculants are not yet commerciallyavailable.

Improve production efÞciency : Anypractice that increases the productivityper animal will reduce CH4 emissionsbecause fewer animals are needed toachieve the same output. For example,giving animals more feed may increaseCH4 production per animal but reducethe amount of CH4 emitted per litre ofmilk or per kilogram of beef. Any otherpractice that promotes efÞciency willlikewise reduce CH4 emission per unit ofproduct.

Many of these practices are alreadypractical and economical. When usedtogether, they can lower loss of energythrough CH4 release from about 5Ð8% ofthe gross feed energy to as low as 2 or3%. Because they increase feedingefÞciency, these practices also often haveeconomic beneÞts. Consequently, theyare already widely used on many farms,

55

especially in dairy herds and beeffeedlots.

Reducing methaneemissions from manures

Most of the CH4 from manure isproduced during storage. When themanure is stored as liquid or in poorlyaerated piles, lack of oxygen preventscomplete decomposition to CO2,resulting in the release of CH4. Mostways of reducing emission, therefore,involve slowing the rate ofdecomposition, providing better aeration,or reducing the length of storage.SpeciÞc methods include the following:

Use solid- rather than liquid-manurehandling systems : Oxygen supply isusually better in solid manure, whichencourages CO2 to form rather than CH4.

Apply manure to land as soon as possible :The longer manure is left in feedlots, instockpiles, or in slurry tanks and lagoons,the more CH4 will be emitted. Frequentapplications to the land can thereforereduce emissions. Unfortunately, storing themanure is sometimes unavoidable becausethe land is frozen, too wet, or planted tocrops.

Minimize amount of bedding in manure :Manure with less bedding, such as straw,contains less C that can be converted toCH4.

Keep storage tanks cool : Lowering thetemperature of tanks, by insulating orplacing them below-ground, slowsdecomposition, thereby reducingemission of CH4.

Burn methane as fuel : Methane is a veryeffective fuel; indeed, it is the main

Cattle management systems

Producers feed and manage their cattle in different ways during different stages of theproduction cycle. The amount of greenhouse gas emitted depends on the system used andthe stage in the cycle. Management systems can be compared in terms of net emissions;for example, grams of CH4 emitted per kilogram of milk or beef produced. Feeding cattlegrain instead of forage reduces CH4 emissions. But feed type is only one factor to beconsidered in selecting a management system. For example, the use of forages in afeeding system encourages land to be used for perennial forage, rather than for annualcrop production which results in greater soil C losses. Manure management and itsgreenhouse gas emissions must also be considered when determining an optimummanagement system.

(P. Strankman, Canadian CattlemenÕs Association and K. Wittenberg, University of Manitoba)

56

Improved manure storage can reduce greenhouse gas emissions

Traditionally, manure is stored during summer and winter and is applied to the Þeld inearly fall or spring. Summer is usually the season of highest gas production because warmtemperatures enhance microbial activity in stored manure. Anaerobic storage favors CH4production, whereas aerobic storage produces CO2 and N2O.

Scientists measured greenhouse gas emissions from beef and dairy manure each stored inthree ways: compost, slurry, and stockpile. Methane and N2O emissions, expressed inCO2 equivalents, were always smaller for compost than for the other storage methods.For dairy manure, slurry emitted 1.9 times more greenhouse gas than the compost.Stockpiled manure emitted 1.5 times more greenhouse gas than the compost. Methanewas the dominant gas in both the slurry and the stockpile. Nitrous oxide represented mostof the compost emissions and a signiÞcant portion of the stockpile emissions.

For beef manure, emissions of CH4 and N2O were much lower than from dairy manure.Emissions of CH4 and N2O were 1.3 times higher from stockpiled beef manure than fromcompost and 4Ð6 times higher from slurry than from compost.

These results indicate that aerobic storage such as composting may limit the greenhousegas emissions. On the other hand, creating fully anaerobic conditions during storagepromotes emission of CH4 that could be collected and used as a fuel.

Bins in which the manure was stored either as slurry, stockpiled, or for composting. Alarge enclosure was installed over each bin, and the gas emissions were monitored for agiven time.

(E. Pattey, AAFC)

constituent of natural gas. In somecountries, CH4 from stockpiled manureis already collected and burned. InCanada, this approach may not yet bewidely practical or economical but isreceiving growing interest. Burning CH4

converts it to CO2, which has a muchlower warming potential.

Avoid landÞlling manure : Althoughmost manure in Canada is applied toland, small amounts are still disposed ofin landÞlls. Because decomposition inlandÞlls is usually oxygen-starved, largeamounts of CH4 can be emitted from thispractice. Furthermore, placing it inlandÞlls wastes valuable nutrients in themanure.

Aerate manure during composting : Tomake it easier to transport, manure issometimes composted before applying itto the land. The amount of CH4 releasedduring composting can be reduced byaerating the stockpiled manure, either byturning it frequently or by providing aventilation system inside the pile.Aeration encourages completedecomposition to CO2 rather than releaseof C as CH4.

These methods can reduce, to someextent, the CH4 emitted from animalmanure. Because of high densities oflivestock in some areas, and the highcost of handling and transportation,managing manure still remains achallenge. Other ways to reduceemissions may still be needed.

57

Reducing nitrous oxideemissionsMuch of the N2O emitted from farmlandis produced when excess NO3

- in soilundergoes denitriÞcation, either onfarmland or after it is leached away.Farmers can reduce these emissions bypreventing build-up of NO3

- or avoidingsoil conditions that favor denitriÞcation.Some N2O is also emitted when NH4

+ isconverted to NO3

- (nitriÞcation). Addingless NH4

+ or slowing the rate ofnitriÞcation can reduce emissions fromthis source. The best way to reduce N2Olosses is to manage the N cycle moreefÞciently, thereby avoiding the buildupof excessive NH4

+ or NO3-.

SpeciÞc ways of reducing N2O emissionvary for farming systems across Canada,but examples include the following:

Match fertilizer additions to plant needs :The best way to reduce N2O emissionmay be to apply just enough N so thatcrops can reach maximum yield withoutleaving behind any available N. A perfectmatch is rarely achievable, but thesynchrony can often be improved bybasing fertilizer rates on soil tests andestimates of N release from residues andorganic matter. In Þelds where fertilityneeds vary, applying N at different ratesacross the landscape (ÒprecisionfarmingÓ) may also improve the matchbetween amount applied and the amounttaken up by crops.

Avoid excessive manure application :Heavily manured land can emit a lot ofN2O because the manure adds N andavailable C, both of which promotedenitriÞcation. Moreover, manure is oftenapplied to land as a means of disposal, sothat rates can be excessive. Applying themanure at rates that just supply plantdemands can greatly reduce N2Oemissions from this source.

New technology of manure treatment

Scientists have introduced a new manure treatment process based on the use of anaerobicmicroorganisms in sequencing batch bioreactors (ASBR). Trials performed in thelaboratory showed that the ASBR technology is very stable and versatile and works wellat low temperatures (between 10 and 20¼C). Furthermore, the bioreactors need to be fedonly once a week, during regular manure removal.

The airtight reservoirneeded to maintainanaerobic conditions inthe bioreactorcompletely eliminatesany emissions ofgreenhouse gas duringtreatment and storage.The biogas can berecovered and used forenergy on the farm.

The technology also hasother interestingbeneÞts. It deodorizesand stabilizes the swinemanure slurry leading tothe degradation of mostof the 150 odor-causingsubstances in themanure. Furthermore,this technology increasesthe availibility ofnitrogen and phosphorusto crops and reduces theneed for chemical fertilizers.

(D. Mass� and F. Croteau, AAFC)

Optimize timing of nitrogen application :When the N is applied is as important asthe rate of addition. Ideally, farmersshould apply N just prior to the time ofmaximum uptake by the crop. Whereverpossible, they should avoid applyingfertilizer and manure in fall. Similarly,they should time the plough-down of N-rich crops, like legumes, so that Nrelease from the residues coincides withsubsequent crop demands.

Anaerobic sequencing batch bioreactor.

58

accumulated NO3-, and, because NH4

+

does not leach easily, it prevents loss ofN into groundwater where denitriÞcationcould occur.

Use cover crops : Where the growingseason is long enough, farmers can sowcrops after harvest to extract excess soilNO3

-, which prevents it from leaching orconverting to N2O.

Lime acid soils : Because it is favored byacidity, N2O emission can be suppressedby applying neutralizing lime to acidsoils.

Reduce tillage intensity : Though resultsare still inconsistent, some studies inCanada suggest that N2O emission maybe lower in no-till than in conventionaltillage. If conÞrmed, this observationmay point to no-till as a method ofreducing emissions, at least in somesoils.

These practices can help reduce N2Oemissions in many settings. BecauseN2O ßuxes are so sporadic, however, allthese practices cannot yet berecommended with conÞdence acrossCanadian soils and cropping systems.But those that improve the efÞciency ofN use are often already justiÞed forreasons quite apart from reduced N2Oemission. Fertilizers account for about9% of production costs on farms, andany method that reduces N losses haseconomic beneÞts.

Improve soil aeration : DenitriÞcation,and hence N2O emission, is favored bythe low oxygen levels that usually occurin saturated soil. As a result, farmers canreduce emission of N2O by managingsoil waterÑdraining soils prone towater-logging, avoiding over-applicationof irrigation water, and using tillagepractices that improve soil structure.

Use improved fertilizer formulations :Some research suggests that certainforms of fertilizer emit more N2O thanothers. Highest emissions may occurfrom anhydrous ammonia; lowest fromforms containing NO3

-. This Þndingsuggests that, by selecting appropriatefertilizers, farmers could reduce N2Orelease. However, the differences amongforms of fertilizer have not yet beenwidely veriÞed in Canada. Anotheroption is to use slow-release fertilizers,such as sulfur-coated urea. These formsrelease available N gradually; they feedthe crop yet prevent available N fromaccumulating. Though effective inreducing N2O emissions, slow-releaseforms may only be economical for high-value crops.

Use appropriate fertilizer placement :Placing fertilizer in close proximity tocrop roots can improve the efÞciency ofnutrient use, allowing the farmer toachieve high yields with lower rates ofapplication. On the other hand, placingthe fertilizer too deep in the soil, orconcentrating forms like urea in bands,may increase N2O emissions.

Use nitriÞcation inhibitors : Certainchemicals, applied with fertilizers ormanures, inhibit the formation of NO3

-

from NH4+. Their use may suppress

N2O formation in several ways: itreduces N2O formation duringnitriÞcation, it prevents denitriÞcation of

59

Putting it all togetherFor simplicity, we often discuss methodsof reducing emissions for each gasseparately. But the C and N cycles aretightly interwoven; a change in farmingpractice that reduces emission of one gasalmost always affects another. Whetheror not a new practice helps alleviate thegreenhouse effect depends on the neteffect on emission of all gases and therelative warming potential of each. Afew examples may help to illustratesome of the complex interactions.

One of the ways to reduce CO2

emissions is to farm more intensively: toeliminate summer fallow, to use higher-yielding varieties, and to aim for higherproductivity. Such practices can increasestored C by producing higher amounts ofresidue that become soil organic matter.At the same time, however, the new,more-intensive system may requirehigher inputs, including fertilizers, tomaximize yields. And those higherinputs of fertilizer may increase N2Oemissions. The overall effect of the newpractice must therefore take into accountthe change in soil C, the CO2 cost ofmaking the added inputs, and anyincrease in N2O emission. Because N2Ois such a potent greenhouse gas, a smallincrease in emission rate (say 1 kg N perhectare per year), will offset acomparatively high rate of soil Caccumulation (~130 kg C per hectare peryear).

The evaluation becomes even morecomplex if we include animals. Suppose,for example, we opt to allocate greaterland area to producing forages. Thiseffect would have pronounced beneÞtsfor storing soil C. Furthermore, it wouldreduce fertilizer requirements (and N2Oemissions from that fertilizer), becausenutrients are effectively recycled back to

the soil as manure. On the other hand,much of the C in that system would befed to animals, and a portion would bereleased as CH4. Furthermore, some CH4

and N2O would be produced frommanure. Thus, with one managementchange, we have affected emission of allthree gases, sometimes both negativelyand positively. And to know the neteffect of the practice, we must considerall three and their relative warmingpotentials.

We cannot yet grasp all the interactionsamong gases, nor are our modelssophisticated enough to predict them. Atpresent, however, it may be sufÞcient torecognize that all are part of a complexweb, and any attempt to reduceemissions of one may affect the others.Often, the net effect may still beoverwhelmingly positive; for example, itmay be that the increased soil C from alivestock-based system more than offsetsany increase in CH4 emission. Indeed,sometimes the effects may even bemutually positive; no-till, for examplemay increase soil C, reduce CO2 fromfossil fuel, and perhaps even reduce N2Oemissions. Similarly, more efÞcient useof manures, can almost certainly reduceN2O and CH4 emissions, while reducingCO2 costs of fertilizer manufacture.

A Þnal consideration is that the variouspractices aimed at reducing greenhousegas emissions may work over differentperiods. For example, efforts to increasesoil C gains may show largest responsein the short term, say one or severaldecades, but rates of C gain maydiminish thereafter because each newincrement of C becomes harder andharder to achieve. In comparison, effortsto reduce CH4 emission from ruminants,N2O emission from soils, or CO2

emission from fossil fuels may have onlysmall effects in the short term but

60

achieve highest effect over manydecades because the beneÞts accrueindeÞnitely.

Other effects ofpractices that reducegreenhouse gasemissionsWe cannot judge the attractiveness ofvarious management practices solely onhow well they reduce greenhouse gasemissions. Other factors that come intoplay include their practical feasibility,economic cost, effect on soil quality, andinßuence on the whole environment(Table 16). When all these factors areconsidered together, many of theproposed practices have favorableratings across the spectrum. Forexample, reducing tillage intensity haseither favorable or neutral effects on all

the criteria (though, clearly, thesetentative ratings will vary for differentareas of the country). Some practices,such as using nitriÞcation inhibitors,have numerous beneÞts but their usemay be limited by cost. Most of theproposed methods of reducinggreenhouse gas emissions have favorableeffects on soil quality and adjacentenvironments.

Many of these other considerations areas important as any beneÞts to theatmosphere. The adoption of proposedpractices will be driven at least as muchby these factors as by the desire toreduce greenhouse gas emissions.

Table 16 Projected effects of various agricultural practices that affectgreenhouse gas emissions

Effect on GHG Other considerationsemission

Practice CO2 CH4 N2O Feasibility Economics Soil Environmentquality

Reduced tillageintensity ++ 0 ? +++ + ++ ++

Reduced summerfallow area +++ + - ++ - ++ +

Improved manuremanagement 0 + ++ ++ -- + ++

Improved feedingrations - ++ 0 + ++ 0 0

Improveddrainage/irrigation + + ++ + + + -

+ beneÞcial 0 no effect - detrimentalnumber of + or - signs indicate magnitude of effect

61

Ozone is a bluish gas, with a sharp,irritating odor. It occurs naturally in theupper atmosphere (ÒstratosphereÓ),where it forms continually fromreactions promoted by the sunÕsradiation. Unlike the more common gasoxygen (O2), O3 is highly unstable,reacting with other molecules in theatmosphere, so that its lifetime is onlyhours or days. The O3 in the upperatmosphere serves a useful function byÞltering out harmful UV radiation.However, pollutants entering the upperatmosphere deplete the O3, therebyincreasing the intensity of UV radiationat the earth's surface.

Ozone also occurs naturally near groundlevel, where it occurs at concentrationsof 25Ð40 parts per billion by volume(ppbv). Along with other pollutants (e.g.,nitrogen oxides, peroxides, peroxyacetylnitrate, and particulate matter), ground-level O3 forms smog. The ill effects ofsmog on human health are reasonablywell known, but its effect on plants hasreceived little publicity. Yet, according tosome estimates, O3 causes tens ofmillions of dollars worth of damage tocrops in Canada annually, mainly in theFraser Valley of British Columbia, theQuebecÐWindsor corridor, and thesouthern Atlantic region.

Thus O3 is unique among atmosphericgases: in the upper layer, it is highlybeneÞcial; near ground level, it is aserious pollutant. Ironically, humanactivity has depleted O3 in the upperatmosphere but increased itsconcentration at ground level. In thissection, we describe the problem ofground-level O3; the problems arisingfrom depleting O3 in the upperatmosphere we discuss later.

Source of ground-level ozoneLow concentrations of O3 occurnaturally at ground level, formed in thepresence of sunlight by reactionsbetween nitrogen oxides and volatileorganic compounds (VOCs) (Fig. 25).Natural sources, such as vegetation andsoils, release these compounds at lowconcentration. But human activities haveincreased the amounts released: VOCsfrom petroleum, chemical industries, andtransportation and nitrogen oxides fromcombustion in power stations andautomobiles. Consequently, O3 is moreconcentrated and more smog occurs indensely populated and industrial regions.The health and environmental hazards ofsmog have prompted federal andprovincial governments to impose limitson emissions of nitrogen oxides andVOCs into the atmosphere.

Ozone concentration measurements

Ozone concentrations are measured at onlyabout 100 locations in Canada as part of theair-quality network operated by theAtmospheric Environment Service ofEnvironment Canada. To see howrepresentative these measurements are,scientists used low-level ßights to measureozone concentrations upwind anddownwind of the city of Montreal.

The ozone concentrations were found to begreater downwind than upwind of the cityat all altitudes. Consequently, ozone concentrations reported by the network may have tobe adjusted depending on the location of the measurement.

(J.I. MacPherson, NRC; R.L. Desjardins, AAFC)

Hei

ght

(m)

2000

1500

1000

500

09070 75 80 85

O3 concentration (ppbv)

Upwind

Downwind

3. Ozone

62

Major Canadian cities now experience, onseveral days each year, O3 levels abovethe maximum acceptable air-quality levelof 82 ppbv for 1 hour. Values of 170 ppbvhave been recorded at several locations inOntario. Stable air conditions duringsummer and fall especially favor theformation of smog. In light winds, smogcan spread over large areas, often affectingregions on both sides of the CanadaÐUSborder. Because it requires sunlight toform, O3 tends to diminish inconcentration at night, whereas othersmog constituents are unaffected.

Volatile organic compounds and agriculture

Volatile organic compounds (VOCs) include natural and artiÞcial chemical compoundsthat contain carbon as a main constituent. Volatile organic compounds and nitrogen oxidescombine in the presence of sunlight to form ozone at ground level. In rural areas, theVOCs are largely contributed by vegetation. Crops that emit VOCs include tomatoes,potatoes, soybeans, wheat, lettuce, and rice. Even if artiÞcial VOCs were eliminatedcompletely, ozone would still form from VOCs released from vegetation.

Air-quality objectives

Air-quality objectives are national goals for outdoor air quality that protect human healthand the environment. These objectives are developed by a working group for variousatmospheric pollutants under the Canadian Environmental Protection Act. The workinggroup reviews the most recent scientiÞc studies.

The current Òmaximum acceptableÓ air-quality objective for ozone is 82 ppbv averagedover a 1-hour period. The current ground-level ozone objective was established in 1976,based on the best scientiÞc information. It was reafÞrmed in 1989, but a new assessmentof the science of ground-level ozone is now nearing completion.

One aspect of the Harmonization Accord, recently signed by the Canadian Council ofMinisters of the Environment, identiÞed ground-level ozone as a priority. Work currentlyunder way will develop a Canada-wide standard for ambient ozone levels.

(M. Shepard, Atmospheric Environment Service, Environment Canada)

Figure 25Conceptual diagram showing ground-levelO3 formation. Volatile organic compounds(VOCs), emitted into the atmosphere fromvegetation and artiÞcial sources, react withNOx in the presence of sunlight to formO3.

Effect of ozone onplantsOzone enters plant leaves via stomata,tiny valved pores on the leaf surface thatregulate the exchange of gas betweenplant and air. During the day, the stomataare normally open to permit entry ofCO2 for photosynthesis. Unfortunately,at this time O3 levels are highest.

Once inside the leaf, O3 oxidizesmolecules in cell membranes, causingthe membranes to break down. BecauseO3 occurs naturally in the atmosphere,plants have evolved some protectivemechanisms, including ÒantioxidantsÓlike vitamins C and E, and specializedproteins (enzymes) that repair injuryfrom O3. But at higher O3 levels, theseprotective mechanisms are inadequate toprevent injury to tissues.

Ozone can cause direct damage to leaftissue, often visible as ßecking,bronzing, water-soaked spotting, andpremature aging of leaves. Furthermore,high O3 concentrations may cause thestomata to close, which cuts the ßow of

NOx

O3

VOC

Industrialand

naturalsources

63

CO2 and shuts down photosynthesis. Asa result of the direct damage and thereduced photosynthesis, yields of someplants can be dramatically reduced bylong-term exposure to elevated O3

levels.

Although scientists have studied theeffects of O3 on various crops in Canadaand elsewhere for more than 40 years,ßuctuations in O3 concentrations inpolluted air pose major difÞculties inproviding reliable estimates of thedamage caused to crops.

Ozone exposureand absorption bycropsAir pollution monitoring sites acrossCanada routinely measure ground-levelO3 concentrations. But concentrationsalone are insufÞcient to evaluatepotential damage to plants. Plants areless sensitive at night and during periodsof slower growth. Temperature andmoisture conditions also affectsensitivity. Consequently, we mustmeasure actual O3 absorption to assesseffects on plants.

One way of estimating O3 absorption is tomeasure the instantaneous O3

concentration in downward- and upward-moving air, using sensors mounted ontowers. If the concentration is greater inair moving down than in air moving up,that indicates O3 absorption: the greaterthe difference, the higher is the absorptionrate. This approach allows almostcontinuous measurement of O3 ßux andprovides daily and seasonal patterns ofabsorption. In one study, for example, theO3 ßux above a soybean Þeld increasedduring the day but then dropped sharplywhen the stomata began closing (Fig. 26).Because the opening and closing of

Ozone and leaf stomata

When plants take in CO2 for photosynthesis through their stomata, ozone can also enter.The ozone causes the cells surrounding the stomata to decrease in turgidity, whichreduces the size of the opening. This closing helps to protect the plant from further ozonedamage. Once inside the leaf, however, ozone is highly reactive and can destroy the leafcells, which can substantially reduce crop yield.

Diagram of O3 ßowing into a leaf via a stomate and causing damage byoxidizing cell walls and mesophyl

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64

Figure 27Estimated O3 absorbed by soybeans in theWindsorÐQuebec corridor, 1988 and 1992.(R.L. Desjardins and Y. Guo, AAFC)

This approach, however, only estimatesaverage absorption over the long term andcannot describe the short-term ßuctuationsassociated with daily changes in moisturestress or plant development. Furthermore,it tends to ÒdiluteÓ relatively briefexposures to high concentrations that arelikely to be most harmful to plants.Nevertheless, these estimates provide auseful indicator of potential plant damage.

stomata is controlled by water stress, thereis a strong relationship between O3

absorption and transpiration (the amountof water lost from the plants).

Figure 26Ozone absorbed and water transpired bysoybean on a sunny day in August inOttawa. (E. Pattey, AAFC)

For larger-scale measurements,instruments can be mounted on aircraft, asdescribed for CO2, N2O, and CH4

measurements. Aerial O3 surveys havealready been made for many crops,weather conditions, and O3

concentrations. One observation from thisapproach is the strong relationshipbetween O3 absorption and the amount ofgreen vegetation.

The scale can be increased still further byusing satellites. Scientists can calculatetranspiration from environmentalconditions and can obtain a ÒgreennessÓindex from satellite images. Because of itsclose relationship to transpiration, O3

absorption can then be estimated for theentire growing season on large areas,using O3 concentrations frommeasurement networks (e.g., Fig. 27).

0.8

0.4

0

-0.4

-0.800:00 24:0012:00

Time (EST)

Ozo

ne fl

ux (p

pbv/

m2 /

s)

Ozone absorptionEvapotranspiration

0.60

0.30

0

-0.30

-0.60 Eva

pora

tion

(mm

/h)

65

Measuring plantresponse to ozoneThe simplest way to measure plantresponse to O3 is to grow them insideopen-top enclosures into which O3 isthen continually released inconcentrations that reßect the dailyvariations. This method allowsresearchers to evaluate the effect ofseveral concentrations of O3 (typicallyup to three times that in outside air) aswell as those of other gases or pollutantsthat can be added simultaneously.

In a less disruptive approach, called theÒzonal air pollution systemÓ (ZAPS), aseries of pipes over the cropcontinuously releases O3 into the plantcanopy at various rates in different plots.This method avoids some of the artiÞcialconditions inside chambers but costsmore. As well, the maximum enrichmentachieved by this technique is not high,because of the continual mixing withuntreated air.

Open-topped enclosures and ZAPS areuseful for detailed research studies, butthey do not provide information on O3

damage over large areas. Networks ofinstruments that continuously record O3

concentrations exist in many populatedregions but are sparse in rural areas. Toprovide O3 information in such areas,scientists use ÒbiomonitorsÓ or ÒpassiveÓmonitors. Biomonitors are plants, like thetobacco variety ÒBel-W3,Ó that arehighly sensitive to O3. They are set outthroughout a region and then inspectedregularly for ßecks of dead-tissue, whichare symptoms of injury from O3. Thebiomonitors therefore provide anestimate of O3 absorption by leaves andindicate potential damage to other less-sensitive crops, even though these mayshow no visual signs of stress. Passivemonitors are simply Þlter papers treated

Open top enclosures, zonal air pollution system, and biomonitors.

66

Examples of cropresponse to ozoneThe effect of O3 has already been widelystudied and some extensive reviews arenow available. Here we present only afew examples to illustrate the nature andobjectives of some recent research.

Effect of ozone on broccoli

Broccoli is a high-value crop that isharvested about 6Ð8 weeks aftertransplanting. Rapid leaf growth aftertransplanting feeds the developing ßowerhead. Any stress on leaves during thistime usually results in smaller heads andlower yield. Studies using a ZAPSshowed that ozone injures leaves in twoways: it kills some of the tissue directly(Fig. 29a) and makes other tissue proneto attack by downy mildew, a fungaldisease (Fig. 29b). Severity of damagewas directly related to O3 enrichment.

with indigo dye. When exposed to O3,the dye changes color. The degree ofcolor change provides an index of thetotal exposure to O3 during the period.

Observations from biomonitors andpassive monitors can be related topotential crop effects by placing thesemonitors inside a ZAPS along with othercrop plants. ÒFleckingÓ of biomonitorleaves or color change in passivemonitors can then be directly related tocrop damage. Using these relationships,scientists can use biomonitors andpassive monitors placed throughout aregion to estimate yield effects of O3

absorption throughout that area.Researchers have used an extensivenetwork of this type to monitor O3

effects on yields in the Fraser Valley, ahighly populated area with intensiveagriculture (Fig. 28).

Agricultural land Instrumental monitoring sites O3 Biomonitor sites

10 kmInternational

Border

Vancouver

Ca

sc

ad

e

Ra

ng

eF r a s e r R

i v e r

49N

123W

122W

Co a s t R a n g

e

Figure 28Map of the ozone monitoring network in the Fraser Valley, B.C. (P. Bowen, AAFC)

Broccoli is sensitive to O3

67

Effect of ozone on orchardgrass

Orchardgrass is the main feed of dairycows in the Fraser Valley. The grass canbe harvested for hay up to Þve times ayear. Loss in yield at any harvestdepends on O3 exposures receivedduring the preceding growing period.One study examined the relationshipbetween level of O3 and orchardgrassyield in a ZAPS (Fig. 30). The data showhow yield decreased as the exposureincreased (reported as the number ofdays during which hourly concentrationsexceeded 50 ppbv). Becauseorchardgrass is a perennial, its earlyspring growth partly depends upon thereserves stored in the roots and stemsduring the previous growing season.Studies over successive years haveshown that exposing plants to O3 in thefall suppresses yield the next spring.

Figure 29Response of broccoli to O3: a) O3 injury to broccoli leaves; b) Leaves infected with downy mildew onbroccoli exposed to O3. (V.C. Runeckles,University of British Columbia, Vancouver,B.C.)

0

5

10

Ozone enrichment level

Ozo

ne-i

njur

ed le

aves

per

pla

nt

a

0

Ozone enrichment level

6

4

2

Infe

cted

leav

es p

er p

lant

b

68

One study measured the effects ofincreasing both O3 and CO2

concentration on alfalfa growth.Increasing the CO2 concentrationactually increased the tolerance of alfalfato high concentrations of O3 (Fig. 31),probably because the stomata arepartially closed at high CO2 levels. ThisÞnding may have important implicationsif, as expected, atmospheric CO2

concentration doubles some time in thenext century.

Figure 31Relative effects of O3 and CO2 on yield ofalfalfa and wheat. The values in the legendindicate concentration relative tobackground. (G. Allard, Universit� Laval)

Differences in ozone toleranceamong varieties

Plants show a wide range of tolerance toO3 in the air, even among varieties of thesame crop. Comparing two alfalfavarieties (ÒApicaÓ and ÒTeamÓ) in open-topped enclosures for 2 years showed thatÒApicaÓ was unaffected by low O3 levelsbut was strongly affected by higherconcentrations in both years. ÒTeamÓ wasalmost unaffected in a cool and rainysummer, but was affected almost asseverely as ÒApicaÓ in a warm and sunnysummer. In a similar study, spring wheatvarieties ÒBlueskyÓ and ÒOpalÓ wereexposed to air with no O3, and 1.0, 1.5,and 3.0 times the ambient O3

Yie

ld (%

of c

ontr

ol)

200

100

02 × CO2+3 × O3

Alfalfa varieties

Wheat varieties

1 × CO2+3 × O3

2 × CO2+1 × O3

Figure 30Dry matter yield of orchardgrass asaffected by exposure to O3. (V.C.Runeckles, University of British Columbia)

Effect of ozone on strawberry

Increased exposure of strawberry plantsto O3 reduces the number and weight ofgood fruit. A network of calibratedpassive monitors in the Fraser Valleyindicated that fruit losses can be as highas 15%.

Effect of ozone on lettuce

Visual appearance of leaves affects themarket value of crops like lettuce. Inozone exposure studies, lettuce leavesshowed no visible symptoms. Even atthe highest exposure levels, the cropappeared healthy. Surprisingly, however,O3 reduced head size and weight,indicating that O3 damage can be subtleand detectable only with careful scrutiny.

Combined effect of ozone andcarbon dioxide on alfalfa

Under high O3 concentrations, alfalfagrows more slowly and competes lessagainst weeds. Like orchardgrass,exposing alfalfa to O3 in the fall of oneyear may reduce its yield the year after.Its ability to survive cold winters,however, does not seem to be affected.

0.25

0.20

0.15

0.10

0.05

0.000 10 20 30 40

Number of days with hourly O3≥50 ppbvD

ry m

atte

r yi

eld

(kg/

m2 )

69

concentration (Fig. 32). ÒOpalÓ appearsmore tolerant to O3. The pattern oftolerance was different for the 2 yearstested. This Þnding suggests not only thatvarieties have different tolerances to O3

but also that weather conditions affectthose tolerances.

Figure 32Yield response of two wheat cultivars toincreasing concentration of O3. (G. Allard, Universit� Laval)

New crop varieties tolerate diseasebetter, are better adapted to localconditions, and generally produce higheryields. Do they also do better underhigher O3 concentrations? One studycompared the O3 tolerance of wheatvarieties released at various times, fromthe 1950s (when O3 concentrations weregenerally lower) to the early 1990s.Under current O3 concentrations, thenewer varieties yielded better, but, athigher O3, they fared worse. Oneexplanation is that newer varieties needmore CO2 to support higher rate ofphotosynthesis; hence, the stomata stayopen longer and absorb more O3.

Gra

in y

ield

(g/p

lant

)

00

1 11.5 1.53 30

6

4

2

Gra

in y

ield

(g/p

lant

)

00

1 11.5 1.53 30

6

4

2

1992

1992

1993

1993

Opal

Bluesky

O3 concentration in multiples of ambient concentration

Another explanation is that the improvedyield of newer varieties results from ahigher ratio of grain to leaf tissue. Withrelatively less leaf area to absorb CO2

for grain production, leaf injury by O3

may be more pronounced.

EnvironmentalinteractionsUnfortunately, crops are rarely exposedto only one pollutant. Plants growing inhigh O3 concentrations may also sufferinjury from sulfur dioxide, nitrogenoxides, acid rain, and UV radiation. Thenet effect of exposing plants to morethan one pollutant may be equal to,greater than, or less than the sum of theirindividual exposures. The effects arefurther complicated by crop type, time ofexposure, weather conditions, previousexposure, and other environmentalstresses. Consequently, recent studieshave only provided some knowledgeabout the potential effects of O3 on a fewmajor crops and regions.

71

concentrations may be much higher,sometimes well above the threshold atwhich it can be detected by smell (~0.6ppmv).

Unlike N2O, NH3 is highly reactive andremains in the atmosphere only a shorttime. It reacts quickly with water,forming ammonium (NH4

+). Thus anymoist surfaceÑsoil, plants, or openwaterÑreadily removes NH3 from theair, as long as the surface is neutral oracidic in pH. In the air, NH3 can dissolvein precipitation and fall to the earth asNH4

+, or it can be oxidized ordissociated by sunlight. As well, NH3

can react with pollutants such as acidicsulfates and nitrates, forming tinyparticles of ammonium nitrate orammonium sulfate. Because NH3 is soreactive, its concentrations are localized:high near sources and almost negligibleelsewhere. In an area near Lethbridge,Alta., for example, high concentrationswere found close to feedlots, butrelatively low values just 1 km away.

Ammonia has many undesirable effectsat high concentrations. Near sources,where concentrations are high, itproduces an unpleasant odor and mayaffect human and animal health. Localdeposition of emitted NH3 mayÒfertilizeÓ the land, but excessiveamounts can result in leaching of N andcontamination of ground- or surface-water. Excessive NH3 may even beconverted to N2O, thus indirectlycontributing to the greenhouse effect.

Though many of the effects of NH3

occur locally, it also has long-rangeeffects. Ammonium particles, formed

Although CO2, N2O, CH4, and O3 haveattracted much attention recently,agriculture also releases other materialsinto the air, including ammonia, otherodors, aerosols, nitrogen oxides, andpesticides. As well, agriculture may beaffected by changes to stratospheric O3.Many of these issues have not yet beenthoroughly studied in Canada. Our mainaim is to identify the potential issues andpoint to some possible effects.

AmmoniaCurrent farming practices rely heavily oninputs of extra N, most of whichultimately derives from atmospheric N2.These high inputs help sustain foodproduction, but they also stress thenatural N cycle, resulting in ÒleaksÓ of Ninto the environment. The release ofN2O is one such leak; another is theemission of ammonia (NH3).

BackgroundGlobally, agriculture is the main sourceof atmospheric NH3 from humanactivity. Much of this NH3 comes fromlivestock production. In parts of Europe,notably the Netherlands, NH3 emissionsfrom animal production are so high thatthey warrant strict regulations. InCanada, the problem is not yet as acute,except perhaps in local areas with highlivestock numbers.

Ammonia is a colorless gas, lighter thanair, with a sharp odor. In remote areas,away from sources, it occurs in theatmosphere at very low concentrations(less than 0.01 ppmv). In areas nearintensive livestock production, however,

4. Other links between agriculture and the atmosphere

72

upon reaction with other N or sulfurcompounds, can be carried longdistances by wind before beingdeposited. Because N is often a growth-limiting nutrient, the deposition of thisNH4

+ can cause undesirable growth inlakes, alter forest growth, or disruptsensitive ecosystems. When deposited onnative grasslands, for example,atmospheric NH3 or (NH4

+) may favorthe growth of some species at theexpense of others, causing a shift in themixture. Atmospheric NH3 can alsoresult in acidiÞcation because itaccelerates the rate at which sulfurdioxide (SO2) converts to sulfuric acid,leading to acid rain. The NH3 itselfproduces acid when it undergoesnitriÞcation, once deposited on soil asNH4

+.

Because of its numerous potentialeffects, both near sources and in remoteareas, NH3 can be a serious pollutant andefforts to reduce its emission arewarranted. Before examining possibleways of reducing emissions, however, itmay be helpful to brießy review thesources of NH3 in agriculture.

Agricultural sources ofammoniaThe three main sources of NH3 on farmsare animal wastes, fertilizers, and cropresidues. The Þrst of these accounts forabout 80% of agricultural emissions.

Of the N consumed by farm animals infeed, only a small proportion (roughlyone-Þfth) is retained by the animal; therest is excreted in feces and urine. Someof this N (especially in urine) occurs asurea, a form easily converted to NH3 andCO2. As a result, a large proportion ofthe N in manure can be lost as NH3 soon

after excretion. On pig farms, forexample, 40Ð95% of the nitrogenexcreted may be lost before the manureis applied to the Þeld. Much of that,perhaps 10Ð40% of the N lost, mayoccur from the barn even before storage.Ammonia losses from cattle manure areoften less than from pig manure,probably amounting to less than 50% ofthe total N content.

Losses of N during storage of manurecan also be high, depending on methodof storage. In a US study, about 60Ð80%of N was lost from pig manure inlagoons exposed to air, compared tolosses of only 30Ð65% from that storedin underground pits and later spread asliquid. Another estimate suggests that theproportion of pig manure N lost as NH3

is less than 10% for anaerobic storage,10Ð25% for semi-aerobic systems, and25Ð85% during composting. Thedifferences reßect the degree of exposureto air and the amount of water and acidpresent.

Some NH3 is also released when manureis applied to land, particularly if a slurryis sprayed into the air. Most loss occursshortly after application. For example, astudy of NH3 loss from cattle manureshowed that about half of the totalemission occurred within 1 day (Fig. 33).

73

Figure 33Pattern of NH3 loss from manure appliedto the surface of soil. (S. McGinn, AAFC)

Another potential source of NH3 isfertilizer. Two forms, both widely usedin Canada, are especially important:anhydrous ammonia (pure NH3) andurea. When anhydrous ammonia isinjected into soil, it normally convertsimmediately to NH4

+ in soil water andthen is held tightly by the soil. If the soilis extremely dry, however, as much as20% of the NH3 can escape. On theother hand, if it is so wet that the soildoes not close up after injection, asmuch as 50% can be lost. Urea fertilizer,like the urea in livestock manure,quickly converts to NH3 and CO2 after itis applied. If the fertilizer is not mixedinto the soil, large amounts of NH3 canbe released to the atmosphere.

A third possible source of NH3 fromfarms is crop residues. Appreciableamounts of NH4

+ can be producedduring the decay of N-rich residues likelegume green manures. If the residuesare allowed to decay on the soil surface,some of this NH4

+ may convert to NH3

and be lost to the atmosphere.

Based on data from 1990, NH3

emissions from all sources in Canadaamount to about 520 Gg (thousand

Cum

ulat

ive

amm

onia

loss

(kg

N/h

a)

10

8

6

4

2

00 40 80 120 160 200 240Hours after manure application

tonnes) of N per year. Of this, about 90%comes from agriculture, largely fromlivestock production (Table 17). Theseestimates, however, are still preliminary.

Table 17 Estimated ammonia emissions from Canadian agriculturein 1990

Source NH3 emission

(Gg N)

Animals

Dairy cattle incl. with beef

Beef cattle 211

Pigs 76

Poultry 88

Sheep/lambs 2

Horses 4

Total animals 381

Fertilizers

Urea 71

Ammonium sulfate 2

Ammonium nitrate 2

Anhydrous ammonia 4

Nitrogen solutions 2

Ammonium phosphates 6

Total fertilizers 87

Total agriculture 468

74

Reducing ammoniaemissionsProducers can reduce the emission ofNH3 from farms in a number of ways. Ingeneral, these methods rely on absorbingNH3 in water or acid, preventingexcessive N excretion by livestock, andminimizing exposure of NH3 sources tothe air. SpeciÞc examples of controlmethods include the following:

Use improved methods of fertilizerapplication : Farmers can reduceammonia loss from fertilizer by ensuringgood contact between the appliedfertilizer and moist soil. They shouldplace urea either below the soil surfaceor till it into the soil immediately afterapplying it to the surface. Injectinganhydrous ammonia into moist soil atsufÞcient depth prevents it fromdiffusing to the surface.

Minimize nitrogen excretion fromlivestock : The most basic way ofreducing NH3 emission from animalwastes is to produce less manure N inthe Þrst place. Although animals cannotavoid excreting N, farmers can reducethe N content of the manure by usingrations with a better N balance, byavoiding excessive N in the diet, or,possibly, by adding bacteria that helpconvert uric acid (a forerunner of urea)to nitrate. Use of these practices couldreduce N excretion by up to 25% incattle, pig, and broiler poultryoperations. Indeed, simulation modelssuggest that, for Quebec conditions,better diets could reduce the N content ofpig manure by up to 60%. Nitrogenexcretion can also be reduced, indirectly,by using breeds of livestock, feedformulations, or other practices thatimprove animal performance and, hence,the product yield per unit of manure N.

Improve manure handling in the barn :Large amounts of NH3 can be emitted inthe barn when the manure is exposed toair. Farmers can minimize this exposureby removing manure frequently; washingbarns with water, which absorbs NH3;collecting liquid wastes in deep, narrowchannels, to reduce surface exposure;and, in poultry barns, maintaining a deeplayer of litter. As well, maintaining cooltemperatures can reduce emission ofgaseous NH3. In Europe, changes inhandling procedures (including diet)have reduced NH3 release from pig barnsby 45%.

Improve manure storage : Farmers canreduce ammonia loss during manurestorage by minimizing exposure to airand lowering temperature. For example,applying a cover of mineral oil, straw, orpeat over lagoons or tanks holding pigmanure can reduce losses. Covers placedon tanks can cut NH3 losses by two-thirds, and a thin layer of mineral oil ona slurry can reduce emissions by morethan 30%. As well, adding acids tomanure or covering composting manureswith mildly acidic peat can minimizeNH3 loss. Ammonia is readily absorbedand held by acid, preventing escape tothe atmosphere. Farmers can achievereductions of at least 75% by using peatmoss, sulfuric acid, or phosphoric acidduring storage.

Use more effective applicationprocedures : Ensuring quick andeffective mixing with soil can minimizelosses of NH3 during application. Forexample, tillage or irrigationimmediately after application drasticallycuts emissions (Fig. 34). Farmers canalso reduce losses by applying manurebefore rain, injecting slurry directly intosoil, or using diluted slurry for irrigation.Where they must apply slurry tograssland, banding it on the surface,

75

rather than spraying it, can reduce losses.Finally, since rate of gaseous loss isrelated to temperature, applying NH3 incool weather (though not on frozen soil)can curtail emission.

Figure 34Proportion of manure NH4

+ volatizedwithin 8 days of application as affected byirrigation or tillage. (S. McGinn, AAFC)

This list shows several ways of cuttingNH3 emissions from agriculture. Not allthese are practical or even advisable inall cases. For example, incorporatingmanure by ploughing is not compatiblewith the no-till systems advocatedelsewhere. Nevertheless, given thenumber of options available, large cutsin emissions are probably easier for NH3

than for some of the other gases, notablyN2O. With increasing attention to health,environmental, and odor issues related toNH3, efforts to achieve such reductionswill likely increase in the future.

The Netherlands has decided that, by2000, NH3 emissions must be no morethan half of those in 1980. There, theannual N deposition has reached 85kg/ha in parts of the country. Thoughdeposition rates in Canada are usuallymuch lower, high rates of depositionmay already occur in local areas ofintensive livestock production.

None Irrigation TillageIncorporation treatment

Am

mon

ia lo

ss (%

of N

H4+ a

pplie

d)

20

10

0

Composting

Gases emitted during composting of organic waste may include CO2, NH3, CH4, N2O,and NO. Smaller quantities of reduced sulfur and nitrogen compounds may also beproduced in anaerobic microsites. The form and quantity of gaseous compounds emittedduring composting depends on the material being composted and the method used. Odor-producing compounds can be virtually eliminated with a properly designed aerationsystem. A bioÞlter system in enclosed composting facilities also ensures odor-free exhaustair.

Methane emission can also be eliminated with adequate aeration. Ammonia emission iscontrolled by the available C:N ratio of the composting material and by the aerationsystem used. When NH3 emission occurs, it is usually early during the compostingprocess. Ammonia may be captured using a scrubber. The factors inßuencing N2O andNO emissions during composting are not well understood. Researchers are workingtoward a better understanding of N2O emissions during composting and strategies tominimize emissions. A well-designed compost facility should not negatively affect thehealth of our air.

(J. Paul, AAFC)

75

rather than spraying it, can reduce losses.Finally, since rate of gaseous loss isrelated to temperature, applying NH3 incool weather (though not on frozen soil)can curtail emission.

Figure 34Proportion of manure NH4

+ volatizedwithin 8 days of application as affected byirrigation or tillage. (S. McGinn, AAFC)

This list shows several ways of cuttingNH3 emissions from agriculture. Not allthese are practical or even advisable inall cases. For example, incorporatingmanure by ploughing is not compatiblewith the no-till systems advocatedelsewhere. Nevertheless, given thenumber of options available, large cutsin emissions are probably easier for NH3

than for some of the other gases, notablyN2O. With increasing attention to health,environmental, and odor issues related toNH3, efforts to achieve such reductionswill likely increase in the future.

The Netherlands has decided that, by2000, NH3 emissions must be no morethan half of those in 1980. There, theannual N deposition has reached 85kg/ha in parts of the country. Thoughdeposition rates in Canada are usuallymuch lower, high rates of depositionmay already occur in local areas ofintensive livestock production.

None Irrigation TillageIncorporation treatment

Am

mon

ia lo

ss (%

of N

H4+ a

pplie

d)

20

10

0

Composting

Gases emitted during composting of organic waste may include CO2, NH3, CH4, N2O,and NO. Smaller quantities of reduced sulfur and nitrogen compounds may also beproduced in anaerobic microsites. The form and quantity of gaseous compounds emittedduring composting depends on the material being composted and the method used. Odor-producing compounds can be virtually eliminated with a properly designed aerationsystem. A bioÞlter system in enclosed composting facilities also ensures odor-free exhaustair.

Methane emission can also be eliminated with adequate aeration. Ammonia emission iscontrolled by the available C:N ratio of the composting material and by the aerationsystem used. When NH3 emission occurs, it is usually early during the compostingprocess. Ammonia may be captured using a scrubber. The factors inßuencing N2O andNO emissions during composting are not well understood. Researchers are workingtoward a better understanding of N2O emissions during composting and strategies tominimize emissions. A well-designed compost facility should not negatively affect thehealth of our air.

(J. Paul, AAFC)

76

and H2S. Given the many compoundsinvolved, odors are not easily measuredand quantiÞed. Indeed, the mostsensitive and reliable sensor is still thehuman nose. One way to measure odorintensity is to count the number of timesan air sample has to be diluted with freshair before its odor becomes nearlyimperceptible. A panel of humanevaluators is used to determine thenumber of Òdilutions to the thresholdÓ(DT), which may range from 0 to 200 ormore. On this scale, a reading of 170 DTor higher would be consideredÒunacceptable.Ó The lowest valueachievable within a feedlot operation isabout 7 DT.

A variation on this approach is tocompare the air sample with knownconcentrations of a reference compound,like butanol. With this method, theintensity of odor is reported in terms ofequivalent concentrations of butanol.The scale normally ranges from 0 to 80ppmv butanol (the highest intensity towhich the nose is responsive). Mostambient odors have a rating of less than60 ppmv butanol.

Researchers have used these techniquesto evaluate the odor from various typesof farms. Odors from pig farms usuallyrate ÒhighÓ to Òvery high,Ó whereaspoultry and cattle operations normallyrate Òhigh,Ó comparable to that of papermills, petrochemical plants, and oilreÞneries. Of course, odor intensityvaries considerably depending on windspeed, air stability, humidity, anddistance from source.

Producers can reduce the intensity ofodors from farms in several ways. Themost obvious, perhaps, is to plan thefarm layout carefully, placing sources ofodor, like barns and lagoons, downwind

Other odorsAmmonia is only one of the gasesreleased from farms that has anunpleasant odor. Many other gases alsoirritate the human nose. Some of theseare not only unpleasant but alsodangerous. Perhaps the most noteworthyis hydrogen sulÞde (H2S), a poisonousgas with the smell of rotten egg. Highconcentrations of this gas can be releasedwhen liquid pig manure in tanks isstirred. It can be fatal to humans, thoughonly at high concentrations producedwhere ventilation is poor. Many othercompounds, although not known to bepoisonous, have an objectionable odor;more than 150 such compounds havebeen identiÞed in pig manure alone.

To date, people have perceived farmodors only as nuisances, but awarenessof this problem is now growing. Indeed,some countries have already establishedregulations regarding allowable odorintensities.

Odor-causing gases can come from manysources. Some of the most offensivearise from organic substances decayingin the absence of oxygen. Thedecomposing matter may be manure,efßuent from manure piles, silage, plantdebris, or a wide range of other organicmaterials. When decomposed without anadequate oxygen supply, they are notcompletely broken down into CO2 andsimple salts but rather are released asvarious intermediates such as organicacids, alcohols, aldehydes, sulÞdes, andCH4. Of these, the compounds with themost offensive odors are the volatileorganic acids.

Many odor-causing compounds comefrom the same source and thereforeoccur together. For example, volatileorganic acids are often found with NH3

77

and far from dwellings. Other methodsinclude cleaning and washing barnsfrequently, aerating stored manure(although this action may favor NH3

release), injecting slurries, andimmediately incorporating solid manuresafter they are applied. Finally, variouschemicals and bacterial cultures havebeen proposed for odor control, but theircost is often high and their efÞcacylimited. One possible approach is to addcalcium bentonite, a clay with highabsorption capacity, to animal diets. Thisadditive has even been found to enhanceweight gain under some conditions.

Nitrogen oxidesNitrogen oxides, upon reaction withvolatile organic carbon (VOC) in thepresence of sunlight, produces O3, themain constituent of smog. Nitrogenoxides come mostly from combustion offossil fuel, and are usually linked toautomobiles and industrial sources. Butfarm machinery also uses a lot of fuel;for example, agriculture accounts forabout 25% of the heavy-duty dieselvehicles in Canada. Although theimportance of farm machinery as asource of nitrogen oxides is not known,its contribution to smog is likelynegligible. Even so, energy-conservingsteps like reduced tillage can reducesomewhat the emissions of nitrogenoxides.

Nitric oxide (NO), like N2O, issometimes produced in soil as a by-product of nitriÞcation anddenitriÞcation. In rural areas, the releaseof NO from this source can rival that ofnitrogen oxides from industrial sources.Using methods similar to those describedfor N2O can probably reduce theemission of NO from agricultural soils.

AerosolsAerosols are solid particles inatmosphere, either formed in the air byreactions among gases or injected intothe air by processes on the ground. Theyconsist of a variety of materials and varyin size from less than 1 micrometre (mm,one-thousandth of a millimetre) to thesize of a sand grain. The main sources ofaerosols are natural events likevolcanoes, sea spray, forest Þres, and soilerosion. But some aerosols are alsoproduced by human activity, likecombustion of fossil fuel.

Particles smaller than 2.5 mm are aserious concern for both visibility andhuman health. Aerosols absorb andreßect light, producing the haze in cities.They can also be breathed in and stay inthe respiratory system causingrespiratory illness and even cancer.

Aerosols also have an important effect onglobal climate. They provide the nucleior ÒseedsÓ that encourage cloud to form.They also reßect solar radiation, therebycooling the earth. In some regions, thecooling effect of aerosols is now aboutthe same as the warming effect of CO2,though it is not expected to increaseenough to offset further increases inCO2.

The amount of aerosols produced byCanadian agriculture has not beenmeasured routinely but is probably small.Nevertheless, farms do emit someaerosols of two types: primary particles,which are released intact into the air (e.g.,Þeld dust, soot, and pesticide crystals);and secondary particles, which are formedin the air from gases emitted byagriculture (e.g., NH4

+ particles fromNH3). Some secondary particles weredescribed earlier; here we focus only onprimary particles.

78

larger soil particles. The detachedparticles travel in three ways: saltation,creep, and suspension. In saltation,particles bounce across the surface; insoil creep, larger particles (0.5Ð1.0 mm)roll and slide after they are hit andaccelerated by ÒbouncingÓ particles.These two processes account for mosterosion. But in Þne-textured soils, withmany particles smaller than 0.1 mm, soilmay be lifted high above the surface(suspended), creating dust clouds thatcan travel for hundreds of kilometres.Eventually, the suspended particles settleout in calm winds or are washed out inrain.

After the bad experience of the 1930s,researchers developed many erosioncontrol measures. Some of these are nowcommonly used: reduced tillage, keepingresidues on soil surface, shelterbelts, andless-frequent use of summer fallow.Consequently, although about half ofCanadaÕs agricultural soil is moderatelyor highly susceptible to wind erosionwhen it is bare, less than 5% ofcultivated land is now at high risk.

Although severe and widespread erosionhas been largely halted, some dust fromfarmland still enters the air throughlocalized erosion events or during tillageand other farm operations. The dustemitted from soils is not just inertmineral particles. It may also containseeds, pollen, and plant tissue, as well asagrochemicals, including pesticides.These materials can cause healthproblems and, in the cases of pesticides,contaminate other environments.

Another agricultural aerosol is smokefrom burning of weeds or straw. Smokecontains soot (particles of carbon) thatcan cause respiratory problems. Untilrecently, burning excess straw wascommonly practiced in areas with high

The most common aerosol fromCanadian farms is probably dust fromsoil erosion. When soil is dry, loose, andwithout plant cover, the wind can pickup surface particles and carry them greatdistances. The problem was most severein the southern prairies during the dirtythirties, when as much as severalcentimetres were lost from some Þelds,obscuring the sky and depositing dusteverywhere. Although conservationmeasures now prevent such large-scaledust storms, occasional erosion episodesstill occur locally.

Erosion occurs in two steps. The windÞrst detaches tiny soil grains (0.1Ð0.5mm), which then act as abrasives on

Aerosol size distribution and global warming

The size distribution of an aerosol is closely related to its source. Coarse particles aregenerated mainly from mechanical processes, such as wind, whereas Þne particles areproduced by chemical reactions. Size distribution and chemical contents of aerosols areimportant factors determining global climate change and visibility. Aerosols have acooling effect, which offsets, in part, the warming effect of greenhouse gases.

(T. Zhu, Ottawa, Ont.)

0.001 0.01 0.1 1 2 10 100

Rainoutand

washout Deposition

AmmoniumSulfateNitrateOrganics

DustPlant particles (pollen)Sea sprayVolcano emissions

Chemically produced : Mechanically generated :

Particle diameter (µm)Fine particles Coarse particles

Secondary aerosol Primary aerosol

Rel

ativ

e nu

mbe

r of

aer

osol

s

Soil erosion in southern Alberta, 1935

79

Because of its vital function, scientists

were alarmed to learn, in recent decades,

that the amount of O3 in the upper

atmosphere is declining; that is, the O3

layer is Òthinning.Ó Worldwide, O3

concentrations have already declined by

an average of 3%. But much of the

depletion has occurred near the poles.

Average values in Canada have declined

by about 6% since 1980. Decreases near

Antarctica have been as high as 60%,

forming the so-called ÒAntarctic ozone

hole.Ó

The thinning of the O3 layer, scientists

now believe, is caused by the release of

various gases from industrial activity.

Most noteworthy of these are the

chloroßuorocarbons (CFCs) that are used

in refrigeration and as a propellant in

aerosol cans. These molecules, which

have a very long life, migrate into the

upper atmosphere where they cause O3

yields, like southern Manitoba. Nowprovincial and municipal regulationshave almost eliminated this practice.Some excess straw now goes toindustrial uses, like Òstrawboard,Ó whicheliminates the health hazard and alsoprovides additional income.

Ultraviolet radiation

BackgroundThe sun produces radiation with a wide

range of wavelengths. Some wavelengths

stimulate receptors in human eyes, so

that we can ÒseeÓ them. Thus, radiation

with a wavelength of about 390 nm (10-9

m) to 760 nm is called Òvisible light.Ó

Within this range, different wavelengths

correspond to various colors: the shortest

wavelengths correspond to violet, the

longest to red. But the sun also produces

radiation outside the visible range.

Radiation of wavelength longer than red

is called infrared radiation; radiation of

wavelength shorter than violet is called

ultraviolet radiation.

The energy of radiation increases as the

wavelength gets shorter. Ultraviolet

radiation, therefore, has much higher

energy than visible light, enough to

cause severe injury to living things. But

little of the sunÕs UV radiation reaches

the earthÕs surface; most is Þltered out by

O3 in the upper atmosphere (the

stratosphere). This effective screening of

UV radiation occurs despite the very low

concentration of O3. If all the O3 were

placed in a layer at the earthÕs surface, it

would be only 3 mm thick. Because it

protects the earthÕs surface from

damaging UV radiation, O3 in the upper

atmosphere (unlike that at ground level)

is essential to life.

80

to break down into O2. Another gas

known to break down O3 is methyl

bromide, used throughout the world as a

fumigant to kill insects and nematodes in

farm Þelds, greenhouses, and food

storage and processing plants. Methyl

bromide accounts for up to 10% of

global O3 losses. Finally, nitric oxide

(NO) can accelerate O3 breakdown. This

gas is produced naturally in the

atmosphere from N2O. Increases in N2O

emissions, therefore, can also indirectly

cause O3 breakdown.

Once they had recognized the cause of

O3 depletion, the international

community set up an agreement

(Montreal Protocol on Substances that

Deplete the Ozone Layer) to curb

emissions of gases like CFCs and methyl

bromide. All developed countries have

agreed to eliminate the use of CFCs by

2000 and the use of methyl bromide by

2015. Canada has committed to

eliminate use of methyl bromide by 2001

(with some exceptions where no

practical alternatives are available).

Already in 1995, the use of methyl

bromide had declined by about 40%

relative to that in 1990. Promising

alternatives to methyl bromide include

using other chemicals, diatomaceous

earth (which physically damages

insects), and integrated pest management

strategies.

By adopting strict controls on CFCs and

other O3-depleting substances, we can

probably halt the continued depletion of

O3 by about 2000. But, because of the

long life of CFCs already in the

atmosphere, it may take until 2060

before O3 concentration returns to its

pre-1980 levels. Consequently, we can

expect high UV intensity for several

more decades and need to consider some

of its effects on agricultural production.

Soybean leaves damage by UV-B radiation

(M. Morrison, AAFC)

Table 18 Sensitivity of Canadian crops to UV-B radiation

Tolerant Intermediate Susceptible

Wheat Barley Oat

Sunßower Rye Pepper

Corn Soybean Cucumber

Tobacco Pea Mustard

Red clover Tomato Canola

Alfalfa Potato

Bluegrass Soft fruit

Orchardgrass

Cabbage

(M. Morrison, AAFC)

81

Effect of ultravioletradiation on cropsBecause some UV radiation reaches theearthÕs surface, terrestrial plants haveevolved protective mechanisms. Someproduce pigments, similar to sun screen,that absorb UV radiation. Others, likesoybean, have UV-absorbing pigments inÞne hairs on the upper surface of leaves(hence, symptoms of UV radiation areoften more severe on the under surfaceof leaves). As well, most plants havesome ability to repair cells and DNAdamaged by excessive UV.

Despite these defense mechanisms, highexposure to UV can injure cellmembranes and DNA within cells.Perhaps its most damaging effect is todisrupt the chloroplasts (the chlorophyll-containing organs where photosynthesisoccurs). Damage to the chloroplastsreduces photosynthesis, which, in turn,can reduce plant growth.

Many recent studies have evaluated theeffects of increased UV on plant growthusing a combination of UV Þlters and UVlamps to produce a range of UVintensities. Much of the research hasfocused on UV-B, a band of wavelengthsfrom 290 to 315 nm. Ultraviolet radiationwith longer wavelengths (UV-A) has lessenergy and is therefore less damaging.Ultraviolet radiation with shorterwavelength (UV-C) is absorbed soeffectively by the atmosphere that it neverreaches the earth's surface.

Scientists have observed plant growth oryield effects from UV-B in numerouscrops, including timothy, soybean,tomato, and canola. The effects of UV-Bon yield are not always consistent,because some varieties yield more withincreased UV-B than without. Studieswith some species (e.g., corn) showed no

damage even at high UV-B levels.Furthermore, as observed with canola andsoybean, the response to UV-B seems tovary among varieties of the same crop.For example, in a study of eight soybeanvarieties, six had lower yield under highUV-B, but two had higher yields.Consequently, though there is goodevidence of potential yield loss fromincreased UV-B intensity, there are manyfactors which complicate the results ofUV-B studies.

To evaluate the potential effects ofincreased UV intensity on agriculture,researchers measured the growth responseof 100 varieties from 12 crops to anincrease in UV corresponding to a 20%reduction in O3. Of these 100 varieties, 40showed no effect. A simple model, basedon these and other data, describes thesensitivity of crops to UV-B (Table 18).ÒTolerantÓ crops would show little yieldloss from an increase in UV-B radiationas high as 20% increase over 1980 levels.Crops with ÒintermediateÓ sensitivity mayhave yields reduced by 1, 2.5, and 5%

82

with increases in UV-B of 5, 10, and20%, respectively; whereas ÒsusceptiblecropsÓ may have yields reduced by 2, 5,and 10% with the same UV-B increments.Using these estimates, we can predictpotential economic losses from increasesin UV-B. For example, a 5% increase inUV would result in crop yield losses ofabout $90 million per year; a 20%increase in losses of about $400 million.

Ultraviolet radiation may also affect cropquality. Exposure may produce surfaceblemishes on vegetables and fruits ormay affect ßavor by causing increasedpigment production. In one study, forexample, amounts of UV-B-absorbingpigments in broccoli were higher withUV-B than without UV-B exposure. Allthese effects can reduce the value of thecrop.

There may also be ecological effects ofUV on plant communities. Under highUV-B, species with higher tolerance may

out-compete susceptible species. Thiseffect could be important in mixedgrasslands or it could alter weed-cropcompetition. Furthermore, elevated UV-B can affect seed production, becauseexposed reproductive parts may beespecially vulnerable.

Aside from effects on yield, quality, andecology of crops, elevated UV-B couldalso have other implications. Forexample, it could affect animal health,plant diseases, pests, and pesticideefÞcacy. These effects have yet to bestudied.

Research into ways of reducing the UV-B effect on crops has made littleprogress as yet. Given the differences inresponse among plant species andvarieties, however, it may be possible tolimit economic losses by selecting UV-Btolerant varieties.

83

Agrochemicals

Agrochemicals, such as insecticides and herbicides, can be released into the environmentby drift, volatilization, and runoff. For example, some have found their way into the GreatLakes. Scientists use a high-volume sampler, installed in an aircraft, to measureagrochemicals ßuxes on a regional scale.

(G. St-Amour, AAFC)

PesticidesMost farms in Canada use somepesticides to control weeds, insects, anddiseases. Many of these pesticides haveat least some toxicity for humans orpotential adverse effects in theenvironment. Pesticides applied to thesoil and crops can either drift whilebeing applied or volatilize afterwards.Once in the air, wind can transport thepesticides long distances beforedepositing them on soil or water.Pesticides deposited in the Great Lakes,for example, have caused concern overwater quality.

Some of the earlier concerns aboutpesticides are no longer as valid todaybecause older, persistent pesticides (likeDDT) are no longer used in Canada.Farmers now usually use newerformulations designed to control speciÞcpests and to be easily degraded by soilmicrobes. Further, pesticides are nowoften applied at much lower rates,typically at grams per hectare rather thankilograms per hectare as in the past.

Despite the improvements in currentpesticides, however, further precautionsmay be helpful to reduce losses to theatmosphere. For example, spraying onlyduring calm conditions and ensuring thatdroplets are large enough to prevent theirsuspension in the air reduces pesticidedrift. In some cases, it may be possibleto reduce rates or frequency of pesticideapplication by relying on other methodsof pest control. For example, biologicalmethods can now control some weedsand insects. The use of ÒIntegrated PestManagementÓ (IPM) techniques, whichrely on optimum combinations ofchemical, biological, and culturalmethods, may provide the best approachto reducing pesticide usage.

Pesticides help to produce high yields onCanadian farms. Given their potentialeffects on human health and theenvironment, however, farmers need tobe vigilant to prevent pesticides fromleaving the target site.

85

The crops, livestock, and soils that makeup our farms are immersed in air. Theygive out gases and particles that changethe airÕs composition, both locally and faraÞeld. At the same time, they take in andare affected by air that has been altered byindustry and other human activity. As aresult, farms are sensitive markers of thehealth of our air.

Current statusOne of the main concerns in recent yearshas been the release of greenhouse gasesinto the atmosphere. We now know thatfarms account for about 10% ofCanadaÕs greenhouse gas emissions.About two-thirds of the emissions are inthe form of N2O and one-third CH4.Livestock and manure account for about58% of these emissions, croppingpractices for 37%. At one time,agriculture was also an important sourceof CO2, mostly from cultivated soils, butthese emissions have abated to almostnegligible levels. Some uncertaintyremains in these emission estimates,particularly for N2O, which is released insporadic bursts, making precise estimatesdifÞcult.

Agriculture also releases other materialsinto the atmosphere. It is the main sourceof atmospheric NH3 and may alsorelease some nitric oxide, dust, andpesticides into the air, though amountsare usually small.

Although farms release some gases intothe air, which affect its composition, theyare also, in turn, inßuenced by emissionsfrom other sectors of society. Oneexample is the ground-level O3 thatcauses crop damage in areas of highpopulation density. This O3 affects the

yield and quality of produce on nearbyfarms, which, because of their proximityto population centres, often grow high-value crops. Another example is thepotential effect of increased UV-Bradiation, which arises when industrialchemicals such as CFCs deplete O3 inthe upper atmosphere. We do not yetknow, precisely, the effects of the higherUV-B on crops and animal health, butsome damage may occur, particularly ifintensity of UV-B continues to increase,as expected.

Opportunities toreduce emissionsThe net release of gasesÑN2O, CO2,CH4, and NH3Ñis usually a symptom ofthe inefÞcient use of resources. Releaseof excessive CH4 from livestock means awaste of feed; loss of N2O or NH3

reßects inefÞcient use of N in fertilizers,crop residues, or manures; and excessiverelease of CO2 reßects inefÞcient use ofsolar energy, stored as fossil fuel or plantC. Farmers can reduce emissions,therefore, by managing the farm N and Ccycles more efÞciently, to prevent gasesfrom leaking into the environment.Because of improved efÞciency, manypractices that reduce emissions also haveother favorable effects: reducingproduction costs, conserving soil andwater, and improving ecosystem health.

Agriculture will always remain a sourceof some gases: CH4, N2O, and NH3.Even the natural ecosystems replaced byfarms release these gases. But, improvedefÞciency of N and C use can minimizethe amounts of emission. Reductions ashigh as 20Ð30% may be possible.Improved farming practices can actually

5. Conclusions

EfÞciency improvements

Market competition makes for more cost-effective production. Energy shortages and costsmake producers more energy conscious. Similarly, faced with the possibility of globalclimate change, producers may be able to further increase their efÞciency in usingresources, thereby increasing the amount of food produced per unit of greenhouse gasemitted.

Examples of increased productivity in Ontario

Crops 1975 1991

Diesel fuel-equivalent of

soybean produced (L/t) 174 95

corn produced (t/ha) 3.4 6.9

Dairy 1951 1991

Animals (million) 1.7 0.9

Milk (billion L) 2.4 2.5

Land area need to produce feed (million ha) 1.1 0.5

Manure generated (million t) 21.4 12.5

Eggs 1951 1991

Eggs produced (million dozen) 107 179

Land area need to produce feed (thousand ha) 129 61

Manure generated (kg/dozen eggs) 7.1 3.4

Chicken 1951 1991

Meat produced (million kg) 45 299

Land area need to produce feed (thousand ha) 96 117

Manure generated (kg/kg meat) 12.6 3.9

These tables show that productivity has increased considerably in the selected periods.Energy per kilogram of soybean has halved in 15 years; and manure per unit of milk,eggs, or chicken has halved in 40 years. It may therefore be expected that fossil CO2 andmanure N2O emissions per unit of production have also decreased.

86

result in net removal of CO2 from theatmosphere, by storing C in soils. Thisincreased storage could even helpCanada meet its targets for reducing thisgreenhouse gas.

Future challengesIn much of this book, we have focusedon current farm practices: how theyaffect our air and how, in turn, thechanging atmosphere affects them. Wehave summarized estimates andprocesses that describe currentagroecosystems. But we know thatagricultural systems are always evolving;that many of the systems we havestruggled to understand here may beobsolete just years from now. Thus, it isimportant to at least point to someimpending changes and speculate abouttheir possible effects.

One important factor is the continuingdrive for higher agricultural productivity.As global population climbs, demand forfarm products increases. Moreover, theeconomic survival of farms oftendepends on ever-higher output ofproducts. The resulting gains inproductivity may have some beneÞts; forexample, they may help to build soil Cby producing more crop residue. At thesame time, however, the higher yieldtargets may require more fertilizers andother inputs that could release moregreenhouse gas.

Economic factors are anotherconsideration. As cost of inputs and priceof products change, farmers alter theirfarming systems to maintain proÞts.Consequently, the area of land devotedto certain crops changes from year toyear, which affects the release ofgreenhouse gases and other emissions.Perhaps the most dramatic example isthe recent shift toward livestock-basedsystems. This change has far-reachingimplications. On the one hand, higherlivestock numbers usually mean moreland in forages, which reduceatmospheric CO2 by storing more C insoil. At the same time, however,

87

increased livestock numbers can lead tomore release of CH4, N2O, and NH3. Ifthe trend toward higher numbers of farmanimals continues, then many of ourcurrent emission estimates will need tobe revised and new measures of reducingemissions may be needed.

But it is not only the farming systemsthat will change. Environmentalconditions that affect farms willthemselves change over the nextdecades. Many scientists believe thatclimate will be noticeably altered by thegreenhouse effect over the next decades;even small changes in temperature orprecipitation would affect Canadianfarms. Another important environmentalcharacteristic has already changedmeasurably: the CO2 concentration,already about 30% higher than in pre-industrial times, will likely double withinthe next century. Since CO2 is the rawmaterial for photosynthesis, this increasemay have important effects on cropyield. Some even predict an increase inyields through ÒCO2 fertilization.Ó Otherenvironmental conditions may change aswell, including concentration of ground-level O3 in populated areas, and theintensity of UV-B radiation. Thesechanges, some of which are not easilypredictable, may affect the way we farmin the next century. As well, they willalter the emissions from farms, therebycontinuing the cycle between farms andthe atmosphere.

Organic farmingÑan alternative approach

Organic farming minimizes the need for off-farm inputs. It employs systems that avoid orlargely exclude the use of synthetic fertilizers, pesticides, growth regulators, and livestockfeed additives. Many believe that greenhouse gas emissions may be less for organicsystems than for conventional agricultural systems.

Organic farms attempt to harmonize with natural systems. They rely on renewableresources and less input from fossil energy. The holistic view of organic farmers follows anatural systems approach to agriculture. Individual growers take daily decisions to make aliving from the land, based on both economic and ecological considerations. With time,agroecosystems reach a steady state, where living and nonliving processes are in balance.For many, it is a way of life as much as a way of making a living.

The holistic systems approach requires an intimate knowledge of the interrelationshipsbetween soil, water, climate, and biology of the agricultural system. In addition, thesesystems also consider off-farm effects such as rural economics and sociology.

Generally, families on organic farms have traditions of environmentalism and are carefulconsumers of all resources. The day-to-day on-farm decisions of organic farmers arecomplex and require an in-depth knowledge of many areas of science. Organic farmersbelieve that their philosophy provides a gentler approach to the earth.

Both conventional and organic systems of agriculture aim to provide society with high-quality food, but some feel that organic farming also attempts to improve quality of theon-farm natural resources and to reduce potential environmental damage.

(J. Dormaar, AAFC)

Carbon dioxide Òfertilization effectÓ

Higher CO2 concentration can enhancecrop yield by increasing photosynthesisand allowing more efÞcient use of water.This CO2 ÒfertilizationÓ is morepronounced in C3 plants (e.g., wheat,soybeans, and most grasses) than in C4(e.g., corn and some grasses). Somescientists think that CO2 fertilization canlargely offset yield losses arising fromclimate change. Others suggest that thebeneÞts may be overstated, because theyoverlook the interaction betweenincreased CO2 and other environmentalconditions. More research on these questions is needed.

Pho

tosy

nthe

sis

(µm

ole

m2 /

s)

30

20

10

00 200 400 600 800 1000

C 3(e.g., wheat)

C4 (e.g., corn)

CurrentCO2

2 × CO2

CO2 (ppmv)

88

well, it will help us to predict betterhow changing farm practices willaffect the environment.

¥ To learn how changes in ouratmosphere will affect Canadianfarming in the future. Of particularimportance may be the effects ofclimate change (temperature andprecipitation), increased CO2

concentration, enhanced UV-Bintensity, and increased ground-levelO3. We need to know how these willaffect yields, crop types, animalproductivity, pests, and productioncosts. As well, we need tounderstand how these changes willalter future emissions fromagriculture to the air.

RemainingquestionsThe unpredictability of future changes infarm ecosystems, along withuncertainties about even our currentestimates of emissions, leave room forfurther study of the ties between ourfarms and the atmosphere. The mosturgent goals may be the following:

¥ To improve further our estimates ofcurrent gas release, especially forN2O. We need better ways of takingdata from local measuring pointsand extending them to larger areas,up to the national level.

¥ To Þnd ways that will help Canadameet its international commitmentsfor reduced emissions of potentiallyharmful gases.

¥ To understand better how C, N, andother elements move through andamong plants, animals, soil, water,and air. Such understanding willshow us how the various gases andenvironmental issues are linkedtogether and how they interact. As

89

6. Bibliography and selected readingActon, D.F. and L.F. Gregorich (eds.). 1995. The health of our soils: towardsustainable agriculture in Canada. Centre for Land and Biological Resources,Agriculture and Agri-Food Canada. Publication 1906/E, 138 pp.

Baht, M.C., B.C. English, A.F. Turhollow, and H.O. Nyangito. 1994. Energy in synthetic fertilizers and pesticides: revisited. Oak Ridge NationalLaboratory, Tennessee, USA. Report ORNL/Sub/90-99732/2.

Canadian Council of Ministers of the Environment. 1997. Ground-level ozone and itsprecursors, 1980Ð1993. Report of the Data Analysis Working Group, CanadianCouncil of Ministers of the Environment. 295 pp.

Coxworth, E., M.H. Entz, S. Henry, K.C. Bamford, A. Schoofs, P.D. Ominski, P.Leduc, and G. Burton. 1995. Study of the effect of cropping and tillage systems on thecarbon dioxide released by manufactured inputs to western Canadian agriculture:identiÞcation of methods to reduce carbon dioxide emissions. Final report forAgriculture and Agri-Food Canada.

Desjardins, R.L. 1998. Agroecosystem greenhouse gas balance indicator: methanecomponent. Report no. 21 to Agri-environmental indicator project. Agriculture andAgri-Food Canada. 19 pp.

Dumanski, J., L.J. Gregorich, V. Kirkwood, M.A. Cann, J.L.B. Culley, and D.R.Coote. 1994. The status of land management practices on agricultural land in Canada.Centre for Land and Biological Resources, Agriculture and Agri-Food Canada.Technical Bulletin 1994-3E, 46 pp.

Duxbury, J.M. and A.R. Mosier. 1993. Status and issues concerning agriculturalemissions of greenhouse gases. Chapter 12 in Agricultural dimensions of globalclimate change, T. Drennen and H.M. Kaiser (eds.); St. Lucie Press, Florida.

ECETOC. 1994. Ammonia emissions to air in Western Europe. European Centre forEcotoxicology and Toxicology of Chemicals. 194 pp.

Ecological StratiÞcation Working Group. 1995. A national ecological framework forCanada. Centre for Land and Biological Resources, Agriculture and Agri-FoodCanada and Ecozone Analysis Branch, State of the Environment Directorate,Environment Canada.

Environment Canada. 1993. A primer on ozone depletion. The environmentalcitizenship series. Environment Canada. 76 pp.

Gribbin, John and Mary. 1996. The greenhouse effect. The New Scientist 2037,Supplement: Inside Science 92:1Ð4.

Houghton, John. 1997. Global warming: the complete brieÞng. Cambridge UniversityPress. 251 pp.

90

McAllister, T.A., E.K. Okine, G.W. Mathison, and K.-J. Cheng. 1996. Dietary,environmental and microbiological aspects of methane production in ruminants.Canadian Journal of Animal Science, 76:231Ð143.

Monteverde, C.A., R.L. Desjardins, and E. Pattey. 1998. Agroecosystem greenhousegas balance indicator: nitrous oxide component. Report no. 20 to Agri-environmentalindicator project; Agriculture and Agri-Food Canada. 29 pp.

Moss, A.R. 1993. Methane: global warming and production by animals. ChalcomePublications, Kingston, UK. 105 pp.

Policy Branch. Canadian fertilizer consumption, shipments and trade. Annualpublications of Policy Branch, Agriculture and Agri-Food Canada. Available on theinternet at www.agr.ca

Shoji S. and H. Kanno. 1994. Use of polyoleÞn-coated fertilizers for increasingfertilizer efÞciency and reducing nitrate leaching and nitrous oxide emissions.Fertilizer Research 39:147Ð152.

Smith, W.N., P. Rochette, C. Monreal, R.L. Desjardins, E. Pattey, and A. Jaques. 1997.The rate of carbon change in agricultural soils in Canada at the landscape level.Canadian Journal of Soil Science, 77:219Ð229.

Stumborg, M.A. (ed.). 1997. Proceeding of the 1997 ethanol research anddevelopment workshop. Natural Resources Canada and Agriculture and Agri-Food Canada.

Surgeoner, G.A. 1995. Sustainable agriculture: Heaven on earth? Agri-food Researchin Ontario. Special edition, July, 29 pp.

Symbiotics Environmental Research and Consulting. 1996. Inventory of technologiesto reduce greenhouse gas emissions from agriculture. Report prepared for Global AirIssues Branch, Environment Canada and Environment Bureau, Agriculture and Agri-Food Canada.

Symbiotics Environmental Research and Consulting. 1997. Agricultural sources,effects and abatement of atmospheric emissions of nitrogen compounds: review ofCanadian science and technology. Report prepared for Environment Bureau,Agriculture and Agri-Food Canada.

Tenuta, M., E.G. Beauchamp, and G.W. Thurtell. 1995. Studies of nitrous oxideproduction and emission from soil: evaluation of N2O release with different methodsand fertilizer sources. Final report to Agriculture and Agri-Food CanadaÕs Trace gasinitiative project.

Tollenaar, M. 1996. Corn production, utilisation and environmental assessmentÑareview. CanadaÕs Green plan, Agriculture and Agri-Food Canada.

Vezina, C. 1997. National ammonia inventory: preliminary emissions. Pollution DataBranch, Environment Canada. (Unpublished draft.)

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Wardle, D.I., J.B. Kerr, C.T. McElroy, and D.R. Francis (eds.). 1997. Ozone science: aCanadian perspective on the changing ozone layer. Environment Canada. 119 pp. See also: http://www.ec.gc.ca/ozone

Working Group 1. Climate change 1995: the science of climate change (Chapter 13:agriculture). Contribution of Working Group 1 to the Second assessment report of theIntergovernmental Panel on Climate Change. World Meteorological Organisation.http://www.ipcc.ch.

The material presented in The Health of Our Air is derived from the work of manyscientists who made valuable contributions to the Agriculture and Agri-Food Canadaresearch initiatives on greenhouse gases and ground-level ozone. These included theprincipal investigators or team leaders, listed here as contributors, along withcollaborators, colleagues, postdoctoral fellows, technicians, and graduate students.Grateful acknowledgment is made to all.

ContributorsAllard, G.; Angers, D.A.; Antoun, H.; Baril, P.; Beauchamp, E.G.; Bordeleau, L.;Bowen, P.A.; Buckley, D.; Burton, D.; Campbell, C.; Chalifour, F.P.; Chiquette, J.; Cho,C.M.; Coxworth, E.; Desjardins, R.L.; Dow, D.; DunÞeld, P.; Ellert, B.; Gleig, D.B.;Grace, B.; Grant, B.; Gregorich, E.; Guo, Y.; Izaurralde, R.C.; Jackson, H.A.; Janzen,H.H.; Kaharabata, S.; Kinsman, R.; Knowles, R.; Lapierre, C.; Lin, M.; Liu, J.;MacDonald, B.; MacLeod, J.; MacPherson, J.I.; Mass�, D.; Mathison, G.W.; Mathur,S.P.; McAllister, T.; McCaughey, W.P.; McGinn, S.; McKenny, D.J.; Merrill, C.;Monteverde, C.; Morrison, M.J.; Paul, R.J.; Pattey, E.; Patni, N.; Prevost, D.; Renaud,J.P.; Richards, J.; Riznek, R.; Rochette, P.; Runeckles, V.C.; Sabourin, D.; Sauer, F.;Schuepp, P.H.; Selles, F.; Smith, W.; St. Amour, G.; Tarnocai, C.; Thurtell, G.W.; Topp,E.; van Bochove, E.; Van Kessel, C.; Wagner-Riddle, C.; Wang, F.; Zhu, T.

A listing of the principal investigators and their project titles are presented inAppendix I for those who wish more detailed information.

ReviewersAcknowledgments are also due to the following individuals for their review of thismanuscript and their helpful comments: T. Daynard, Ontario Corn Producers; S. Forsyth, National Agriculture Environment Committee; P. Strankman, CanadianCattlemenÕs Association; K. Whittenberg and D. Burton, University of Manitoba; V.Runeckles, University of British Columbia; J. Farrell, Fertilizer Institute of Canada; E.Beauchamp, University of Guelph; G. Hamblin, Canadian Organic Advisory Board;and E. Gregorich and K. Beauchemin, Agriculture and Agri-Food Canada.

Production teamThe editors wish to thank the following for their valuable technical assistance inpreparing the manuscript and Þgures for publication: S. Rudnitski, J. Sylvestre-Drouin, R. Riznek, and C. Merrill of Agriculture and Agri-Food Canada, and J.T. Buckley (Gilpen Editing Service).

Acknowledgments

92

93

Program ManagementAcknowledgment is due to J.M.R. Asselin, L. Bordeleau, G. den Hartog, R.L. Desjardins, B. Grace, H.H. Janzen, C.W. Lindwall, G.A. Neish, and A. St-Yves for their contribution to the management of the AAFC research programon greenhouse gases and ozone. We would also like to acknowledge the contributionof the Program for Energy Research and Development (PERD), managed by NaturalResources Canada.

95

Appendix IThe principal investigators and their project titles are listed here for those who wishmore detailed information.

Investigators and projectsAllard, G. Ozone damage on agricultural speciesTel.: 418-656-2131 x 2706Fax: 418-656-7856E-mail: Guy.allard@plg.ulaval.ca

Angers, D.A. Agriculture management effects on carbon Tel.: 418-657-7980 x 270 sequestration in eastern Canada Fax: 418-648-2402E-mail: Angersd@em.agr.ca

Antoun, H. Physical, chemical, and biological soil Tel.: 418-656-3650 factors that affect N2O and CH4 emissionFax: 418-656-7176E-mail: Antoun@rsvs.ulaval.ca

Baril, P. Development of a plan to reduce Tel: 418-871-1851 greenhouse gas emissions from the animal Fax: 418-871-9625 sectorE-mail: BPRQuebec@groupe-BPR.com

Beauchamp, E.G. Measurement of ßuxes of N2O from (see Thurtell) agricultural sites in Ontario

Bowen, P.A. Ozone impacts to Fraser Valley crops Tel.: 604-796-2221 x 225 grown under Þeld conditionsFax: 604-796-0359E-mail: Bowenp@em.agr.ca

Chalifour, F.P. EfÞciency of N use to limit N2O emission Tel.: 418-656-2131 x 2306 in cerealÐlegume cropping systemsFax: 418-656-7856E-mail: Francois-p.chalifour@plg.ulaval.ca

Chiquette, J. GHG production from ruminants: a Tel.: 819-565-9171 x 249 system approachFax: 819-564-5507E-mail: Chiquettej@em.agr.ca

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Cho, C.M. Investigation on stability, persistence, and Tel.: 204-474-6045 ßux of N2O in laboratory and Þeld soil Fax: 204-275-8099 proÞles

Coxworth, E. Study of the effects of cropping and tillage Tel.: 306-343-9281 systems on the carbon dioxide released by Fax: 306-665-2128 manufactured inputs to western Canadian

agriculture

Desjardins, R.L. Assessment of ozone uptake by Tel.: 613-759-1522 agricultural crops in critical areas along Fax: 613-996-0646 the WindsorÐQuebec corridorE-mail: Desjardins@em.agr.ca

Ellert, B. Contribution of representative prairie Tel.: 403-327-4561 agroecosystems to greenhouse gas Fax: 403-382-3156 emissionsE-mail: Ellert@em.agr.ca

Izaurralde, R.C. QuantiÞcation of nitrous oxide, methane, Tel.: 403-492-5104 and carbon dioxide ßuxes over managed Fax: 403-492-1767 and natural ecosystems of AlbertaE-mail: Cizzaurra@rr.ualberta.ca

Jackson, H.A. Methane and carbon dioxide emissions (see Sauer, F.) from farm animals and manure

Knowles, R. Methane and nitrogen cycle interactions Tel.: 514-398-7890 in agriculture systemsFax: 514-398-7990E-mail:

Lapierre, C. Contribution of liming and tillage to N2O Tel.: 418-657-7980 x 269 and CO2 emissions in eastern CanadaFax: 418-648-2402E-mail: Lapierre@em.agr.ca

MacDonald, B. Characterization of agroecosystems in Tel.: 519-826-2086 eastern Ontario for their potential to act Fax: 519-826-2090 as sources or sinks of greenhouse gasesE-mail: Macdonaldb@em.agr.ca

MacLeod, J. Nitrogen cycling in potato systemTel.: 902-566-6848Fax: 902-566-6821E-mail: Macleodj@em.agr.ca

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Mathison, G.W. Strategic approach to quantifying and Tel.: 403-492-7666 reducing CH4 production by animalsFax: 403-492-9130E-mail: Mathison@afns.ualberta.ca

McCaughey, W.P. Methane production by beef cattleTel.: 204-726-7650 x 211Fax: 204-728-3858E-mail: Pmccaughey@em.agr.ca

McKenny, D.J. Effects of conservation and conventional Tel.: 514-253-4232 x 280 tillage practices with/without Fax: 514-973-7098 subirrigation/controlled drainage on

greenhouse gas emissions from corn production systems in southwestern Ontario

Morrison, M.J. IdentiÞcation of corn and soybean Tel.: 613-759-1556 cultivars with tolerance to atmospheric Fax: 613-952-6438 ozone pollutionE-mail: Morrisonmj@em.agr.ca

Paul, R.J. Nitrous oxide and methane emissions in Tel.: 604-796-2221 x 215 dairy and hog manure management Fax: 604-796-0359 systemsE-mail: Paulj@em.agr.ca

Pattey, E. Impact of agricultural management on Tel.: 613-759-1523 greenhouse gas ßuxesFax: 613-996-0646E-mail: Patteye@em.agr.ca

Prevost, D. Mechanisms involved during burst of Tel.: 418-657-7980 x 239 N2O emissionsFax: 418-648-2402E-mail: Prevostd@em.agr.ca

Richards, J. Losses of fertilizer and soil N by Tel.: 709-772-4619 denitriÞcation in podzolic soilsFax: 709-772-6064E-mail: Richardsj@em.agr.ca

Rochette, P. Contribution of agricultural practices to Tel.: 418-657-7980 the atmospheric increase of greenhouse Fax: 418-648-2402 gasesE-mail: Rochettep@em.agr.ca

Runeckles, V.C. Ozone impacts to Fraser Valley crops Tel.: 604-822-6829 grown under Þeld conditionsFax: 604-822-8640E-mail: userapol@mtsg.ubc.ca

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Schuepp, P.H. Scaling up of GHG emission models on Tel.: 514-389-7935 the basis of land-use mapping and Fax: 514-398-4853 airborne ßux observation

Selles, F. Comparison of present and future crop Tel.: 306-778-7245 management practices on the emission of Fax: 306-773-9123 greenhouse gases in the semi-arid prairiesE-mail: Selles@em.agr.ca

Smith, W. Modelling CO2 and N2O ßuxes for Tel.: 613-256-7093 agroecosystems in CanadaFax: 613-996-0646E-mail: smithw@comnet.ca

Tarnocai, C. Amount of organic carbon in Canadian soilsTel.: 613-759-1857Fax: 613-759-1926E-mail: Tarnocaict@em.agr.ca

Thurtell, G.W Measurements of ßuxes of N2O from Tel.: 519-824-2453 agricultural sites in OntarioFax: 519-824-5730E-mail: Gthurtell@lrs.uo.guelph.ca

Van Kessel, C. Landscape-scale ßuxes of CO2 and N2O Tel.: 306-966-6854 in the PrairiesFax: 306-966-6881E-mail: Vankessel@sask.usask.ca

Zhu, T. Improving ßux-measuring technology Tel.: 613-759-1889 based on the relaxed eddy accumulation Fax: 613-996-0646 techniqueE-mail: Zhut@em.agr.ca

Grace, B. Program CoordinationTel.: 250-494-7711Fax: 250-494-0755E-mail: Graceb@em.agr.ca