soil horizons newsletter, issue 4, january 2000...issue 4, january 2000 printed on recycled paper...

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Soil Horizons Issue 4, January 2000

What's inside

Sindi is alive! ................................. 1

Fate of a microbial tracer applied toeffluent-irrigated soils ................ 2

Land-based effluent treatment:Which soils are best? ................... 3

Stormwater management — a newresearch initiative ........................ 4

Groundwater treatment wall ... 5

Sorption of copper and cadium byallophane-humic complexes ..... 6

Factors controlling soil organiccarbon in pasture soils ................ 7

Withholding fertilizer influencessoil organic matter and nutrientdynamics ....................................... 8

Soil biological parameters inagroforestry regimes ................... 9

Soil bactrium to degrade DDE ... 10

The role of wheel trackcompaction on cropland erosionat Pukekohe ............................... 11

Snow fields ................................. 12

SOILHORIZONS

Issue 4, January 2000

Printed on recycled paper

ISSN 1174-0922

Manaaki WhenuaLandcare Research

A newsletter communicating our work

in soil related research to end-users,

customers and colleagues

Sindi is alive!Sindi (Soil indicatorassessment tool) went live atthe end of August. Sindi is asoftware tool designed toallow users to compare andassess the quality of their soil.It builds on work by GrahamSparling to identify andcharacterise a set of 10 keysoil indicators, and by AllanHewitt to set standards forsoil quality (see Soil Horizons,Issue 3). Implementation ofSindi, on the World WideWeb, by Roger Gibson, was

an ideal way to make the toolwidely available.

The 10 soil indicators are:bulk density, macroporosity,total porosity, pH, Olsen P,base saturation, total carbon,total nitrogen, CEC, andanaerobic nitrogen. Sindiexplains what each indicatoris, how it is measured, andwhy it is important. A userenters measured values forany soil sample of interestonto the web page.

Two means of comparison areprovided. One allows the

Example web page showing bar graphs for the four components ofsoil quality


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Fate of a microbialtracer applied toeffluent-irrigatedsoilsLand treatment of animal orhuman waste can result inchemical and microbialcontamination of shallowgroundwater and/orwaterways. Research haslargely focussed on the fate ofthe chemical load of sucheffluents. Using 50 cmdiameter undisturbed soilcores, Malcolm McLeod, andco-workers, investigated thefate of a host-specificSalmonella bacteriophage thatwas applied to four widelydistributed soils commonlyused for land disposal ofeffluent. The soils were apoorly drained clayey GleySoil from Hauraki Plains; awell-drained Pumice Soil fromTaupo; a well-drainedAllophanic soil developed involcanic ash from nearMatamata; and a well-drainedRecent Soil developed in dunesand from Waitarere. Thirtymm of water containing thebacteriophage and a non-reactive chemical bromidetracer (Br-) were applied to thesoil at a rate of 5 mm/hr,followed by simulated rainfallat the same rate. Leachatecollected at a depth of 70 cmwas analysed for thebacteriophage and bromide

tracer. Collection of leachatecontinued until at least 1 porevolume ( approx. 80 L) hadbeen collected. Bromidemoved uniformly throughthe Allophanic and PumiceSoils with peak concentrationappearing at about 1 porevolume, but thebacteriophage was presentonly at very low levels. Incontrast, both the Br- andbacteriophage tracers movedrapidly through the Gley andRecent soils, appearing in theleachate at high levels withinthe first five litres and thentailing off. Such flow patternsare indicative of preferentialflow. Coarse, well-definedsoil structure in the Gley Soiland finger flow due to waterrepellency in the sandyRecent Soil are considered tobe responsible for thepreferential flow observed inthese two soils. Allophanicand Pumice soils have finer,more porous soil structure,leading to a predominance ofmatrix flow over preferentialflow. Furthermore, allophanicclay has a very high surfacearea and a positive chargethat adsorbs negativelycharged bacteriophage.

This study highlights theneed to select soils carefullywhen land treatment ofeffluent is being considered.

For more information contactMalcolm McLeod, ph (07) 858 3700,e-mail [email protected]

user to see how their samplerelates to data from theNational Soils Database(NSD – see Soil Horizons, Issue1) by displaying a box plot. Asoil indicator in the lower (orhigher) quartile of similarsoils signals a possible soilquality problem. However,the NSD does not containdata for all indicators andpossible land uses.

The alternative is to view bargraphs, which illustrate anexpert interpretation of thefour components of soilquality, given soil order andland use. The soil qualityrating is adjusted dependingon the soil order and landuse. If one component is ofparticularly high or lowquality, the user can thenchoose to view bar graphs ofthe indicators that make upthis component. Informationon why a specific indicatormight be of low quality, andsome management strategiesto improve it, can then beviewed.

The team is currentlyplanning the next version ofSindi, so feedback on whatyou would find useful on thiswebsite is very welcome. Alink to Sindi can be found onhttp://sindi.landcare.cri.nz/soilquality

For more information contactLinda Lilburne, ph (03) 325 6700,e-mail [email protected]

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Land-basedeffluent treatment:Which soils arebest?In New Zealand, land-basedeffluent treatment systemshave become increasinglypopular. However, it is notalways clear which soilsshould be used in thesesystems, or why. Landapplication of effluent canadversely affect ground andsurface water quality ifeffluent constituents, inparticular nitrogen andphosphorus, are notremoved by the soil beforethey reach ground water.Louise Barton and LandcareResearch co-workers haveinitiated a new study thatinvestigates the ability ofdifferent soils to removenitrogen and phosphorusfrom domestic, secondary-treated effluent.

They irrigate large, intact soillysimeter cores withsecondary-treated domesticeffluent on a weekly basis,and test the resultant soilleachate for nitrogen andphosphorus. Soil lysimetershave been collected fromfour soils (Pumice,Allophanic, Gley and Recentsoils) and transported toHamilton where they arehoused in a facility adjacentto the Temple View

treatment ponds. These soiltypes range in texture,carbon content andphosphorus retention.Pumice, Allophanic, Gleyand Recent soils are eithercurrently being used, orbeing considered for use,in land treatment systemsin New Zealand.

At the lysimeter facility,the lysimeters have beenoversown with ryegrass,and fitted with a drainagecollection system at thebase. Half the lysimetersare irrigated weekly witheffluent from the nearbypond (50 mm week-1, i.e.450 kg N ha-1 yr-1 and 110 kgP ha-1 yr-1), while theremaining lysimeters receiveonly rainfall. By comparingnitrogen and phosphorusloading rates with leachatequality, Louise andcolleagues will be able torank the ability of the soils totreat effluent. In addition,soil properties thought toinfluence effluent-N andeffluent-P renovation will bemonitored and comparedwith leachate quality. Incollaboration with LincolnEnvironmental, these datawill be used to test modelsthat will help predict howother soil types, not includedin the study, renovateeffluent.

When the study is completethe group will have

identified soil types andcharacteristics that maximiseeffluent treatment, and henceminimise groundwaterpollution.

Lysimeter faculty

This information will helpland treatment systemdesigners assess thesuitability and capability of aparticular soil to treatdomestic effluent.

For more information contactLouise Barton, ph (07) 858 3700,e-mail [email protected]

SOILHORIZONSVisit the Landcare Research

Soil Quality website at http://

www.landcare.cri.nz/science/soilquality


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Groundwatertreatment wallA treatment wall is astructure for removingcontaminants from water. Asthe contaminated waterpasses through the wall, thecontaminants are trapped byor transformed into harmlesssubstances. These walls maybe made with differentmaterials as some fillings arebetter at removing differenttypes of contaminant, e.g.,groundwater treatment wallsfilled with limestoneeffectively remove the metalslead and chromium, whilethose filled with irongranules are effective atremoving chlorinatedsolvents. Different types offilling work throughdifferent chemical, physicaland biological processes.Another virtue of treatmentwalls is that they are cheapto run — they require nomechanical equipment, suchas pumps.

Treatment walls may also beused to treat surface wastewater and other types ofcontamination, for example,road runoff. The ability offive different fillings toremove heavy metals andtrace organics from artificialroad runoff has beenexamined by MatthewTaylor, Surya Pandey andBob Lee (Landcare Research).

Stormwatermanagement anew researchinitiativeWhen rain falls in urbanregions, impervious surfacessuch as car parks, roads androofs generate enormousvolumes of surface runoffover a short time. Usuallythis runoff is dischargedthrough stormwater drainagenetworks directly into urbanstreams and waterways, withnumerous adverseconsequences. Where runoffvolumes combine furtherdownstream, typical resultscan be flash-flooding,scouring and erosion ofwaterways, and thedestruction of natural aquatichabitats. Over time, thereduced volume ofinfiltrating rainwater leads todepleted groundwaterreserves, and to reducedstream flows during dryperiods. Urban stormwateralso contains variouspollutants (heavy metals,bacteria, pesticides andherbicides, oil and otherhydrocarbons) detrimental tothe health of the receivingwaterways.

While engineering providessome solutions to theseproblems, it is possible to usenatural soil-plant systems(e.g., grassed or forested

areas) to receive and treatstormwater close to itssource. This approach is usedin New Zealand alongsidesome motorways. As well asreducing infrastructure costsand maintenance, there is thebenefit of groundwaterrecharge and the reduction ofthe impact on urban streams.Furthermore, the provisionfor natural ecosystems in theurban environment willbenefit birds and localwildlife.

Little is currently know aboutthe fate of stormwatercontaminants once they enterthe soil-plant ecosystem. Aspart of the NIWA leadprogramme, Stormwater andtransport effects on urbanaquatic ecosystems, a newresearch initiative based inAuckland will investigate theeffectiveness of soil-plantsystems for removingpollutants from stormwater.The intention is to focus onsoil processes and theirability to removecontaminants from road-runoff, with emphasis oncomparing grassed andnative vegetation systems.This work will provide thescientific basis for usingsoil-plant systems to reducethe effects of stormwaterrunoff.

For more information contactJulie Zanders, ph (07) 858 3700,e-mail [email protected]

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The fillings tested werecommercially availableSphagnum moss, crushedlimestone, waste wood pulp,fly ash and waste wool felt.

Wall fillings were tested bypacking 600 by 60 mmdrainpipes with the differentfillings to make a column.Artificial road runoff wasapplied at


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Sorption of copperand cadmium byallophane-humiccomplexesIncreased emphasis onresearch into sustainablemanagement of NZ soilsand their role inenvironmental protectionhas focused attention onprocesses that occur intopsoils. One importantprocess is the interaction oftopsoils with introducedheavy metals such as copper(Cu) and cadmium (Cd)from domestic, industrial,and agricultural sources.Sorption by soil particlesaffects retention andmobility of heavy metals insoils. Clay and humus(organic matter) fractions ofsoil are the most reactiveconstituents. These areclosely associated to form aclay-organic complex, sotheir contributions to heavymetal sorption capacity ofsoil are difficult to assessseparately.

Guodong Yuan, HarryPercival and Benny Thenghave prepared clay-organiccomplexes with a widerange of organic surfacecoverage that can act assurrogates for topsoils inmetal sorption experiments.Allophane was selected asthe clay component. This

clay-size aluminosilicate is atypical short-range ordermineral that is widespread inNZ soils. Allophane is veryreactive to heavy metals butthe underlying mechanism isnot well understood. Humicacid (HA) was used for theorganic component as itrepresents one of the mostabundant fractions oforganic matter in soil.Allophane-HA complexeswith organic carbon (OC)contents of 14–123 g/kgwere prepared.

The sorption of Cu and Cdmetal ions (at 2 mmole/Lconcentration) by allophanealone and allophane-HAcomplexes was carried out atpH 5.0, 5.5 and 6.0.Substantial amounts of Cuwere sorbed (Fig. 1). At eachpH, Cu sorption increaseslinearly with the OC contentof the allophane-HAcomplexes. Cd showed asimilar behaviour but muchless was sorbed under thesame experimentalconditions (Fig. 2). Sorptionof both Cu and Cd is pH-

dependent and increaseswith pH.

There appears to be a levelof organic surface coveragebelow which clay mineralexerts a dominantinfluence, and above whichit plays only a subsidiaryrole. These levels vary withthe pH. At pH 5.5, forexample, the allophane andHA components wouldmake an equal (50/50)contribution to Cu sorptionwhen the complex containsabout 87 g OC/kg. For Cd,the equivalent value isabout 6 g OC/kg. Abovethese values, the HA in thecomplex contributes moreto Cu or Cd sorption thandoes the allophanecomponent.

Sorption patterns arecurrently being interpretedin terms of surface charge,complex formation, and Cuand Cd ion in aqueoussolution.

For more information contactHarry Percival, ph (06) 356 7154,e-mail [email protected]

0 20 40 60 80 100 120 140

Sorbed HA (g OC/kg allophane)

Sor

bed

Cd

(mm

ol/k

g al

loph

ane) 3

00

250

200

150

100

50

0

pH 6.0

pH 5.5

pH 5.0

0 20 40 60 80 100 120 140

Sorbed HA (g OC/kg allophane)

Sor

bed

Cu

(mm

ol/k

g al

loph

ane) 700

600

500

400

300

200

100

0

pH 6.0

pH 5.5

pH 5.0

Figure 1: Sorption of copper byallophane and allophane-humiccomplexes

Figure 2: Sorption of cadium byallophane and allophane-humiccomplexes

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Factors controllingsoil organic carbonin pasture soilsSoil organic carbon (OC) is akey indicator of soil quality,and important forsustaining agriculturalproduction. Harry Percival,Roger Parfitt and Neal Scotthave used the National SoilsDatabase (NSD - see SoilHorizons Issue 1) todetermine factorscontrolling long-term soilOC accumulation inpastures across a range ofclimatic conditions and soiltypes.

In the NSD there were datafor 122 soils underpermanent grass that couldbe used to calculate OCcontent and contents ofselected soil properties in t/ha. For each soil, data wereextracted to give a singlevalue for the 0–20 cm layer.Soil properties were levelsof clay, silt, extractable ironand silicon, and bothpyrophosphate and oxalate-extractable aluminium (Al).The correspondingconcentrations (g/kg soil) ofOC and the soil propertieswere also calculated,weighted over the 0–20 cmdepth. There were 167pedons for whichconcentration data wereavailable. The relationships

between OC content orconcentration and climaticand soil properties wereanalysed using linear andmultiple regressiontechniques for bothindividual and combinedsoil orders.

Overall, clay and silt contentand climatic factors(precipitation andtemperature) by themselvesrelated poorly to OC acrossall soils and within each soiltype, and were not goodpredictors of OC. Clay andsilt concentration alsocorrelated poorly with OC.Pyrophosphate-extractableAl in the form of Al 'gel'related the most strongly toOC, and explained thelargest amount of variationin OC content (55 %) andconcentration (60%) acrossall soils. It also explainedthe greatest amount of OCvariation within each soiltype. For all the soilscombined, multipleregression analysis did notgreatly improve the amountof variation in OC contentor concentration explainedby extractable Al alone. ForAllophanic Soils and GleySoils alone there were somesignificant improvementsbut for all other soil ordersno multiple regressionmodel was significant.

The extractable Al 'gel' is

thought to arise from thedissolution and dispersionof Al and organic matterfrom Al-humus complexes.Much of the NZ landscapewas previously forested,with substantialreplacement of forest bygrasses and legumes in the1800s. Most of the soilscould therefore have hadhigh extractable Al becauseof their low pH and thelarge quantity of Al-bearingminerals. The soils generallyare acid, and pH values ofthe uppermost soil horizonsare commonly between 5and 6.5. Soils with one ofthe subsoil horizons havinga pH less than 5.0 occur inabout 65% of NZ’s landarea. Under theseconditions, Al dissolvesfrom soil minerals and isavailable for complexationwith soil organic matter.

This study suggests that inNZ soils, chemicalstabilization of organicmatter is the key processcontrolling OCaccumulation, and that claycontent or concentrationrelates poorly to long-termOC accumulation. It alsoemphasizes the importanceof land-use history as afactor influencing OCstorage.

For more information contactHarry Percival, ph (06) 356 7154,e-mail [email protected]


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Withholdingfertilizer influencessoil organic matterand nutrientdynamicsIn NZ, superphosphate hashistorically played a vital rolein sustaining productivity oflegume-based pastures. Inthe mid-1980s, depressedagricultural commodityprices and sharp increases insingle superphosphate (SSP)prices resulted in reducedfertilizer inputs anddecreased potentialproductivity. What happensto soil organic matterdynamics and nutrientsustainability when fertilizerapplications are withheld?

Surinder Saggar, LandcareResearch, examined the effectof soil P status and Naddition on thedecomposition of 14C-labelledglucose so that thisinformation could be used todetermine whetherwithholding fertilizerapplications influenced the'quality' of organic matterinputs. The two soils used inthis study varied widely in Pfertility (640 and 820 mg P kg-1) and productivity (4868 and14120 kg DM ha-1), due to thewithholding of SSP fertilizerinputs from one of them.

Withholding fertilizer P had

a significant impact on Cmineralization in thesepasture soils. The 14CO2evolution rates were higherin High Fertility (HF) soilthan LF soil; the addition ofN further enhanced 14CO2evolution rates in both soils(Fig. 1a). A higher proportionof 14C substrate wastransformed to microbialbiomass in LF soil than in HFsoil, suggesting that initialgrowth of soil microbes wasnot restricted by the supplyof inorganic P and N (Fig.1b). However, the turnoverof microbial biomass did notfollow quite the samerelationship and was stronglyinfluenced by P and Navailability. Constraints onglucose decomposition in LFsoil may be due to a lack of Pand N for microbes.Fluctuations in inorganic P

were small in LF soilscompared with those in HFsoils (Fig. 2a), and suggestthat P was cycling moreslowly and tightly throughmicrobes in LF soil. Net Nmineralisation andnitrification rates were alsolow in LF soils (Fig. 2b), andsupport the concept of a slowturnover of microbes in LFsoils because of limitednutrient supply.

The degree of accumulationof organic matter depends onthe difference between Cinput and decompositionrates. In the present study,the soils differed only infertilizer application, whichresulted in widely differentpasture production. Otherrecent studies on these soilsby Surinder Saggar and co-workers showed thatwithholding fertilizer

Figure 1: a) Residual 14C and b)14C microbial biomass at variousstages of decomposition in Lowand High Fertility soils with andwithout N addition.

Figure 2: a) Extractable inorganicP (Pi) and b) net mineralised Nconcentrations in Low and HighFertility soils with and without Naddition.

Net

N m

iner

aliz

ed n

(m

g N

kg-

1 so

il)N

et N

min

eral

ized

N (

mg

NO

3 kg

-1 s

oil)

300

250

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150

100

50

0

-50

300

250

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100

50

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-50

0 14 28 42 56 70 84 98 112 126 140 154 168

Incubation period (days)

a

b

LF, -N

LF, +N

HF, -N

HF, +N

0 14 28 42 56 70 84 98 112 126 140 154 168

Incubation period (days)

30

25

20

15

5

0

a

b200

175

150

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LF, -NLF, +NHF, -NHF, +N

Mic

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al P

(m

g kg

-1 s

oil)

Ext

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able

inor

gani

c P

i (m

g kg

-1 s

oil)

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reduced C inputs to soil.Furthermore, the 'quality' ofinputs was different betweenthe two sites. Twice as muchC was incorporated in HFsoils as in LF soils, but theirtotal C concentrations werefractionally different. Thissuggested that a higherproportion of C inputsdecomposed in HF than in LFsoils. Withholding fertilizerrestricted nutrients supply tomicrobes and slowed organicmatter decomposition. Net Nmineralization rates were alsolow, and little P wasmineralized in LF soil.Addition of N to regularlyfertilized HF soil increased Nand P mineralization.

This research demonstratedthat reduced nutrient supplycan restrict microbial turnover,lower the rate of C loss andslow nutrient cycling.

For further information contactSurinder Saggar, ph (06) 356 7154,e-mail [email protected]

Soil biologicalparameters inagroforestryregimesAgroforestry profitability inNZ varies as relative returnsfrom livestock farming andforestry alter. Additionally,agroforestry seems to improvehill country stability. Researchby several groups of NZscientists has shown thatagroforestry has the potentialto alter microclimate andacidify soils.

Recent investigations bySurinder Saggar and GregorYeates show that 25 years ofagroforestry declined soilorganic C (~15%) and N(~25%), and had a significantinfluence on soil biologicalparameters. They foundmicrobial soil processes

under agroforestry to beprofoundly different fromthose under adjacentgrassland. There was lessmicrobial C (mostly bacteriaand fungi but including somemicrofauna and algae) and Nin soils under P. radiata thanin soils under grassland.With increasing tree stockingrate there was a shift in thecomposition of nematodefauna. Bacterial feedingnematodes dominatedgrasslands, and theproportion of fungal feedingnematodes doubled as treestocking rates increased.

Soil C decomposition andmicrobial activity weremeasured by trappingcarbon dioxide produced bysoils over a 60-week period.Results (Fig. below) showedsoil C decomposition rateswere one and a half times as

Carbon mineralisation in soil (0-10 cm) after 25 years of Pinus radiataagroforestry regimes. The lines represent the accumulated amount of soilcarbon respired during a laboratory incubation.

6000

5000

4000

3000

2000

1000

0

0 5 10 15 20 25 30 35 40 45 50 55 60

Incubation period (weeks)

Re

spire

d C

(m

g C

O2kg

-1 s

oil)

0 stem‘Control’ pasture

200 - 400 stems

50 - 100 stems

Soil Horizons is on the webhttp://

www.landcare.cri.nz/information_services/

publications/newsletters/soilhorizons/

SOILHORIZONS


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much (ca 15 mg CO2-C kg–1

soil) in grassland as in 50–100trees/ha (ca 10 mg CO2-C kg

–

1 soil), and were furtherreduced to one half (ca 5.5 mgCO2-C kg

–1 soil) in 200–400trees/ha. Soils under P.radiata gave off less carbondioxide per unit of biomass(the metabolic quotient) thansoils under grassland. Inother words, there werefewer 'bugs' in soils under P.radiata, and those bugs wereworking slowly. These shiftsin microfauna, microbes andmetabolic quotients appear tobe associated with differencesin the quantity and 'quality'of inputs, and SOMdecomposition rates.Populations of topsoil-mixingearthworms showed similartrends.

This research demonstratedthat increased tree density canalter microbial processes andlower the turnover rate of Cand nutrient cycling. Giventhe ability of soil microfaunaand microbes to recolonisedepopulated areas after treeharvest, no problems are seenin restoring populations ofthese soil organisms after treefelling, provided adequateadjustments to soil pH aremade.

For further information contactSurinder Saggar, ph (06) 356 7154,Gregor Yeates, ph (06) 356 7154e-mail [email protected] [email protected]

Soil bacterium todegrade DDEThe insecticide DDT wasonce used extensively tocontrol both agriculturalpests and disease vectors.Although many countriesstopped using DDT over 20years ago, its residues stillpersist in the environment,predominantly as DDE,DDD, and 2,4'-DDT, as wellas unchanged DDT. Amongthese residues, DDE is themost toxicologicallysignificant as it blocks theaction of androgens in rats,and has been implicated inmale reproductiveabnormalities in alligators. InNew Zealand topsoils, thepresence of DDT residues,often as DDE, limits land-useoptions and may impact onthe trade of agriculturalproducts. Cost-effectiveremediation methods forwidespread low levelcontamination are thereforerequired. While bacterialdegradation of DDE hasrecently been reported in arecombinant strain thatdegrades biphenyl, it is notknown how widespread thiscapacity is in nature. SoJackie Aislabie attempted toisolate DDE-metabolisingbacteria from DDTresidue-contaminatedagricultural soils. Thepresence of organisms with

this degradative capacity incontaminated soil may bebeneficial in developingtreatment options.

A Gram-positive bacterium,Terrabacter sp. strain DDE-1,which is able to metaboliseDDE when induced withbiphenyl, was isolated. Thisis only the second report ofextensive metabolism ofDDE, and the first report ofthis metabolism in a Gram-positive organism. Thisstudy provides evidence forthe usefulness ofbiphenyl-metabolisingbacteria for in situremediation ofDDE-contaminatedagricultural soils. To enhancedegradation of DDE in soils,information is needed on thein situ abundance anddistribution of bacteria withthis type of metabolism, andon the substrates that willinduce the expression of thecatabolic pathway for DDEdegradation. While biphenylhas proved a useful substratefor isolating Terrabacter sp.strain DDE-1, the applicationof biphenyl to soils is notdesirable as it is on the USEPA priority pollutant list.Fortunately, a number ofplant terpenes, includingcymene and limonene, havebeen shown to stimulatedegradation ofpolychlorinated biphenyls.Whether these compounds

11


also induce DDE degradationin Terrabacter sp. DDE-1 hasyet to be determined.

Although remediation ofDDT residue-contaminatedsoils is difficult, the isolationof a bacterium with theability to degrade DDE fromsuch a site indicates thatmicrobially mediatedprocesses for cleanup of DDTresidue-contaminated soilsare worth furtherinvestigation.

For more information contactJackie Aislabie, ph (07) 858 3700,e-mail [email protected]

The role of wheeltrack compactionon cropland erosionat PukekoheErosion is a major challengeto sustainable vegetableproduction at Pukekohe. Pastresearch has established therates of soil redistributionwithin fields, and the net lossof soil into streams. Presentresearch aims to understandthe mechanisms of erosionand provide vegetablegrowers with practicalmanagement techniques forreducing erosion.

Many of the vegetable cropsare grown in beds, and thewheel tracks between thebeds appear to be the keyzones for initiation of surface

runoff and erosion becausethey are sites of waterconvergence, and the soils arehighly compacted, whichlimits water infiltration.Control of water movementalong the wheel tracks maybe a key to reducing rates oferosion.

In the spring and summer of1998, Les Basher and CraigRoss (Landcare Research)undertook the firstinvestigation in the Pukekohearea of the role of wheeltracks in the management oferosion. They comparederosion and infiltration rateson cultivated anduncultivated wheel tracks attwo fields. While the trial wasable to demonstrate howcultivating wheel tracks canimprove water infiltration,there were few significantstorms during the trialperiod, and so no conclusionwas reached on the value ofthis practice in reducingerosion. Cultivating thewheel tracks increasedinfiltration rates by twoorders of magnitude.Infiltration rates in theuncultivated tracks were onaverage 350 mm/hr.Although there was a verywide range for individual

infiltration ratemeasurements (4–1715 mm/hr), most measurementsexceeded 100 mm/hr andwere greater than typicalrainfall intensities.

A current investigation aimsto confirm the role ofcultivating wheel tracks inimproving infiltration, and tomeasure differences inerosion rates through thewinter and spring periodsthat were not sampled in the1998 trial. Observations in theprevious trial and after arainstorm in January 1999suggest that the effect ofcompaction of wheel trackson infiltration (and hencerunoff and erosion) reducesas the surface soil dries andcracks. For example, in thesevere storm in January 1999,wheel tracks did not appearvery significant in theinitiation of erosion. Thissuggests that once surfacecracking occurs there may beno need to cultivate wheeltracks to improve infiltration.

Infiltration rates will bemeasured at three times totest this idea. Results ofinfiltration ratemeasurements in June andOctober clearly demonstratedthe low infiltration rates inwheel tracks, how trackcultivation can increase theserates, and that infiltrationrates in the onion beds arewell in excess of rainfall


12

Editors: Peter StephensAnne Austin

Layout: Nicolette Faville

© Landcare ResearchNew Zealand Ltd 2000

Unless otherwise stated, funds for the research described in this newsletter wereprovided by the Foundation for Research, Science and Technology.

Published byLandcare ResearchPrivate Bag 11052Palmerston NorthNew Zealand

Telephone: (06) 356 7154Facsimile: (06) 355 9230

SOILHORIZONSContacts

Snow fieldsLandcare Research staff BarryFahey and Kate Wardleassessed the extent to whichvegetation and soil have beendisturbed by snow groomingat Treble Cone ski field near

Wanaka for the Departmentof Conservation. Field visitsto three west Otago ski fields(Coronet Peak, TheRemarkables, and TrebleCone) showed that cushionfields appeared to be themost vulnerable to damagefrom snow grooming.Permanent 30-m transectswere established acrosscushion fields and inter-tussock areas that weregroomed and skied, and alsoon non-groomed slopes toserve as controls. Data werecollected to estimatepercentage ground cover forsix vegetation classes, and toestablish the percentagefrequency of plant species ineach class. Information wasalso collected on the depth ofthe A-horizon, bulk density,and penetration resistance.

Although there were nodifferences in speciescomposition, groomed slopeshad a higher proportion ofbare ground. There were nostatistically significant

differences in soil bulkdensities or penetrationresistance. A subsequentinvestigation in the winter onsome of the same transectsshowed the snow pack ongroomed slopes to havehigher densities andequivalent water contentsthan the snowpack on non-groomed slopes.

Provisional results indicatethat disturbance to vegetationat Treble Cone is notexcessive. However, it iswidespread, and may be on-going. Overseas researchsuggests that the types ofchanges in snow propertieson groomed slopes observedat Treble Cone may besufficient to inhibit soilbacteria and litterdecomposition in the near-surface soil horizons.Temporal trends invegetation disturbance arebeing investigated.

For further information contactBarry Fahey, ph (03) 325 6700,e-mail [email protected]

intensities. Results alsosuggest infiltration rates inthe uncultivated wheeltracks are increasing.Preliminary measurementsshow erosion rates from theuncultivated tracks are atleast 10 times higher thanfrom cultivated wheel tracks.Shallow cultivation of wheeltracks using a single tyne isan effective and practicaltechnique to reduce erosionrates.

The Franklin SustainabilityProject and the Foundationfor Research, Science andTechnology have funded thiswork.

For more information contactLes Basher, ph (03)325 6700,e-mail [email protected]

soil horizons newsletter, issue 4, january 2000...issue 4, january 2000 printed on recycled paper...

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