Soil degradation:

a major threat to humanity

Soildegradation:amajorthreattohumanity

WrittenandresearchedbyRichardYoungandStefanoOrsini,withIanFitzpatrick

PublishedbytheSustainableFoodTrust

38RichmondStreet

Bristol

UK

http://sustainablefoodtrust.org

December2015

Coverimage:EffectsofsoilerosiononfarmlandinShottishamnearWoodbridgeSuffolk,UK,2007.©ClyntGarnhamEnvironmental/AlamyStockPhoto

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Soildegradation–amajorthreattohumanity

Summary

• Soildegradationneedstoberecognised,alongsideclimatechange,asoneofthemostpressing

problemsfacinghumanity.Solutionsneedtobedevelopedandintroducedwhichaddressbothissuessimultaneously.

• ResearchbytheEconomicsofLandDegradationInitiativein2015calculatedthatsoildegradationiscostingbetween$6.3and10.6trilliondollarsperyearglobally,butthesecostscouldbereducedbyenhancingsoilcarbonstocksandadoptingmoresustainablefarmingmethods.

• AresearchgroupatCranfieldUniversityestimatedthatinEnglandandWalessoildegradationcosts£1.33billionannually.Halfofthiscostrelatestolossofsoilorganiccarbon(SOC),andtheintensityoffarmingisamajorcauseofsoilcarbonloss.

• Landusechangecansignificantlyreducesoilorganiccarbonandincreasecarbondioxide(CO2),nitrousdioxide(NO2)andmethane(CH4)emissions.ChanginglandusefrompasturetocroplandresultsinthegreatestlossofSOC.

• Farmingpracticescanbeemployedtoimprovesoilqualityandincreasesoilcarbon,includingoptimalfertilisation,crop-grasslandrotation,hedgerowplantingandanimalmanureapplication.TheeffectsofotherpracticestoSOCstocks,likeno-tillandgreenmanures,aredebated:recentstudiesshowthattheircontributionisoftenlimited,andinmanysituationsno-tillactuallyleadstoyielddeclinescomparedwithconventionaltillagesystems.

• Inaridandsemi-aridregions,salt-inducedsoildegradationisoneofthemostwidespreadsoildegradationprocesses.Ithasbeenestimatedthatoverthelast20years,2,000hectaresofagriculturallandperday,anareathesizeofFrance,hasbeenlostduetosalinisation.Thisisequivalenttoaglobaleconomiclossof$27.3billionperyear.Efficientwatermanagement,alongwithbetterfertiliseruseandimprovedcropvarietiescouldsignificantlyreducethenegativeeffectsofsalt-inducedsoildegradation.

• Giventhetechnologicaladvancesthathavebeenmadeinrecentyearsandthegreaterscientificunderstandingoftheissuestoday,alltypesofsoildegradationarepotentiallyreversible,aslongasthereissufficientpublicsupport,understandingandpoliticalwill.

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1.IntroductionSoilisavitalresourceforthefutureofhumanity.Itneedstobeprotectedandenhanced.Instead,morethanhalf(52%)ofallfertile,food-producingsoilsgloballyarenowclassifiedasdegraded,manyofthemseverelydegraded(UNCCD2015).Throughouthumanhistory,atleasttwelvepastcivilisationshavefloweredonfertilesoilsandmadehugeadvances,suchasthedevelopmentofwrittenlanguage,mathematicsandfinancialsystems,onlytodisappearovertimeastheirsoilsprogressivelydegradedandcouldnolongerfeedtheirpopulations.Thesecivilisationsoccupied,ordependedon,definedgeographicalregions:theSumeriansinMesopotamia,theRomanEmpire’sexploitationoftheoncehighlyfertilesoilsofnorthAfrica,partsofancientGreece,China,CentralAmerica,Indiaandelsewhere.Thedamagedonetosoilsintheseregionsisstillpresenttoday,butnewcivilisationswereabletospringupelsewhere,convertingforestsandnativegrasslandstoagricultureandthrivingonthefertilitythathadbuiltupoverthousandsofyearsinthesoil.Today,however,duetotheglobaltradeinfood,theglobaladoptionofexploitativefarmingmethodsandtheextenttowhichforestsandnaturalgrasslandshavealreadybeenconvertedtocropproduction,itistheentireglobalcivilisationthatisthreatedbyprogressivesoildegradation.

Soildegradationisacriticalandgrowingglobalproblem,withimplicationsforanumberofkeypolicyareas,includingfoodsecurity,climatechange,floodriskmanagement,droughttolerance,drinkingwaterquality,agriculturalresilienceinthefaceofnewcropdiseases,biodiversityandfuturegeneticresources.Discussionsaroundclimatechangeseldomrefertosoil,eventhoughthesoilsglobally,downtoonemetreindepth,contain1,500billionsoftonnesoforganiccarbon,overthreetimesasmuchcarbonastheatmosphere1.Soildegradationaddscarbon(C)andreactivenitrogen(N)totheatmosphere,increasingglobalwarming,whichinturnacceleratesdegradationprocesses(Lal2004).Moreover,soilcontainsover98%ofthegeneticdiversityinterrestrialecosystems(Fiereretal.2007).Soilbiodiversity,however,isnotmentionedintheGlobalBiodiversityOutlookfromtheUNConventiononBiologicalDiversity(CBD),itisnotreferredtointheInternationalUnionforConservationofNatureRedListofThreatenedSpecies,andithasnotbeenconsideredbytheUKNaturalCapitalCommittee.

Agricultureandthefoodweeatdependonsoil.Underappropriatemanagementsoilsareaninfinitelyrenewableresource,whileunderinappropriatemanagementtheyareeffectivelyaveryfiniteresource.Undernaturalconditionitcantake500-1,000yearstoformaninchofsoilfromparentrock.

1Theglobalsoilcarbonpoolis2,500billionsoftonnesandincludes1,550billionsoftonnesofsoilorganiccarbon(SOC)and950billionsoftonnesofsoilinorganiccarbon(SIC).Thesoilcarbonpoolis3.3timesthesizeoftheatmosphericpool(760billionsoftonnes)(Lal,2004).

SoildegradationexplainedSoilisacomplexmixtureofmineralsobtainedfromthebreakdownofunderlyingrocksorsub-soils,organicmatterobtainedfromthedecayofplantandanimalmaterial,water,airandothergases,plusbiologicallifeintheformofworms,insectsandmicrobes.Soilsvarywidelyintheircompositionandsomearemuchmoreresilientthanothers.Soildegradationisthedeclineinanyorallofthecharacteristicswhichmakesoilsuitableforproducingfood.Soildegradationoccursthroughthedeteriorationofthephysical,chemicalandbiologicalpropertiesofsoilthatresultsinsoilcompaction,salinisation,acidification,andsoillossfromwindandwatererosion.

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Recentestimatesindicatethateveryyear:

• Soildegradationaffects1.9billionhectares• 12millionhectares(23hectaresaminute)oflandislosttofoodproduction• 24billiontonnesoffertilesoilisirretrievablywashedorblownaway(3.4tonnesforeveryhumanon

theplanet).

Itisalsoprojectedthatthiswillleadtoa12%declineinglobalfoodproductionoverthenext25years,resultingina30%increaseinworldfoodprices(UNCCD2015).Thiswilloccurduringaperiodwhenmoreisdemandedofsoilsthaneverbefore,duetothegrowingglobalpopulationandclimatechange.Themostlikelyreactiontothiswillbetoencourageevengreaterintensificationoffoodproduction(andthisisalreadyhappening),butwhilethismaybringsomeshorttermincreasesinproductivityanddelaythedevelopmentoffoodshortages,allindicationssuggestitwillonlyincreasetherateofsoildegradation,makingtheunderlyingproblemevenworse(UNCCD2015).Thiswillincreasepressureforthelastremainingareasofnaturalgrasslandandforesttobeconvertedtofoodproduction,somethingthatwouldhaveadevastatingimpactonthenaturalworldandplanetarysystems.Assuch,theissueofsoildegradationneedstobegivenahighprioritynow,beforewereachacrisispoint,andbeconsideredalongsideclimatechangeasoneofthegreatestthreatsfacinghumanity.Thisisalsoimportantbecausesomeofthemostactivelypromotedmeasuresformitigatingclimatechangewillactuallyincreasesoildegradation.Itisthereforeessentialthatthesetwoglobalthreatsareconsideredtogether,sosolutionscanbedevelopedwhichaddressthemsimultaneously.2.Soilorganiccarbon(SOC)anditsimportanceforsoilqualitySoilorganiccarbonisthemajorcomponent(approximately58%)ofsoilorganicmatterandplaysavitalroleinagriculturalproductionandinarangeofothersoilfunctions.Itisalsorecognisedasthemostusefulsingleindicatorofsoilquality(SoilCarbonInitiative2011).Formanyyearsitwasassumedbymanyadvocatesofintensivefarmingthatsoilcarbonwasnotalimitingfactorincropyieldsbecausestudiesfailedtoshowotherwise,butmorerecentresearchhasshownthatoncesoilcarbonfallstoolow,maximumcropyieldscannotbeachievednomatterhowmuchfertiliserisused(Johnstonetal2009).Soilorganiccarbon(SOC)alsoimprovesthephysicalpropertiesofsoilthatincreasetheextenttowhichitcanabsorbrainfallandholdwater,makingitavailableforlatercropuse.OtherimportantpropertiesinfluencedbySOCinclude:theabilityofsoilstoabsorbminerals,includingfertilisernitrogen,withoutbecomingmoreacidic,itsroleasamajorsourceofnutrients,abilitytoreduceleaching,andtheextenttowhichitenhancesmicrobialbiomassactivityandspeciesdiversityofsoilfaunaandflora.Cropsgrowninsoilswithaloworganiccarbonlevelarealsomoresusceptibletodisease(Barrios2007;Stoneetal.2004;USDA2004;AltieriandNicholls2003).Depletionoforganiccarboninagriculturalsoilsisexacerbatedby,andinturnexacerbates,soildegradation(UNCCD2015).Between42─78billionsoftonnesofcarbonhavebeenlostfromsoilsoverthelastcenturyduetodegradation,mostlyemittedtotheatmosphereascarbondioxide(CO2)andothergreenhousegases(GHGs)withnegativeimplicationsforclimatechangeandfoodproduction(Lal2004).InEnglandandWalesthetotalestimatedorganiccarbonlossfromthesoileachyearis5.3milliontonnes,correspondingtoameanrateof0.6%oftheexistingsoilcarboncontent(Gravesetal.2015;Bellamyetal.2005).Soilmicrobiallifeonlymakesup5%ofsoilorganicmatter,butitisneverthelessvitallyimportantinrelationtomaintainingorenhancingSOClevels,becauseitisresponsibleforthedecompositionofcropandlivestock

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wasteandanyaddedorganicmaterial,whichoncefullydecomposedconstitutesthesoil’sstableorganicmatter.Thiswillslowlyincreaseordecreaseovertimeaccordingtofarmingmethods,temperature,soiltypeandotherfactors,butundecomposedorganicmatterrapidlyoxidisestocarbondioxideandisthereforeoflittlevalue(Johnstonetal.2009).Thisisalsoimportantbecausesomeofthemostactivelypromotedmeasuresformitigatingclimatechangewillactuallyincreasesoildegradation.Examplesofthisincludethepromotionofchickenoverredmeat(Esheletal.2014)andthepublishedpapersclaimingthatfeedlotbeefintheUSismoreenvironmentallyfriendlythanbeeffromcattleraisedongrass(KoneswaranandNierenberg2008).Inbothcasesthefocusisonthedirectgreenhousegasemissionsfromtheanimalsandfailstoincludetheemissionsassociatedwithlandusechangeandthenongoingcontinuouscropproductiontogrowthefeed.Oncetheseareincludedtheoveralldifferencesinemissionsisverysmallbutwiththegrain-fedmeatalsocomesincreasedlanddegradation(VanMiddelaaretal.2013;AveryandAvery2008).Itisthereforeessentialthatthesetwoglobalthreatsareconsideredtogether,sosolutionscanbedevelopedwhichaddressthemsimultaneously.3.Landusechangeandinappropriatelandmanagementpractices-majorcausesofsoildegradation,SOCreductionandGHGemissions.Becausethereismorethanthreetimesasmuchorganiccarboninthesoilasthereiscarbonintheatmosphere(Lal2004),smallchangesinSOClevelscanhavealargeimpactonatmosphericcarbondioxideconcentrations(Stockmannetal.2013).LandusechangefromnativeforestsandnaturalgrasslandstoagriculturecontributestoGHGemissionsthroughthemineralisationofsoilcarbon2anddecompositionofvegetation.Landusechangeisestimatedtohavecontributed55-135billiontonsofcarbonemissionasCO2

duringtheperiod1850-1998(Lal2004).WhileitmightbeassumedthatthegreatestlossesofSOCresultfromconversionofforeststoagriculture,andthisistruewhenthebiomassofthetreesisincluded,ameta-analysisof74studies(GuoandGifford2002)foundthatinrelationtoSOC,grasslandsoilsconvertedtocropproductionlost59%oftheircarbon,whileforestsoilslost42%oftheirs.Reversingtheselandusechangesleadsovertimetoreversalinthesetrends,returningcarbonfromtheatmospheretothesoil.Incontrast,conversionofcroplandtograsslandleadstoanaverage19%increaseinSOC,butanothersurprisingfinding,onebasedon170separateobservations,isthattheconversionofnativeforesttograsslandleadstoanaverage8%increaseinSOC,(GuoandGifford2002,Stockmannetal.2013).InsoilswithahighclayfractionSOCcarbonlevelsdonotfallindefinitely,butinsteadfalltoalowerequilibriumlevel.Incontrast,lightandsandysoilscontaininglittleclayarelessabletoresistsoilcarbondeclinewhichcanfalltoextremelylowlevelswherethesoilisincapableofsupportinglivestockorgrowingcrops(Johnstonetal.2009,Johnston2011).Thisisaveryimportantpoint,whichappliesinparticulartomanysoilsinAfrica,whichunderinappropriatefarmingsystemshavethepotentialtodegraderapidly(FAO1995,EliasandFantaye2000).LandusechangeandsoilnitrogenlossesWhileithasreceivedlessattentionthancarbon,landusechangealsoresultsinthelossofnitrogenfromsoils,andincontrasttocarbon,whichislostannuallyover50-100years,nitrogenislostveryrapidlyintheformofnitrousoxide,themostpersistentandpotent,inglobalwarmingterms,ofallthemajorgreenhousegases.AstudyundertakenintheNetherlandsfoundthattakingnitrogenlossesintoaccountincreasedthenetGHG

2Inthiscontext‘mineralisation’principallyreferstotheoxidationofcarbontoproducecarbondioxide

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impactofconvertinggrasslandtocropproductionby50%,withanassociatedsoilfertilitydecreaseanddisruptionofnutrientcycles(Vellingaetal.2004).Incontrasttocarbon,whichisbothlostandsequesteredprincipallyascarbondioxide,nitrogenislostasreactivenitrousoxidebutsequesteredmostlyfromtheunreactivedi-nitrogengaswhichmakesup80%oftheairwebreath,suggestingthatinglobalwarmingtermsthelossofsoilnitrogenisverymuchhardertoreversethanthelossofsoilcarbon.ThesoilmethanesinkLandusechangecanalsoreducethesoilmethanesink-thecapacityofcertaintypesofsoilbacteria(methanotrophs),whichusemethaneasanenergysource,tonegatemethaneemissionsfromothersoilmicrobesandalsobreakdownatmosphericmethanetocarbondioxideandwater(Tate2015;Nazariesetal.2013).Thegreatestdamagetothesoilmethanesinkoccurswhenforestlandorestablishedgrasslandsareconvertedtocroplandsandwhenammonia-basednitrogenfertilisersareused(ActonandBaggs,2011;ReayandNedwell,2004;Willisonetal1995).Atmosphericmethanelevelsareaseriousglobalconcernbecauseconcentrationsareapproximately2.5timeshigherthanbeforetheindustrialrevolution.Becauseofthehugescaleofthemethaneproblem,moststudieshavetendedtoignoreorunderestimatethesignificanceofthesoilmethanesink,becauseitonlybreaksdown5%ofglobalmethaneemissions(therestmostlybeingbrokendownintheatmosphereover10-12years).Butsinceglobalmethanelevelshavebeenincreasingby0.1%peryearoverthelastdecade(Nazariesetal.2013)itcanbeseenthatevenasmallreductioninthesoilmethanesinkcouldmakeamajorcontributiontooverallmethaneincreasesovertime.SoilcarbonsequestrationSoilcarbonsequestrationisthereverseofSOCloss.Itisthenetremovalofcarbonfromtheatmosphereanditsstorageinsoil.Allplantstakecarbondioxidefromtheatmosphereandcombineitwithwaterthroughphotosynthesistoproducecarbohydrates,thebuildingblocksofplantgrowth.Thedifferencebetweenfarmingmethodswhichresultinanetlossofcarbonandthosewhichresultinanetgainrelateslargelytotheextenttowhichplantandanimalresiduesareabletodecomposeandturnintohumus,thesoil’smoststablecarbonreserves,ratherthanrapidlyturningbacktocarbondioxide.Thisisinfluencedbyfactorssuchascroppingpatterns,methodandextentofcultivationandlivestockgrazingdensity,comparedwithgrasslandproductivity.Grasslandsoilsunderextensivemanagementconditionsaresignificantstoresofcarbon,withglobalcarbonstocksestimatedat50%morethantheamountstoredinallforests(FAO2010).Allsoilsalsohaveanequilibriumpoint(whichdependsonsoiltypeandotherfactors)abovewhichcarbonlevelscannotbeincreased,butthecarbonsequestrationpotentialoftheworld’sgrasslandshasbeenestimatedat10─300millionsoftonnesofcarbonperyear(Lal2004).4.Thefinancialcostofsoildegradation,globallyandintheUKDeterminingthevalueofsoilasnaturalcapitalwouldhelptotranslatethegreaterunderstandingwehavetodayofsoildegradation,throughadvancesinsoilscience,intopolicyforsustainabledevelopment.Thisrequiresamultidisciplinaryapproachengagingthefieldsofecologyandeconomics.ArecentstudybytheEconomicsofLandDegradationInitiative(ELD)calculatedthatglobalsoildegradationcostsusbetweenUS$6.3andUS$10.6trillion(£4.4to£7trillion)peryear.TheELDstudyalsoestimatedthat

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US$480billion(£317billion)couldbegeneratedbyenhancingcarbonstocksinsoils,andthatbyadoptingmoresustainablefarmingpracticesincreasedcropproductionworthanUS$1.4trillion(£900million)couldbeachieved(ELD2015).ThecostofsoildegradationinEnglandandWalesSomeideaoftherelativesignificanceofsoildegradationcomparedwiththemuchmoreeasilyunderstoodissueofsoilerosion(whichisjustonemanifestationofsoildegradation)canbeseenbycomparingthecostsofsoilerosioninEngland,estimatedat£45millionperyear,whichincludes£9millionpoundsinlostproduction(Defra,2009)withthecostsofsoildegradationinEnglandandWaleswhichhasrecentlybeencalculatedbyaresearchgroupatCranfieldUniversity.TheyestimatethevalueofquantifiablesoildegradationinEnglandandWalestobeapproximately£1.33billionperyear(Gravesetal.2015).Thisvalue─whichdoesnotincludethecostsassociatedwithlossofculturalservices3,soilbiotaorsoilsealing4─ismainlylinkedto:lossoforganiccontentofsoil(47%oftotalcost);compaction(39%);anderosion(12%).Theestimatedcostsofsoildegradationarepositivelycorrelatedwiththeintensityoffarming.OtherstudiesalsoshowthatsoildegradationinBritainhasoftenbeenlinkedwithintensiveagricultureandinappropriatelandusepractices.Forexample,asaconsequenceoftheconversionoflargeareasofextensivegrasslandtointensivearablefields,thehighesterosionrateseverrecordedintheUKoccurredintheSouthDownsofSouthEastEngland(Boardmanetal.2003).InthecontextoftheUKNaturalEnvironmentWhitePaper(2011)andtheEUThematicStrategyforSoilProtection(2006),theevaluationexerciseconductedbyCranfieldUniversityprovidesaneconomicargumenttohighlighttheneedtoreducesoilcompactionanderosiononintensivelyfarmedsoilsandmaintaintheorganiccontentofsoilsingeneral.5.Salt-inducedsoildegradationSalinisationisoneofthemostwidespreadsoildegradationprocesses,whichcommonlyoccursinaridandsemi-aridregions,whererainfallistoolowtomaintainadequatepercolationofrainwaterthroughthesoilandirrigationispracticedwithoutnaturalorartificialdrainagesystems.Irrigationpracticeswithoutappropriatedrainageresultintheaccumulationofsaltsintherootzone,affectingseveralsoilpropertiesandcropproductivitynegatively.Salinisationalsooccursintheseregionsbecauseevaporationratesareextremelyhigh,meaningthatwithcommonlyusedirrigationsystemsmuchofthewaterevaporatesrapidly,leavingdissolvedmineralsaltinthetoppartofthesoilprofile,whichbuildupprogressivelyovertime.Ithasbeenestimatedthisresultsin2,000hectaresofonceproductivefarmlandeverydayforthelast20yearsreachingthepointwheredegradationbecomessoseverethatfurthercropproductionhastobeabandoned(Qadiretal.2014).OverthelasttwodecadesanareathesizeofFrance,i.e.about62millionhectares,hasbeenlostinthisway,withanestimatedglobaleconomiclossof$27.3billionperyear(Qadiretal.2014).Improvedwatermanagement,alongwithbetterfertiliseruseandimprovedcropvarietieshavethepotentialtoreducethenegativeeffectsofsalt-

3Lossofculturalservicesandrelatedcostscouldbesubstantialinlocationswhere,forexample,soilrelatedlossofwaterqualityinriverscompromisesrecreationalinterestsandwhereerosionaffectsthevaluetopeopleoflandscapes,habitatsandbiodiversity(Gravesetal.2015).4Thelossoflandunderconcrete,tarmacandnewbuildings

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inducedsoildegradationconsiderably,butinsomeareasfundamentalconsiderationneedstobegiventowhetherthecultivationofcropsrequiringirrigationisthebestandmostreliablewaytoproducefoodorwhetherthelandwouldbetterbereturnedtolowintensitygrazingusingdeep-rootinggrassesandherbs,duetotheirgreaterdroughtresistance,beforethepointofno-returnisreached.6.CombattingsoildegradationSomeagriculturalpracticesareknowntosequesterSOC,reduceGHGemissionsandreversesoildegradation,whileothersarewellknowntoreduceSOCandincreasedegradation.Inbetweenthesecomeanumberoftechniques,somewidelyemployed,whereevidenceoftheirsequestrationpotentialiseitherdisputedorhasrecentlybeencalledintoquestionbynewresearch.Adetailedconsiderationoftheseisoutsidethescopeofthispaper,butinthissectionwewillsummarisesomeofthekeyissuesanddilemmas.Akeypointtonoteisthatbecausesoilsvarysomuchbetweencountries,regions,farmsandevenindividualfields,nosinglesolutionfitsallcases.Insteadsolutionsneedtobetailoredtoeachsituation.Reversinglanddegradationisalsopotentiallymucheasierinsomeregionsandwithsomefarmingsystemsthanwithothers.Assuchthereisacaseforpickingofftheeasieronesfirst.Oneofthesemightbeintensivehorticulturalsystems,suchasthoseinpartsofCalifornia,SpainandSouthernEngland.Giventhepoliticalwill,SOCcouldbeincreasedrapidlyinthesesystemsthroughtheuseofcarefullypreparedcompostderivedfromurbanwasteandthereplacementofnitrogenfertiliserwithcompostandfertilitybuildingbreakcrops,suchascloverandotherlegumes.Therelativelyhighprofitabilityofthesesystemsperhectaremeansthatmostproducerscouldaffordtointroducemorediverserotationsifthiswere,forexamplearequirementofcrosscomplianceandGoodAgriculturalandEnvironmentalConditions,whichproducersneedtosatisfyinordertoreceivedsubsidypaymentsthroughtheBasicPaymentsScheme.SoildegradationintemperategraingrowingregionssuchasnorthernEurope,partsofRussiaandtheUSaswellasmuchofAsiacouldbereducedbyimprovementsonexistingsystemsandrotationsbutonlyreversedbythere-introductionofmixedfarmingsystems.However,theseregionshaveadequaterainfallandsufficientlymoderatetemperaturestomakesuchchangespossible.Moredifficultwouldbeaddressingthedegradationthathasoccurredinextensivegraingrowingregions,suchaspartsofAustralia,thesouthernUS,partsoftheformerSovietUnionandpartsofSouthAmerica,drylands,suchasthoseinNamibia,whereprecipitationislimitedorerratic,orclimateextremes,suchashardfrostslimitoptions.Hardeststillistoreversethedegradationinsub-SaharanAfrica,otherdesertregions,suchasintheMiddleEast,inmountainsoilsandotherareaswherelargepopulationsaretryingtosurviveinsomeofthemostdifficultconditionsontheplanet.However,inalloftheseareasthereareinspiringexamplesofwhatcanbeachievedwithdeterminationandimagination.Whatisneededisforgreaterpublicandpoliticalunderstandingoftheissues,theextenttowhichourshoppingchoicescontributetotheproblemsandanunderstandingofwhytheconstantfinancialpressurepushingproducersworldwidetointensifyandincreasethesizeoftheirholdingsorleavetheindustryisnotinthebestinterestsofconsumers.

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Despite,theneedfordifferentapproachessomegeneralpointscanbemade.LongtermexperimentsintheUKatRothamstedExperimentalStation5showthatSOCdeclinessteadilywheregrasslandsoilisploughedandconvertedtocontinuousarablecropproduction,whileitincreasessteadilywherearablecroplandissowntograss(Johnstonetal.2009).ThesestudiesalsoshowthatrotationsalternatingbetweencroplandandgrasslandhavethepotentialtomaintainorslowlyincreaseSOClevelsprovidingthecorrectbalanceisachievedbetweencarbonbuildingundergrassandcarbonexploitationundercropping.ThereappearshowevertobeverylittlemainstreamresearchlookingatSOCtrendsintraditionalmixedfarmingsystemsintheUK,suchasthoseusedbymanyorganicandbiodynamicfarmers,whereinadditiontorotationsalternatinglandusebetweengrassandcropproduction,animalmanurescombinedwithstrawandotherbeddingmaterialproducetraditionalfarmyardmanurewhichisthenthoroughlycompostedbeforebeingappliedtotheland.However,areviewofstudiesquantifyingsoilcarbonlevelsinorganicandequivalentintensivelyfarmedsoils,bytheSoilAssociation,foundthatonaverageorganicallyfarmedsoilscontained28%morecarbon(Azeez2009).IntheUK,however,overthelastdecadetherehasbeenacontinuingconversionofgrasslandtocropland,driveninlargepartbythelowprofitabilityinthegrazinglivestocksectorwiththeoverallnumberofgrazinganimalsdeclining.Amongstlandmanagementpractices,severalstudiesshowthatasanalternativetoconventionaltillage,no-tillisbeneficialtothefunctioningandqualityofsoil:ithelpstoincreaseresiliencetoweathervariabilityandcontributestoclimatechangeadaptationinsomesituations(Garcia-Francoetal.2015;Powlsonetal.2014;UNEP2013;Lal2012).However,ameta-analysisoftheevidenceundertakenintheUKbyPowlsonetal.(2014),showedthatmitigationpotentialofno-tillthroughsoilcarbonsequestrationisactuallyverylimitedandhasbeenwidelyoverstated,withanymeasurableincreaseinSOCinthetopfewinchesofsoildoingnomorethancompensatingforSOClossesatgreaterdepths.Thestudiesconcludethatinregionswhereno-tillisappropriateanddoesnotnegativelyimpactcropyieldsitshouldbepromotedforitsbeneficialeffectstosoilstructureandresilience,butnotonthebasisofequivocalevidenceforclimatechangemitigation.However,areviewof610studiescomparingno-tillwithconventionaltillageundertakenbyresearchersattheUniversityofCaliforniaDavisfoundthatwhiletherearesituationsinwhichno-tillcanincreaseproduction,overallitreducedyieldsby6-9%.Takentogetherthesetwostudiescallintoquestionthevalueofno-tillsystemsforincreasingSOCandcropyields.Theuseofgreenmanuresisincreasinglyadvocatedinconjunctionwithno-till.Evidencesuggeststhatwheresuchcropscanbeestablishedafterharvest,theyhavethepotentialtoreducenutrientleachingoverwinterandimprovesoilworkability.However,thefactthatsuchcropsremaininthegroundonlyforafewwintermonthsduringwhichtheygrowonlyalittlesuggeststheyhavelittlepotentialtosequesterSOCorreversesoildegradationinallarableproductionsystems.AccordingtoresearchconductedatRothamstedExperimentalStation,thecontributionofgreenmanurestoSOCstockissmallandhasbeenwidelyoverstated(Johnstonetal.2009).Moreover,mostoftheexperimentalstudiesthatfocusedontheimpactsofgreenmanureonSOCdynamicshavebeenperformedunderextensivecerealsandirrigatedcrops.OtherpracticeswiththepotentialtoincreaseSOCandreducesoildegradationincludingoptimalfertilisation,carefulcalculationofappropriategrazinganimalstockingdensity,theuseofmoreproductiveandmorenutritiousgrasses,deeper-rootinggrasses,foragelegumes,hedgerowsandtreeplantingandcarefullytimed

5Longtermexperimentssince1843atRothamstedprovidethelongestdatasetsontheeffectofsoil,crop,manuring,andmanagementonchangesinsoilorganicmatterundertemperateclimaticconditions.

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animalmanureapplication(Lemaireetal.2015;SoussanaandLemaireetal.2014;Strassburgetal.2014;FranzluebbersandDoraiswamy2007).7.ConclusionsLossofsoilorganiccarbonandnitrogenrelatedtodegradationreducesfoodsecurityandsignificantlycontributestoclimatechange.Whilemanyadvanceshaverecentlybeenmade,morestudiesareneededtounderstandsoilprocessesfordifferentsites,landuses,managementsystemsandscales.Forinstance,itisstilldisputedexactlyhowsoilcanbestbeusedtomitigateGHGemissionsand,atthesametime,meetourneedsforotherecosystemservices,especiallyincreasedfoodproduction.Determiningtheeconomicvalueofthenaturalcapitalofsoilincludingalltheecosystemservicesitprovidesandthecostsofdegradationiscrucialinordertoimplementeffectivepolicyinstrumentsandstimulatepublicinterestinreducingsoildegradation.Itisbecomingincreasinglyevidentthatallowingsoiltodegradeisexpensive,bothtoproducersandsocietyingeneral,especiallyinthelong-term,andthatthecostsofinvestinginpreventionwillbemuchlowerthanthecostsoflettingdegradationcontinueandintensify(ELD2015).SoildegradationispotentiallyreversiblethroughplannedecosystemrestorationandbyintroducingagriculturalsystemsandpracticesthatregeneratesoilbybuildingfertilityandincreasingbiologicalactivityandSOC.Itisessentialthatsoilhealthshouldbegivenacentralpositionindecisionsmadetocombatclimatechangeandthatitisrecognisedasavitalresourceforthefutureofhumanity.

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