1476J.Limpensetal.:Peatlandsandthecarboncycle–asynthesis - PDF document

1476J.Limpensetal.:Peatlandsandthecarboncycle–asynthesis theCbalanceoftheseecosystems(Moore,2002;Pageetal.,2002).Despitetheirprovenroleinpastandpresentglobalcoolingandtheirpotentialforlargepositivefeedbackstotheclimatesystemthroughsequestrationandemissionofgreen-housegases,peatlandsarenotexplicitlyincludedinglobalclimatemodelsandthereforeinpredictionsoffutureclimatechange(IPCC,2007).InApril2007asymposiumwasheldinWageningen,theNetherlands(Wiederetal.,2007),toadvanceourunder-standingofpeatlandCcyclingthroughintegrationacrossdisciplinesandresearchapproaches,andtodevelopamoresyntheticpictureofthepresentandfutureroleofpeatlandsintheglobalCcycleandtheirinteractionswiththeclimatesystem.Thispaperaimstosynthesizethemainndingsofthesymposium.Withaprocessbasedapproachwehopetocontributetotheintegrationofresearchresultsacrosspeatlandtypesandcli-maticzones,improvingfuturepredictionsoftheresponseofthepeatlandCbalancetoenvironmentalandclimatechange.Thispaperpresentsanewsynthesison(i)small-scalepro-cesses,(ii)Cuxesatthelandscapescale,and(iii)peatlandsinthecontextofclimatechange.Thepaperconcludeswithdirectionsfornewresearchtoreducekeyuncertaintiesinourknowledgeinordertofacilitatetheexplicitinclusionoftheseecosystemsinanewgenerationofearthsystemmodels.1.1GeneralisationsaccrosspeatlandtypesThenecessityoffocusingonprocessesbecomesclearwhenweconsiderthelargevariationinpeatlandtypes.Peatlandscomprisemanydifferentecosystems(RydinandJeglum,2006)withandwithoutatreeand/ormosslayer.Fortunately,therearesomeunifyingconceptsthatapplyacrossdifferentpeatlandtypesalthoughSphagnum-dominatedpeatlandsarethemoststudiedcomparedtootherpeatlandtypes.There-doxpotentialisdecisiveformanybiogeochemicalprocessesinthepeat,andthisislargelygovernedbythepositionofthewatertable.Theupperpeatlayer(about5–40cm)isun-saturatedwithwaterandoxicduringthegrowingseasonandsupportsmostbiologicalactivity,whereasthelayerbelowiswaterloggedandanoxic.Theoxic-anoxicboundaryshiftsasaresultofwatertableuctuations.Anotherkeydistinc-tionismadeonthedominantwatersourceinuencingtheorganicsoillayer.Peatlandsthatreceivemostoftheirwaterfromprecipitationarereferredtoasombrotrophicpeatlandsorbogs,whereaspeatlandsthataremainlyfedbywaterthathasbeenintocontactwiththemineralsoilarereferredtoasminerotrophicpeatlandsorfens.2Smallscaleprocessesandplant-soilfeedbacksUnderstandingthemechanismscontrollingaerobicandanaerobicrespirationatsmallspatialscalesisparticularlyimportantforimprovingourpredictionsofpotentialeffectsofclimatechangesandperturbationsonpeatlandCux.Anexampleofakeyprocessisnitrogen(N)fertilizationduetoanthropogenic-drivenNdeposition.Feedbacksbetweenveg-etation,soilphysicalprocessesandaerobicandanaerobicrespirationwillfurtherhelpunderstandingthehighspatialandtemporalvariabilityofpeatlandCuxestowaterwaysandtheatmosphere,inparticulartheproductionandoxida-tionofCH4(Sects.3and4)andgenerationandexportofdissolvedorganiccarbon(DOC,Sects.3and4).2.1BiogeochemistryThedecompositionofcomplexorganicmattertonalprod-ucts,i.e.CO2,CH4andDOC,istheresultofacloseinterac-tionofmicroorganismactivity,physicallycontrolledtrans-portprocessesofelectronacceptorsandnutrients,andthequalityoftheorganicmatteritself.Iaddition,thedecompo-sitionprocessisregulatedbytheavailabilityandefciencyofextracellularenzymes,whichbreakdowncomplexbio-moleculesintomonomersthatcanbeutilizedbymicroor-ganisms.Changesintheenvironmentcaneitherdirectlyaf-fectmicrobialactivitybyactingontheavailabilityofnu-trientsandelectronacceptorsorindirectlythroughchangesinvegetationorsoilphysicalstructure.Directimpactsare,forexample,causedbythedepositionofNandsulphur(S),oroxygenationafterwatertabledraw-downduringsummerdrought.Indirectimpactscomprisethechangetowardsplantspecieswithmoredecomposablelitterormorerootexudates(seeSect.2.2).2.1.1BiogeochemicalprocessesanddriversBelow-groundCcycling,irrespectiveofpeatlandtype,canbeconceptualizedintermsofelectrontransferprocesses(Fig.1a).Primaryproductionprovidesreducedcompoundsthatareburiedaslitterorreleasedbelow-groundbyvascularroots,creatingaredoxgradienttotheatmosphere.Thisgra-dientresultsinO2transferbygasdiffusion,advectionandaerenchymatictransportinrootsandalsoinuencesbelow-groundoxygen(O2/consumptionitself(ShannonandWhite,1994;ChantonandWhiting,1996).Oxidativecapacitycanfurtherbestoredintheformofnitrate,ferricironhydroxides,sulfate(SO4/,andhumicsubstancesthatcanbeusedasal-ternativesforO2inheterotrophicrespiration.Whichoxidantisusedbytheresidentmicrobialcommunityisbroadlyregu-latedbydifferencesintheGibbsfreeenergyoftherespectiverespirationprocesses(Fig.1d)andtheconcentrationsofelec-tronacceptorsanddonors(Achtnichetal.,1995;KellerandBridgham,2007).Inpeatlands,electrontransferprocessesproceedinastronglyverticallystructuredenvironment,whichisillus-tratedinFig.1b.Fromthepeatsurfacedownwardsfourmaincharacteristicschange:(1)therelativeimportanceofrootrespirationversusheterotrophicrespirationdecreaseswithdepth.Finerootsofshrubs,forexample,arelargely Biogeosciences,5,1475– 1491 ,2008www.biogeosciences.net/5/1475/2008/ J.Limpensetal.:Peatlandsandthecarboncycle–asynthesis1479 2.2Vegetation-mediatedfeedbacksAtanecosystemscale,waterlevelandpHcontrolthevegeta-tioncomposition.InturnthevegetationcompositionaffectstheCbalancethroughitseffectsonnetprimaryproductivity(NPP),theamountandratioofCO2:CH4releasedintotheatmosphereandpeatphysicalproperties(Fig.5).Asplantsandvegetationtypesgenerallyformeasilyrecognizableunitsforbothland-basedandairborneassessmentsandarecloselycoupledtosoilprocesses,vegetationchangesarebothconve-nientpredictorsforenvironmentalchangestocomeaswellasmonitoringtoolsforchangesunderway.Plantsdifferinimportantcharacteristicssuchaspro-ductivity,litterdecomposabilityandassociationwithfungiwhichinturnaffectstheCbalanceofpeatlandsatlocaltoecosystemscales.Thisresultsinanumberofvegetation-biogeochemistryfeedbacks:(1)Thereisageneraltrade-offbetweenhummockandhollowinhabitingSphagnumspecies,withlowerdecomposabilityamongtheformer,andhigherproductivityamongthelatter(Rydinetal.,2006);(2)Om-brotrophicgrowingSphagnumspeciesareusuallydifculttodecomposecomparedwithmostco-inhabitingvascularplants(LimpensandBerendse,2003;Dorrepaaletal.,2005);(3)ThedecreaseinlitterC:NratiofollowingfromNdepo-sitionmayenhancedecomposition(LimpensandBerendse,2003;Bragazzaetal.,2006);(4)Plantsproducepeatwithdifferentstructureandhydraulicconductivity,featuresthatinturnaffectredoxstatusandotherphysico-chemicalcon-ditions.Forinstance,forthesamedegreeofhumica-tion,thehydraulicconductivitygenerallyincreasesintheor-derSphagnumpeatCarexpeatwoodypeat(P¨aiv¨anen,1982);(5)Plantdifferintherateanddegradabilityoftheir(root)exudates(CrowandWieder,2005);and(6)Anin-creasingsedgeorgraminoidcoverwillincreaseCH4emis-sionthroughaerenchymatictissue(Thomasetal.,1996;Nilssonetal.,2001).Onthewhole,thevegetationthatdominatesapeatlandwillnotonlyaffectphotosyntheticandrespirationrates,butalsotheamountofCavailableformobilisationasDOCfromthepeatland.Forexample,vegetationtypesinAlaskamaybemoreimportantthanregionalclimateindeterminingDOCuxesfrompeatlands(NeffandHooper,2002).Figure2presentsresultsfromasurveyacrossUKblanketpeatsshow-ingthatDOCconcentrationsaresignicantlygreaterwherewoodyCallunadominateswhencomparedtoEriophorum-dominatedsitesorSphagnum-Eriophorumsites.Finally,large-scalechangesinshrubandtreecover,goingfromtundratotaiga(Chapinetal.,2005)orfromtaigatobog(Vygodskayaetal.,2007),involvesmarkedchangesinalbedo,inturnaffectingtheamountofsolarenergyabsorbedbythesoil,withimportantpotentialfeedbackstopermafrostdepthandlocalwatertable. Fig.2.DOCconcentrationsinstreamwaterfromUKblanketpeat-landcatchments(meanSE)dominatedbyparticularvegetationtypes.Noburningoccurredonthecatchmentsusedtoderivedatainthegure.CollectionofthedatawasperformedbyDr.AlonaArmstrongandwasfundedbyYorkshireWater. 2.2.1VegetationresponsestoenvironmentalchangeApartfromtheobviousdestructionofpeatlandsbydrainage,climaticchangesthataffectwateravailabilityare,togetherwithNdeposition,themostimportantfactorsalteringpeat-landvegetationintemperateandborealregions.Thereisquiteagoodunderstandingfromecologicalexperimentshowdifferentpeatlandplantspeciesreacttowatertableposi-tion,wetness(RydinandJeglum,2006)andNdeposition(Limpensetal.,2006).Precipitationfrequencyduringthegrowingseasonisaparticularlyimportantfactorincontrol-lingSphagnumphotosynthesis(Robroeketal.,2008),whichconstitutesanimportantpartoftheNPPinmanypeatlands.Ingeneral,decreasingwetnessduringthegrowingseasonaswellasincreasesinNdepositionstimulatevascularplantgrowthatthecostofSphagnum,withthemagnitudeofef-fectsgreatlydependingonthestartingconditions.TheshiftfromaSphagnumdominatedtovascularplantdominatedvegetationtypecoincideswithincreasesinlitterdecom-posability(Dorrepaaletal.,2005),heterotrophicrespiration(Bubieretal.,2007),hydrologicalconductivityoftheupperpeatlayer(BelyeaandBaird,2006)andageneraldeclineofCsequestrationinthelongerterm.Thefuturechallengeistounderstandhowvariousclimatescenariosaffectthewet-nessofthepeatlands,andhowthisinturnleadstovegetationchangesthroughdifferentialgrowthandinterspeciccom-petition.HowthelatterisaffectedbydifferentNdepositionloadsisparticularlyofinterestinareaswithNdepositionincreasingabovec.1–1.5gm�2yr�1.Thisresearcheld,requiringdynamicvegetationmodelsthatallowforcontin-uousfeedbacksbetweenvegetationandenvironment,isjuststartingtoemerge(Heijmansetal.,2008;St-Hilaireetal.,2008).Ndepositionabovec.1–1.5gm�2yr�1generallyin-creasesvascularplantcover,anddecreasesSphagnumcover,althoughchangesmaybeslow,andexperimentallydifcult www.biogeosciences.net/5/1475/2008/Biogeosciences,5,1475– 1491 ,2008 J.Limpensetal.:Peatlandsandthecarboncycle–asynthesis1481 Fig.3.Anaturalpeatlandpipeshowing(A)theoutletonastreambank(withcompassforscale)and(B)theinternalfeaturesofthepipechannelwhichis27cmtall. thatover30%ofrunoffinfensandblanketpeatsintheUKmovesthroughmacropores,resultinginwaterandnutrientsbeingtransferredbetweendeepandshallowlayersofthepeatprole.Thereisevenatheorythatpipescanexplainsomeoftheadditionaldegassingfrombogpoolsthatdoesnotseemtobeaccountedforbybogpoolsizealone.Manydrainedpeatpoolshavebeenfoundtohaveanaturalpeatpipeontheiroororwall(Rapsonetal.,2006).Whatisoftenignoredonalandscapescaleisthevariabil-ityinwatertableandrunoffthatcanoccurwhereverpeat-landsaresubjecttoanysignicanttopographicvariation.Underestimatedisalsothespeedwithwhichthedominantoverlandandnear-surfaceowinmanypeatlandsmayaccel-eratepeatexposureafterdegradationofthevegetationcover,resultinginenhancedPOCefuxandreducedNPPasare-sultofvegetationloss(Holdenetal.,2007a).3.1.1DOCuxesDOCisimportantinpeatlandsbecauseanychangeintheuxofDOCwillresultinasignicantregionalredistribu-tionofterrestrialC.Indownstreamecosystems,DOCexertssignicantcontroloverproductivity,biogeochemicalcyclesandattenuationofvisibleandUVradiation(Pastoretal.,2003).Inaddition,DOCimpactswaterqualityintermsofcolor,taste,safety,andaestheticvalue,aswellasalteringtheacid-baseandmetalcomplexationcharacteristicsofsoilwa-terandstreamwater.TogetherwiththeClossesconsiderablenutrientexportmayoccuraswell,potentiallyincreasingim-pactsonaquaticdiversitydownstream(Waldronetal.,2008).DOCaccumulatesinpeatporewatersandisushedoutbywatermovement,withconcentrationsoftengreatestfollow-ingperiodsofwarm,dryconditionswhenDOChashadtimetoaccumulate.DOCconcentrationsareusuallybetween20and60mgl�1innorthernpeatlands(Blodau,2002)butcon-centrationsarehigherduringlowowperiods.DespitethisthetotaluxofDC(DC=DOC+DIC)exportedislikelytobehigherduringstormowsbutmanysamplingpro-grammesdonottakethisintoaccount(Schiffetal.,1998).Recentstronginterestinwater-borneCexportsfrompeat-landshasfocusedmainlyonconcentrationsanduxesofC,especiallyDOCwithinthedrainagesystemofpeatdomi-natedcatchments(Dawsonetal.,2002;Billettetal.,2006).Buffametal.(2007)reportedDOCexportfor15Swedishpeatlandstreamsat2–10gCm�2yr�1whileDICwasbe-tween0.2and2gCm�2yr�1.However,westillknowlit-tleaboutwhatcontrolsthetransportofDOCandparticulateCreleasewithinpeatlandsthemselvesandthehydrologicalprocessesleadingtotheirdeliverytorivers.3.2Land-atmosphereCuxesThemassexchangeofCbetweenpeatlandsandtheatmo-sphereisdominatedbytheuxesofCO2andCH4,butbe-causeofthedifferentradiativepropertiesofCO2andCH4themuchsmallermassexchangeofCH4canhaveadispro-portionaleffectonclimateforcing.3.2.1NetEcosystemExchangeofCO2(NEE)NEEfrombogs,asdeterminedbyeddyco-variancesystems,rangesfromanetlossof�100gCm�2yr�1forapermafrostdominatedSiberianbog(Friborgetal.,2003)andRussianEuropeanborealbogs(Arnethetal.,2002)tolargeuptakesofabout90gCm�2yr�1(Almetal.,1997).However,moststudiesreportmeanannualNEEinamuchsmallerrangewithmultipleyearaveragesbetween20and60gCm�2yr�1(SottocornolaandKiely,2005;Dunnetal.,2007;Rouletetal.,2007).TheNEEfromminerotrophicpeatlands(fens),particularlymineralpoorfens,showsasimilarrangeandin-terannualvariabilityasthatofbogs(Shurpalietal.,1995;Joineretal.,1999;Aurelaetal.,2001,2007).Whileallthesestudiesshowlargedifferencesamongyearsandpeat-landtypesinannualNEEtheobservedrangesarerelativelysmallerthanthoseobservedamongothermajorecosystemtypes.Humphreysetal.(2006)didacomparativeanalysisofNEEfromsevencontrastingbogsandfensandfoundthat,despitelargedifferencesinwatertabledepth,waterchem-istry,andplantcommunitystructure,thatthesummerdailyNEEwasabout1.5gCm�2yr�1forallpeatlands.Thisre-sultimpliesthatthedifferencesincumulativeannualNEEbetweenindividualpeatlandsaremoreafunctionofbroadscalegeographiclocationandphysicalsettingratherthaninternalfactorssuchasthehydrology,communitystruc-tureandbiogeochemistry.Morelong-termandcontinuousrecordsofNEEforawiderrangeofpeatlandtypesandge-ographicallocationsarerequiredtosupporttheabovecon-clusion.AnanalysisbyLindrothetal.(2007)forfourdiffer-entpeatlandsinSwedenandFinlandshowsthatphotosyn-theticradiation(PAR)wasthedominantcontrolonthegrossandnetuptakeofCO2.Frolkingetal.(1998)foundthatin www.biogeosciences.net/5/1475/2008/Biogeosciences,5,1475– 1491 ,2008 1482J.Limpensetal.:Peatlandsandthecarboncycle–asynthesis comparisonwithotheruplandecosystems(e.g.closedforest,grasslandsandcroplands),oneNEE-PARrelationshipwithsmallerecosystemrespirationandNEEratesappliedtothetwobroadgroupsofombrotrophicandminerotrophicpeat-lands.Atalandscapescale,internalfactorssuchasmois-ture,temperature,vegetationcompositionandnutrientstatusallseemtoplaysecondaryroles.ThismaybepartlyduetotheconfoundingeffectsthatsomeenvironmentalfactorshaveonthecomponentsofNEE.Amodestincreaseintem-perature,forexample,maybothstimulateCassimilationandCrespiration,diminishingorevencancellingoutitsnetef-fectonNEE.IfitisconrmedthattherangeofNEEamongpeatlandtypesismuchsmallerthaninitiallyexpectedthenitwillsimplifythetaskofmodelingpeatlandCO2exchangeatcoarsescalesinglobalmodels.3.2.2ExchangeofCH4betweenpeatlandsandtheatmo-sphereTherearemanycomprehensivereviewsofCH4uxfrompeatlands(Mooreetal.,1998).Theseshowthatthetem-poralandspatialvariabilityofCH4exchangeismuchmorevariable,byseveralordersofmagnitude,thanthatofpeat-landCO2-NEE.Drierbogswithpersistentlylowwaterta-bles,withorwithoutpermafrost,haveverysmallCH4uxoflessthan1gCm�2yr�1(Rouletetal.,2007;Christensenetal.,2004)whilewetterbogscanhaveintermediateuxesof5–8gCm�2yr�1(Laineetal.,2007).Inwetterfens,particu-larlywhenthereissignicantcoverofsedges,theannualuxcanexceed15gCm�2yr�1(Shurpalietal.,1993;Suykeretal.,1996).Inaboreallandscape,Bubieretal.(2005)estimatedaregionalCH4uxof7gCm�2yr�1,buttheuxesfromindividualpeatlandstypesdifferedbyafactor10ormore(seealsoSchrieretal.,2008).Backcalculat-ingfromtherecentestimatesoftheatmosphericburdenofCH4fromhighlatitudewetlands(8–201012gCH4yr�1,MikaloffFletcheretal.,2004;ChenandPrinn,2006)yieldsanaverageemissionof2–5gCm�2yr�1.3.3Puttingtheuxestogether:theannualCbudgetofpeat-landsThereareveryfewexamplesofnetecosystemCbalances(NECB)forpeatlands,whereboththeatmosphericuxesofCO2andCH4andthewaterborneuxesofDOC,POCandDIChavebeenmeasuredatthesametime.InthisassessmentoftheCbalanceofapeatlanditisimportanttobeveryclearontheterms.NECBrequiresacleardenitionofterminol-ogyandweadoptinthispapertheterminologydiscussedbyLovettetal.(2006)andChapinetal.(2006):1orgC(Lovettetal.,2006)orNECB(Chapinetal.,2006)istheaccumula-tionofCinthesedimentsofanecosystemafteralltheinputsandoutputsarebalanced.TheNECBfornorthernpeatlandsfrommeasurementsisquiteconservativerangingbetween10to30gCm�2yr�1.NEEisthelargesttermrangingbetween0and100gCm�2yr�1inputsandtheapproximatelossesviatheCH4uxandDCrangerespectivelybetween0and12,and2and70gCm�2yr�1(Fig.4).However,thehighendoftherangeofDCexportcomesfrompeatlandsthathavebeendisturbedorusedextensivelythroughoutpartoftheirhistoryforgrazingand/orsubjectedtootherformsofland-use(Wor-ralletal.,2003).ExaminingtheNECBwithallitstermsmakesitclearthattheexclusionofCH4andDClossescouldleadtoanerrorinexcessof100%.ThismeansnotonlydoestheCH4andDCuxneedtobemeasuredbutthesimulationoftheresponseofnorthernpeatlandstoenvironmentalchangeswillalsoneedtoincludetheselossmechanismstomakethepredictionsofuse.3.4PerturbationsMainperturbationsofpeatlandCstorageandsequestrationinvolvelarge-scaleland-usechanges,particularlyforthetropicalpeatlands,changesinaerialnutrientinput,suchasNforthetemperatepeatlands(discussedinSect.2.2)andclimatechangeforpeatlandsinthenorthernhighlatitudes.Furthermore,combinedperturbationsmayhaveanadditionalaggravatingeffect,leadingtohigherlossesthanexpectedfromeachdisturbanceseparately.Fortropicalpeatlands,forexample,theinteractionbetweendrought(broughtbytheSouthernOscillation),deforestation,andreisrespon-sibleforthedramaticlossesofCtotheatmosphere.Like-wise,feedbacksbetweenpermafrostmelting,re,vegetationchangesandtheireffectsonalbedomayhavefar-reachingconsequencesforsoilCstoresinpermafrostpeatlandsattheothersideoftheequator.3.4.1LandusechangesCurrently,peatlandsinmanypartsoftheworldareun-dergoingmajortransformationduetohumanalteration,of-teninvolvingdrainageandburning,particularlypeatswampforestsinSoutheastAsia.Thesepeatlandsstoreatleast42Pg(1Petagram=1015g)ofsoilCbutareincreasinglyun-derthreatfromdrainageandresassociatedwithplantationssuchaspalmoilcrops(Pageetal.,2002)andAcaciaforpulp.Hooijeretal.(2006)estimatedthatcurrentCO2emissionre-sultingfromthedestructionofpeatlandsinSoutheastAsia(90%emittedbyIndonesia)throughdrainageandburningaloneisequalto8%ofglobalfossilfuelemissions.Further-more,theburningofpeatandvegetationinIndonesiaduringtheElNi˜no-SouthernOscillation(ENSO)in1997releasedanestimated0.81–2.57PgC,anequivalentamountof13to40%ofthemeanannualglobalCemissionsfromfossilfuels(Pageetal.,2002).Manytemperatepeatlandshavebeendrainedforcommer-cialexploitationsuchasagriculture,forestryormorere-centlywindmillfarming(Waldronetal.,2008).Thishasresultedinprofoundeffectsonpeatlandbiogeochemistry, Biogeosciences,5,1475– 1491 ,2008www.biogeosciences.net/5/1475/2008/ 1484J.Limpensetal.:Peatlandsandthecarboncycle–asynthesis 1998).TheeffectsofpermafrostthawingonpeatlandC-accumulationisequallyuncertain.Ontheonehandthewaterreleasedbythemeltingoftheicemayincreasewaterresi-dencetime(seeSect.2.1.3),promotingpeatformationandthuslocalC-accumulationbutincreasingCH4emission.In-deed,theC-sequestrationrateofcollapsescarsisamongthehighestreportedforpeatlands(Meyers-Smithetal.,2008).Ontheotherhand,ifthepermafrostlayeractsasawaterim-permeablelayer,thethawingwillleadtodrainage,stimulat-ingdecompositionprocesses(seeSect.2.1.3).Inpeatlandswithdiscontinuouspermafrost,severereeventsmaycon-tributetopermafrostthawingleadingtomorepermanentveg-etationchanges,potentiallyincreasingC-accumulationinthelongerterm(Kuhry,1994;Myers-Smithetal.,2008;Schuuretal.,2008).3.4.3RestorationManydegradedpeatlandsarethesubjectofrestorationprojects.Managementpracticesarevariedbutincluderais-ingthewatertablethroughgullyandditchblocking(Evansetal.,2005)andreseedingorplantingbaresurfaces(Petroneetal.,2004).ThesemanagementinterventionscanhaveanimmediateimpactontheexportofCtostreamsandrivers.Holdenetal.(2007b)forexample,demonstratedanalmosttwoordersofmagnitudedecreaseinPOCexportforarestorationprojectinnorthernEnglandwhileWallageetal.(2006)andWorralletal.(2007)haveshownthatDOCuxesaresignicantlyreducedthroughdrainblockingandwatertablerecovery.Quickre-vegetationofdegradedpeatisoftenpossibleandpeatformationcanbefastingulliesanddrains,evenwithoutwatertablerestorationmeasures,therebystillhavingpositiveeffectsonecosystemCstorage(EvansandWarburton,2007).Re-vegetationoutsidegulliesisoftenaimedatrestoringavascularplantcover(oftenEriophorumsp.),orapplyingalayerofstrawmulchtoprovideasuitablemicroclimateforSphagnummossestore-establish(Grosvernieretal.,1995;Rochefortetal.,2003).Thiscreatesnewbiomassbutalsoprimesthepeatthroughinputoffreshorganicmatter,leadingtoatransientincreasesinpeatdecompositionandincreasedmethaneemissionrates(Chojnickietal.,2007),beforethenormalcoverofmossesandshrubsdevelops.4PeatlandsandclimateClimatechangeislargelydrivenbytheaccumulationofCintheatmospherewhichisthenetbalancebetweenemis-sionsfromhumanactivitiesandtheremovalofCbynaturalsinksonlandandintheocean.Overthelast50years,theefciencyofnaturalCO2sinkshasbeendeclining(Canadelletal.,2007)andmodelprojectionsshowafurtherdeclinethroughoutthe21stcentury(Friedlingsteinetal.,2006).HighlatitudeandtropicalpeatlandshavebeenidentiedaspotentialkeyvulnerabilitiesoftheterrestrialCcyclelikelytoleadtopositivefeedbackstoclimatechange(Freemanetal.,1993;Fenneretal.,2005;Canadelletal.,2004).GlobalwarmingforthehighlatitudepeatlandsanddeforestationintropicalpeatlandsaremajordriversofthenetCbalanceoftheseregions.4.1Modellingvegetation–climatefeedbacksOverthelasteightyearstherehasbeenconsiderableefforttocoupleglobalclimatemodelswithglobalCmodelstoexam-inethemagnitudeandrateofpotentialfeedbacksasaresultofchangesinthecyclingofCintheterrestrialbiosphereandtheoceans.Untilnow,allthesimulationsagreeonapos-itivefeedbackbetweentheterrestrialbiosphere,theoceansandclimate.Thestrengthofthisfeedback,however,variesconsiderably,andresultsrangebetween20and200ppmofadditionalatmosphericCO2by2100,usingtheA2-IPCCemissionscenario(Friedlingsteinetal.,2006).Climate-carbonmodelersacknowledgethattherearemajorcompo-nents,suchasdynamicland-usechange,Ncycling,re,andwetlandsthatcanleadtolargefeedbacksbutarecurrentlyignored.Theuncertaintyintheprojectionscausedbytheseomissionsisconsiderable.Recentcarbon-nitrogen-climatesimulationsforexampleshowthatresultsaredramaticallydifferentbetweenrunsthatincludeNandthosethatdonot(Thorntonetal.,2007).IfNavailabilityisincluded,theca-pacityoftheterrestrialbiospheretotakeupCandrespondtochangesintemperatureandprecipitationvariationsisre-duced,makingthecarbon-climatefeedbackmorepositive.Thefeedbackpotentialofwetlandsisequallylarge.Apre-liminaryestimatesuggeststhatupto100PgCofCO2equiv-alentscouldbereleasedtotheatmospherefromwetlandsandpeatlandsoverthenext100years(Gruberetal.,2004).4.1.1IncludingpeatlandsinearthsystemmodelsTheinclusionofwetlands,particularlypeatlands,inglobalmodelsisseverelyconstrainedbythewayhydrologyistreatedbythemodels,andlessbytheuncertaintyconcerningtheirexactlocation(Krankinaetal.,2008).Ingeneral,theyareconcentratedinareaswhereprecipitationexceedsevapo-transpiration.However,theactualhydrologyofwetlandsthatcontrolsecosystemstructureandfunctionislargelyafunc-tionofphysicalsetting,substrates,andtopographiclocationatalandscapelevel.Atpresent,thesefactorsaretoolocaltobeincluded,andtheresearchcommunityisstillsearchingforanadequatewaytoparameterizetheissue.Therehavebeenattemptstouseatopographicwetnessindextosimu-latenorthernpeatlanddistribution(Kirkbyetal.,1995)andthishasbeenextendedgloballybyGedneyandCox(2003).However,thisapproachonlysimulateswheresurfacesatura-tionoccurs,whereasmuchofthebiogeochemicalprocessesinwetlandsareafunctionoftheseasonalwatertablechangesoverarangeof0.3–0.4m(Mooreetal.,1998). Biogeosciences,5,1475– 1491 ,2008www.biogeosciences.net/5/1475/2008/ 1486J.Limpensetal.:Peatlandsandthecarboncycle–asynthesis 5.1Waysforward – Betterunderstandingofthecouplingbetweensoilphys-icalstructureandaerobicandanaerobicrespiration,withfurthermodeldevelopmentatlocalandregionalscales. – DetermininghowvegetationchangealterstheCcy-cleandCmobilizationindifferentforms(DOC,POC,gases). – Understandinghowvariousclimatescenarioswillaffectthehydrologyofpeatlands,andhowthiswillleadtovegetationchangesandassociatedchangesinCuxesandstocks. – Analyzingandestablishingprocessstudiesincombina-tionwithecosystemuxmeasurementstounderstandthekeycontrolsonuxesinandoutoftheecosystems(watertable,temperature,etc.). – IncorporatinginbiogeochemicalmodelskeyprocessescontrollinglandscapeCuxessuchasresregimesandvegetationshifts. – IdentifyingpossiblethresholdsinCdynamicsofpeat-landstopreventlargescalevulnerabilityofCstocksandassociatedpositivefeedbackstotheclimatesystem. – DeterminingamoreaccurateglobalrepresentationofthespatialdistributionofC-poolsandstocksinpeat-lands,withparticularfocusonpeatdepth,peatbulkdensityandthedifferentiationbetweenombrotrophicandminerotrophicpeatlands. – Includingthetreatmentoforganicsoils,andparticularlypeat,intheCcyclecomponentsoffullycoupledclimatemodels. Acknowledgements. WeareindebtedtotheEditorsofBiogeo-sciencesfortheircontinuoussupportinputtingtogetherthisspecialissue.WeacknowledgePeatnet( www.peatnet.siu.edu )andtheGlobalCarbonProject( www.globalcarbonproject.org )forco-sponsoringtheFirstInternationalSymposiumonCarboninPeatlandsheldinWageningen,theNetherlands,inApril2007,andfortheirnancialcontributiontothisspecialissue.Finally,wethanktwoanonymousreviewersandTimMooreforhelpfulcommentstoimprovetherstdraft.Editedby:T.LaurilaReferences Achtnich,C.,Bak,F.,andConrad,R.:Competitionforelectron-donorsamongnitratereducers,ferricironreducers,sulfatere-ducers,andmethanogensinanoxicpaddysoil,Biol.Fert.Soils,19,65–72,1995. Albers,D.,Migge,S.,Schaefer,M.,andScheu,S.:Decomposi-tionofbeechleaves(Fagussylvatica)andspruceneedles(Piceaabies)inpureandmixedstandsofbeechandspruce,SoilBiol.Biochem.,36,155–164,2004. 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