Authors:
StephenM.Ogle,ColoradoStateUniversity(LeadAuthor)PatrickHunt,USDAAgriculturalResearchServiceCarlTrettin,USDAForestService
Contents:
4 QuantifyingGreenhouseGasSourcesandSinksinManagedWetlandSystems................4‐34.1 Overview...........................................................................................................................................................4‐3
4.1.1 OverviewofManagementPracticesandResultingGHGEmissions...........4‐44.1.2 SystemBoundariesandTemporalScale................................................................4‐74.1.3 SummaryofSelectedMethods/ModelsandSourcesofData........................4‐74.1.4 OrganizationofChapter/Roadmap..........................................................................4‐8
4.2 ManagementandRestorationofWetlands........................................................................................4‐84.2.1 DescriptionofWetlandManagementPractices..................................................4‐84.2.2 Land‐UseChangetoWetlands..................................................................................4‐13
4.3 EstimationMethods...................................................................................................................................4‐144.3.1 BiomassCarboninWetlands....................................................................................4‐144.3.2 SoilC,N2O,andCH4inWetlands..............................................................................4‐17
4.4 ResearchGapsforWetlandManagement.........................................................................................4‐21Chapter4References.............................................................................................................................................4‐23
SuggestedChapterCitation:Ogle,S.M.,P.Hunt,C.Trettin,2014.Chapter4:QuantifyingGreenhouseGasSourcesandSinksinManagedWetlandSystems.InQuantifyingGreenhouseGasFluxesinAgricultureandForestry:MethodsforEntity‐ScaleInventory.TechnicalBulletinNumber1939.OfficeoftheChiefEconomist,U.S.DepartmentofAgriculture,Washington,DC.606pages.July2014.Eve,M.,D.Pape,M.Flugge,R.Steele,D.Man,M.Riley‐Gilbert,andS.Biggar,Eds.
USDAisanequalopportunityproviderandemployer.
Chapter 4
Quantifying Greenhouse Gas Sources and Sinks in Managed Wetland Systems
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Acronyms,ChemicalFormulae,andUnits
C CarbonCH4 MethaneCO2 CarbondioxideCO2‐eq CarbondioxideequivalentsDNDC Denitrification‐DecompositionEPA EnvironmentalProtectionAgencyFVS ForestVegetationSimulatorGHG Greenhousegasha HectareIPCC IntergovernmentalPanelonClimateChangeN NitrogenN2O NitrousoxideNOx Mono‐nitrogenoxidesNRCS USDANaturalResourcesConservationServiceP PhosphorousSOC SoilorganiccarbonTg TeragramsUSDA U.S.DepartmentofAgricultureUSDA‐ARS U.S.DepartmentofAgriculture, AgriculturalResearchService
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4 QuantifyingGreenhouseGasSourcesandSinksinManagedWetlandSystems
Thischapterprovidesmethodologiesandguidanceforreportinggreenhousegas(GHG)emissionsandsinksattheentityscaleformanagedwetlandsystems.Morespecifically,itfocusesonmethodsformanagedpalustrinewetlands.1Section4.1providesanoverviewofwetlandsystemsandresultingGHGemissions,systemboundariesandtemporalscale,asummaryoftheselectedmethods/models,sourcesofdata,andaroadmapforthechapter.Section4.2presentsthevariousmanagementpracticesthatinfluenceGHGemissionsinwetlandsystemsandland‐usechangetowetlands.Section4.3providestheestimationmethodsforbiomasscarboninwetlandsandforsoilcarbon,N2O,andCH4emissionsandsinks.Finally,Section4.4includesadiscussionofresearchgapsinwetlandmanagement.
4.1 OverviewWetlandsoccuracrossmostlandforms,existingasnaturalunmanagedandmanagedlands,restoredlandsfollowingconversionfromanotheruse(typicallyagriculture),andasconstructedsystemsforwatertreatment,suchasanaerobiclagoons.AllwetlandssequestercarbonandareasourceofGHGs.Table4‐1providesadescriptionofthesourcesofemissionsorsinksandthegasesestimatedinthemethodology.
Table4‐1:OverviewofWetlandSystemsSourcesandAssociatedGreenhouseGases
SourceMethodforGHGEstimation Description
CO2 N2O CH4
Biomasscarbon
Provisionsforestimatingabovegroundbiomassforwetlandforestsandaboveandbelowgroundbiomassandcarbonareincludedforshrubandgrasswetlandsinthischapter.Abovegroundbiomassforforestedwetlandsandshrubandgrasswetlandsincludeslivevegetation,trees,shrubs,andgrasses,standingdeadwood(deadbiomass),anddowndeadorganicmatter—litterlayer(deadbiomass).
SoilC,N2O,andCH4inwetlands
Theproductionandconsumptionofcarbon inwetland‐dominatedlandscapesareimportantforestimatingthecontributionofGHGs,includingCO2,CH4,andN2Oemittedfromthoseareastotheatmosphere.ThegenerationandemissionofGHGsfromwetland‐dominatedlandscapesarecloselyrelatedtoinherentbiogeochemicalprocesses,whichalsoregulatethecarbonbalance(RoseandCrumpton,2006).However,thoseprocessesarehighlyinfluencedbythelanduse,vegetation,soilorganisms,chemicalandphysicalsoilproperties,geomorphology,andclimate(SmemoandYavitt,2006).
1Palustrinewetlandsincludenon‐tidalandtidalwetlandsthatareprimarilycomposedoftrees,shrubs,persistentemergent,emergentmosses,orlichens,wheresalinityduetoocean‐derivedsaltsisbelow0.5‰(partsperthousand).Palustrinewetlandsalsoincludethosewetlandslackingvegetationthathavethefollowingfourcharacteristics:(1)arelessthan20acres;(2)donothaveactivewave‐formedorbedrockshorelines;(3)haveamaximumwaterdepthoflessthan6.5ft.atlowwater;and(4)haveasalinityduetoocean‐derivedsaltslessthan0.5%(StedmanandDahl,2008).
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4.1.1 OverviewofManagementPracticesandResultingGHGEmissions
ThischapterprovidesmethodsforestimatingcarbonstockchangesandCH4andN2Oemissionsfromnaturallyoccurringwetlands2andrestoredwetlandsonpreviouslyconvertedwetlandsites.Constructedwetlandsforwatertreatment,includingdetentionponds,areengineeredsystemsthatarebeyondthescopeconsideredherebecausetheyhavespecificdesigncriteriaforinfluentandeffluentloads.Inaddition,themethodsarerestrictedtoestimationofemissionsonpalustrinewetlandsthatareinfluencedbyavarietyofmanagementoptionssuchaswatertablemanagement,timber,orotherplantbiomassharvest,andwetlandsthataremanagedwithfertilizerapplications.ThemethodsarebasedonestablishedprinciplesandrepresentthebestavailablescienceforestimatingchangesincarbonstocksandGHGfluxesassociatedwithwetlandmanagementactivities.However,giventhewidediversityofwetlandstypesandthevarietyofmanagementregimes,thebasisforthemethodsprovidedinthissectionarenotaswell‐developedasothersectionsinthisguidance(i.e.,CroplandandGrazingLands,AnimalProduction,andForestryMethods).Table4‐2providesasummaryofthemethodsandtheircorrespondingsectionforthesourcesofemissionsestimatedinthisreport.
Table4‐2:OverviewofWetlandSystemsSources,Method,andSection
Section Source Method
4.3.1Biomasscarbon
MethodsforestimatingforestvegetationandshrubandgrasslandvegetationbiomasscarbonstocksuseacombinationoftheForestVegetationSimulator(FVS)modelandlookuptablesfordominantshrubandgrasslandvegetationtypesfoundinChapter3,Cropland,andGrazingLand.Ifthereisaland‐usechangetoagriculturaluse,methodsforcroplandherbaceousbiomassareprovidedinChapter3.
4.3.2SoilC,N2O,andCH4inwetlands
TheDenitrification‐Decomposition (DNDC) process‐basedbiogeochemicalmodelisthemethodusedforestimatingsoilC,N2O,andCH4emissionsfromwetlands.DNDCsimulatesthesoilcarbonandnitrogenbalanceandgeneratesemissionsofsoil‐bornetracegasesbysimulatingcarbonandnitrogendynamicsinnaturalandagriculturalecosystems(Lietal.,2000;Miehleetal.,2006;Stangetal.,2000)andforestedwetlands(Daietal.,2011;Zhangetal.,2002),usingplantgrowthestimatedasdescribedinSection4.3.1.
4.1.1.1 DescriptionofSector
TheNationalWetlandsInventorybroadlyclassifieswetlandsintofivemajorsystems:(1)marine,(2)estuarine,(3)riverine,(4)lacustrine,and(5)palustrine(Cowardinetal.,1979).Fourofthosesystems(marine,estuarine,riverine,andlacustrine)areopen‐waterbodiesandnotconsideredwithinthemethodsdescribedinthisguidance.Palustrinewetlandsencompassthewetlandtypesoccurringonthelandandarefurtherclassifiedbymajorvegetativelifeformandwetnessorfloodingregime.CommonpalustrinewetlandsareillustratedinFigure4‐1.Forexample,forestedwetlandsareoftenclassifiedaspalustrine—forested.Similarly,mostgrasswetlandsareclassifiedaspalustrine—emergent,reflectingemergentvegetation(e.g.,grassesandsedges).Wetlandsalsovarygreatlywithrespecttogroundwaterandsurfacewaterinteractionsthatdirectlyinfluence
2WetlandsaredefinedinChapter7,LandUseChange.Wetlandsthatareconvertedtoanon‐wetlandstatusshouldbeconsideredintheappropriatechapter(e.g.,CroplandandGrazingLands,AnimalProduction,andForestryMethods).
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hydroperiod(i.e.,thelengthoftimeandportionoftheyearthewetlandholdswater),waterchemistry,andsoils(Cowardinetal.,1979;Winteretal.,1998).AllthesefactorsalongwithclimateandlandusedriversinfluencetheoverallcarbonbalanceandGHGfluxes.
Figure4‐1:PalustrineWetlandClassesBasedonVegetationandFloodingRegime
Source:Cowardinetal.(1979).
GrasslandandforestedwetlandsaresubjecttoawiderangeoflanduseandmanagementpracticesthatinfluencethecarbonbalanceandGHGflux(Faulkneretal.,2011;Gleasonetal.,2011).Forexample,forestedwetlandsmaybesubjecttosilviculturalprescriptionswithvaryingintensitiesofmanagementthroughthestandrotation;hence,thecarbonbalanceandGHGemissionsshouldbeevaluatedonarotationbasis,whichcouldrangefrom20tomorethan50years.Incontrast,grasswetlandsmaybegrazed,hayed,ordirectlycultivatedtoproduceaharvestablecommodityannually.WhileeachmanagementpracticemayinfluencecarbonsequestrationandGHGfluxes,theeffectisdependentonvegetation,soil,hydrology,climatologicalconditions,andthemanagementprescriptions.Thissectionfocusesonrestorationandmanagementpracticesassociatedwithpalustrinewetlandsthataretypicallyforestedorgrassland.
4.1.1.2 ResultingGHGEmissions
GHGemissionsfromwetlandsarelargelycontrolledbywatertabledepthanddurationaswellasclimateandnutrientavailability.Underaerobicsoilconditions,whicharecommoninmostuplandecosystems,organicmatterdecompositionreleasesCO2,andatmosphericCH4canbeoxidizedinthesurfacesoillayer(Trettinetal.,2006).Incontrast,theanaerobicsoilsthatcharacterizewetlandscanproduceCH4(dependingonthewatertableposition)inadditiontoemittingCO2.Accordingly,wetlandsareaninherentsourceofCH4,withgloballyestimatedemissionsof55to150teragrams(Tg)ofCH4peryear(Blainetal.,2006).
Toaccommodateentity‐scalereportingintheUnitedStatesforagriculturalandforestryoperations,Tier2and3methodsaddresspalustrinewetlandscontainingbothorganicandmineralhydricsoils.Thesewetlandsmaybeinfluencedbyagriculturalandforestrymanagement,andmethodsarecurrentlyavailableforbothtypesofmanagement.Thischapterprovidesmethodologiesforthefollowingwetlandsourcecategories:
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1. Biomasscarboninforested,shrub,andgrasswetlands;2. Soilcarbonsinksinwetlands;and3. N2OandCH4emissionsinwetlands.
Biomasscarboncanchangesignificantlywithmanagementofwetlands,particularlyinforestedwetlands,changesfromforesttowetlandsdominatedbygrassesandshrubs,oropenwater.Inforestedwetlands,therecanalsobesignificantcarbonindeadwood,coarsewoodydebris,andfinelitter.Harvestingpracticeswillalsoinfluencethecarbonstocksinwetlandstotheextentthewoodiscollectedforproducts,fuel,orotherpurposes.
WetlandsarealsoasourceofsoilN2Oemissions,primarilybecauseofnitrogenrunofffromadjoininguplandsandleachingintogroundwaterfromagriculturalfieldsand/oranimalproductionfacilities.N2OemissionsfromwetlandsduetonitrogeninputsfromsurroundingfieldsoranimalproductionareconsideredindirectemissionsofN2O(deKleinetal.,2006).MethodologiesforestimatingindirectN2Oareprovidedintherespectivesourcechapter(i.e.,Chapter3,CroplandandGrazingLands,orChapter5,AnimalProduction).However,directN2Oemissionsoccurinwetlandsifmanagementpracticesincludenitrogenfertilization,hence,guidanceisprovidedforthissourceofemissions.
4.1.1.3 RiskofReversals
Wetlandsinherentlyaccumulatecarboninthesoilsduetoanaerobicconditions,andtheyarenaturalsourcesofCO2andCH4totheatmosphere.Managementmayalterconditionsthataffectboththepoolsandfluxes.Forexample,accumulatedsoilcarboncanbereturnedtotheatmosphereifthewetlandisdrained(ArmentanoandMenges,1986).Incontrast,silviculturalwatermanagementinwetlandscanleadtohigherbiomassproduction,whichmaypartiallyoffsetincreasedsoilorganicmatteroxidation.Conversely,thesoilcarbonpoolinconvertedwetlandsistypicallylowerthantheunmanagedsoil,andrestoringwetlandconditionsmayincreasecarbonstorageovertimeifinherenthydricsoilconditionsaremaintainedwithconsistentorganicmatterinputs.
Reversalsofemissiontrendscanoccurifamanagerrevertstoapriorconditionoranearlierpractice.Forexample,anentitymaydecidetoreturnawetlandthathadbeendrainedandcroppedbacktoaforestedwetlandcondition.Anothercommonexamplewouldbeifarestoredforestedwetlandisrevertedbacktoagriculture.ThesereversalsdonotnegatethemitigationofCH4orN2Oemissionstotheatmospherethathadoccurredpreviously,totheextentthatwetlandrestorationorchangeinmanagementcanreduceorchangetheseemissions.Correspondingly,thestartingpointfromthereversionwilldeterminetheeffectoncarbonsequestrationandGHGflux.Forexample,inarestoredforestedwetland,reversionofthesitetocropproductionwouldreturncarbonsequesteredduringtherestorationperiodtotheatmosphereovertime.
Thereisatrade‐offinCH4andN2Oemissionswithmanagementofthewatertableposition.WetlandswithanaerobicsoilconditionsthatarepersistentnearthesurfaceforalongerperiodduringtheyearwilltendtohavehigherCH4emissionsandloweremissionsofN2O.N2Oemissionsaregreatlyreducedifsoilsaresaturatedbecausethereislittleinherentnitrification,anddenitrificationwillleadtoN2production(Davidsonetal.,2000).Forexample,restorationofwetlandswillnormallyleadtoahigherwatertableforalongerperiodoftheyear,andthuscontributetohigheremissionsofCH4butloweremissionsofN2O.Thesetrendscanbereversedifthewatertableisloweredthroughmanagementordrought,whichwilltendtoenhanceN2Oemissionsifthereisasourceofnitrate,whilereducingemissionsofCH4.Figure4‐2providesanillustrationofthecarboncycletypicallyfoundinwetlandforestandgrasslandwetlandsandrepresentsthescopeofthemethodspresentedinthisguidance.
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Figure4‐2:CarbonCycleforForestandGrasslandWetlands
Source:TrettinandJurgensen(2003).
4.1.2 SystemBoundariesandTemporalScale
Systemboundariesaredefinedbythecoverage,extent,andresolutionoftheestimationmethods.ThelocationofthewetlandsmaybeapproximatedbyuseoftheNationalWetlandsInventory,3thelocationofhydricsoilsasconveyedbytheNRCSsoilsmap,orthroughdirectdelineationofwetlands.Thecoverageofthemethodscanbeusedtoestimateavarietyofemissionsources,includingemissionsassociatedwithbiomassC,litterC,andsoilscarbonstockchangesandCO2,CH4,andN2Ofluxesfromsoils.Systemboundariesarealsodefinedbytheextentandresolutionoftheestimationmethod.Themethodsprovidedforwetlandshaveaspatialextentthatwouldincludeallwetlandsintheentity’soperation,withestimationoccurringattheresolutionofanindividualwetland.EmissionsareestimatedonanannualbasisforasmanyyearsasneededforGHGemissionsreporting.
4.1.3 SummaryofSelectedMethods/ModelsandSourcesofData
TheIPCC(2006)hasdevelopedasystemofmethodologicaltiersforestimatingGHGemissions.Tier1representsthesimplestmethodsusingdefaultequationsandfactorsprovidedintheIPCCguidance.Tier2usesdefaultmethodsbutemissionfactorsthatarespecifictodifferentregions.Tier3utilizesaregion‐specificestimationmethod,suchasaprocess‐basedmodel.Highertiermethodsareexpectedtoreduceuncertaintiesintheemissionestimatesifthereissufficientinformationandtestingtodevelopthesemethods.Inthisguidance,biomass,litter,andsoilcarbonstockchanges,inadditiontosoilN2OandCH4emissions,areestimatedusingTier2and3methods.
Thedatarequiredtoapplythesemethodsrangefrombasicinformationonsoils,vegetation,weather,landuse,andmanagementhistorytodataonfertilizationratesordrainageconditions.Whilesomeofthesedataareoperation‐specificandmustbeprovidedbytheentity,otherdatacanbeobtainedfromnationaldatabases,suchasweatherdataandsoilcharacteristics.
3SeeNationalWetlandsInventoryhttp://www.fws.gov/wetlands/.
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4.1.4 OrganizationofChapter/Roadmap
Thewetlandssectionofthisreportisorganizedintothreeprimarysections.Section4.2providesadescriptionofwetlandmanagementeffectsonGHGemissions,elaboratingonthescientificbasisforhowvariouspracticesinfluenceGHGemissions.Section4.3providesarationalefortheselectedmethod,adescriptionofthemethod,includingageneraldescription(withequationsandfactors),activitydatarequirements,ancillarydatarequirements,limitationsofthemethod,anduncertaintiesassociatedwiththeestimation.Asinglemethodisprovidedforeachsourcepresentedinthischapter(i.e.,biomasscarboninforested,shrub,andgrasswetlands;soilcarbonandCH4inwetlands;anddirectN2Oemissionsinwetlands).Asinglemethodwasselectedtoensureconsistencyinemissionestimationbyallreportingentities,andtheselectedmethodisconsideredthebestoptionamongpossibilitiesforentity‐scalereporting.Methodsmayberefinedinthefutureastheyarefurtherdeveloped.Thelastsectionprovidesasummaryofselectedresearchgaps.
4.2 ManagementandRestorationofWetlandsHowwetlandsaremanagedcanhaveasignificanteffectonGHGemissionsandsinks,whichareprimarilyinfluencedbythedegreeofwatersaturation,climate,andnutrientavailability.Inamajorityofwetlands,90percentofcarboningrossprimaryproductionisreturnedtotheatmospherethroughdecay,andtheremaining10percentaccumulatesinthebottomofthewaterbodyaccumulatingonpreviouslydepositedmaterials(Blainetal.,2006).ManagementofthewatertablewithinawetlandwillresultinbothlowerCH4emissionsduetodecreasedproductionandoxidationofCH4producedinthesubsoilandanincreaseinCO2emissionsduetoincreasedoxidationofsoilorganicmatter.N2Oemissionsfromwetlandsaretypicallylow,unlessananthropogenicsourceofnitrogenentersthewetland.Indrainedwetlands,N2Oemissionsarelargelycontrolledbythefertilityofthesoilandwatermanagementregime.Incontrast,restoredandconstructedwetlandsgeneratehigherlevelsofCH4andlowerlevelsofCO2becauseofthechangeinawatertabledepth(Blainetal.,2006).
4.2.1 DescriptionofWetlandManagementPractices
ThissectionprovidesadescriptionofmanagementpracticesinwetlandsthatinfluenceGHGemissions(CH4orN2O)orcarbonstocks.Individualsectionsdealwithforestedandgrasswetlandsthatcouldoccurinagriculturalandforestryoperations.Itisimportanttonotethatdrainageofwetlandsforcommodityproduction,suchasannualcrops,orforotherpurposesarenotconsideredwetlandsintheseguidelines.MethodsfordrainedwetlandscanbefoundinChapter3,CroplandsandGrazingLands,orChapter6,ForestLands,dependingonthelanduseafterdrainageofthewetland.
4.2.1.1 SilviculturalWaterTableManagement
Silviculturalwatermanagementsystemsareprincipallyusedtoregulatethewatertabledepthinordertoreducesoildisturbanceassociatedwithharvestingoperationsandalleviatestressfromsaturatedsoilconditionsonartificiallyregeneratedplantations.Thesilviculturalwatermanagementsystemshouldnoteliminatethewetlandconditionsofthesite.
SilviculturalwatermanagementsystemsaffectthecarbonbalanceandGHGemissionsfromthesite(Bridghametal.,2006).Typicallyorganicmatterdecompositionisenhancedwiththeimpositionofadrainagesystem,CH4emissionsarereduced,andN2Oemissionsmayincrease(Lietal.,2004).Carbonsequestrationinbiomassmaybeenhancedonsiteswithsilviculturaldrainagesystemsduetoincreasedtreeproductivity(MinkkinenandLaine,1998).
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4.2.1.2 ForestHarvestingSystems
Therearetwogeneraltypesofsystemsusedtoharvesttreesfromforestedwetlands:partialcuttingandclearcutting.Apartialcutinvolvestheremovalofselectedtreesfromthestand.Thenumberoftreesremovedortheresidualdensityofthestandwilldependonthestandtype,species,intendedproduct(s),andstandage.Theamountoftreebiomassremovedduringthepartialcutmayalsovary;topsmaybeleftonsiteifonlylogsareremoved,ortheymaybeconcentratedinalandingifwhole‐treeharvestingisused.Withthelattersystem,thetopsmayalsobeutilizedandremovedfromthesite.Partialcuttingistypicallyusedinriparianzonesandsitesthataremanagedforsolidwoodproducts.Clearcuttingresultsintheremovalofalloverstorytreesfromthesite.ClearcuttingistypicallyusedonnaturalstandsoccurringinfloodplainsofthesoutheasterncoastalplainandlacustrineandoutwashplainsoftheupperMidwest.Clearcuttingisalsothetypicalsystememployedtoharvestconiferandhardwoodplantations.
Partialcuttingaffectsthecarbonbalanceofthesitebydirectremovalofbiomass;increasedbiomassontheforestfloor,whichisthensubjecttodecayprocesses;andincreasedgrowthoftheremainingtreesforseveralyears.Decompositionofdeadbiomasswithinthestandmaybeacceleratedtemporarilyduetothechangesinambientconditionsandtheaddedresiduefromtheharvest.
Clearcuttingaffectscarbonstocksofthesitebydirectlyremovingthebiomass;increasingamountsofbiomassaddedtotheforestfloor;alteringthecarbonsequestrationforseveralyears,dependingonthetypeofregeneration;andalteringtherateoforganicmatterdecompositionintheforestfloorandsoil(Lockabyetal.,1999).Clearcuttingaffectstheambientconditionsofthesitebecauseoftheremovaloftheoverstoryvegetation.Italsoaltersthewaterbalanceofthewetlandduetothereductioninevapotranspirationfollowingharvesting.Typically,asaresultoflowerevapotranspiration,thewatertablerises,andthesitewillexhibitlongerperiodsofsaturation.ThischangeinthewatertablepositionhasdirecteffectsontheproductionofCH4andN2Oandsubsequentfluxestotheatmosphere(Lietal.,2004).
4.2.1.3 ForestRegenerationSystems
Therearetwobasicforestregenerationsystems,characterizedas(a)naturalregeneration,and(b)artificialregeneration.Naturalregeneration,asthenameimplies,reliesuponregenerationofthetreesfromseedorsproutsthatareleftbyharvestedtrees.Naturalregenerationisusedinbothpartial‐cutandclear‐cutharvestsystems.Naturalregenerationwillleadtoeven‐agedstandsofshade‐intolerantorearlysuccessionalcommunities,typicallyinfloodplainsinthesoutheasternUnitedStatesandtheconiferousplainsoftheupperMidwest.
Artificialregenerationresultsfromplantingseedlingsonapreparedsite.Thesitepreparationpracticesmayinvolveremovaloftheharvestresiduebiomass,mechanicalscarificationand/ortheapplicationofherbicidetotemporarilyreduceweedcompetitionwithseedlings,andthecreationofplantingbeds.
TheeffectoftheforestregenerationsystemoncarbonstocksandtraceGHGemissionsdependsonthetypeofharvestingsystemthatwasused(Lockabyetal.,1999;Trettinetal.,1995).Thecombinationofpartialcuttingandnaturalregenerationhaslittleadditiveeffectbecausetheextentofregenerationistypicallyquitelowfollowingapartialcutthatremoveslessthanhalfofthebasalarea.Carbonstocksfollowingclear‐cutharvestingwithnaturalregenerationisaffectedbytherateofgrowthoftheregeneration,changesinambientconditions,andchangesinthesoilwaterregime.Thosefactorsalsoaffectartificialregenerationsystems;additionally,thetypeandextentofsitepreparationalsoaffectsthecarbonstocks.
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4.2.1.4 Fertilization
Fertilizationisusedprimarilyinforestedwetlands,suchastreeplantations,toenhancegrowth(Albaughetal.,2004).Grasswetlandsalsoreceivefertilizerasaresultofadjacentagriculturalactivities,andwhendryconditionspermit,aredirectlytilled,planted,andfertilized.Nitrogenisthemostcommonlyappliedfertilizer,andincreasednitrogeninputsareknowntoincreaseemissionsofN2O(Bedard‐Haughnetal.,2006;Davidsonetal.,2000;Gleasonetal.,2009;Merbachetal.,2002;PhillipsandBeeri,2008;ThorntonandValente,1996).NitrogenfertilizerswillalsoenhanceN2Oemissionsbothdirectlyonthesiteandindirectlyifnitrogenislostfromthesiteasnitrateingroundwaterorrunoff,aswellasvolatilizationofnitrogenasammoniaorNOx.TheindirectlosseswillcontributetoN2Oemissionsatothersites.
Theeffectoffertilizationoncarbonstocksisprincipallyrealizedthroughchangesintreegrowthrates.Theeffectwouldresultfromnitrogenfertilizers,butphosphorusmayalsobeappliedinthesoutheasternUnitedStates.
4.2.1.5 ConversiontoOpen‐WaterWetland
Theconversionofwetlandtoopenwateroccursprimarilyasaresultofbeaverimpoundmentsandtoalesserdegreeimproperlyinstalledroadsorotherartificialembankmentsthroughawetlandthatimpedesnaturaldrainage.Theconversiontoopenwatersignificantlyreducescarbonsequestrationthroughplantgrowth,becauseuptakeislimitedtosubmergedaquaticvegetation.ThehigherwatertableforalongerperiodoftheyearwillalsotendtoincreaseCH4flux.
4.2.1.6 ForestTypeChange
Changingamanagedforesttoacharacteristicnativeconditionisalsoconsideredaformofrestoration.TheeffectoftherestorationactivitiesonthecarbonstocksandCH4emissionsdependsontheextentofthehydrologicmodificationsthatwereemployedintheprevioussilviculturalsystem.Thetwomostcommonsituationsareasitethathasbeenmanagedforaparticularspeciesorproductwithouthydrologicmodification;theothercommonsituationiswherethesitehasbeenmanagedforplantationforestryandthehydrologyandvegetationhavebeenextensivelymodified.
4.2.1.7 WaterQualityManagement
Riparianzonesalongstreams,rivers,andlakesmaybemanagedtoprotectwaterqualitybymitigatingnonpointsourcepollution(Balestrinietal.,2011;Chaubeyetal.,2010).4Pollutantsareremovedbyphysicalfiltration,chemicaladsorption,plantuptake,andmicrobialtransformations(Abu‐Zreigetal.,2003;Borinetal.,2005).5However,riparianbuffersarelimitedintheiradsorptioncapacitiesforsomeconstituents,whichmaythenflowintowaterways.Thebufferzonesizeandconfigurationvariesaccordingtorunoffpatternsofthesite,phosphorus/nitrogeninputs,hydrologicconnectivity,organiccarbon,mineralcontent,andoxidative/reductivestate(Abu‐Zreigetal.,2003;Hoffmannetal.,2009;Novaketal.,2002;YoungandBriggs,2008).
Riparianbufferzonesarecomprisedofnativeandnon‐nativevegetationormayalsocontaincultivatedplantsinsomecases.Managementactivitiesofthenativevegetationbufferzonesaretypicallyconstrainedorlimitedtosmallremovals.Inthecaseofforestriparianbuffers,aselective‐4Additionalreferencesinclude(Choetal.,2010;Fliteetal.,2001;Hoffmannetal.,2009;Huntetal.,2004;Leeetal.,2004;Lowranceetal.,2007;Montreuiletal.,2010;PeterjohnandCorrell,1984;RanalliandMacalady,2010;Schoonoveretal.,2005;Tabacchietal.,1998;YoungandBriggs,2008).5Additionalreferencesinclude(Dillahaetal.,1989;Dillahaetal.,1988;Hoffmannetal.,2009;Jordanetal.,2003;Kellyetal.,2007;Novaketal.,2002;Vellidisetal.,2003;YoungandBriggs,2008).
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harvestregimewouldbeusedthatinfluencesbothcarbonstocksandGHGemissions.Inmixedbuffers(i.e.,grassstripsfollowedbyforest),themanagementofthecultivatedbufferwouldlargelydeterminetheeffectofthepractice,whichwillbeanalogoustohaycultivation.Riparianzonesmaycontainamosaicofhydric(wetland)andnon‐hydricsoils;accordingly,thedistributionofsoiltypesisimportantforassessingtheeffectofthemanagementactivity.
Whereasriparianbuffersoccupylowlandscapepositionsandaretypicallywet,theyareoftenveryeffectiveinremovingnitrogenviadenitrification(Ambus,1991;Davisetal.,2008;Dodlaetal.,2008;Hilletal.,2000;Huntetal.,2007;Jordanetal.,1998;Roobroecketal.,2010;Smithetal.,2006;Stoneetal.,1998;Woodwardetal.,2009),whichleadstoindirectN2Oemissions(Jetten,2008).Denitrificationinriparianbuffersisoftenspatiallyunevenbecauseriparianbuffersvaryconsiderablyintheirsizeandlandscapepositionsaswellastheirsoil,vegetative,andhydrologicalconditions(Bowdenetal.,1992;BrulandandMacKenzie,2010;Fliteetal.,2001;Hilletal.,2000).StudieshavesuggestedthatN2Oemissionsinriparianzoneswerenotasignificant“pollution‐swappingphenomenon”(Dhondtetal.,2004;Kimetal.,2009a;Kimetal.,2009b).Significantemissionsarelikelytobelimitedtospatialandtemporalhotspots(Groffmanetal.,2000;Huntetal.,2007;Kimetal.,2009b).Moreover,someriparianwetlandsystemscanserveassinksfornitrogen(Roobroecketal.,2010).WhilemanyfactorsaffectthemicrobialproductionofN2O,oneofthemostdominatingfactorsisthecarbontonitrogenratio;largerratiosgenerallyhavelowN2Oemissionsbecausenitrogenisimmobilizedinthesoilorganicmatter(Huntetal.,2007;Klemedtssonetal.,2005).However,itisimportanttonotethatindirectN2Oemissionsareattributedtothesourceofthenitrogen,whichcanbeaneighboringfieldorlivestockfacility;sothemethodstoestimateindirectN2Oemissionsareprovidedinothersectionsofthisreport(i.e.,Chapter3,CroplandandGrazingLands,orChapter5,AnimalProduction).
RiparianbufferscanserveasbothsourcesandsinksofCH4(Hopfenspergeretal.,2009;Soosaaretal.,2011).TheirhydrologyandbiogeochemicalcharacteristicsexhibitsignificantinfluenceonthenetCH4emission.Thesecharacteristicsincludewatertableposition,temperature,oxidative/reductivepotential,andplantcommunitycompositions(Pennocketal.,2010;Whalen,2005).Moreover,N2Oemissionsfromdenitrificationcanbesignificantlyinfluencedbymethanotrophs(Costaetal.,2000;Knowles,2005;Modinetal.,2007;Osakaetal.,2008).
Similarbuffersexistforgrasswetlands,eitheraspartofaconservationprogramorasanaturallyoccurringareaaroundawetlandwheremoist‐soilconditionspreventtillage.Grassbuffersreducerunoffandinterceptsedimentsthatwouldaffectwaterqualitybyincreasingturbidityandinputsoffertilizersandagrichemicals.Moreover,plantingtheentirecatchmentwithgrasscanreduceCH4emissionsbydecreasingtheartificiallyhighwaterlevelsandextendedhydroperiodsthatoftenareassociatedwithcroplandsites(EulissJrandMushet,1996;Gleasonetal.,2009;vanderKampetal.,2003).
4.2.1.8 WetlandManagementforWaterfowl
Wetlandsmaybemanagedforwaterfowlhabitat.ActivitiesthatarespecifictowetlandwaterfowlmanagementhavedirectinfluencesoncarbonstocksandGHGemissions,includingregulationofthewaterregime,specificallydepthanddurationofinundation,aswellasplantingandcultivationofcropsforfoodandhabitat.Waterregimesimposedforwaterfowlmanagementmaybedifferentthanthenaturalwatertablecycleofthesite.Accordingly,changingthewatertablealterstheperiodsofsoilaerationandsaturationinfluencingratesofCH4andN2O,aswellascarbonstockchangesintimberstandsandotherwetlandvegetation.CultivatingcropsinwetlandsmanagedforwaterfowlwillalsoinfluencecarbonstocksandN2Oemissionsbasedonselectionofcropsand/orrotationpractice,tillage,liming,andnutrientmanagement.
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4.2.1.9 ConstructedWetlandsforWastewaterTreatment,SedimentCapture,andDrainageWaterAbatement
Constructedwetlandsareengineeredsystemsforwastewatertreatment,captureofsediments,anddrainagewaterabatementinagriculturalandforestryoperations(Chenetal.,2011;Elgoodetal.,2010).Surface‐flowandsubsurfaceflowsystemsarethetwoprincipaltypesofconstructedwetlands(KadlecandKnight,1996).Theprincipaldifferencebetweenthesetwotypesofconstructedwetlandsisthewaterflowpath.Inthecaseofthesubsurfaceflowwetlands,allthewaterflowsarebeneaththesoilsurface;thesurface‐flowsystemshaveflowbothaboveandwithinthesoil.
Thesubsurfacewetlandstypicallyconsistofwetlandplantsgrowinginabedofhighlyporousmediasuchasgravelorwoodchipsthathaveawatertablefromonetotwometersabovethesoilsurfacewitharectangularshape.ThereislackofagreementabouttherelativeimpactofmicrobialandplantprocessesinthefunctionofsubsurfacewetlandsincludingGHGemissions.However,plantsandmicrobesaretypicallyinterdependentlyinvolvedintheprocessesthatcontributetoemissions(Faubertetal.,2010;Luetal.,2010;Piceketal.,2007;TannerandHeadley,2011;Wangetal.,2008;Zhuetal.,2007).WhilethemicrobialcommunitydrivesthebiogeochemicalprocessesthatspecificallyemitGHGs(Dodlaetal.,2008;Faulwetteretal.,2009;Huntetal.,2003;Tanneretal.,1997;Zhuetal.,2010),theplantcommunitymodifiestheenvironmentalconditionscontributingtoemissionrates,includingtransportingoxygenintothedepthofthewetlands,providingrootsurfacesforrhizospherereactions,andventinggasestotheatmosphere.Theplantprocessesaresignificantlyimpactedbyplantcommunitycompositionandweatherconditions(Steinetal.,2006;SteinandHook,2005;Tayloretal.,2010;Towleretal.,2004;Wangetal.,2008;Zhuetal.,2007).
SurfaceflowwetlandshaveamuchmoredirectexchangeofoxygenandGHGswiththeatmosphere.Theycanbevariableinshapeandaregenerallylessthan0.5metersindepth.Surfacewetlandsminimizecloggingproblems,buttheycanhaveasignificantlossoftreatmentasaresultofchannelflow.Theyaretypicallydesignedforeithercarbonornitrogenremoval(Steinetal.,2006;Steinetal.,2007;Stoneetal.,2002;Stoneetal.,2004),includingthepreventionofexcessiveammoniaemissions(Poachetal.,2004;Poachetal.,2002).
Constructedwetlandsaretypicallycreatedinuplandsettings(e.g.,non‐wetland);accordingly,thesiteassumesthesamebiogeochemicalprocessesthatareinherenttonaturalwetlands.CarbonstocksandGHGemissionsareaffectedbythetypeandquantityofeffluentbeingtreated,thetypeofvegetationinthewetlandcells,andmanagementofthehydrologicregimeswithinthecells.ThemanagementofCH4andN2OfromconstructedwetlandsissomewhatsimilartomanagingGHGemissionsfromwetlandricesystems(Feyetal.,1999;Freemanetal.,1997;Johanssonetal.,2003;Maltais‐Landryetal.,2009;Manderetal.,2005a;Manderetal.,2005b;Piceketal.,2007;Tanneretal.,1997;TeiterandMander,2005;Wuetal.,2009).Ofparticularimportanceisthemaintenanceofwetlandoxidative/reductivepotentialsthataresufficientlypositivetoavoidCH4production(InsamandWett,2008;SeoandDeLaune,2010;Tanneretal.,1997).Thisrequireshigherlevelsofoxygenandlowerlevelsofavailablecarbon.ThemanagementofN2Oemissionsiscomplicatedbythefactthatnitratesareoftenpresentinthewastewatersordrainagewaters,andsoGHGemissionscanbereducedintheconstructedwetlandsifN2gasisemittedinsteadofN2O.CompletedenitrificationtoN2gasrequireshighercarbon/nitrogenratios(Huntetal.,2007;Hwangetal.,2006;Klemedtssonetal.,2005).Thus,thereisanimportantbalancebetweensufficientcarbonforcompletedenitrificationandcopiouscarbonthatdriveswetlandsintothelowredoxconditionsassociatedwithCH4production.
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Thissectionisincludedforcompleteness,butnomethodforconstructedwetlandsisprovidedinthissection.Section5.4.10inChapter5,AnimalAgriculture,providesaqualitativediscussionofestimatingemissionsfromliquidmanurestorageandtreatment‐constructedwetlands.However,Chapter5doesnotprovidemethodstoestimategreenhousegasemissionsfromconstructedwetlands.
4.2.2 Land‐UseChangetoWetlands
Conversionoflandtowetlandsmayinvolverestoringagriculturallandintoafunctioningwetland.However,wetlandscanberestoredfrompreviouslydrainedforestorgrasslands,andthechangetendstovaryfordifferentregionsoftheUnitedStates.Wetlandscanalsobeconstructedinanylocationforwastewatertreatment.Theoriginalconversionofwetlandstoanotherusetypicallyinvolvesanalterationofthenaturalwetlandhydrology.Chapter7,LandUseChange,addressesthistypeofconversion.Restorationofwetlandsentailsreestablishmentoftherequisitehydrologytosupportforest,scrub‐shrub,sedge,oremergentwetlandplantcommunitiesandoccursinfloodplains,riparianzones,depressions,andslopesandvalleys.
4.2.2.1 ActivelyRestoringWetlands
TheeffectofrestoringbothforestedandgrasswetlandswillleadtocarbonsequestrationandCH4emissionsthatwouldbecharacteristicforthatwetlandtype.However,theextenttowhichcarbonsequestration,organicmatterturnover,andgasfluxesreturntoratestypicalforthewetlandtypedependsonmanyfactors,particularlythedegreeofalteration,timesincerestoration,hydrology,anddevelopmentofthevegetation.Ingeneral,restoredsiteswillbecarbonsinksduetosequestrationinthedevelopingbiomass(e.g.,foreststand)andsoils(EulissJretal.,2008).Soilcarbonisexpectedtoincreaseslowlyinforestedsettingsandsomewhatmorerapidlyingrasslandsites(Gleasonetal.,2009);however,theextentandratesofchangeareuncertain.ReestablishmentofthewetlandhydrologywillalsoaltertheCH4fluxfromtherestoredsitesincehydrologicmodificationsforotherlanduseswilltypicallyinvolvedrainageordiversions.RaisingthewatertableandincreasingtheperiodoftimethatthesoilsurfaceiscoveredwithwaterwillincreaseCH4production.However,manyrestoredgrasslandsitesarenotdirectlydrained,andreestablishmentofgrassesinthecatchmentcanshortenthehydroperiod(VanDerKampetal.,1999;Voldsethetal.,2007),thusreducingCH4production.
Conversionofscrub‐shrubwetlandstypicallyinvolvesdrainagetoanon‐wetlandstate,andtheimpositionofcultivationorotherpracticesdependingonthelanduse.Accordingly,therestorationofprior‐convertedscrub‐shrubwetlandstypicallyinvolvesreestablishmentofthenaturalwetlandhydrologyandselectiveplantingtoestablishnativevegetation.ThedevelopmentofthecharacteristicwetlandhydrologyistheprincipalfactoraffectingthecarbonstocksandGHGemissionsfromthesitefollowingconversion,butthetypeofvegetationandtimesinceestablishmentwillalsohavesomeinfluence.
4.2.2.2 CreatedWetlands
Createdwetlandsareengineeredintonon‐wetlandoruplandsites.Typicalexamplesincludemitigationsites,anaerobiclagoons(SeeSection5.4.10inChapter5,AnimalAgriculture)onlivestockoperations,andstormwaterdetentionbasins.TheprincipalactivityaffectingthecarbonstocksandGHGemissionsistheimpositionofahydrologicregimethatinduceshydricsoilpropertiesandsupportshydrophyticplants,inadditiontoclearingofthepreviousvegetationthatmayleadtoachangeinbiomasscarbonstocks.
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4.2.2.3 PassiveRestorationofWetlands
Allowinganareatoregeneratethroughnaturalsuccessionisalsoconsideredaformofrestoration.TheeffectoftherestorationactivitiesonthecarbonstocksandCH4emissionsdependsonwhethertherewashydrologicremediationandthedegreeofvegetationchangeovertime.
4.3 EstimationMethodsSection4.3.1providesmethodsforestimatingliveanddeadbiomassinforested,shrub,andgrasslandwetlands.Section4.3.2providesmethodsforestimatingsoilC,N2O,andCH4emissionsfrommanagednaturallyoccurringwetlands.
4.3.1 BiomassCarboninWetlands
4.3.1.1 RationaleforSelectedMethod
Variousapproachesareusedforestimatingtreebiomasscarbon,butultimatelyeachreliesonallometricrelationshipsdevelopedfromacharacteristicsubsetoftrees.TheForestVegetationSimulator(FVS)hasbeenselectedasthemethodtoestimatetreebiomass.FVSismodel‐basedapproachthatisspecifictoU.S.conditionsandaTier3methodasdefinedbytheIPCC.ThesimulatoristhemostcompletemodelintheUnitedStatestoestimatetreebiomass.RegionalversionsofFVShavebeenrefinedbasedonlargedatabasesdevelopedfrommanyyearsofdatacollectiononforeststandsthroughouttheUnitedStates,therebyprovidingimprovedestimateswhilerequiringfewinputparametersfromtheuser.
BothIPCC(2006)andEPA(2011)considerherbaceousbiomasscarbonstockstobeephemeral,andrecognizethattherearenonetemissionstotheatmospherefollowinggrowthandsenescence.However,withrespecttochangesinlanduse(e.g.,foresttocropland),theIPCC(Lascoetal.,2006)recommendsthatgrazinglandbiomassbecountedintheyearthatlandconversionoccurs(Verchotetal.,2006).AccordingtotheIPCC,accountingfortheherbaceousbiomasscarbonstockduringchangesinlanduseisnecessarytoaccountfortheinfluenceofherbaceousplantsonCO2uptakefromtheatmosphereandstorageintheterrestrialbiosphere.ThemethodisconsideredaTier2methodasdefinedbytheIPCCbecauseitincorporatesfactorsthatarebasedonU.S.specificdata.
Themethodspresentedinthissectionarebasedonthefollowingdefinitions.
Livevegetationbiomass:Livevegetationincludestrees,shrubs,andgrasses.Thetreecarbonpoolincludesabovegroundandbelowgroundcarbonmassoflivetrees,asdefinedinSection6.2.3.1,andtheabovegroundbiomassoftheforestunderstoryisdefinedinSection6.2.3.2.Themethodstoestimatefull‐treeandabovegroundbiomassfortreesgreaterthanoneinchindiameteratbreastheightarebasedonthemodelsprovidedintheforestsection.
MethodforEstimatingLiveandDeadBiomassCarboninWetlands
MethodsforestimatingforestvegetationandshrubandgrasslandvegetationbiomasscarbonstocksuseacombinationoftheForestVegetationSimulatormodelandthebiomasscarbonstockchangesmethodinSection3.5.1ofChapter3,CroplandandGrazingLand.Ifthereisaland‐usechangetoagriculturaluse,methodsforcroplandherbaceousbiomassareprovidedinChapter3.
Thesemethodswerechosenbecausetheyofferthemostconsistentapproachwithinthecontextofthisreport.
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Theforestunderstoryvegetationincludesallbiomassofundergrowthplantsinaforest,includingwoodyshrubsandtreeslessthanoneinchindiameteratbreastheight.
Standingdeadwood(deadbiomass):ThecarbonpoolofstandingdeadwoodinaforestedwetlandisdefinedandestimatedaccordingtothemethodsinSection6.2.3.3ofChapter6,Forestry.
Downdeadorganicmatter—litterlayer(deadbiomass):Downdeadorganicmatterincludesthelitterlayercomposedofsmallpiecesofdeadwood,branches,leaves,androotsinvariousstagesofdecay.Thislayeristypicallydesignatedastheorganiclayerofthesoil.Thispoolalsoincludeslogsinvariousstagesofdecaythatlieonthesoilsurface(e.g.,Section6.2.3.4,down‐deadwood,andSection6.2.3.5,forestfloororlitter).
4.3.1.2 DescriptionofMethod
Provisionsforestimatingabovegroundbiomassforwetlandforestsandaboveandbelowgroundbiomassandcarbonareincludedforshrubandgrasswetlandsinthissection.Sincethevegetativecoveronwetlandsmayvaryfromnaturalcommunitiestoagriculturalcrops,cross‐referencesaremadetoensurecongruitywithSection3.5.1ofChapter3,Croplands,andGrazingLands,andSection6.2.3ofChapter6,Forestry.
Forestvegetation:BiomasscarbonstocksareestimatedforforestsinwetlandsusingthemethodsdescribedinSection6.2.3ofChapter6,Forestry.TheapproachusestheFVS,whichisasystemofgrowthandyieldmodelsthatestimategrowthandyieldforU.S.forests.FVSisanindividualtreemodelandcanestimatebiomasscarbonstockchangefornearlyanytypeofforeststand.TheFireandFuelsExtensiontoFVScanbeusedtogeneratereportsofallliveanddeadbiomasscarbonpoolsinadditiontoharvestedwoodproducts.RegionalvariantsareavailableforFVSthatallowforregion‐specificfocusonspeciesandforestvegetationcommunities.Thedriverforproductivityistheavailabilityofsiteindexcurves,6andtheregionalvariantsincludemanywetlandtreespecies.RegionalvariantsofFVSmayalsoprovideprovisionsforrefiningthebasisforestimatingproductivitybyclassifyingtheareaofinterestintoecologicalunits,habitattype,orplantassociations.However,ifaspecies‐specificcurveisnotavailable,thenadefaultfunctionisusedtoestimatecarbonstockchanges.
Grasslandvegetation:Thechangeincarbonstockforgrasswetlandsisgenerallysmallunlesstherearedroughtconditionsortheareaisactivelymanaged.Incaseswherereportingisrequired,biomasscarbonstockchangescanbeestimatedfollowingalandusechangeusingthemethodinSection3.5.1ofChapter3,CroplandsandGrazingLands.Therearenomethodscurrentlyavailabletoestimatetheshrubcover.
4.3.1.3 ActivityData
Forestedwetlands:ThedataandrequirementsforestimatingthechangesincarbonstocksinwetlandforestsarethesameasthosedescribedforuplandforestsinSection6.2.3.
Grasslandvegetation:ThedataandrequirementsforestimatingthechangesincarbonstocksingrasslandvegetationarethesameasthosedescribedfortotalbiomasscarbonstockchangespresentedintheCroplands/GrazingLandsSections3.5.1.
6Siteindexisthemeasureofaforest’spotentialproductivity.Theheightofthedominantorco‐dominanttreesataspecifiedageinastandarecalculatedinanequationthatusesthetree’sheightandage.Siteindexequationsdifferbytreespeciesandregion.Siteindexcurvesareconstructedbyusingthetreeheightsatabaseageandanequationisderivedfromthecurvestoestimatethesiteindexwhenanindividualtree’sageisnotthesameasthebaseage(Hansonetal.,2002).
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4.3.1.4 ModelOutput
Changeinabovegroundcarbonpoolsassociatedwithwetlandforestsareprovidedforlivevegetation,standingdeadbiomass,anddowndeadbiomass.Changeinlivebiomasscarbonisalsoprovidedforbelowgroundbiomass.Theunitsofreportingaremetrictonnesha‐1CO2‐eq.
4.3.1.5 LimitationsandUncertainty
Estimatesoftheforestbiomasscarbonpoolsinwetlandsareconstrainedbylimiteddataonproductivityresponsetomanagementandaresensitivetothewidearrayofcharacteristicvegetativecommunitiesandsoiltypes.AlthoughFVSisthemostinclusivemodelavailable,manyresultsforwetlandswillstillbebasedondefaultmodelfunctions,becausethereislimiteddataonthegrowthofspecificwetlandspeciesunderparticularmanagementregimes.Accordingly,theresultswillprovidearelativebasisfortrackingchangesovertimeinbiomasscarbon.Table4‐3summarizesadditionallimitationsinthecurrentapproach.
Table4‐3:KeyLimitationstoEstimatingBiomassCarbonPoolsinForestWetlandVegetation
Consideration Limitation
Ratioforbelowgroundbiomass
Aratioisused toestimatebelowgroundbiomassinuplandandwetlandforestsbasedonabovegroundbiomass.Whileacommonratiowillprovideabasisforestimatingrelativechange,itwilllikelyoverorunderestimateactualstocksinmanywetlands.
Responsetomanagementorclimaticconditions
Wetlandvegetationisknowntorespondtomanagementpractices,soil, andclimaticconditions.ThoserelationshipsarenotnecessarilyreflectedinFVSbecausethereisinsufficientbasisforgeneralizedassessmentpurposes.Forexample,inresponsetodynamicwater‐levelfluctuationsduringwetanddrycycles,wetlandsoftenexhibitmajorintraandinterannualshiftsinvegetativestructure,rangingfromopenwatertoemergentherbaceousvegetation.Correspondingly,thealteredsiteconditionsunderthemanagementregimeandthegeneticqualityoftheplantedtreesmayexhibitresponsesthatarenotcapturedbytheexistingallometricrelationshipsinFVS.
ThisshrubandgrasslandmethodisbasedontheassumptionsfoundinChapter3,CroplandandGrazingLand.Essentially,themethodassumesthathalfofthecropbiomassatharvestorpeakforage/shrubbiomassprovidesanaccurateestimateofthemeanannualcarbonstock.Thisassumptionwarrantsfurtherstudy,andthemethodmayneedtoberefinedinthefuture.
Majorsourcesofuncertaintyincludebelowgroundbiomass,vegetationresponsetomanagement,andhydrologicregime(e.g.,seasonalhydroperiod).Uncertaintyinherbaceouscarbonstockchangeswillresultfromlackofprecisionincroporforageyields,residue‐yieldratios,root‐shootratios,andcarbonandcarbonfractions,aswellastheuncertaintiesassociatedwithestimatingthebiomasscarbonstocksfortheotherlanduses.
Measurement,sampling,andregression/modelingerrorsareallpartoftheestimationprocessinFVS.Somesimilarmeasureoftherepresentativenessofselectedforestinventoryandanalysisplotstotheentities’forestsisneeded.Uncertaintiesaboutcarbonconversionfactorsarealsosignificantinsomecases.
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4.3.2 SoilC,N2O,andCH4inWetlands
4.3.2.1 RationaleofMethod
Theproductionandconsumptionofcarboninwetland‐dominatedlandscapesareimportantforestimatingthecontributionofGHGs,includingCO2,CH4,andN2Oemittedfromthoseareastotheatmosphere.ThegenerationandemissionofGHGsfromwetland‐dominatedlandscapesarecloselyrelatedtoinherentbiogeochemicalprocessesthatalsoregulatethecarbonbalance(RoseandCrumpton,2006).However,thoseprocessesarehighlyinfluencedbythelanduse,vegetation,soilorganisms,chemicalandphysicalsoilproperties,geomorphology,andclimate(SmemoandYavitt,2006).
Giventhiscomplexity,aprocess‐basedmodelingapproachisdesirablebecausetheseapproachestypicallyaccountformoreofthevariabilitythansimpleremissionfactormethods(IPCC,2006).However,fewprocess‐basedmodelshavebeentestedsufficientlytobeusedforoperationalreportingofGHGemissions.OneofthemorewidelytestedmodelsforestimatingGHGfluxesfromwetlandsistheDNDCmodel.DNDCisaprocess‐basedbiogeochemicalmodelthatisusedtopredictplantgrowthandproduction,carbonandnitrogenbalance,andgenerationandemissionofsoil‐bornetracegasesbymeansofsimulatingcarbonandnitrogendynamicsinnaturalandagriculturalecosystems(Lietal.,2000;Miehleetal.,2006;Stangetal.,2000)andforestedwetlands(Zhangetal.,2002).Themodelisdesignedtoexplicitlyconsideranaerobicbiogeochemicalprocesses,whicharefundamentaltoaddressingsoilcarbondynamicsandtraceGHGdynamicsinwetlands(Trettinetal.,2001).Itintegratesdecomposition,nitrification–denitrification,photosynthesis,andhydro‐thermalbalancewithintheecosystem.Thesecomponentsaremainlydrivenbyenvironmentalfactors,includingclimate,soil,vegetation,andmanagementpractices.
DNDChasbeentestedandusedforestimatingGHGemissionsfromforestedecosystemsinawiderangeofclimaticregions,includingboreal,temperate,subtropical,andtropical(Kesiketal.,2006;Kieseetal.,2005;Kurbatovaetal.,2008;Lietal.,2004;Stangetal.,2000;Zhangetal.,2002),andsimilarlyforgrasslandsandcultivatedwetlands(Giltrapetal.,2010;Rafiqueetal.,2011).
4.3.2.2 DescriptionofMethod
Themethodconsistsofusingtheprocess‐basedmodel—DNDC—toestimatethechangesinsoilorganiccarbon(SOC)stocks,CH4,andN2Oemissions,basedonthestandingbiomassandplantgrowththatareprovidedbythevegetationmethodoutlinedabove(Section4.3.1),wetlandcharacteristics,andtheplannedmanagementactivities.ThemodelsimulatesSOCstocks,CH4,andN2Oemissionsatthebeginningofthereportingperiodbasedonanassessmentofinitialconditionsatthesite;thenthemodelsimulatesthereportingperiodbasedonthecurrent/recentmanagementactivityandanychangesinthewetlandconditions.Thisinformationcharacterizesthephysicalandchemicalsoilpropertiesthatinturninteractwiththeclimaticregime,managementpractices,and
MethodforEstimatingSoilC,N2OandCH4 inWetlands
TheDNDCprocess‐basedbiogeochemicalmodelisthemethodusedforestimatingsoilC,N2O,andCH4emissionsfromwetlands.
DNDCpredictssoilcarbonandnitrogenbalanceandgenerationandemissionofsoil‐bornetracegasesbysimulatingcarbonandnitrogendynamicsinnaturalandagriculturalecosystems(Lietal.,2000;Miehleetal.,2006;Stangetal.,2000)andforestedwetlands(Daietal.,2011;Zhangetal.,2002),usingplantgrowthestimatedasdescribedinSection4.3.1.
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thevegetationresponse.Thereportedemissionsforthelandparcelmustreflectthetotalfortheentirelandarea.Accordingly,theper‐unitareaemissionratesfromDNDCareexpandedbasedonthetotalwetlandareaforthelandparceltoestimatetotalemissions.
Equation4‐1isusedtoestimateSOCstockchangesfromaparceloflandinawetland:
Equation4‐2isusedtoestimateCH4emissionsfromaparceloflandinawetland:
N2OemissionsareestimatedforalandparcelinawetlandusingEquation4‐3:
Equation4‐1:ChangeinSoilOrganicCarbon StocksforWetlands
ΔCSoil=(SOCt‐SOCt‐1)xAxCO2MW
Where:
ΔCSoil =Annualchangeinmineralsoilorganiccarbonstock(metrictonsCO2‐eqyear‐1)
SOCt =Soilorganiccarbonstockattheendoftheyear(metrictonsCha‐1)
SOCt‐1 =Soilorganiccarbonstockatthebeginningoftheyear(metrictonsCha‐1)
A =Areaofparcel(ha)
CO2MW =RatioofmolecularweightofCO2toC=44/12(metrictonsCO2(metrictonsC)‐1)
Equation4‐2:MethaneEmissionsfromWetlands
CH4=ERxAxCH4MWxCH4GWP
Where:
CH4 =TotalCH4emissionsfromthelandparcel(metrictonsCO2‐eqyear‐1)
ER =Emissionrateonaperunitwetlandarea(metrictonsCH4ha‐1year‐1)
A =Area(ha)
CO2MW =RatioofmolecularweightofCH4toC=16/12(metrictonsCH4(metrictonsC)‐1)
CH4GWP =GlobalwarmingpotentialofCH4
Equation4‐3:NitrousOxideEmissionsfromWetlands
N2O=ERxAxCO2MWxCH4GWP
Where:
N2O =TotalN2Oemissionsfromthelandparcel(metrictonsCO2‐eqyear‐1)
ER =Emissionrateonaperunitlandarea(metrictonsN2Oha‐1year‐1)
A =Area(ha)
CO2MW =RatioofmolecularweightofN2OtoN=44/28
(metrictonsN2O(metrictonsN2O‐N)‐1)
CH4GWP =GlobalwarmingpotentialofN2O
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ToestimatetheSOCstockchanges,CH4,andN2Oemissions,DNDCrequiresaconsiderableamountofinformationtocharacterizetheplantproduction(Section4.3.1),wetlandcharacteristics,andmanagementactivities.TheinitialstepinapplyingthemethodistoparameterizeDNDCusingthebaselinesoilconditions,alongwiththecorrespondingforestorgrasslandconditions.Forexample,ifaforestplantationistobeharvestedandregeneratedduringthereportingperiod,theinitialconditionsshouldreflectthepre‐harvestconditions.Basedontheinitialconditions,themodelsimulatesbaselinefluxesandtheSOCstockpriortothereportingperiodfortheentity.Subsequently,theentityspecifiesthetypeofmanagementactivity(s)changesthatoccurredduringthereportingperiod(ifanyoccurred).Provisionsareavailabletohavemultiplemanagementactivitiesonasingletractifthereweremixedactivities.Climaticfactors,especiallyprecipitation,canaffectcarbonturnoverandwetlandconditions.Consequently,weatherdataareakeyinputtoDNDC,andwillbeprovidedfromaclimatologicaldataset.
ThesimulationoutputattheendofeachyearisusedtoestimatechangeinSOCstocksandthetotalamountofCH4andN2Oemissionsfortheyear.AnnualchangesinSOCcanbeestimatedbasedonthedifferencebetweenyears,andthetotalchangeinemissionscanbeestimatedbycombiningthechangesinSOCpoolswiththeannualCH4andN2Oflux.
4.3.2.3 ActivityData
ActivitydatafortheapplicationofDNDCaresummarizedinTable4‐4.Vegetationmanagementinformationaffectstheamountoforganicmatterthatisavailablefordecompositionprocesses.Watermanagementinformationconveyshowthedrainagesystemaffectsthesoilwatertabledynamicascomparedtoanundrainedcondition.Thesoiltillageinformationisusedtoconveywhenthesurfacesoilisdisturbedoritselevationchangedbecauseoftheassociatedeffectsondecomposition.ThefertilizationinformationisneededbecausetheadditionofnitrogengreatlyaffectsdecompositionandN2Oproduction.Inaddition,landusehistoryinfluencestheamountofsoilorganiccarbon.Ifanentityiscomposedofdifferentwetlandtypes,itisrecommendedthatseparateestimatesbepreparedbecausethecarbonturnoverrateandGHGemissionscanvarywidelydependingonhydricsoilpropertiesandthetypeofvegetation.
Table4‐4:ActivityDataforApplicationofDNDC
Category ManagementPractice Data
Vegetationmanagement
Grazingormanagementeventsshouldbeincludedtocapturetheinfluenceoncarboninputtosoilsandsubsequenteffectsonthesoilcarbonstocks.
Harvesting:date,harvest,orcutfraction Understorythinningorchopping:date,
choppedfraction Prescribedfire:date,proportionofforest
floor,andunderstoryconsumed Treeplanting:date,species,density
Watermanagementregime
Watertableresponsetothedrainagesystem,dailydata.
Drainagesystem:date,controlledwatertableelevation
Soilmanagement
Applicationofsoilamendmentsorsitepreparationpracticesfortreeplanting. Typeofsitepreparation
Fertilizationpractices
ApplicationsofmineralororganicnitrogenfertilizerswillbeneededtosimulatetheeffectonN2Oemissions.
Fertilizationfrequency,date,applicationrate(N,Pkgha‐1)
Landusehistory
Summaryoflandusepracticesoverthepast5years.Forassessingifprioruseaffectsparameterization.Thetimesinceachangeinlandmanagementpracticeforassessingeffectsondecomposition.
Fertilizationregimes,drainageregimes,cropping,orforestmanagementhistory
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4.3.2.4 AncillaryData
TheDNDCmodelrequiresrelativelydetailedinformationaboutthesite(Table4‐5).Whiledefaultvaluesareavailableformostparameters,someentity‐specificdataareneededtoproducereasonableestimates.Mostoftherequiredsoilsinputdataareavailablefromthenationalsoilsdatabase.7Similarly,climatedataareavailablefromtheNationalClimateDataCenter.8
Table4‐5:InputInformationNeededfortheApplicationofDNDC
Category Data
ClimateDailymaximumandminimumtemperature,dailyrainfall; nitrogendepositioninrainfall,orusedefaultvalue.
Vegetation StandingbiomassandbiomassanddetritalinputsprovidedinSection4.3.1;belowgroundbiomassestimatedbasedonabovegroundbiomass.
Soil
Hydraulicparametersandphysicalandchemicalcomponents, includingthickness;layers;hydraulicconductivity;porosity;fieldcapacity;wiltingpoint;carboncontent;pH;organicmatterfractions;contentofstone,sand,silt,andclay;andbulkdensityformajorsoillayers.
Hydrology WatertablebelowsurfaceasdailyinputorstartingpositionandDNDCcanestimateGHGemissionsandsinksusingempiricalfunctions.
4.3.2.5 ModelOutput
ModeloutputincludesannualestimatesofCH4,N2Oemissions,andchangesinsoilorganiccarbonstocks.TheunitsofreportingaremetrictonsCO2‐eqha‐1.
4.3.2.6 LimitationsandUncertainty
Themodelstoestimatecarbonsequestrationinvegetationarerobustwithrespecttospeciesandcommunitycomposition.However,uncertaintiesmaybehigherthanforuplandsbecauseoflimitedbackgroundinformation.ThemeritoftherecommendedapproachisthatitensuresconsistencyforestimatingchangesinthevegetativecarbonpoolamonglandtypesandusesbyusingcommonmethodsasdescribedinSection4.3.1.However,thisapproachcomplicatestheapplicationofDNDCforestimatingchangesinsoilcarbonpoolsandfluxesbecauseitcontainsprovisionsforsequesteringcarbonincrops,grasslands,andforestvegetation.Accordingly,DNDCwouldhavetoundergosubstantialrevisionstoaccommodatethevegetativecomponentasaninputvariablebecausethevegetationgrowthfunctionsareintegralwiththeconsiderationofhydrologicprocesses(especiallyevapotranspiration)andbiogeochemicalprocesses.TheDNDCmodelcouldbeusedasastand‐alonetoolforwetlands,butunfortunately,theproductionorcarbonsequestrationfunctionshavenotbeenvalidatedformanyofthewetlandplantcommunities.
Theavailabilityofwatertabledataisessentialtomodelingthecarboncycleinwetlandsoils.Sincethelackofsite‐specificwatertabledataforasufficientperiodislikelyaconstraintformostentities,anapproachincorporatingahydrologicmoduleorlook‐uptableisneeded.Hydrologicmodelsthatprovideinformationonwatertabledynamicsareinherentlycomplex,buttheycanbeeffective(Dai.etal.,2010).Accordingly,thedevelopmentofcharacteristicwatertableconditionsforarangeofclimatologicalandsoilsettingswouldbeaviableapproachthatcanalsoincorporatewatermanagementeffects(e.g.,Skaggsetal.,2011).
7SeeNationalCooperativeSoilSurveySoilCharacterizationdatahttp://soils.usda.gov/survey/nscd/.8SeeNOAANationalClimaticDataCenterhttp://www.ncdc.noaa.gov/.
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Tidalfreshwaterforestedwetlands,whichoccurtoalimitedextentalongtheAtlantic,Gulf,andPacificcoasts,areaspecialcase.Thetidalinfluenceonwatertabledynamicscanmakecharacterizingthewatertableregimeofsuchsitesmoredifficult.ForDNDCtosimulatethecarbondynamicswouldrequiredetaileddataondailywatertabledynamics,andsuchdetaileddataareunavailable.
WhiletheeffectsofthevariousmanagementregimesonsoilcarbonpoolsandGHGfluxeshavenotbeenwidelystudied,thisismoreofaconsiderationwithrespecttouncertaintiesintheestimatesasopposedtoalimitationtoitsapplication.TheDNDCframeworkisrobustbecauseitisaprocess‐basedmodelthathasbeenvalidatedinawidevarietyofwetlandtypesandsoils.However,ithasnotbeenextensivelytestedonHistosolsorpeatsoils,especiallywithrespecttochangesinsoilcarbonstocks.ThemodelwasvalidatedsuccessfullyforestimatingCH4frommicotopographicpositionsinapeatland(Zhangetal.,2002),butadditionalworkisneededtobetteraddressthewidearrayofmanagedHistosolsthatexistacrossthecountry.
Similarly,thismethodisnotapplicabletoconstructedwetlands,impoundments,orshallowreservoirsystemsthathaveextendedperiodsofponding;thosesiteswouldtendtohavedynamicsmoresimilartoalakeorpondasopposedtoaterrestrialecosystem.
Withrespecttotheforestmodel,accuracyoftheestimatesisdependentonapplicabilityoftheavailablesiteindexcurves.Whilethegeneralcurvesareavailableforallspecies,theymaynotaccuratelyrepresentthesiteortheentity’smanagementregime.ProvisionsareincludedwithinFVSforcustomizingthetreesiteindexcurves,whichcouldbeimportantforanentityespeciallyifgenetically‐improvedplantingstockandfertilizationregimesareemployed.
Detritalorganicmatteristhesourcefordecompositionprocesses.Theeffectofvegetationonwetlandcarbondynamicsispromulgatedthroughtheamountoforganicmatterandthewaterregime(e.g.,evapotranspiration).Accordingly,theaccuracyofthevegetationproductivityandturnoverwillaffecttheestimatesofthesoilcarbonpoolsandGHGflux.
WatertablepositionisthemostcriticalfactoraffectingCH4andN2Ofluxfromthewetlandsoil(Trettinetal.,2006).Accordingly,considerationstoimprovethatestimateasdiscussedinSection4.3.2willimprovetheestimatesofGHGemissionsfromthesoil.Thereareotheruncertaintiesintheactivityandancillarydata,aswellasmodelstructurethatcancreatebiasandimprecisionintheresultingestimates.Wetlandstypicallyexistinamosaicwithuplandforests,grasslands,andcultivatedlands.Accordingly,theaccuracyofpartitioningtheentityintoupland(agriculture,forest)andwetlandswillaffecttheaccuracyoftheestimates.
4.4 ResearchGapsforWetlandManagementWetlandmanagement,anditsinfluenceonGHGemissions,isnotaswellstudiedassomeoftheothermanagementpracticesinthisreport,suchastillageincroplandsorforestharvestingpracticesinuplands.ThereisthepotentialforimprovingtheestimationofGHGemissionsassociatedwithdifferentmanagementpracticesinthefutureiftherearemonitoringactivitiesandstudiestofillinformationgaps.Aselectnumberofinformationneedsandresearchgapsareidentifiedhere.
The2013Supplementtothe2006IntergovernmentalPanelonClimateChange(IPCC)Guidelinesprovidenewguidanceforestimatingemissionsfromdrainedinlandorganicsoils,rewettedorganicsoils,coastalwetlands,inlandwetlandmineralsoils,andconstructedwetlandsforwastewatertreatment(Blainetal.,2013).Thesenewlydevelopedguidelineswillbecomparedtothetechnicalmethodsprovidedinthisreport.
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Watertablepositionistheprincipalfactoraffectingcarbondynamicsinwetlands;unfortunatelythereisalackoflong‐termdata,whichisneededtocharacterizethewatertableresponsetoamanagementregimeandtoprovideabasisforvalidatingassessmenttools.EstablishmentofanetworkofwatertablemonitoringsiteswithinselectedUSDAForestServiceexperimentalforestsandrangesandUSDA‐AgriculturalResearchService(ARS)experimentstationscouldprovidethecontinuityinmeasurementsandlinkageswithcommonmanagementpracticestorepresentthemajorsoilandclimaticconditionintheUnitedStates.
Improvingmodelingcapabilitiesthatintegratesurroundingareaswiththewetlandsthatreceivesurfaceandsubsurfacedrainagewaterswillallowformodelingtheflowsofnutrientsandorganicmatterintowetlandsandsubsequentlossestootherwetlandsbeyondtheentity’soperation.Thistypeofassessmentframeworkisusedinseveralestablishedspatially‐explicithydrologicmodels;theneedistointegratethebiogeochemistry.Linkedmodelscanbeusedatpresent;butdevelopmentofafunctionally‐integratedsystemisneededtosupportbroad‐basedapplications.
Thereisaneed,generally,forimprovedinformationonbiomassproductionandallocationinmanagedwetlands.ThesedatacouldbeobtainedthroughacoordinatedmonitoringprogramemployingUSDA‐ForestServiceexperimentalforestsandranges,USDA‐ARSexperimentstations,andU.S.DepartmentoftheInteriorwildliferefugestomonitorproductionofkeyspeciesorvegetationtypesinassociationwithcommonmanagementprescriptions.Thereisalsoneedformoredetailedmechanisticresearchtoprovideinformationonenergy,water,andGHGdynamicsonselectedmanagedsites;thisinformationiscriticalforvalidatingprocess‐basedmodels.
Field‐basedstudiesareneededtodevelopmorecompletedatabasesthatprovideancillarydataforGHGestimation,particularityCH4emissionsforDNDCorsimilarprocess‐basedmodels,ratherthanrelyingonentityinput,whichwilllikelybechallenging.Akeyattributeofthisworkshouldbetheconsiderationoftheinherentspatialandtemporalvariabilitywithinasite.
FurtherquantificationofthecontrollingandthresholdparametersandassociateduncertaintywithinDNDCorsimilarprocess‐basedmodelstoestimatetracegasemissionsiswarranted.ThisworkcouldalsosuggestapathtowardsdevelopmentofanassessmenttoolthatwasnotreliantonawidearrayofparameterstoeffectivelysimulatetheGHGdynamicsofthesite.
AmorerobustandextensivedatabaseonGHGemissionsfromfreshwatertidal(salinity<0.5‰)palustrinewetlandsisneededtomorefullyunderstandthedriversofemissions,inadditiontoprovidingamorecompletedatasetforparameterizationandevaluationofprocess‐basedmodels.
Studiesonindividualsitesandmeta‐analysesofexistingdataareneededtofullyevaluatethenetGHGfluxforCH4,N2O,andsoilcarbon.MoststudiesonlyconsideroneoftheGHGsandmaymasksomeofthedifferencesinfluxesamongtheGHGsassociatedwithamanagementactivity.
ConstructedwetlandsarediscussedqualitativelyinSection5.4.10ofChapter5,AnimalProductionSystemsforLiquidManureStorageandTreatmentinConstructedWetlands.Moreresearchisneededinthisareatoaccuratelyestimateemissionsfromconstructedwetlands.
ThislistisnotexhaustivebutisintendedtoprovidesomedirectionforimprovingtheestimationmethodsforGHGemissionfromwetlands.
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