Quantifying Greenhouse Gas Sources and Sinks in Managed ...

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

Chapter 4: Quantifying Greenhouse Gas Sources and Sinks in Managed Wetland Systems

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

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).

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).

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:

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.

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/.

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).

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.

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).

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.

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.

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.

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.

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).

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.

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.

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

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

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.

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/.

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.

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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