Journal of Ecology 2018106655ndash670 wileyonlinelibrarycomjournaljec emsp|emsp655copy 2017 The Authors Journal of Ecology copy 2017 British Ecological Society

Received23May2017emsp |emsp Accepted17October2017DOI1011111365-274512901

R E S E A R C H A R T I C L E

Direct and indirect controls on organic matter decomposition in four coastal wetland communities along a landscape salinity gradient

Camille L Stagg1 emsp|emspMelissa M Baustian2 emsp|emspCarey L Perry34emsp|emspTim J B Carruthers2emsp|emsp Courtney T Hall1

1USGeologicalSurveyWetlandandAquaticResearchCenterLafayetteLAUSA2TheWaterInstituteoftheGulfBatonRougeLAUSA3CoalitiontoRestoreCoastalLouisianaBatonRougeLAUSA4LouisianaDepartmentofWildlifeandFisheriesBatonRougeLAUSA

CorrespondenceCamilleLStaggEmailstaggcusgsgov

Funding informationUSGeologicalSurveyClimateResearchandDevelopmentProgramUSGeologicalSurveyEcosystemsProgramUSGeologicalSurveyLandCarbonProgramScienceandEngineeringProgramofTheWaterInstituteoftheGulfLouisianaCoastalProtectionandRestorationAuthority(CPRA)BatonRougeAreaFoundation(BRAF)RestoretheMississippiRiverDeltaCampaignviaCoalitiontoRestoreCoastalLouisiana(CRCL)

HandlingEditorAmyZanne

Abstract1 CoastalwetlandsstoremorecarbonthanmostecosystemsgloballyAssealevelriseschangesinfloodingandsalinitywillpotentiallyimpactecologicalfunctionssuch as organicmatter decomposition that influence carbon storageHoweverlittle isknownabout themechanisms thatcontrolorganicmatter loss incoastalwetlandsatthelandscapescaleAssealevelriseshowwilltheshiftfromfreshtosalt-tolerant plant communities impact organicmatter decompositionDo long-termplant-mediatedeffectsofsea-levelrisedifferfromdirecteffectsofelevatedsalinityandflooding

2 Weidentifiedinternalandexternalfactorsthatregulatedindirectanddirectpath-waysofsea-levelriseimpactsrespectivelyalongalandscape-scalesalinitygradi-entthatincorporatedchangesinwetlandtype(fresholigohalinemesohalineandpolyhalinemarshes)Wefoundthatindirectanddirectimpactsofsea-levelrisehadopposingeffectsonorganicmatterdecomposition

3 Salinityhadanindirecteffectonlitterdecompositionthatwasmediatedthroughlitter qualityDespite significant variation in environmental conditions along thelandscapegradientthebestpredictorsofabove-andbelow-groundlitterdecom-positionwereinternaldrivers initial litternitrogencontentandinitial litter lignincontentrespectivelyLitterdecayconstantsweregreatestintheoligohalinemarshanddeclinedwithincreasingsalinityandthefractionof litterremaining(asymp-tote)wasgreatestinthemesohalinemarshIncontrastdirecteffectsofsalinityandfloodingwerepositiveExternaldriverssalinityandfloodingstimulatedcellulyticactivitywhichwashighestinthepolyhalinemarsh

4 SynthesisOurresultsindicatethatassealevelrisesinitialdirecteffectsofsalinitywillstimulatedecayoflabilecarbonbutovertimeasplantcommunitiesshiftfromfresh topolyhalinemarsh litterdecaywill decline yieldinggreaterpotential forlong-termcarbonstorageThesefindingshighlighttheimportanceofquantifyingcarbon loss atmultiple temporal scales notonly in coastalwetlandsbut also inother ecosystems where plant-mediated responses to climate change will have significantimpactsoncarboncycling

656emsp |emsp emspenspJournal of Ecology STAGG eT Al

1emsp |emspINTRODUCTION

Climatechange-inducedshiftsinvegetationcommunitycompositionwillhaveimportantimplicationsforecologicalfunction(Woltersetal2000) and ultimately carbon cycling (Jobbagy amp Jackson 2000) Interrestrial ecosystems shifts in vegetation community compositionhavebeenobservedinresponsetoelevatedatmosphericcarbondi-oxide (Leadley Niklaus Stocker amp Korner 1999 Owensby CoyneHamAuenampKnapp1993)elevatedairsoilandwatertemperatures(AlwardDetling ampMilchunas 1999Harteamp Shaw 1995) alteredprecipitation patterns (Sternberg Brown Masters amp Clarke 1999)andtheirinteractions(Kardoletal2010)Coastalmarshessituatedbetweenterrestrialandmarineecosystemsareexposednotonlytotheseclimaticdrivers(Oslandetal2016OslandEnwrightampStagg2014)butalsotoanthropogenicpressuresalongthelandwardbound-aryandrisingsealevelsalongtheseawardboundary(Dayetal2008SmallampNicholls2003)

Sea-level rise can force shifts in wetland vegetation commu-nity composition by altering flooding and salinity regimes (BurdickampMendelssohn 1987 DeLaune Patrick amp Pezeshki 1987 HesterMendelssohn amp McKee 2001 Krauss etal 2009 McKee ampMendelssohn1989)Assalinityincreaseswithsea-levelrisemarshhab-itatswillconverttocommunitiesdominatedbymoresalt-tolerantplantspecies(SharpeampBaldwin2012WarrenampNiering1993)Moreoveranthropogenic restrictions toupslopemigration (EnwrightGriffithampOsland2016)inconjunctionwithsea-levelrisemayresultintheex-pansionof saltmarshesat theexpenseofotherwetland typesyield-ing anoverall shift tomore saline conditions (VisserDuke-SylvesterCarterampBroussard 2013)Changes inwetlandecosystem structuremayeventuallyreflectalteredecological functionandecosystemser-vicesIntidalfreshwaterforestedwetlandsimpactedbysea-levelrisethetransitiontoherbaceousoligohalinemarshresultedingreaternitro-genandphosphorusmineralization fluxesandturnover (NoeKraussLockabyConnerampHupp2013)Similarlythetransitionofherbaceoussaltmarshtomangroveforestsignificantlyalteredecosystemfunctionwith greater carbon sequestration rates and lignin storage rates in mangrovescomparedtosaltmarshes(Bianchietal2013)

Coastalwetlands provide numerous ecosystem services (Barbieretal2011)includingsignificantcarbonstorageinlivingandnon-livingbiomassandinfloodedsoils(McCleodetal2011)Furthermoreunliketerrestrialsoilsthatmaybecomecarbon-saturatedovertime(StewartPaustianConantPlanteampSix2007)coastalwetlandscontinuallyac-cretemineralsedimentsandorganicmattertokeeppacewithsea-levelrise(Reed1995)thusthepotentialforcarbonstorageinwetlandsoilsincreasesovertime(ChmuraAnisfeldCahoonampLynch2003)

Carbon storage in wetlands is the net result of organic matterproduction and organic matter loss for example decomposition

Decompositionoforganicmatterisinfluencedbyinternalandexternaldrivers(GodshalkampWetzel1978)Whenconsideringtheeffectofanultimatedriversuchassea-levelriseonorganicmatterdecomposi-tioninternalandexternaldriverswillhaveindirectanddirectimpactsrespectivelyonthefateoforganicmatter Internaldriversarechar-acteristicsof theorganicmatter itself and includequalitiesofplantmorphologyandchemicalcompositionof theplantmaterial (McKeeampSeneca1982MelilloNaimanAberampLinkins1984)Incontrastexternaldriversofdecompositionarecharacteristicsoftheenviron-mentandincludesoilmicrobeanddetritivorecommunitycomposition(MorriseyBerrierNeubauerampFranklin2014Valielaetal1985)andabiotic conditions suchas soil temperature flooddurationand fre-quencyandsalinity(ReddyampPatrick1975WestonDixonampJoye2006)Therefore sea-level rise has the potential to impact organicmatterdecompositionindirectlybyforcingshiftsinplantcommunitycompositionandlitterquality(internalcontrols)anddirectlythroughalteringsalinityandflooding(externalcontrols)(StaggSchoolmasterKrauss Cormier amp Conner 2017) Furthermore these indirect anddirect impacts are resolved at different spatial and temporal scales(Herbertetal2015NeubauerFranklinampBerrier2013)thereforeitiscriticalthatwequantifybothinternalandexternalcontrolsonor-ganicmatterdecompositionifwehopetoaccuratelypredictthefateoforganicmatterinafuturewithsea-levelrise

Although there has been much recent progress in elucidatingfine-scale mechanisms of soil respiration in response to elevatedsalinity (Chambers Osborne amp Reddy 2013 Chambers Reddy ampOsborne 2011 Neubauer 2013 Neubauer etal 2013 WestonVileNeubauerampVelinsky2011Westonetal2006)westilllackacomprehensiveunderstandingofthemechanismsthatcontrolorganicmatterdecompositionincoastalwetlandsatthelandscapescale(butseeJanouseketal2017WestonNeubauerVelinskyampVile2014)To address thiswemeasured decomposition of in situ litter and astandardizedcarbonsourceacrossalandscape-scalesalinitygradientthatincorporatedchangesinvegetationcommunitytoaddressthefol-lowingresearchquestions(1)Whatdriversinfluenceorganicmatter decompositionalongalandscapesalinitygradientincoastalmarshes(2)Howdo long-term indirect impactsof sea-level risediffer fromshort-termdirectimpactsofsea-levelriseonorganicmatterdecom-positionand(3)Whataretheimplicationsforlong-termcarbonstor-ageinestuarinecoastalwetlandsimpactedbysea-levelrise

2emsp |emspMATERIALS AND METHODS

21emsp|emspStudy sites

Above- and below-ground decomposition was measured in es-tuaries along a landscape-scale (c 65km) salinity gradient that

K E Y W O R D S

carboncellulosedecaycoastalwetlandslandscapelitterdecompositionplantcommunityplant-climateinteractionssea-levelrise

emspensp emsp | emsp657Journal of EcologySTAGG eT Al

incorporatedfresh(0ndash05ppt)oligohaline(05ndash5ppt)mesohaline(5ndash18 ppt) and polyhaline (gt18 ppt) coastal marshes as definedby Cowardin Carter Golet and LaRoe (1979) (Figure1) Due tohightemporalvariationinsalinitywetlandcommunitytypesweredefined not only bymeasured salinity but also by dominant veg-etation species known to be associatedwith specific salinity andhydrologicalregimes(VisserSasserChabreckampLinscombe2002)FreshsitesweredominatedbyPanicum hemitomon and Typha lati-folia oligohaline siteswere dominated by Sagittaria lancifolia and Schoenoplectus americanus mesohaline sites were dominated bySpartina patens and S americanusandpolyhalinesitesweredomi-natedbySpartina alterniflora and Juncus roemerianusWithineachofthefourwetlandtypessixreplicatesiteswereestablishedacrosstwohydrologicbasinsTerrebonneandBaratariaBasinsforatotalof24sites(Figure1Baustianetal2017Staggetal2017)

22emsp|emspResponse variables

221emsp|emspLitter decomposition

Rates of above- and below-ground organic matter decompositionweremeasuredusingthelitterbagtechnique(HackneyampdelaCruz1980) which integrates short- and long-term decomposition pro-cesses associatedwith labile and refractory organicmatter (Valielaetal1985)Litterbagscontainingsite-specificabove-groundbiomassand litterbags containing site-specific below-groundbiomass or lit-terwere installed inthreereplicateplots ineachsiteLivebiomassof each site-specific dominant specieswas collected from a subsetofsiteswithineachwetlandtypeandhydrologicbasin(subsetn=8)in July2014Above-groundbiomassofeachdominantspecieswascollectedfromamonospecificstandclippedat thesoilsurfaceandsortedintoliveanddeadpoolsuponreturningtothelaboratoryTo

collectbelow-groundbiomass in largequantities 20-cmwidetimes30-cmdeepsodswereharvestedfrommonospecificstandsrepresentingeachdominantspeciesThesodsweretransportedbacktothelabora-torywherethebelow-groundbiomasswasrinsedovera1-mmsieveto remove soil particles The remaining macro-organic matter wasseparated into live and dead components Live roots and rhizomesweredistinguishedfromdeadrootsandrhizomesbycolourturgidityandstructuralintegrity(SchubauerampHopkinson1984)Above-andbelow-groundlivebiomasswasallowedtoair-drytoaconstantmassforatleast1weekbeforeplacementinlitterbagsLiveair-driedlitterwas used in place of senesced litter to captureweight loss associ-atedwithinitialdecayprocessessuchasleachingthatwouldhaveal-readyoccurredinsenescedmaterialcollectedfromthefield(McKeeampSeneca1982)Above-groundlitterbagswerepreparedbyfillingmeshbags(20cmlongtimes20cmwidetimes15mmopening)withliveair-driedleafandstemmaterial(20gbag)Below-groundlitterbagswerepre-paredbyfillingmeshbags(8cmlongtimes20cmwidetimes10mmopening)with live air-dried roots and rhizomes (5gbag) Generally above-groundlitterbagshavealargermeshopeningthanbelow-groundlit-terbags(HalupaampHowes1995HemmingaKokampdeMunck1988)toallowforthepassageofsmallandyounginvertebrates(McKeeampSeneca1982)Asubsampleofthe initialair-driedmaterial foreachspecieswasweighedoven-driedat60degCandreweighedtocalculateamoisturecorrectionfactorwhichwasappliedtothestartingmass(w0)ofair-driedlitterusedinsubsequentcalculations

Above- and below-ground litterbag transects were establishedperpendicular to thewater body and included three replicate plotslocated 10 25 and 40m from the shorelineAbove-ground litterb-agsweresecuredonthesoilsurfacewithlandscapepinsandbelow-ground litterbags were inserted into the soil to a depth of 10cmFourlitterbagswereinstalledineachreplicateplot inOctober2014(n=288 above-ground litterbags n=288 below-ground litterbags)

F IGURE 1emspDecompositionstudysiteslocatedalongalandscape-scalesalinitygradientspanningfourwetlandtypesincoastalLouisianaUSA

658emsp |emsp emspenspJournal of Ecology STAGG eT Al

andindividual litterbagswereretrievedfromeachplotatfour inter-vals(136and12monthsafterinstallation)tofollowamodelofex-ponential decayAfter retrieval the above- and below-ground litterbagsweregentlyrinsedwithdeionizedwaterovera1-mmsieveandremaininglitterwasoven-driedforatleast48hrtoaconstantmassat60degC(HalupaampHowes1995)Thelitterwasthenweighedandre-tainedforfurtherchemicalanalysesPercentmassremaining(MR)wascalculatedusingthefollowingequation

where w0isdryweightattimezeroandwtisdryweightattimet(daysafterinstallation)

The proportion of mass remaining over timewas used to esti-mate two parameters that describe the decomposition process (1)thedecayrateorexponentialdecayconstantand(2)theasymptoteornon-decomposable fractionTheexponentialdecayconstantwas derivedusingasinglenegativeexponentialdecaymodel

where X ispercentmassremainingaftertimet (daysafter installa-tion)andminuskistheinstantaneousdecayconstant(perday)(WeiderampLang1982)Additionallyweusedanasymptoticmodel toestimatethenon-decomposablefractionorasymptote

where Caistheasymptoteorfractionofmaterialremaining(WeiderampLang1982)Becausethesingleexponentialdecaymodelprovidedabetterfitfordecayconstantestimatesweonlyusedtheasymptoticmodeltoestimateasymptotesnotdecayconstants

222emsp|emspCellulose decay

Inadditiontolitterdecompositionwealsomeasuredcellulyticactivityusingthecottonstriptechniquewhichprovidesameasureofshort-termlossoflabilecarbon(Maltby1988)Cottonstripsaremadefromartistcanvaswhichiscomprisedof98holocelluloseandbyusingastandardizedcarbonsourcewewereabletoisolateexternalfactorsthat influencemicrobial activity (Mendelssohn etal 1999 SlocumRobertsampMendelssohn2009)

Cottonstripswereinstalledinthreereplicateplotsalongatran-sect parallel to the litterbag transects Below-ground cotton strips(10-cmwidetimes30-cmlong)wereinsertedverticallyintothesoiltoadepthof25cmThecottonstripsweredeployedfourtimesseason-allyandretrievedafter12ndash14daysinthesoildependingonthewatersurfacetemperature(Slocumetal2009)Above-groundcottonstrips(20cmtimes20cm)weredeployedinOctober2015securedtothesoilsurfaceusinglandscapepinsandretrieved14dayslaterThreecot-tonstrips(twoteststripsandonereferencestrip)wereplacedineachreplicatedplotReferencestripsusedtoquantifythetensilestrengthofnon-decomposedmaterialwerehandledexactlythesameastheteststripsbutretrievedimmediatelyafterdeployment

After retrieval cotton strips were rinsed gently with deionizedwatertoremoveallsoilandextraneousmaterialandthecottonstripswereallowedtoair-dryforatleast48hrCottonstripswerecutinto

2-cm substrips along the vertical profile and decomposition of the2-cm substripswasmeasured as tensile strength lost compared tothe reference substrip using a Dillon Quantroltrade Snapshot TensionCompressionMotorizedTestStandtensometerconnectedtoaDillonQuantroltrade Advanced Force Gauge (Slocum etal 2009) Cellulosedecay ratewascalculatedasper cent cellulose tensile strength lostperday(CTSLperday)

whereT is the force (N) requiredto tear thetestsubstripsR is theforce(N)requiredtotearthereferencesubstripsandtistime(days)inthemarsh

23emsp|emspPredictive variables

231emsp|emspExternal drivers

All study sites were located within the 1-km2 boundary of aCoastwide Reference Monitoring Systems station (httplacoastgovcrms2homeaspx) where surface water salinity and surfacewater elevation are measured hourly Marsh surface elevation of15plotswithineachsite (n=360)weresurveyedusingRealTimeKinematicmethodology(GaoAbdel-SalamChenampWojciechowski2005)withaTrimbleR10GNSSSystem(TrimbleNavigationLimitedUSAChenetal2011)andrectifiedtotheNorthAmericanVerticalDatumof1988(NAVD88)usingTrimbleBusinessCenter25soft-ware for data post-processing (Trimble Navigation Limited USA)We usedmarsh elevation data in conjunctionwith surfacewaterelevationdatatocalculateflooddepthanddurationforeachof15plotsineachsite

Discrete soil and porewater sampleswere taken to measure asuiteofenvironmentalparameters inDecember2014Ateachsitetwosoilcores(10-cmdiametertimes30-cmlength)werecollectedneareachofthethreereplicatedplotsalongthebelow-ground litterbagtransect(n=72)Aftercollectionthesoilcoresweresectionedintotwoincrements(0ndash15cmand16ndash30cm)andimmediatelyplacedonice in the fieldand transportedback to the laboratorywhere theywerehomogenizedThefirstcorewasusedforanalysisofsoilbulkdensity (Blake 1965) moisture (Blake 1965) organic mat-ter (OliverLotterampLemcke2001)andelectricalconductivity (ECRhodes1996)

ThesecondsoilcorewasusedtomeasuresoilpH(Thomas1996)soiltotalCNandPsoilextractablenutrients(PO4-PandNH4-N)andotherelementsofinterest(CaCuFeKMgNaNiP)Thehomog-enizedsoilwasdriedtoaconstantweightat60degCgroundinaWileyMill(Model420mesh850μm)andseparatedintoseveralscintil-lationvials formultipleanalysesSoil totalNandtotalCweremea-suredusingaCostechreg4010ElementalCombustionanalyzer(Nelsonamp Sommers 1982 EPAMethod 440) Extractionswere performedforthefollowinganalysessoiltotalP(HClAspilaAgemianampChau1976)PO4-P(Bray-2OlsenampSommers1982)NH4-N(KClKeeneyampNelson1982)andotherparametersofinterest(H2NO3AmericanPublic Health Association 2005a) Soil total P PO4-P samples and

MR= (wt∕w0)times100

X=eminuskt

X=Ca+ (1minusCa)eminuskat

CTSL per day= [1minus (T∕R)times100]∕t

emspensp emsp | emsp657Journal of EcologySTAGG eT Al

incorporatedfresh(0ndash05ppt)oligohaline(05ndash5ppt)mesohaline(5ndash18 ppt) and polyhaline (gt18 ppt) coastal marshes as definedby Cowardin Carter Golet and LaRoe (1979) (Figure1) Due tohightemporalvariationinsalinitywetlandcommunitytypesweredefined not only bymeasured salinity but also by dominant veg-etation species known to be associatedwith specific salinity andhydrologicalregimes(VisserSasserChabreckampLinscombe2002)FreshsitesweredominatedbyPanicum hemitomon and Typha lati-folia oligohaline siteswere dominated by Sagittaria lancifolia and Schoenoplectus americanus mesohaline sites were dominated bySpartina patens and S americanusandpolyhalinesitesweredomi-natedbySpartina alterniflora and Juncus roemerianusWithineachofthefourwetlandtypessixreplicatesiteswereestablishedacrosstwohydrologicbasinsTerrebonneandBaratariaBasinsforatotalof24sites(Figure1Baustianetal2017Staggetal2017)

22emsp|emspResponse variables

221emsp|emspLitter decomposition

Rates of above- and below-ground organic matter decompositionweremeasuredusingthelitterbagtechnique(HackneyampdelaCruz1980) which integrates short- and long-term decomposition pro-cesses associatedwith labile and refractory organicmatter (Valielaetal1985)Litterbagscontainingsite-specificabove-groundbiomassand litterbags containing site-specific below-groundbiomass or lit-terwere installed inthreereplicateplots ineachsiteLivebiomassof each site-specific dominant specieswas collected from a subsetofsiteswithineachwetlandtypeandhydrologicbasin(subsetn=8)in July2014Above-groundbiomassofeachdominantspecieswascollectedfromamonospecificstandclippedat thesoilsurfaceandsortedintoliveanddeadpoolsuponreturningtothelaboratoryTo

collectbelow-groundbiomass in largequantities 20-cmwidetimes30-cmdeepsodswereharvestedfrommonospecificstandsrepresentingeachdominantspeciesThesodsweretransportedbacktothelabora-torywherethebelow-groundbiomasswasrinsedovera1-mmsieveto remove soil particles The remaining macro-organic matter wasseparated into live and dead components Live roots and rhizomesweredistinguishedfromdeadrootsandrhizomesbycolourturgidityandstructuralintegrity(SchubauerampHopkinson1984)Above-andbelow-groundlivebiomasswasallowedtoair-drytoaconstantmassforatleast1weekbeforeplacementinlitterbagsLiveair-driedlitterwas used in place of senesced litter to captureweight loss associ-atedwithinitialdecayprocessessuchasleachingthatwouldhaveal-readyoccurredinsenescedmaterialcollectedfromthefield(McKeeampSeneca1982)Above-groundlitterbagswerepreparedbyfillingmeshbags(20cmlongtimes20cmwidetimes15mmopening)withliveair-driedleafandstemmaterial(20gbag)Below-groundlitterbagswerepre-paredbyfillingmeshbags(8cmlongtimes20cmwidetimes10mmopening)with live air-dried roots and rhizomes (5gbag) Generally above-groundlitterbagshavealargermeshopeningthanbelow-groundlit-terbags(HalupaampHowes1995HemmingaKokampdeMunck1988)toallowforthepassageofsmallandyounginvertebrates(McKeeampSeneca1982)Asubsampleofthe initialair-driedmaterial foreachspecieswasweighedoven-driedat60degCandreweighedtocalculateamoisturecorrectionfactorwhichwasappliedtothestartingmass(w0)ofair-driedlitterusedinsubsequentcalculations

Above- and below-ground litterbag transects were establishedperpendicular to thewater body and included three replicate plotslocated 10 25 and 40m from the shorelineAbove-ground litterb-agsweresecuredonthesoilsurfacewithlandscapepinsandbelow-ground litterbags were inserted into the soil to a depth of 10cmFourlitterbagswereinstalledineachreplicateplot inOctober2014(n=288 above-ground litterbags n=288 below-ground litterbags)

F IGURE 1emspDecompositionstudysiteslocatedalongalandscape-scalesalinitygradientspanningfourwetlandtypesincoastalLouisianaUSA

658emsp |emsp emspenspJournal of Ecology STAGG eT Al

andindividual litterbagswereretrievedfromeachplotatfour inter-vals(136and12monthsafterinstallation)tofollowamodelofex-ponential decayAfter retrieval the above- and below-ground litterbagsweregentlyrinsedwithdeionizedwaterovera1-mmsieveandremaininglitterwasoven-driedforatleast48hrtoaconstantmassat60degC(HalupaampHowes1995)Thelitterwasthenweighedandre-tainedforfurtherchemicalanalysesPercentmassremaining(MR)wascalculatedusingthefollowingequation

where w0isdryweightattimezeroandwtisdryweightattimet(daysafterinstallation)

The proportion of mass remaining over timewas used to esti-mate two parameters that describe the decomposition process (1)thedecayrateorexponentialdecayconstantand(2)theasymptoteornon-decomposable fractionTheexponentialdecayconstantwas derivedusingasinglenegativeexponentialdecaymodel

where X ispercentmassremainingaftertimet (daysafter installa-tion)andminuskistheinstantaneousdecayconstant(perday)(WeiderampLang1982)Additionallyweusedanasymptoticmodel toestimatethenon-decomposablefractionorasymptote

where Caistheasymptoteorfractionofmaterialremaining(WeiderampLang1982)Becausethesingleexponentialdecaymodelprovidedabetterfitfordecayconstantestimatesweonlyusedtheasymptoticmodeltoestimateasymptotesnotdecayconstants

222emsp|emspCellulose decay

Inadditiontolitterdecompositionwealsomeasuredcellulyticactivityusingthecottonstriptechniquewhichprovidesameasureofshort-termlossoflabilecarbon(Maltby1988)Cottonstripsaremadefromartistcanvaswhichiscomprisedof98holocelluloseandbyusingastandardizedcarbonsourcewewereabletoisolateexternalfactorsthat influencemicrobial activity (Mendelssohn etal 1999 SlocumRobertsampMendelssohn2009)

Cottonstripswereinstalledinthreereplicateplotsalongatran-sect parallel to the litterbag transects Below-ground cotton strips(10-cmwidetimes30-cmlong)wereinsertedverticallyintothesoiltoadepthof25cmThecottonstripsweredeployedfourtimesseason-allyandretrievedafter12ndash14daysinthesoildependingonthewatersurfacetemperature(Slocumetal2009)Above-groundcottonstrips(20cmtimes20cm)weredeployedinOctober2015securedtothesoilsurfaceusinglandscapepinsandretrieved14dayslaterThreecot-tonstrips(twoteststripsandonereferencestrip)wereplacedineachreplicatedplotReferencestripsusedtoquantifythetensilestrengthofnon-decomposedmaterialwerehandledexactlythesameastheteststripsbutretrievedimmediatelyafterdeployment

After retrieval cotton strips were rinsed gently with deionizedwatertoremoveallsoilandextraneousmaterialandthecottonstripswereallowedtoair-dryforatleast48hrCottonstripswerecutinto

2-cm substrips along the vertical profile and decomposition of the2-cm substripswasmeasured as tensile strength lost compared tothe reference substrip using a Dillon Quantroltrade Snapshot TensionCompressionMotorizedTestStandtensometerconnectedtoaDillonQuantroltrade Advanced Force Gauge (Slocum etal 2009) Cellulosedecay ratewascalculatedasper cent cellulose tensile strength lostperday(CTSLperday)

whereT is the force (N) requiredto tear thetestsubstripsR is theforce(N)requiredtotearthereferencesubstripsandtistime(days)inthemarsh

23emsp|emspPredictive variables

231emsp|emspExternal drivers

All study sites were located within the 1-km2 boundary of aCoastwide Reference Monitoring Systems station (httplacoastgovcrms2homeaspx) where surface water salinity and surfacewater elevation are measured hourly Marsh surface elevation of15plotswithineachsite (n=360)weresurveyedusingRealTimeKinematicmethodology(GaoAbdel-SalamChenampWojciechowski2005)withaTrimbleR10GNSSSystem(TrimbleNavigationLimitedUSAChenetal2011)andrectifiedtotheNorthAmericanVerticalDatumof1988(NAVD88)usingTrimbleBusinessCenter25soft-ware for data post-processing (Trimble Navigation Limited USA)We usedmarsh elevation data in conjunctionwith surfacewaterelevationdatatocalculateflooddepthanddurationforeachof15plotsineachsite

Discrete soil and porewater sampleswere taken to measure asuiteofenvironmentalparameters inDecember2014Ateachsitetwosoilcores(10-cmdiametertimes30-cmlength)werecollectedneareachofthethreereplicatedplotsalongthebelow-ground litterbagtransect(n=72)Aftercollectionthesoilcoresweresectionedintotwoincrements(0ndash15cmand16ndash30cm)andimmediatelyplacedonice in the fieldand transportedback to the laboratorywhere theywerehomogenizedThefirstcorewasusedforanalysisofsoilbulkdensity (Blake 1965) moisture (Blake 1965) organic mat-ter (OliverLotterampLemcke2001)andelectricalconductivity (ECRhodes1996)

ThesecondsoilcorewasusedtomeasuresoilpH(Thomas1996)soiltotalCNandPsoilextractablenutrients(PO4-PandNH4-N)andotherelementsofinterest(CaCuFeKMgNaNiP)Thehomog-enizedsoilwasdriedtoaconstantweightat60degCgroundinaWileyMill(Model420mesh850μm)andseparatedintoseveralscintil-lationvials formultipleanalysesSoil totalNandtotalCweremea-suredusingaCostechreg4010ElementalCombustionanalyzer(Nelsonamp Sommers 1982 EPAMethod 440) Extractionswere performedforthefollowinganalysessoiltotalP(HClAspilaAgemianampChau1976)PO4-P(Bray-2OlsenampSommers1982)NH4-N(KClKeeneyampNelson1982)andotherparametersofinterest(H2NO3AmericanPublic Health Association 2005a) Soil total P PO4-P samples and

MR= (wt∕w0)times100

X=eminuskt

X=Ca+ (1minusCa)eminuskat

CTSL per day= [1minus (T∕R)times100]∕t

658emsp |emsp emspenspJournal of Ecology STAGG eT Al

andindividual litterbagswereretrievedfromeachplotatfour inter-vals(136and12monthsafterinstallation)tofollowamodelofex-ponential decayAfter retrieval the above- and below-ground litterbagsweregentlyrinsedwithdeionizedwaterovera1-mmsieveandremaininglitterwasoven-driedforatleast48hrtoaconstantmassat60degC(HalupaampHowes1995)Thelitterwasthenweighedandre-tainedforfurtherchemicalanalysesPercentmassremaining(MR)wascalculatedusingthefollowingequation

where w0isdryweightattimezeroandwtisdryweightattimet(daysafterinstallation)

The proportion of mass remaining over timewas used to esti-mate two parameters that describe the decomposition process (1)thedecayrateorexponentialdecayconstantand(2)theasymptoteornon-decomposable fractionTheexponentialdecayconstantwas derivedusingasinglenegativeexponentialdecaymodel

where X ispercentmassremainingaftertimet (daysafter installa-tion)andminuskistheinstantaneousdecayconstant(perday)(WeiderampLang1982)Additionallyweusedanasymptoticmodel toestimatethenon-decomposablefractionorasymptote

where Caistheasymptoteorfractionofmaterialremaining(WeiderampLang1982)Becausethesingleexponentialdecaymodelprovidedabetterfitfordecayconstantestimatesweonlyusedtheasymptoticmodeltoestimateasymptotesnotdecayconstants

222emsp|emspCellulose decay

Inadditiontolitterdecompositionwealsomeasuredcellulyticactivityusingthecottonstriptechniquewhichprovidesameasureofshort-termlossoflabilecarbon(Maltby1988)Cottonstripsaremadefromartistcanvaswhichiscomprisedof98holocelluloseandbyusingastandardizedcarbonsourcewewereabletoisolateexternalfactorsthat influencemicrobial activity (Mendelssohn etal 1999 SlocumRobertsampMendelssohn2009)

Cottonstripswereinstalledinthreereplicateplotsalongatran-sect parallel to the litterbag transects Below-ground cotton strips(10-cmwidetimes30-cmlong)wereinsertedverticallyintothesoiltoadepthof25cmThecottonstripsweredeployedfourtimesseason-allyandretrievedafter12ndash14daysinthesoildependingonthewatersurfacetemperature(Slocumetal2009)Above-groundcottonstrips(20cmtimes20cm)weredeployedinOctober2015securedtothesoilsurfaceusinglandscapepinsandretrieved14dayslaterThreecot-tonstrips(twoteststripsandonereferencestrip)wereplacedineachreplicatedplotReferencestripsusedtoquantifythetensilestrengthofnon-decomposedmaterialwerehandledexactlythesameastheteststripsbutretrievedimmediatelyafterdeployment

After retrieval cotton strips were rinsed gently with deionizedwatertoremoveallsoilandextraneousmaterialandthecottonstripswereallowedtoair-dryforatleast48hrCottonstripswerecutinto

2-cm substrips along the vertical profile and decomposition of the2-cm substripswasmeasured as tensile strength lost compared tothe reference substrip using a Dillon Quantroltrade Snapshot TensionCompressionMotorizedTestStandtensometerconnectedtoaDillonQuantroltrade Advanced Force Gauge (Slocum etal 2009) Cellulosedecay ratewascalculatedasper cent cellulose tensile strength lostperday(CTSLperday)

whereT is the force (N) requiredto tear thetestsubstripsR is theforce(N)requiredtotearthereferencesubstripsandtistime(days)inthemarsh

23emsp|emspPredictive variables

231emsp|emspExternal drivers

All study sites were located within the 1-km2 boundary of aCoastwide Reference Monitoring Systems station (httplacoastgovcrms2homeaspx) where surface water salinity and surfacewater elevation are measured hourly Marsh surface elevation of15plotswithineachsite (n=360)weresurveyedusingRealTimeKinematicmethodology(GaoAbdel-SalamChenampWojciechowski2005)withaTrimbleR10GNSSSystem(TrimbleNavigationLimitedUSAChenetal2011)andrectifiedtotheNorthAmericanVerticalDatumof1988(NAVD88)usingTrimbleBusinessCenter25soft-ware for data post-processing (Trimble Navigation Limited USA)We usedmarsh elevation data in conjunctionwith surfacewaterelevationdatatocalculateflooddepthanddurationforeachof15plotsineachsite

Discrete soil and porewater sampleswere taken to measure asuiteofenvironmentalparameters inDecember2014Ateachsitetwosoilcores(10-cmdiametertimes30-cmlength)werecollectedneareachofthethreereplicatedplotsalongthebelow-ground litterbagtransect(n=72)Aftercollectionthesoilcoresweresectionedintotwoincrements(0ndash15cmand16ndash30cm)andimmediatelyplacedonice in the fieldand transportedback to the laboratorywhere theywerehomogenizedThefirstcorewasusedforanalysisofsoilbulkdensity (Blake 1965) moisture (Blake 1965) organic mat-ter (OliverLotterampLemcke2001)andelectricalconductivity (ECRhodes1996)

ThesecondsoilcorewasusedtomeasuresoilpH(Thomas1996)soiltotalCNandPsoilextractablenutrients(PO4-PandNH4-N)andotherelementsofinterest(CaCuFeKMgNaNiP)Thehomog-enizedsoilwasdriedtoaconstantweightat60degCgroundinaWileyMill(Model420mesh850μm)andseparatedintoseveralscintil-lationvials formultipleanalysesSoil totalNandtotalCweremea-suredusingaCostechreg4010ElementalCombustionanalyzer(Nelsonamp Sommers 1982 EPAMethod 440) Extractionswere performedforthefollowinganalysessoiltotalP(HClAspilaAgemianampChau1976)PO4-P(Bray-2OlsenampSommers1982)NH4-N(KClKeeneyampNelson1982)andotherparametersofinterest(H2NO3AmericanPublic Health Association 2005a) Soil total P PO4-P samples and

MR= (wt∕w0)times100

X=eminuskt

X=Ca+ (1minusCa)eminuskat

CTSL per day= [1minus (T∕R)times100]∕t

emspensp emsp | emsp659Journal of EcologySTAGG eT Al

NH4-N were measured on a segmented flow AutoAnalyzer (FlowSolution IVAutoAnalyzer O-I Analytical USA EPAMethod 3655EPAMethod 3501)The remaining extractswere analysedwith aninductivelycoupledargonplasmaopticalemissionspectrometer(ICP-OES)(Varian-MPXAgilantUSAAmericanPublicHealthAssociation2005b)

Simultaneously four separate aliquots of porewaterwere col-lectedfromadepthof10cmusingthesipper-tubemethod(VasilasampVasilas2013)Onealiquotofwaterwasused tomeasurepore-water pH (EPA Method 1501) and salinity (EPA Method 1201)The second porewater sample was used to measure porewatertotalNand totalP followingpersulfateoxidation (DrsquoEliaSteudlerampNathaniel1977EbinaTsutsuiampShirai1983)onasegmentedflowAutoAnalyzer (Flow Solution IVAutoAnalyzerO-IAnalyticalUSA)Thethirdaliquotwasfilteredthrougha045-μmfiltertomea-sureNH4-NandPO4-PusingasegmentedflowAutoAnalyzer(FlowSolution IVAutoAnalyzerO-IAnalytical USA EPAMethod3655EPAMethod3501)Thefourthaliquotwasfirstfiltered(45μm)andthenacidifiedtopHlt2tomeasureotherelementsofinterestusingan inductively coupled argon plasma optical emission spectrome-ter (ICP-OES) (Varian-MPX Agilant USA American Public HealthAssociation2005b)

232emsp|emspInternal drivers

Above- and below-ground biomass from each wetland type alongthesalinitygradientwascollectedtocharacterizechemicalcomposi-tionofthelitterinJuly2013and2014InJuly2014subsamplesofinitialabove-andbelow-groundair-driedbiomasscollectedforeachrelevantdominantspecieswithineachwetlandtypewereanalysedfor lignin contentusing theacid-detergent fibreandacid-insolubleash techniques (Van Soest amp Wine 1968) Because insufficientinitial biomass remained for further analyses separate vegetationsamplespreviouslycollectedfromthestudysitesandanalysedfortotalCandtotalNinJuly2013servedasaproxyforthelitterusedin the litterbags In July 2013 above-ground biomasswas clippedat thesoil surface from025-m2quadrats separated into total liveandtotaldeadcomponentsandweighedafterdryingtoaconstantmassat60degC(Mendelssohn1979)Afterabove-groundbiomasswasremovedfromtheplotasharpened10-cmPVCcorerwasusedtocollectbelow-groundbiomass from thecentreof thequadratThecoresweretakentoamaximumdepthof30cmortheentirerootmatthicknessandwerewashedina1-mmsievetoremovesoilpar-ticlesLiverootsandrhizomeswereseparatedfromdeadrootsandrhizomes and the remainingmatrixof deadorganicmaterial baseduponbiomasscolourturgorandbuoyancy(SchubauerampHopkinson1984)Allmaterialwasdriedat60degCtoaconstantmassandweighedAbove-andbelow-groundvegetationsampleswerethengroundinaWileyMill (Model420mesh850μm)oven-driedat60degCandanalysed for totalCand totalNusingaCostechreg 4010ElementalCombustionanalyzer(NelsonampSommers1982EPAMethod440)Only values for live biomass samples were used in subsequent statisticalanalyses

24emsp|emspStatistical analysis

We used a nonlinear regression to estimate decay constants andasymptotes from single exponential decay models and asymptoticmodelsrespectivelyNonlinearregressionmodelsweredevelopedforeach plot in each site (above-groundn=72 below-groundn=72)Onlyestimates frommodels thatsuccessfullyconvergedwereusedinsubsequentstatisticaltestsWeusedamixed-modelANOVAwitharandomizedcompleteblockdesignwithsamplingtocomparevari-ation in response variables (decay constants asymptotes cellulosedecay rates) The fixed effect of wetland type was the treatmenteffect basins represented error associatedwith blocking and threesiteswithineachbasinbywetlandtypetreatmentcombinationrep-resentedsite-levelerrorResponsevariablesweremeasuredinthreeplots within each site which represented sampling error Principalcomponentanalysis(PCA)wasusedtoexplainvariationintheenvi-ronmentalparameterdatasetandananalysisofsimilarity(ANOSIM)wasperformedtodeterminewhethertheprincipalcomponents(PCs)variedsignificantlyamongthetreatmentgroups (wetlandtype)Wecalculated correlation coefficients tomeasure the linear associationbetweenredoxpotentialandPCfactorscoresFinallyweperformedmultiple linear regression analysis using the lm function to identifysignificantpredictorsofdecompositionForeach responsevariableweidentifiedafullmodelapriorithatincludedexplanatoryvariablesofknownimportanceandrelevanceThelitterdecompositionmodelspredictedabove-orbelow-groundlitterdecompositionrateasafunc-tionoflitterqualitysoilandporewaterphysico-chemistryandflood-ing The cellulose decaymodels predicted above- or below-groundcellulose decay rate as a function of soil and porewater physico-chemistryandfloodingWeusedprincipalcomponentfactorsasex-planatoryvariablestorepresentporewaterandsoilphysico-chemistryand flooding Because not all litter quality parameterswere signifi-cantlycorrelatedwithaprincipalcomponentweincludedlitterligninandnitrogencontentasexplanatoryvariablesinthemultipleregres-sionanalysesusing theobserveddata inplaceof the factorscoresThe following analyses were performed in SAS 93 software (SASInstitute Inc 2011) nonlinear regression (proc nlin) ANOVA (procmixed) and correlation analysis (proc corr) The following analyseswereperformedusingRsoftware(RDevelopmentCoreTeam2013)PCAANOSIMandmultiplelinearregression

3emsp |emspRESULTS

31emsp|emspResponse variables

311emsp|emspLitter decomposition

Inallwetlandtypesabove-andbelow-groundlittersignificantlyde-clinedwith time (Table1Figure2)Therewasa significant interac-tionbetweenabove-andbelow-grounddecayrateandwetlandtype(p=0004 df = 3 F=646) however regardless of wetland typeabove-ground litterdecomposed faster thanbelow-ground litter Inbothabove-andbelow-groundlitterpoolsthedecayratewasgreatest

660emsp |emsp emspenspJournal of Ecology STAGG eT Al

intheoligohalinemarshIntheabove-groundlitterpooldecayrateinthepolyhalinemarshwassignificantlygreaterthandecayratesinthefreshandmesohalinemarshes(Figure2a)Incontrastbelow-groundlitterdecayratesdidnotvarysignificantlyamongthefreshmesoha-lineandpolyhalinemarshes(Figure2b)

Therewas a significant interaction between above- and below-ground asymptotes and wetland type (p = 0356 df = 3 F=304Figure2) Above-ground litter decomposition was more complete(smallerasymptote)thanbelow-groundlitterdecompositioninallwet-landtypeswiththeexceptionof theoligohalinemarshwheretheywereequivalentWithin the above-ground litter pool therewasnosignificantvariationinthefractionoflitterremainingamongthefourwetlandtypes (Figure2a) Incontrastbelow-groundthefractionoflitterremainingwassignificantlyhigherinthemesohalinemarshcom-paredtoallotherwetlandtypes(Figure2b)

312emsp|emspCellulose decay

Cellulose decay was greater below-ground than above-ground(Figure3)Furthermoretrendsincellulosedecayalongthelandscapesalinity gradientwere different between above- and below-groundpools (p = 002 df = 3 F=488)Above-groundcellulosedecaywasgreatest in the polyhaline marsh but otherwise similar among theother wetland types Below-ground cellulose decay significantly increasedalongthegradientfromfreshtopolyhalinemarsh

Additionallytherewasasignificantinteractionbetweenwetlandtype and depth (plt0001df=42F=398 FigureS1)At the sur-facecellulosedecayratesweresimilaralongthelandscapegradientAsdepthbelowthesoilsurfaceincreasedtherewasadivergenceincellulosedecayamongthewetlandtypesandoverallratesofdecayweregreaterinthemesohalineandpolyhalinemarshascomparedtothefreshandoligohalinemarsh

32emsp|emspPredictive variables

The PCA generated three PCs that cumulatively explained 59 ofthe variance in the predictive variable dataset (TableS1) The firstPC(PC1mdashPhysico-chemical)explained39ofthevarianceandwasdefinedbyporewaterandsoilphysico-chemicalpropertiesincludingtemperaturesalinityandnutrientparametersAnnualsurfacewatertemperaturewas positively associatedwith PC1 (R2=65) aswereporewaterandsoilsalinityparameterssuchasporewaterandsoilEC(R2=90and 89 respectively)SoilnutrientparameterssuchassoiltotalnitrogenandtotalphosphoruswerenegativelycorrelatedwithPC1 (R2=minus90andminus67respectively)ThesecondPC (PC2mdashLignin)explainedc13ofthevariationandwasdefinedbylignincontentofabove-ground(leaf)andbelow-ground(root)litterLeaflignincontentwaspositivelycorrelatedwithPC2(R2=55)androotlignincontentwasnegativelycorrelatedwithPC2(R2=minus55)whereasleafandrootlitternitrogencontentwerenotsignificantlycorrelatedwith thisoranyotherPC

The third PC (PC3ndashFlooding) explained 7 of the databasevariance andwas defined by elevation and flood duration parame-tersWetland surface elevationwas negatively correlatedwith PC3(R2=minus55)andannualpercenttimefloodedwaspositivelycorrelatedwithPC3(R2=54)

TherewasaseparationamongthewetlandtypesalongboththePC1(Physico-chemical)andPC2(Lignin)axes(R = 212 p = 001 and R=092 p=001 for PC1 and PC2 respectively)As expected thefour wetland types separated by salinity (Figure4ab) Additionallywetlandstypeswithhigh leaf lignincontentalsohadlowroot lignincontent(Figure4ac)

Due tomissing redoxpotential data thisparameterwasnot in-cludedinthePCAorinthesubsequentmultipleregressionanalysesHowever redox potential varied significantly among the wetland

Wetland type Pool Parameter Estimate SE t- value p- value

Fresh Above k 0003 910E-05 32 lt0001

Oligohaline Above k 0005 000041 131 lt0001

Mesohaline Above k 0003 000012 208 lt0001

Polyhaline Above k 0004 000012 337 lt0001

Fresh Below k 0002 000011 156 lt0001

Oligohaline Below k 0003 000024 129 lt0001

Mesohaline Below k 0001 00001 128 lt0001

Polyhaline Below k 0001 00001 133 lt0001

Fresh Above a 2199 627 351 00008

Oligohaline Above a 2302 543 424 lt0001

Mesohaline Above a 2197 711 309 003

Polyhaline Above a 767 941 082 42

Fresh Below a 4413 627 704 lt0001

Oligohaline Below a 3185 521 611 lt0001

Mesohaline Below a 6174 543 1137 lt0001

Polyhaline Below a 4418 595 743 lt0001

TABLE 1emspNonlinearregressionestimatesofsingleexponentialdecayconstants(minuskperday)andasymptotes (a)forabove-andbelow-groundlitterdecomposition

emspensp emsp | emsp661Journal of EcologySTAGG eT Al

types andwas highest in the freshmarsh and lowest in themeso-halineandpolyhalinemarshes(plt0001df = 3 F=5282Figure5)Additionallyredoxpotentialwashighlycorrelatedwithseveralwell-characterized parameters associated with PC1 (Physico-chemical)such as porewater EC and porewater sulphur (R2=minus63 plt0001R2=minus46p=002respectively)

33emsp|emspMultiple regression analysis

Initialnitrogencontentwas theonly significantpredictorofabove-groundlitterdecomposition(Table2)whichincreasedwithincreasinglitternitrogencontent(Figure6a)Initiallignincontentwasasignifi-cantpredictorofbelow-groundlitterdecomposition(Table2)whichdeclinedwithincreasinglignincontent(Figure6b)

NeitherPC1(Physico-chemical)norPC3(Flooding)hadasignifi-canteffectonabove-groundcellulosedecay(Table2)IncontrastPC1(Physico-chemical) andPC3 (Flooding)were importantpredictorsof

below-groundcellulosedecay(Table2)whichincreasedwithincreas-ingsalinityand floodinganddecreasingsoilnutrientconcentrations(Figure7ab)

4emsp |emspDISCUSSION

To improveourunderstandingofhoworganicmatterdecomposi-tionandthefateofcarbonincoastalwetlandswillbeimpactedbysea-level rise we identified the internal and external drivers thatinfluencedecompositionincoastalwetlandsacrossanestuarinesa-linitygradientspanningfreshtopolyhalinewetlandtypes Internaldriverssuchaslitterqualityarecharacteristicsoftheorganicmat-ter itselfwhereas external drivers such as hydrologic conditionsare characteristics of the environment (Aerts 1997 Webster ampBenfield 1986) and their effects on decomposition may be ex-pressed through both direct and indirect pathways For example

F IGURE 2emspEstimatesof(a)above-groundlitterdecayconstantsandasymptotesalongthelandscapegradientand(inset)relativeabove-groundbiomassremainingovertime(b)below-groundlitterdecayconstantsandasymptotesalongthelandscapegradientand(inset)relativebelow-groundbiomassremainingovertimeBarsrepresentmeans(n=18)anderrorbarsrepresentSEsCapitallettersdenotestatisticalsignificanceofpost-hocmultiplecomparisonsamongasymptotemeansfrombothabove-andbelow-groundpoolsLowercaselettersdenotestatisticalsignificanceofpost-hocmultiplecomparisonsamongdecayconstantmeansfrombothabove-andbelow-groundpools(FisherrsquosProtectedLSDα=005)

(a)

(b)

662emsp |emsp emspenspJournal of Ecology STAGG eT Al

sea-level rise can impact soil organic matter decomposition indi-rectly through changes in internal drivers such as plant commu-nity composition and litter qualitywhich control litter decay rate(StaggSchoolmasterKraussetal2017) Inthisstudywefoundthatdespitesignificantvariationinenvironmentalconditionsalongthisgradientexternaldriversincludingporewatersalinityandflooddurationhadnosignificantpredictivecapacityforeitherabove-orbelow-ground litterdecompositionThereforeour results indicatethattheindirectpathwaymediatedthroughchangesinlitterqualityan internaldriverwasmore important incontrolling litterdecom-positionthandirecteffectsofexternaldriversalongthislandscapegradient

Previous research has documented variation in lignin contentamongdifferentwetlandplantspecies(Buth1987GuoLuTongampGuohua2008)andourmeasurementsoflitterlignincontentforfresholigohalinemesohalineandpolyhalinespeciesweresimilartothosereported in the literature (Table3) However decay constants frommixed-specieslittermaterialwilldiffersignificantlycomparedtolittercomprisedofasinglespecies(ChapmanNewmanHartSchweitzerampKoch2013)Thereforeinsitumeasurementsoflittermixturesthatrepresentthevegetationcommunityarecriticalforachievingaccurateestimates of litter decay and identifying patterns of decompositionamongdifferentwetlandtypes

Because lignincontentcandiffersignificantlybetweentwospe-cieswithin the samewetland type or salinity zone it is difficult toidentifyuniversal patternsof lignin content alonga landscape-scalesalinity gradient Furthermore changes in plant diversity along thelandscapegradientmayimpactratesofdecompositionOdum(1988)observedgreaterplantdiversity infreshmarshescomparedtomoresalinemarsheswhichmayaffecttheproportionofdifferentspeciesand overall litter quality in a litter mixture Very few studies havecompared litter quality and decomposition along a salinity gradientthatincorporateschangesincomposition(GallagherKibbyampSkirvin

1984LopesMartinsRicardoRodriguesampQuintino2011ScartonDayampRismondo2002Windham2001)andasfarasweknowthisisthefirststudytoquantifylitterdecompositiondynamicsofdiffer-entwetlandtypesacrosstheentirecoastalmarsh landscapesalinitygradientOdumrsquos (1988) comparative review of freshvs polyhalinetidal marshes reports that freshwater macrophytes from the lowerintertidal zone such asSagittaria latifolia tend to have lower lignincontenthighernitrogencontentandhigherdecayratescomparedto

F IGURE 3emspCellulosedecayamongdifferentwetlandtypesBarsrepresentmeans(n=540)anderrorbarsrepresentSEsLettersdenotestatisticalsignificanceofpost-hocmultiplecomparisonsofmeans(FisherrsquosProtectedLSDα=005)

F IGURE 4emspPrincipalComponentAnalysisbiplotsofobservationprojectionsorfactorscoresincomponentspaceforallcomparisonsbetween(a)principalcomponent(PC)1andPC2(b)PC1andPC3and(c)PC2andPC3IneachplotfactorscoresarecolouredbywetlandtypeAsubsetofhighlycorrelatedvectorsfromeachPCoverlaythefactorscores

minus6 minus4 minus2 0 2 4 6

minus6minus4

minus20

24

6

PC

2 (L

igni

n)

FreshOligohaline Mesohaline Polyhaline

minus6 minus4 minus2 0 2 4 6

minus6minus4

minus20

24

6

PC

3 (F

lood

ing)

FreshOligohaline Mesohaline Polyhaline

minus6 minus4 minus2 0 2 4 6

minus6minus4

minus20

24

6

PC2 (Lignin)

PC

3 (F

lood

ing)

FreshOligohaline Mesohaline Polyhaline

(a)

(b)

(c)

PC1 ( )

PC1 ( )

emspensp emsp | emsp663Journal of EcologySTAGG eT Al

polyhalineorsaltmarshspeciesIncontrastfreshwatermacrophytesfrom the upper intertidal such asT latifolia more closely resemble typicalsaltmarshplantsintheirligninandnitrogencontentandexhibitslowerratesofdecaythanplantsfromthelowerintertidalfreshzoneInsupportofOdumrsquosconclusionswefoundthatlitterdecompositionwas greatest in the oligohalinemarsheswhichwere dominated byS lancifolia and S americanusandalsohadthelowestlignincontentThefreshmarshdominatedbyT latifolia and P hemitomon was similar inlignincontentanddecayratetothemesohalinemarshesdominatedby S patensandthepolyhalinemarshesdominatedbyS alterniflora and J roemerianus Thus although landscape-scale salinity patternsoflignincontentandlitterdecompositionarelargelyspecies-specific

(AertsampdeCaluwe1997)itisclearthatsea-levelrisehasthepoten-tialtoalterthequalityofcarbonandindirectlyimpactdecompositionalongthisgradient

Similarlyinitiallitterqualityintheformofnitrogencontentwasthebestpredictorof above-ground litter decomposition (MarinucciHobbie amp Helfrich 1983 Taylor Parkinson amp Parsons 1989)Althoughsomeresearchidentifiestheratiooflignin-to-nitrogeninini-tiallitterqualityasanimportantpredictorofdecomposition(MelilloAberampMuratore1982Valielaetal1984)ourresultsindicatethatonlyoneofthesevariableswasasignificantpredictoreithernitrogencontent or lignin content of above- or below-ground litter decom-positionrespectivelyLikewiseastudybyMelilloNaimanAberandEshleman(1983)identifiedeitherligninaloneorincombinationwithnitrogencontentasasignificantpredictorofdecompositionThesig-nificanceofonepredictoroveranothermaybeduetointeractionsbe-tweeninitiallitterqualityandenvironmentalconditionsIngeneralifexogenousnitrogen(egfromsoilorwatercolumn)isreadilyavailabletomicrobesandorinitiallitterlignincontentishightheninitiallitternitrogencontentmayhave little impactondecayrate (Melilloetal1982 1984) Interestingly despite these differences among above-andbelow-groundlittercontentthepatternofdecayalongtheland-scapegradientwassimilarwiththegreatestdecompositionoccurringin the oligohaline marsh Although salinity is a known regulator ofplantnitrogendynamics (BradleyampMorris1991Morris1980)wedidnotobserveasimplelineardeclineinlitterqualitywithincreasingsalinityThusitisclearthatlitterdecompositioniscontrolledthroughan indirectpathwaymediatedby internaldriversbutmoreresearchisneededtoidentifytheultimatedriversthatgenerateoptimallitterqualityforenhanceddecayintheoligohalinemarsh

To characterize the direct effects of sea-level rise ondecompo-sitionwe controlled for the influenceof litter qualitybymeasuringthe decay rate of a standard carbon source cellulose across thelandscape-scalesalinitygradient(Mendelssohnetal1999)Celluloseisalabilecompoundthatrapidlydecaysduringtheinitialphasesofde-compositionandservesasareadilyavailablefuelformicrobialactivity

TABLE 2emspResultsofmultiplelinearregressionanalysisforlitterdecompositionandcellulosedecay

Response variable Predictive variable Parameter estimate SE t- value p- value Model R2

Above-groundlitter PC1 minus177E-05 565E-05 minus0314 755 121

Decomposition PC3 minus495E-05 131E-04 minus0378 707

LeafN 00003 104E-03 268 009

LeafLignin minus736E-05 218E-04 minus0338 736

Below-groundlitter PC1 431E-05 338E-05 127 207 395

Decomposition PC3 507E-05 676E-05 0750 456

RootN 353E-04 313E-04 113 265

RootLignin minus217E-04 363E-05 minus598 110E-07

Above-ground PC1 0042 0032 130 198 027

Cellulose decay PC3 minus0021 0075 minus0280 780

Below-ground PC1 0039 0012 324 002 195

Cellulose decay PC3 0066 0028 236 021

plt05plt01plt001

F IGURE 5emspVariationinsoilredoxpotentialalonglandscapegradientBoxplotboundariesclosesttozerorepresentthe25thpercentilethelinewithintheboxesindicatesthemedianandboundariesfarthestfromzerorepresentthe75thpercentile(n=90)Whiskersindicatethe90thand10thpercentilesBlackdotsrepresentoutlyingpointsLettersdenotestatisticalsignificanceofpost-hocmultiplecomparisonsofmeans(FisherrsquosProtectedLSDα=005)

664emsp |emsp emspenspJournal of Ecology STAGG eT Al

(Hodson Chrsitian amp Maccubbin 1984) Therefore in addition toidentifyingexternalcontrolsondecaywewerealsoabletocharac-terizethedecompositiondynamicsoftheisolatedlabilecarbonpool

We found that below-ground cellulosedecay increasedwith in-creasingsalinitywhichsupportsfindingsfromrecentstudiesthatsoilrespirationisstimulatedbysalinityinshort-termexposuresthatdonotincorporatechangesincarbonsource(Chambersetal2011Westonetal2006)Whileincreasingsalinitycanhavedirectimpactsonor-ganicmattermineralizationthroughalteringthesoilchemicalcompo-sitionandreleasingpreviouslysoil-boundorganiccarbon(DouPingGuoampJorgenson2005)thepatternofcellulosedecayalongthissa-linitygradientwaslikelyinfluencedbydifferencesamongthemicrobialcommunities(Chambersetal2013)Assalinitycontinuestoincreasesulphate reducersout-competemethanogensandoverall anaerobicmetabolism isgreaterwhensulphate is thedominant terminalelec-tronacceptor(Sutton-GrierKellerKochGilmourampMegonigal2011Westonetal2006)

In contrast to salinity soil nutrients were negatively correlatedwith below-ground cellulose decay Although Mendelssohn etal

(1999)foundthatsoilnutrientshadasignificantpositiveinfluenceoncellulose decay in a Phragmites australis-dominatedwetlandthistrendisnotuniversalamongallwetlandtypesForexamplethereviewbyRybczyk Garson andDay (1996) illustrates varying impacts of soilnutrientsonsoilorganicmatterdecompositionEvensoitisunlikelythathighsoilnutrientswoulddirectlyinhibitcellulosedecayRatherweproposethatthestimulatoryeffectofelevatedsalinityandgreatersulphate availability overcame the potential negative effects of lowsoilnutrientconcentrations

Floodingwasalsoa strongpredictorofdecomposition andhada direct positive effect on below-ground cellulose decay Althoughgreaterflooddurationcanleadtoanaerobicsoilconditions(GambrellampPatrick1978Ponnamperuma1984)whichcan limit the rateofdecomposition (Day amp Megonigal 1993 McKee amp Seneca 1982WhiteampTrapani1982)decompositionisnotalwaysslowerunderan-aerobicconditionsForexampleKirwanLangleyGuntenspergenand

F IGURE 6emspLinearregressionof(a)above-groundlitterdecompositionandlitternitrogencontentand(b)below-groundlitterdecompositionandlitterlignincontentFresholigohalinemesohalineandpolyhalinewetlandtypesdifferentiatedbyshapesymbols

F IGURE 7emspLinearregressionofbelow-groundcellulosedecayand(a)PC1(Physico-chemical)and(b)PC3(Flooding)Fresholigohalinemesohalineandpolyhalinewetlandtypesdifferentiatedbyshapesymbols

(a)

(b)

emspensp emsp | emsp665Journal of EcologySTAGG eT Al

Megonigal(2013)observedastimulatoryfloodingeffectonrootandrhizomedecompositionFurthermore soil redoxpotentialalong thislandscapegradientwasmorecloselycorrelatedwithporewatersalin-itythanwithfloodduration(PwECR2=minus62plt0001AnnFloodR2=minus39p=01) indicating that the stimulating effect of floodingwasnotstronglyassociatedwithanaerobicconditionsThereforeweproposethatthepositiverelationshipbetweenfloodingandcellulosedecayillustratesthewell-documentedobservationthatmicrobialac-tivity ismoisture-limited (FrascoampGoode 1982HalupaampHowes

1995NewellArsuffiampPalm1996ReiceampStiven1983)at leastinthetop20cmofthesoilprofileAtfurtherdepthscellulosedecaybelowthesoilsurfacewaslikelyoxygenlimitedassoilsbecomemorereduced with increasing depth (Maltby 1988 Schipper amp Reddy1995)Similartoourfindingslowoxygenavailabilityhasbeeniden-tifiedasaprimaryinhibitorofcellulosedecayatdepthsbelow22cm(Mendelssohnetal1999)

Surprisingly none of the environmental drivers we measuredhad predictive capacity for above-ground cellulose decayAlthough

TABLE 3emspLitterlignincontentofdominantspeciesfromeachwetlandtypeLitterconditionidentifiedaslive(L)dead(D)orunknown(U)

Species Litter type Lignin content () Study

Typha latifolia Leaves 122L Currentstudy

58L MoranampHodson(1989)

395ndash427D WelschampYavitt(2003)

182D PoideNeiffNeiffampCasco (2006)

Roots 73L Currentstudy

Panicum hemitomon Leaves 64ndash73L Currentstudy

64L MoranampHodson(1989)

59D OsborneInglettampReddy(2007)

Roots 88ndash122L Currentstudy

Sagittaria lancifolia Leaves 77ndash77L Currentstudy

18U Laursen(2004)

Roots 54ndash74L Currentstudy

26U Laursen(2004)

Schoenoplectus americanus Leaves 79ndash89L Currentstudy

205D BallampDrake(1997)

Roots 47ndash75L Currentstudy

1443ndash2650L SaundersMegonigalampReynolds(2006)

Spartina patens Leaves 78ndash92L Currentstudy

142D BallampDrake(1997)

Roots 148ndash186L Currentstudy

2695ndash3041L Saundersetal(2006)

Juncus roemerianus Leaves 99L Currentstudy

6U Benneretal(1987)

Roots 137L Currentstudy

51U Benneretal(1987)

Spartina alterniflora Leaves 55ndash57L Currentstudy

151L MaccubbinampHodson(1980)

131ndash168L Hodsonetal(1984)

117U Wilson(1985)

11ndash12D WilsonBuchsbaumValielaampSwain(1986)

43ndash61U Benneretal(1987)

Roots 137ndash152L Currentstudy

122ndash193U Hodsonetal(1984)

93U Benneretal(1987)

666emsp |emsp emspenspJournal of Ecology STAGG eT Al

above-groundcellulosedecayvariedsignificantlyacrossthelandscape-scalesalinitygradient(higherinpolyhalinemarsh)salinitypersewasnotasignificantpredictorofdecay(sensuMendelssohnetal1999)Several recentstudieshave illustratedthatsalinitycan indirectlyaf-fect soil respiration throughchanges inmicrobial function resultinginhigherratesofcarbonmineralizationathighersalinities(Chambersetal2011Neubauer2013Sutton-Grieretal2011Westonetal2006)Thuswehypothesizethatanunidentifiedmediatingfactorthatvarieswithwetlandtypesuchasmicrobialfunction(CaponeampKiene1988) is regulating cellulose decayWe suggest that future studiesinclude focusedmeasuresofmicrobial structure and function alongthislandscape-scalegradienttoconfirmthemechanismofindirectef-fectsofsalinityoncellulosedecay(HopfenspergerBurginSchoepferampHelton2014Morriseyetal2014NeubauerGivlerValentineampMegonigal2005)

Insummaryourstudyshowedthat the indirecteffectofsea-levelriseonlitterdecompositionwasmediatedthroughchangesinplantcommunitycompositionand litterqualitywhich resulted indecliningratesoflitterdecompositionalongthegradientfromoli-gohalinetopolyhalinemarshesHoweverwhenwecontrolledforchangesinlitterqualitywefoundthatincreasingsalinityandflood-ingstimulateddecayoflabilecarbon(cellulose)Wehaveidentifiedtwomechanismsoforganicmatterlossoperatingatdifferenttem-poralscalesthatprovideinsighttothepotentialforlong-termcar-bonstorageassea-levelrisesOurresultsindicatethatassealevelrises initial direct effectsof salinitywill stimulatedecayof labilecarbonbutovertimeasvegetationcommunitycompositionshiftstomoresalinewetlandtypes litterdecay (decayofrefractilecar-bon)willdeclineyieldinggreaterpotential for long-termsoilcar-bonstoragethroughnetaccretion(LoomisampCraft2012)Recentstudieshavehighlightedtheimportanceofscaleinconsideringtheinfluence of sea-level rise on carbon loss fromwetland systems(Herbertetal2015Neubaueretal2013)Ourresearchprovidesanexampleofhowmultiplemechanismsofcarbon lossoperatingatdifferentscalescanyielddifferentratesandpatternsoforganicmatter decompositionwhich is relevant not only in coastalwet-landsbutalsoinotherecosystemssuchasgrasslandsshrublandsandforestswhereplant-mediatedresponsestoclimatechangewillhavesignificant impactsoncarbondynamics (JobbagyampJackson2000)

ACKNOWLEDGEMENTS

The authors acknowledge themany people who assisted in fieldsamplecollectionandsampleprocessingAdaDizEvanBergeronKelly Darnell Lindsey Hebert Samantha Humphrey JamalMathurinAlyssaMitchellLelandMossBrettPattonSaraiPiazzaCaitlinPinsonatJacyReynoldsJacksonRollingsandRachelVillaniWe thank Thomas Blanchard and Sara Gay at Louisiana StateUniversityWetlandBiogeochemistryAnalyticalServicesandUttamKumarSahaandDavidParksatUniversityofGeorgiaCooperativeExtension for performing chemical analyses We are grateful toChristopherSwarzenski forhishelp insiteselectionBrettPatton

for rectifyingelevationdatasetsLelandMoss fordevelopmentofFigure1LaurenLeonpacherfordevelopmentofTable3andmanu-scripteditingWealsothankJoshuaJonesChristopherSwarzenskiandanonymouspeersfortheirreviewofthismanuscriptThisre-searchwasfundedbytheUSGeologicalSurveyClimateResearchand Development Program US Geological Survey EcosystemsProgram the US Geological Survey LandCarbon program theScienceandEngineeringProgramofTheWaterInstituteoftheGulfwith funds fromtheLouisianaCoastalProtectionandRestorationAuthority (CPRA) and the Baton Rouge Area Foundation (BRAF)andtheRestoretheMississippiRiverDeltaCampaignviaCoalitionto Restore Coastal Louisiana (CRCL) Any use of trade firm orproductnamesisfordescriptivepurposesonlyanddoesnotimply endorsementbytheUSGovernment

AUTHORrsquoS CONTRIBUTIONS

CLSMMBandCLPconceivedtheideasanddesignedmethod-ology CLSMMB CLP TJBC andCTH collected the dataCLSMMBandCTHanalysedthedataCLSledthewritingofthe manuscript All authors contributed critically to the drafts andgavefinalapprovalforpublication

DATA ACCESSIBILITY

Allof thesupportingdatapresented in thispaperhavebeenpubli-callyarchivedatsciencebasegovhttpsdoiorg105066f7639mvk(StaggBaustianPerryCarruthersampHall2017)

ORCID

Camille L Stagg httporcidorg0000-0002-1125-7253

Melissa M Baustian httporcidorg0000-0003-2467-2533

Courtney T Hall httporcidorg0000-0003-0990-5212

REFERENCES

AertsR(1997)ClimateleaflitterchemistryandleaflitterdecompositioninterrestrialecosystemsAtriangularrelationshipOikos 79439ndash449httpsdoiorg1023073546886

AertsRampdeCaluweH(1997)Nutritionalandplant-mediatedcontrolson leaf litterdecompositionofCarex speciesEcology 78 244ndash260httpsdoiorg1018900012-9658(1997)078[0244NAPMCO]20 CO2

Alward R Detling J amp Milchunas D (1999) Grassland vegetationchangesandnocturnalglobalwarmingScience 283118ndash231httpsdoiorg101126science2835399229

AmericanPublicHealthAssociation(2005a)3030EnitricaciddigestionInADEatonampMAFranson(Eds)Standard methods for the exam-ination of water and wastewater (21stedpp325ndash326)WashingtonDCAmericanPublicHealthAssociation

AmericanPublicHealthAssociation(2005b)3120metalsbyplasmaemis-sionspectroscopy(85)InADEatonampMAFranson(Eds)Standard methods for the examination of water and wastewater(21stedpp377ndash386)WashingtonDCAmericanPublicHealthAssociation

660emsp |emsp emspenspJournal of Ecology STAGG eT Al

intheoligohalinemarshIntheabove-groundlitterpooldecayrateinthepolyhalinemarshwassignificantlygreaterthandecayratesinthefreshandmesohalinemarshes(Figure2a)Incontrastbelow-groundlitterdecayratesdidnotvarysignificantlyamongthefreshmesoha-lineandpolyhalinemarshes(Figure2b)

Therewas a significant interaction between above- and below-ground asymptotes and wetland type (p = 0356 df = 3 F=304Figure2) Above-ground litter decomposition was more complete(smallerasymptote)thanbelow-groundlitterdecompositioninallwet-landtypeswiththeexceptionof theoligohalinemarshwheretheywereequivalentWithin the above-ground litter pool therewasnosignificantvariationinthefractionoflitterremainingamongthefourwetlandtypes (Figure2a) Incontrastbelow-groundthefractionoflitterremainingwassignificantlyhigherinthemesohalinemarshcom-paredtoallotherwetlandtypes(Figure2b)

312emsp|emspCellulose decay

Cellulose decay was greater below-ground than above-ground(Figure3)Furthermoretrendsincellulosedecayalongthelandscapesalinity gradientwere different between above- and below-groundpools (p = 002 df = 3 F=488)Above-groundcellulosedecaywasgreatest in the polyhaline marsh but otherwise similar among theother wetland types Below-ground cellulose decay significantly increasedalongthegradientfromfreshtopolyhalinemarsh

Additionallytherewasasignificantinteractionbetweenwetlandtype and depth (plt0001df=42F=398 FigureS1)At the sur-facecellulosedecayratesweresimilaralongthelandscapegradientAsdepthbelowthesoilsurfaceincreasedtherewasadivergenceincellulosedecayamongthewetlandtypesandoverallratesofdecayweregreaterinthemesohalineandpolyhalinemarshascomparedtothefreshandoligohalinemarsh

32emsp|emspPredictive variables

The PCA generated three PCs that cumulatively explained 59 ofthe variance in the predictive variable dataset (TableS1) The firstPC(PC1mdashPhysico-chemical)explained39ofthevarianceandwasdefinedbyporewaterandsoilphysico-chemicalpropertiesincludingtemperaturesalinityandnutrientparametersAnnualsurfacewatertemperaturewas positively associatedwith PC1 (R2=65) aswereporewaterandsoilsalinityparameterssuchasporewaterandsoilEC(R2=90and 89 respectively)SoilnutrientparameterssuchassoiltotalnitrogenandtotalphosphoruswerenegativelycorrelatedwithPC1 (R2=minus90andminus67respectively)ThesecondPC (PC2mdashLignin)explainedc13ofthevariationandwasdefinedbylignincontentofabove-ground(leaf)andbelow-ground(root)litterLeaflignincontentwaspositivelycorrelatedwithPC2(R2=55)androotlignincontentwasnegativelycorrelatedwithPC2(R2=minus55)whereasleafandrootlitternitrogencontentwerenotsignificantlycorrelatedwith thisoranyotherPC

The third PC (PC3ndashFlooding) explained 7 of the databasevariance andwas defined by elevation and flood duration parame-tersWetland surface elevationwas negatively correlatedwith PC3(R2=minus55)andannualpercenttimefloodedwaspositivelycorrelatedwithPC3(R2=54)

TherewasaseparationamongthewetlandtypesalongboththePC1(Physico-chemical)andPC2(Lignin)axes(R = 212 p = 001 and R=092 p=001 for PC1 and PC2 respectively)As expected thefour wetland types separated by salinity (Figure4ab) Additionallywetlandstypeswithhigh leaf lignincontentalsohadlowroot lignincontent(Figure4ac)

Due tomissing redoxpotential data thisparameterwasnot in-cludedinthePCAorinthesubsequentmultipleregressionanalysesHowever redox potential varied significantly among the wetland

Wetland type Pool Parameter Estimate SE t- value p- value

Fresh Above k 0003 910E-05 32 lt0001

Oligohaline Above k 0005 000041 131 lt0001

Mesohaline Above k 0003 000012 208 lt0001

Polyhaline Above k 0004 000012 337 lt0001

Fresh Below k 0002 000011 156 lt0001

Oligohaline Below k 0003 000024 129 lt0001

Mesohaline Below k 0001 00001 128 lt0001

Polyhaline Below k 0001 00001 133 lt0001

Fresh Above a 2199 627 351 00008

Oligohaline Above a 2302 543 424 lt0001

Mesohaline Above a 2197 711 309 003

Polyhaline Above a 767 941 082 42

Fresh Below a 4413 627 704 lt0001

Oligohaline Below a 3185 521 611 lt0001

Mesohaline Below a 6174 543 1137 lt0001

Polyhaline Below a 4418 595 743 lt0001

TABLE 1emspNonlinearregressionestimatesofsingleexponentialdecayconstants(minuskperday)andasymptotes (a)forabove-andbelow-groundlitterdecomposition

emspensp emsp | emsp661Journal of EcologySTAGG eT Al

types andwas highest in the freshmarsh and lowest in themeso-halineandpolyhalinemarshes(plt0001df = 3 F=5282Figure5)Additionallyredoxpotentialwashighlycorrelatedwithseveralwell-characterized parameters associated with PC1 (Physico-chemical)such as porewater EC and porewater sulphur (R2=minus63 plt0001R2=minus46p=002respectively)

33emsp|emspMultiple regression analysis

Initialnitrogencontentwas theonly significantpredictorofabove-groundlitterdecomposition(Table2)whichincreasedwithincreasinglitternitrogencontent(Figure6a)Initiallignincontentwasasignifi-cantpredictorofbelow-groundlitterdecomposition(Table2)whichdeclinedwithincreasinglignincontent(Figure6b)

NeitherPC1(Physico-chemical)norPC3(Flooding)hadasignifi-canteffectonabove-groundcellulosedecay(Table2)IncontrastPC1(Physico-chemical) andPC3 (Flooding)were importantpredictorsof

below-groundcellulosedecay(Table2)whichincreasedwithincreas-ingsalinityand floodinganddecreasingsoilnutrientconcentrations(Figure7ab)

4emsp |emspDISCUSSION

To improveourunderstandingofhoworganicmatterdecomposi-tionandthefateofcarbonincoastalwetlandswillbeimpactedbysea-level rise we identified the internal and external drivers thatinfluencedecompositionincoastalwetlandsacrossanestuarinesa-linitygradientspanningfreshtopolyhalinewetlandtypes Internaldriverssuchaslitterqualityarecharacteristicsoftheorganicmat-ter itselfwhereas external drivers such as hydrologic conditionsare characteristics of the environment (Aerts 1997 Webster ampBenfield 1986) and their effects on decomposition may be ex-pressed through both direct and indirect pathways For example

F IGURE 2emspEstimatesof(a)above-groundlitterdecayconstantsandasymptotesalongthelandscapegradientand(inset)relativeabove-groundbiomassremainingovertime(b)below-groundlitterdecayconstantsandasymptotesalongthelandscapegradientand(inset)relativebelow-groundbiomassremainingovertimeBarsrepresentmeans(n=18)anderrorbarsrepresentSEsCapitallettersdenotestatisticalsignificanceofpost-hocmultiplecomparisonsamongasymptotemeansfrombothabove-andbelow-groundpoolsLowercaselettersdenotestatisticalsignificanceofpost-hocmultiplecomparisonsamongdecayconstantmeansfrombothabove-andbelow-groundpools(FisherrsquosProtectedLSDα=005)

(a)

(b)

662emsp |emsp emspenspJournal of Ecology STAGG eT Al

sea-level rise can impact soil organic matter decomposition indi-rectly through changes in internal drivers such as plant commu-nity composition and litter qualitywhich control litter decay rate(StaggSchoolmasterKraussetal2017) Inthisstudywefoundthatdespitesignificantvariationinenvironmentalconditionsalongthisgradientexternaldriversincludingporewatersalinityandflooddurationhadnosignificantpredictivecapacityforeitherabove-orbelow-ground litterdecompositionThereforeour results indicatethattheindirectpathwaymediatedthroughchangesinlitterqualityan internaldriverwasmore important incontrolling litterdecom-positionthandirecteffectsofexternaldriversalongthislandscapegradient

Previous research has documented variation in lignin contentamongdifferentwetlandplantspecies(Buth1987GuoLuTongampGuohua2008)andourmeasurementsoflitterlignincontentforfresholigohalinemesohalineandpolyhalinespeciesweresimilartothosereported in the literature (Table3) However decay constants frommixed-specieslittermaterialwilldiffersignificantlycomparedtolittercomprisedofasinglespecies(ChapmanNewmanHartSchweitzerampKoch2013)Thereforeinsitumeasurementsoflittermixturesthatrepresentthevegetationcommunityarecriticalforachievingaccurateestimates of litter decay and identifying patterns of decompositionamongdifferentwetlandtypes

Because lignincontentcandiffersignificantlybetweentwospe-cieswithin the samewetland type or salinity zone it is difficult toidentifyuniversal patternsof lignin content alonga landscape-scalesalinity gradient Furthermore changes in plant diversity along thelandscapegradientmayimpactratesofdecompositionOdum(1988)observedgreaterplantdiversity infreshmarshescomparedtomoresalinemarsheswhichmayaffecttheproportionofdifferentspeciesand overall litter quality in a litter mixture Very few studies havecompared litter quality and decomposition along a salinity gradientthatincorporateschangesincomposition(GallagherKibbyampSkirvin

1984LopesMartinsRicardoRodriguesampQuintino2011ScartonDayampRismondo2002Windham2001)andasfarasweknowthisisthefirststudytoquantifylitterdecompositiondynamicsofdiffer-entwetlandtypesacrosstheentirecoastalmarsh landscapesalinitygradientOdumrsquos (1988) comparative review of freshvs polyhalinetidal marshes reports that freshwater macrophytes from the lowerintertidal zone such asSagittaria latifolia tend to have lower lignincontenthighernitrogencontentandhigherdecayratescomparedto

F IGURE 3emspCellulosedecayamongdifferentwetlandtypesBarsrepresentmeans(n=540)anderrorbarsrepresentSEsLettersdenotestatisticalsignificanceofpost-hocmultiplecomparisonsofmeans(FisherrsquosProtectedLSDα=005)

F IGURE 4emspPrincipalComponentAnalysisbiplotsofobservationprojectionsorfactorscoresincomponentspaceforallcomparisonsbetween(a)principalcomponent(PC)1andPC2(b)PC1andPC3and(c)PC2andPC3IneachplotfactorscoresarecolouredbywetlandtypeAsubsetofhighlycorrelatedvectorsfromeachPCoverlaythefactorscores

minus6 minus4 minus2 0 2 4 6

minus6minus4

minus20

24

6

PC

2 (L

igni

n)

FreshOligohaline Mesohaline Polyhaline

minus6 minus4 minus2 0 2 4 6

minus6minus4

minus20

24

6

PC

3 (F

lood

ing)

FreshOligohaline Mesohaline Polyhaline

minus6 minus4 minus2 0 2 4 6

minus6minus4

minus20

24

6

PC2 (Lignin)

PC

3 (F

lood

ing)

FreshOligohaline Mesohaline Polyhaline

(a)

(b)

(c)

PC1 ( )

PC1 ( )

emspensp emsp | emsp663Journal of EcologySTAGG eT Al

polyhalineorsaltmarshspeciesIncontrastfreshwatermacrophytesfrom the upper intertidal such asT latifolia more closely resemble typicalsaltmarshplantsintheirligninandnitrogencontentandexhibitslowerratesofdecaythanplantsfromthelowerintertidalfreshzoneInsupportofOdumrsquosconclusionswefoundthatlitterdecompositionwas greatest in the oligohalinemarsheswhichwere dominated byS lancifolia and S americanusandalsohadthelowestlignincontentThefreshmarshdominatedbyT latifolia and P hemitomon was similar inlignincontentanddecayratetothemesohalinemarshesdominatedby S patensandthepolyhalinemarshesdominatedbyS alterniflora and J roemerianus Thus although landscape-scale salinity patternsoflignincontentandlitterdecompositionarelargelyspecies-specific

(AertsampdeCaluwe1997)itisclearthatsea-levelrisehasthepoten-tialtoalterthequalityofcarbonandindirectlyimpactdecompositionalongthisgradient

Similarlyinitiallitterqualityintheformofnitrogencontentwasthebestpredictorof above-ground litter decomposition (MarinucciHobbie amp Helfrich 1983 Taylor Parkinson amp Parsons 1989)Althoughsomeresearchidentifiestheratiooflignin-to-nitrogeninini-tiallitterqualityasanimportantpredictorofdecomposition(MelilloAberampMuratore1982Valielaetal1984)ourresultsindicatethatonlyoneofthesevariableswasasignificantpredictoreithernitrogencontent or lignin content of above- or below-ground litter decom-positionrespectivelyLikewiseastudybyMelilloNaimanAberandEshleman(1983)identifiedeitherligninaloneorincombinationwithnitrogencontentasasignificantpredictorofdecompositionThesig-nificanceofonepredictoroveranothermaybeduetointeractionsbe-tweeninitiallitterqualityandenvironmentalconditionsIngeneralifexogenousnitrogen(egfromsoilorwatercolumn)isreadilyavailabletomicrobesandorinitiallitterlignincontentishightheninitiallitternitrogencontentmayhave little impactondecayrate (Melilloetal1982 1984) Interestingly despite these differences among above-andbelow-groundlittercontentthepatternofdecayalongtheland-scapegradientwassimilarwiththegreatestdecompositionoccurringin the oligohaline marsh Although salinity is a known regulator ofplantnitrogendynamics (BradleyampMorris1991Morris1980)wedidnotobserveasimplelineardeclineinlitterqualitywithincreasingsalinityThusitisclearthatlitterdecompositioniscontrolledthroughan indirectpathwaymediatedby internaldriversbutmoreresearchisneededtoidentifytheultimatedriversthatgenerateoptimallitterqualityforenhanceddecayintheoligohalinemarsh

To characterize the direct effects of sea-level rise ondecompo-sitionwe controlled for the influenceof litter qualitybymeasuringthe decay rate of a standard carbon source cellulose across thelandscape-scalesalinitygradient(Mendelssohnetal1999)Celluloseisalabilecompoundthatrapidlydecaysduringtheinitialphasesofde-compositionandservesasareadilyavailablefuelformicrobialactivity

TABLE 2emspResultsofmultiplelinearregressionanalysisforlitterdecompositionandcellulosedecay

Response variable Predictive variable Parameter estimate SE t- value p- value Model R2

Above-groundlitter PC1 minus177E-05 565E-05 minus0314 755 121

Decomposition PC3 minus495E-05 131E-04 minus0378 707

LeafN 00003 104E-03 268 009

LeafLignin minus736E-05 218E-04 minus0338 736

Below-groundlitter PC1 431E-05 338E-05 127 207 395

Decomposition PC3 507E-05 676E-05 0750 456

RootN 353E-04 313E-04 113 265

RootLignin minus217E-04 363E-05 minus598 110E-07

Above-ground PC1 0042 0032 130 198 027

Cellulose decay PC3 minus0021 0075 minus0280 780

Below-ground PC1 0039 0012 324 002 195

Cellulose decay PC3 0066 0028 236 021

plt05plt01plt001

F IGURE 5emspVariationinsoilredoxpotentialalonglandscapegradientBoxplotboundariesclosesttozerorepresentthe25thpercentilethelinewithintheboxesindicatesthemedianandboundariesfarthestfromzerorepresentthe75thpercentile(n=90)Whiskersindicatethe90thand10thpercentilesBlackdotsrepresentoutlyingpointsLettersdenotestatisticalsignificanceofpost-hocmultiplecomparisonsofmeans(FisherrsquosProtectedLSDα=005)

664emsp |emsp emspenspJournal of Ecology STAGG eT Al

(Hodson Chrsitian amp Maccubbin 1984) Therefore in addition toidentifyingexternalcontrolsondecaywewerealsoabletocharac-terizethedecompositiondynamicsoftheisolatedlabilecarbonpool

We found that below-ground cellulosedecay increasedwith in-creasingsalinitywhichsupportsfindingsfromrecentstudiesthatsoilrespirationisstimulatedbysalinityinshort-termexposuresthatdonotincorporatechangesincarbonsource(Chambersetal2011Westonetal2006)Whileincreasingsalinitycanhavedirectimpactsonor-ganicmattermineralizationthroughalteringthesoilchemicalcompo-sitionandreleasingpreviouslysoil-boundorganiccarbon(DouPingGuoampJorgenson2005)thepatternofcellulosedecayalongthissa-linitygradientwaslikelyinfluencedbydifferencesamongthemicrobialcommunities(Chambersetal2013)Assalinitycontinuestoincreasesulphate reducersout-competemethanogensandoverall anaerobicmetabolism isgreaterwhensulphate is thedominant terminalelec-tronacceptor(Sutton-GrierKellerKochGilmourampMegonigal2011Westonetal2006)

In contrast to salinity soil nutrients were negatively correlatedwith below-ground cellulose decay Although Mendelssohn etal

(1999)foundthatsoilnutrientshadasignificantpositiveinfluenceoncellulose decay in a Phragmites australis-dominatedwetlandthistrendisnotuniversalamongallwetlandtypesForexamplethereviewbyRybczyk Garson andDay (1996) illustrates varying impacts of soilnutrientsonsoilorganicmatterdecompositionEvensoitisunlikelythathighsoilnutrientswoulddirectlyinhibitcellulosedecayRatherweproposethatthestimulatoryeffectofelevatedsalinityandgreatersulphate availability overcame the potential negative effects of lowsoilnutrientconcentrations

Floodingwasalsoa strongpredictorofdecomposition andhada direct positive effect on below-ground cellulose decay Althoughgreaterflooddurationcanleadtoanaerobicsoilconditions(GambrellampPatrick1978Ponnamperuma1984)whichcan limit the rateofdecomposition (Day amp Megonigal 1993 McKee amp Seneca 1982WhiteampTrapani1982)decompositionisnotalwaysslowerunderan-aerobicconditionsForexampleKirwanLangleyGuntenspergenand

F IGURE 6emspLinearregressionof(a)above-groundlitterdecompositionandlitternitrogencontentand(b)below-groundlitterdecompositionandlitterlignincontentFresholigohalinemesohalineandpolyhalinewetlandtypesdifferentiatedbyshapesymbols

F IGURE 7emspLinearregressionofbelow-groundcellulosedecayand(a)PC1(Physico-chemical)and(b)PC3(Flooding)Fresholigohalinemesohalineandpolyhalinewetlandtypesdifferentiatedbyshapesymbols

(a)

(b)

emspensp emsp | emsp665Journal of EcologySTAGG eT Al

Megonigal(2013)observedastimulatoryfloodingeffectonrootandrhizomedecompositionFurthermore soil redoxpotentialalong thislandscapegradientwasmorecloselycorrelatedwithporewatersalin-itythanwithfloodduration(PwECR2=minus62plt0001AnnFloodR2=minus39p=01) indicating that the stimulating effect of floodingwasnotstronglyassociatedwithanaerobicconditionsThereforeweproposethatthepositiverelationshipbetweenfloodingandcellulosedecayillustratesthewell-documentedobservationthatmicrobialac-tivity ismoisture-limited (FrascoampGoode 1982HalupaampHowes

1995NewellArsuffiampPalm1996ReiceampStiven1983)at leastinthetop20cmofthesoilprofileAtfurtherdepthscellulosedecaybelowthesoilsurfacewaslikelyoxygenlimitedassoilsbecomemorereduced with increasing depth (Maltby 1988 Schipper amp Reddy1995)Similartoourfindingslowoxygenavailabilityhasbeeniden-tifiedasaprimaryinhibitorofcellulosedecayatdepthsbelow22cm(Mendelssohnetal1999)

Surprisingly none of the environmental drivers we measuredhad predictive capacity for above-ground cellulose decayAlthough

TABLE 3emspLitterlignincontentofdominantspeciesfromeachwetlandtypeLitterconditionidentifiedaslive(L)dead(D)orunknown(U)

Species Litter type Lignin content () Study

Typha latifolia Leaves 122L Currentstudy

58L MoranampHodson(1989)

395ndash427D WelschampYavitt(2003)

182D PoideNeiffNeiffampCasco (2006)

Roots 73L Currentstudy

Panicum hemitomon Leaves 64ndash73L Currentstudy

64L MoranampHodson(1989)

59D OsborneInglettampReddy(2007)

Roots 88ndash122L Currentstudy

Sagittaria lancifolia Leaves 77ndash77L Currentstudy

18U Laursen(2004)

Roots 54ndash74L Currentstudy

26U Laursen(2004)

Schoenoplectus americanus Leaves 79ndash89L Currentstudy

205D BallampDrake(1997)

Roots 47ndash75L Currentstudy

1443ndash2650L SaundersMegonigalampReynolds(2006)

Spartina patens Leaves 78ndash92L Currentstudy

142D BallampDrake(1997)

Roots 148ndash186L Currentstudy

2695ndash3041L Saundersetal(2006)

Juncus roemerianus Leaves 99L Currentstudy

6U Benneretal(1987)

Roots 137L Currentstudy

51U Benneretal(1987)

Spartina alterniflora Leaves 55ndash57L Currentstudy

151L MaccubbinampHodson(1980)

131ndash168L Hodsonetal(1984)

117U Wilson(1985)

11ndash12D WilsonBuchsbaumValielaampSwain(1986)

43ndash61U Benneretal(1987)

Roots 137ndash152L Currentstudy

122ndash193U Hodsonetal(1984)

93U Benneretal(1987)

666emsp |emsp emspenspJournal of Ecology STAGG eT Al

above-groundcellulosedecayvariedsignificantlyacrossthelandscape-scalesalinitygradient(higherinpolyhalinemarsh)salinitypersewasnotasignificantpredictorofdecay(sensuMendelssohnetal1999)Several recentstudieshave illustratedthatsalinitycan indirectlyaf-fect soil respiration throughchanges inmicrobial function resultinginhigherratesofcarbonmineralizationathighersalinities(Chambersetal2011Neubauer2013Sutton-Grieretal2011Westonetal2006)Thuswehypothesizethatanunidentifiedmediatingfactorthatvarieswithwetlandtypesuchasmicrobialfunction(CaponeampKiene1988) is regulating cellulose decayWe suggest that future studiesinclude focusedmeasuresofmicrobial structure and function alongthislandscape-scalegradienttoconfirmthemechanismofindirectef-fectsofsalinityoncellulosedecay(HopfenspergerBurginSchoepferampHelton2014Morriseyetal2014NeubauerGivlerValentineampMegonigal2005)

Insummaryourstudyshowedthat the indirecteffectofsea-levelriseonlitterdecompositionwasmediatedthroughchangesinplantcommunitycompositionand litterqualitywhich resulted indecliningratesoflitterdecompositionalongthegradientfromoli-gohalinetopolyhalinemarshesHoweverwhenwecontrolledforchangesinlitterqualitywefoundthatincreasingsalinityandflood-ingstimulateddecayoflabilecarbon(cellulose)Wehaveidentifiedtwomechanismsoforganicmatterlossoperatingatdifferenttem-poralscalesthatprovideinsighttothepotentialforlong-termcar-bonstorageassea-levelrisesOurresultsindicatethatassealevelrises initial direct effectsof salinitywill stimulatedecayof labilecarbonbutovertimeasvegetationcommunitycompositionshiftstomoresalinewetlandtypes litterdecay (decayofrefractilecar-bon)willdeclineyieldinggreaterpotential for long-termsoilcar-bonstoragethroughnetaccretion(LoomisampCraft2012)Recentstudieshavehighlightedtheimportanceofscaleinconsideringtheinfluence of sea-level rise on carbon loss fromwetland systems(Herbertetal2015Neubaueretal2013)Ourresearchprovidesanexampleofhowmultiplemechanismsofcarbon lossoperatingatdifferentscalescanyielddifferentratesandpatternsoforganicmatter decompositionwhich is relevant not only in coastalwet-landsbutalsoinotherecosystemssuchasgrasslandsshrublandsandforestswhereplant-mediatedresponsestoclimatechangewillhavesignificant impactsoncarbondynamics (JobbagyampJackson2000)

ACKNOWLEDGEMENTS

The authors acknowledge themany people who assisted in fieldsamplecollectionandsampleprocessingAdaDizEvanBergeronKelly Darnell Lindsey Hebert Samantha Humphrey JamalMathurinAlyssaMitchellLelandMossBrettPattonSaraiPiazzaCaitlinPinsonatJacyReynoldsJacksonRollingsandRachelVillaniWe thank Thomas Blanchard and Sara Gay at Louisiana StateUniversityWetlandBiogeochemistryAnalyticalServicesandUttamKumarSahaandDavidParksatUniversityofGeorgiaCooperativeExtension for performing chemical analyses We are grateful toChristopherSwarzenski forhishelp insiteselectionBrettPatton

for rectifyingelevationdatasetsLelandMoss fordevelopmentofFigure1LaurenLeonpacherfordevelopmentofTable3andmanu-scripteditingWealsothankJoshuaJonesChristopherSwarzenskiandanonymouspeersfortheirreviewofthismanuscriptThisre-searchwasfundedbytheUSGeologicalSurveyClimateResearchand Development Program US Geological Survey EcosystemsProgram the US Geological Survey LandCarbon program theScienceandEngineeringProgramofTheWaterInstituteoftheGulfwith funds fromtheLouisianaCoastalProtectionandRestorationAuthority (CPRA) and the Baton Rouge Area Foundation (BRAF)andtheRestoretheMississippiRiverDeltaCampaignviaCoalitionto Restore Coastal Louisiana (CRCL) Any use of trade firm orproductnamesisfordescriptivepurposesonlyanddoesnotimply endorsementbytheUSGovernment

AUTHORrsquoS CONTRIBUTIONS

CLSMMBandCLPconceivedtheideasanddesignedmethod-ology CLSMMB CLP TJBC andCTH collected the dataCLSMMBandCTHanalysedthedataCLSledthewritingofthe manuscript All authors contributed critically to the drafts andgavefinalapprovalforpublication

DATA ACCESSIBILITY

Allof thesupportingdatapresented in thispaperhavebeenpubli-callyarchivedatsciencebasegovhttpsdoiorg105066f7639mvk(StaggBaustianPerryCarruthersampHall2017)

ORCID

Camille L Stagg httporcidorg0000-0002-1125-7253

Melissa M Baustian httporcidorg0000-0003-2467-2533

Courtney T Hall httporcidorg0000-0003-0990-5212

REFERENCES

AertsR(1997)ClimateleaflitterchemistryandleaflitterdecompositioninterrestrialecosystemsAtriangularrelationshipOikos 79439ndash449httpsdoiorg1023073546886

AertsRampdeCaluweH(1997)Nutritionalandplant-mediatedcontrolson leaf litterdecompositionofCarex speciesEcology 78 244ndash260httpsdoiorg1018900012-9658(1997)078[0244NAPMCO]20 CO2

Alward R Detling J amp Milchunas D (1999) Grassland vegetationchangesandnocturnalglobalwarmingScience 283118ndash231httpsdoiorg101126science2835399229

AmericanPublicHealthAssociation(2005a)3030EnitricaciddigestionInADEatonampMAFranson(Eds)Standard methods for the exam-ination of water and wastewater (21stedpp325ndash326)WashingtonDCAmericanPublicHealthAssociation

AmericanPublicHealthAssociation(2005b)3120metalsbyplasmaemis-sionspectroscopy(85)InADEatonampMAFranson(Eds)Standard methods for the examination of water and wastewater(21stedpp377ndash386)WashingtonDCAmericanPublicHealthAssociation

emspensp emsp | emsp661Journal of EcologySTAGG eT Al

types andwas highest in the freshmarsh and lowest in themeso-halineandpolyhalinemarshes(plt0001df = 3 F=5282Figure5)Additionallyredoxpotentialwashighlycorrelatedwithseveralwell-characterized parameters associated with PC1 (Physico-chemical)such as porewater EC and porewater sulphur (R2=minus63 plt0001R2=minus46p=002respectively)

33emsp|emspMultiple regression analysis

Initialnitrogencontentwas theonly significantpredictorofabove-groundlitterdecomposition(Table2)whichincreasedwithincreasinglitternitrogencontent(Figure6a)Initiallignincontentwasasignifi-cantpredictorofbelow-groundlitterdecomposition(Table2)whichdeclinedwithincreasinglignincontent(Figure6b)

NeitherPC1(Physico-chemical)norPC3(Flooding)hadasignifi-canteffectonabove-groundcellulosedecay(Table2)IncontrastPC1(Physico-chemical) andPC3 (Flooding)were importantpredictorsof

below-groundcellulosedecay(Table2)whichincreasedwithincreas-ingsalinityand floodinganddecreasingsoilnutrientconcentrations(Figure7ab)

4emsp |emspDISCUSSION

To improveourunderstandingofhoworganicmatterdecomposi-tionandthefateofcarbonincoastalwetlandswillbeimpactedbysea-level rise we identified the internal and external drivers thatinfluencedecompositionincoastalwetlandsacrossanestuarinesa-linitygradientspanningfreshtopolyhalinewetlandtypes Internaldriverssuchaslitterqualityarecharacteristicsoftheorganicmat-ter itselfwhereas external drivers such as hydrologic conditionsare characteristics of the environment (Aerts 1997 Webster ampBenfield 1986) and their effects on decomposition may be ex-pressed through both direct and indirect pathways For example

F IGURE 2emspEstimatesof(a)above-groundlitterdecayconstantsandasymptotesalongthelandscapegradientand(inset)relativeabove-groundbiomassremainingovertime(b)below-groundlitterdecayconstantsandasymptotesalongthelandscapegradientand(inset)relativebelow-groundbiomassremainingovertimeBarsrepresentmeans(n=18)anderrorbarsrepresentSEsCapitallettersdenotestatisticalsignificanceofpost-hocmultiplecomparisonsamongasymptotemeansfrombothabove-andbelow-groundpoolsLowercaselettersdenotestatisticalsignificanceofpost-hocmultiplecomparisonsamongdecayconstantmeansfrombothabove-andbelow-groundpools(FisherrsquosProtectedLSDα=005)

(a)

(b)

662emsp |emsp emspenspJournal of Ecology STAGG eT Al

sea-level rise can impact soil organic matter decomposition indi-rectly through changes in internal drivers such as plant commu-nity composition and litter qualitywhich control litter decay rate(StaggSchoolmasterKraussetal2017) Inthisstudywefoundthatdespitesignificantvariationinenvironmentalconditionsalongthisgradientexternaldriversincludingporewatersalinityandflooddurationhadnosignificantpredictivecapacityforeitherabove-orbelow-ground litterdecompositionThereforeour results indicatethattheindirectpathwaymediatedthroughchangesinlitterqualityan internaldriverwasmore important incontrolling litterdecom-positionthandirecteffectsofexternaldriversalongthislandscapegradient

Previous research has documented variation in lignin contentamongdifferentwetlandplantspecies(Buth1987GuoLuTongampGuohua2008)andourmeasurementsoflitterlignincontentforfresholigohalinemesohalineandpolyhalinespeciesweresimilartothosereported in the literature (Table3) However decay constants frommixed-specieslittermaterialwilldiffersignificantlycomparedtolittercomprisedofasinglespecies(ChapmanNewmanHartSchweitzerampKoch2013)Thereforeinsitumeasurementsoflittermixturesthatrepresentthevegetationcommunityarecriticalforachievingaccurateestimates of litter decay and identifying patterns of decompositionamongdifferentwetlandtypes

Because lignincontentcandiffersignificantlybetweentwospe-cieswithin the samewetland type or salinity zone it is difficult toidentifyuniversal patternsof lignin content alonga landscape-scalesalinity gradient Furthermore changes in plant diversity along thelandscapegradientmayimpactratesofdecompositionOdum(1988)observedgreaterplantdiversity infreshmarshescomparedtomoresalinemarsheswhichmayaffecttheproportionofdifferentspeciesand overall litter quality in a litter mixture Very few studies havecompared litter quality and decomposition along a salinity gradientthatincorporateschangesincomposition(GallagherKibbyampSkirvin

1984LopesMartinsRicardoRodriguesampQuintino2011ScartonDayampRismondo2002Windham2001)andasfarasweknowthisisthefirststudytoquantifylitterdecompositiondynamicsofdiffer-entwetlandtypesacrosstheentirecoastalmarsh landscapesalinitygradientOdumrsquos (1988) comparative review of freshvs polyhalinetidal marshes reports that freshwater macrophytes from the lowerintertidal zone such asSagittaria latifolia tend to have lower lignincontenthighernitrogencontentandhigherdecayratescomparedto

F IGURE 3emspCellulosedecayamongdifferentwetlandtypesBarsrepresentmeans(n=540)anderrorbarsrepresentSEsLettersdenotestatisticalsignificanceofpost-hocmultiplecomparisonsofmeans(FisherrsquosProtectedLSDα=005)

F IGURE 4emspPrincipalComponentAnalysisbiplotsofobservationprojectionsorfactorscoresincomponentspaceforallcomparisonsbetween(a)principalcomponent(PC)1andPC2(b)PC1andPC3and(c)PC2andPC3IneachplotfactorscoresarecolouredbywetlandtypeAsubsetofhighlycorrelatedvectorsfromeachPCoverlaythefactorscores

minus6 minus4 minus2 0 2 4 6

minus6minus4

minus20

24

6

PC

2 (L

igni

n)

FreshOligohaline Mesohaline Polyhaline

minus6 minus4 minus2 0 2 4 6

minus6minus4

minus20

24

6

PC

3 (F

lood

ing)

FreshOligohaline Mesohaline Polyhaline

minus6 minus4 minus2 0 2 4 6

minus6minus4

minus20

24

6

PC2 (Lignin)

PC

3 (F

lood

ing)

FreshOligohaline Mesohaline Polyhaline

(a)

(b)

(c)

PC1 ( )

PC1 ( )

emspensp emsp | emsp663Journal of EcologySTAGG eT Al

polyhalineorsaltmarshspeciesIncontrastfreshwatermacrophytesfrom the upper intertidal such asT latifolia more closely resemble typicalsaltmarshplantsintheirligninandnitrogencontentandexhibitslowerratesofdecaythanplantsfromthelowerintertidalfreshzoneInsupportofOdumrsquosconclusionswefoundthatlitterdecompositionwas greatest in the oligohalinemarsheswhichwere dominated byS lancifolia and S americanusandalsohadthelowestlignincontentThefreshmarshdominatedbyT latifolia and P hemitomon was similar inlignincontentanddecayratetothemesohalinemarshesdominatedby S patensandthepolyhalinemarshesdominatedbyS alterniflora and J roemerianus Thus although landscape-scale salinity patternsoflignincontentandlitterdecompositionarelargelyspecies-specific

(AertsampdeCaluwe1997)itisclearthatsea-levelrisehasthepoten-tialtoalterthequalityofcarbonandindirectlyimpactdecompositionalongthisgradient

Similarlyinitiallitterqualityintheformofnitrogencontentwasthebestpredictorof above-ground litter decomposition (MarinucciHobbie amp Helfrich 1983 Taylor Parkinson amp Parsons 1989)Althoughsomeresearchidentifiestheratiooflignin-to-nitrogeninini-tiallitterqualityasanimportantpredictorofdecomposition(MelilloAberampMuratore1982Valielaetal1984)ourresultsindicatethatonlyoneofthesevariableswasasignificantpredictoreithernitrogencontent or lignin content of above- or below-ground litter decom-positionrespectivelyLikewiseastudybyMelilloNaimanAberandEshleman(1983)identifiedeitherligninaloneorincombinationwithnitrogencontentasasignificantpredictorofdecompositionThesig-nificanceofonepredictoroveranothermaybeduetointeractionsbe-tweeninitiallitterqualityandenvironmentalconditionsIngeneralifexogenousnitrogen(egfromsoilorwatercolumn)isreadilyavailabletomicrobesandorinitiallitterlignincontentishightheninitiallitternitrogencontentmayhave little impactondecayrate (Melilloetal1982 1984) Interestingly despite these differences among above-andbelow-groundlittercontentthepatternofdecayalongtheland-scapegradientwassimilarwiththegreatestdecompositionoccurringin the oligohaline marsh Although salinity is a known regulator ofplantnitrogendynamics (BradleyampMorris1991Morris1980)wedidnotobserveasimplelineardeclineinlitterqualitywithincreasingsalinityThusitisclearthatlitterdecompositioniscontrolledthroughan indirectpathwaymediatedby internaldriversbutmoreresearchisneededtoidentifytheultimatedriversthatgenerateoptimallitterqualityforenhanceddecayintheoligohalinemarsh

To characterize the direct effects of sea-level rise ondecompo-sitionwe controlled for the influenceof litter qualitybymeasuringthe decay rate of a standard carbon source cellulose across thelandscape-scalesalinitygradient(Mendelssohnetal1999)Celluloseisalabilecompoundthatrapidlydecaysduringtheinitialphasesofde-compositionandservesasareadilyavailablefuelformicrobialactivity

TABLE 2emspResultsofmultiplelinearregressionanalysisforlitterdecompositionandcellulosedecay

Response variable Predictive variable Parameter estimate SE t- value p- value Model R2

Above-groundlitter PC1 minus177E-05 565E-05 minus0314 755 121

Decomposition PC3 minus495E-05 131E-04 minus0378 707

LeafN 00003 104E-03 268 009

LeafLignin minus736E-05 218E-04 minus0338 736

Below-groundlitter PC1 431E-05 338E-05 127 207 395

Decomposition PC3 507E-05 676E-05 0750 456

RootN 353E-04 313E-04 113 265

RootLignin minus217E-04 363E-05 minus598 110E-07

Above-ground PC1 0042 0032 130 198 027

Cellulose decay PC3 minus0021 0075 minus0280 780

Below-ground PC1 0039 0012 324 002 195

Cellulose decay PC3 0066 0028 236 021

plt05plt01plt001

F IGURE 5emspVariationinsoilredoxpotentialalonglandscapegradientBoxplotboundariesclosesttozerorepresentthe25thpercentilethelinewithintheboxesindicatesthemedianandboundariesfarthestfromzerorepresentthe75thpercentile(n=90)Whiskersindicatethe90thand10thpercentilesBlackdotsrepresentoutlyingpointsLettersdenotestatisticalsignificanceofpost-hocmultiplecomparisonsofmeans(FisherrsquosProtectedLSDα=005)

664emsp |emsp emspenspJournal of Ecology STAGG eT Al

(Hodson Chrsitian amp Maccubbin 1984) Therefore in addition toidentifyingexternalcontrolsondecaywewerealsoabletocharac-terizethedecompositiondynamicsoftheisolatedlabilecarbonpool

We found that below-ground cellulosedecay increasedwith in-creasingsalinitywhichsupportsfindingsfromrecentstudiesthatsoilrespirationisstimulatedbysalinityinshort-termexposuresthatdonotincorporatechangesincarbonsource(Chambersetal2011Westonetal2006)Whileincreasingsalinitycanhavedirectimpactsonor-ganicmattermineralizationthroughalteringthesoilchemicalcompo-sitionandreleasingpreviouslysoil-boundorganiccarbon(DouPingGuoampJorgenson2005)thepatternofcellulosedecayalongthissa-linitygradientwaslikelyinfluencedbydifferencesamongthemicrobialcommunities(Chambersetal2013)Assalinitycontinuestoincreasesulphate reducersout-competemethanogensandoverall anaerobicmetabolism isgreaterwhensulphate is thedominant terminalelec-tronacceptor(Sutton-GrierKellerKochGilmourampMegonigal2011Westonetal2006)

In contrast to salinity soil nutrients were negatively correlatedwith below-ground cellulose decay Although Mendelssohn etal

(1999)foundthatsoilnutrientshadasignificantpositiveinfluenceoncellulose decay in a Phragmites australis-dominatedwetlandthistrendisnotuniversalamongallwetlandtypesForexamplethereviewbyRybczyk Garson andDay (1996) illustrates varying impacts of soilnutrientsonsoilorganicmatterdecompositionEvensoitisunlikelythathighsoilnutrientswoulddirectlyinhibitcellulosedecayRatherweproposethatthestimulatoryeffectofelevatedsalinityandgreatersulphate availability overcame the potential negative effects of lowsoilnutrientconcentrations

Floodingwasalsoa strongpredictorofdecomposition andhada direct positive effect on below-ground cellulose decay Althoughgreaterflooddurationcanleadtoanaerobicsoilconditions(GambrellampPatrick1978Ponnamperuma1984)whichcan limit the rateofdecomposition (Day amp Megonigal 1993 McKee amp Seneca 1982WhiteampTrapani1982)decompositionisnotalwaysslowerunderan-aerobicconditionsForexampleKirwanLangleyGuntenspergenand

F IGURE 6emspLinearregressionof(a)above-groundlitterdecompositionandlitternitrogencontentand(b)below-groundlitterdecompositionandlitterlignincontentFresholigohalinemesohalineandpolyhalinewetlandtypesdifferentiatedbyshapesymbols

F IGURE 7emspLinearregressionofbelow-groundcellulosedecayand(a)PC1(Physico-chemical)and(b)PC3(Flooding)Fresholigohalinemesohalineandpolyhalinewetlandtypesdifferentiatedbyshapesymbols

(a)

(b)

emspensp emsp | emsp665Journal of EcologySTAGG eT Al

Megonigal(2013)observedastimulatoryfloodingeffectonrootandrhizomedecompositionFurthermore soil redoxpotentialalong thislandscapegradientwasmorecloselycorrelatedwithporewatersalin-itythanwithfloodduration(PwECR2=minus62plt0001AnnFloodR2=minus39p=01) indicating that the stimulating effect of floodingwasnotstronglyassociatedwithanaerobicconditionsThereforeweproposethatthepositiverelationshipbetweenfloodingandcellulosedecayillustratesthewell-documentedobservationthatmicrobialac-tivity ismoisture-limited (FrascoampGoode 1982HalupaampHowes

1995NewellArsuffiampPalm1996ReiceampStiven1983)at leastinthetop20cmofthesoilprofileAtfurtherdepthscellulosedecaybelowthesoilsurfacewaslikelyoxygenlimitedassoilsbecomemorereduced with increasing depth (Maltby 1988 Schipper amp Reddy1995)Similartoourfindingslowoxygenavailabilityhasbeeniden-tifiedasaprimaryinhibitorofcellulosedecayatdepthsbelow22cm(Mendelssohnetal1999)

Surprisingly none of the environmental drivers we measuredhad predictive capacity for above-ground cellulose decayAlthough

TABLE 3emspLitterlignincontentofdominantspeciesfromeachwetlandtypeLitterconditionidentifiedaslive(L)dead(D)orunknown(U)

Species Litter type Lignin content () Study

Typha latifolia Leaves 122L Currentstudy

58L MoranampHodson(1989)

395ndash427D WelschampYavitt(2003)

182D PoideNeiffNeiffampCasco (2006)

Roots 73L Currentstudy

Panicum hemitomon Leaves 64ndash73L Currentstudy

64L MoranampHodson(1989)

59D OsborneInglettampReddy(2007)

Roots 88ndash122L Currentstudy

Sagittaria lancifolia Leaves 77ndash77L Currentstudy

18U Laursen(2004)

Roots 54ndash74L Currentstudy

26U Laursen(2004)

Schoenoplectus americanus Leaves 79ndash89L Currentstudy

205D BallampDrake(1997)

Roots 47ndash75L Currentstudy

1443ndash2650L SaundersMegonigalampReynolds(2006)

Spartina patens Leaves 78ndash92L Currentstudy

142D BallampDrake(1997)

Roots 148ndash186L Currentstudy

2695ndash3041L Saundersetal(2006)

Juncus roemerianus Leaves 99L Currentstudy

6U Benneretal(1987)

Roots 137L Currentstudy

51U Benneretal(1987)

Spartina alterniflora Leaves 55ndash57L Currentstudy

151L MaccubbinampHodson(1980)

131ndash168L Hodsonetal(1984)

117U Wilson(1985)

11ndash12D WilsonBuchsbaumValielaampSwain(1986)

43ndash61U Benneretal(1987)

Roots 137ndash152L Currentstudy

122ndash193U Hodsonetal(1984)

93U Benneretal(1987)

666emsp |emsp emspenspJournal of Ecology STAGG eT Al

above-groundcellulosedecayvariedsignificantlyacrossthelandscape-scalesalinitygradient(higherinpolyhalinemarsh)salinitypersewasnotasignificantpredictorofdecay(sensuMendelssohnetal1999)Several recentstudieshave illustratedthatsalinitycan indirectlyaf-fect soil respiration throughchanges inmicrobial function resultinginhigherratesofcarbonmineralizationathighersalinities(Chambersetal2011Neubauer2013Sutton-Grieretal2011Westonetal2006)Thuswehypothesizethatanunidentifiedmediatingfactorthatvarieswithwetlandtypesuchasmicrobialfunction(CaponeampKiene1988) is regulating cellulose decayWe suggest that future studiesinclude focusedmeasuresofmicrobial structure and function alongthislandscape-scalegradienttoconfirmthemechanismofindirectef-fectsofsalinityoncellulosedecay(HopfenspergerBurginSchoepferampHelton2014Morriseyetal2014NeubauerGivlerValentineampMegonigal2005)

Insummaryourstudyshowedthat the indirecteffectofsea-levelriseonlitterdecompositionwasmediatedthroughchangesinplantcommunitycompositionand litterqualitywhich resulted indecliningratesoflitterdecompositionalongthegradientfromoli-gohalinetopolyhalinemarshesHoweverwhenwecontrolledforchangesinlitterqualitywefoundthatincreasingsalinityandflood-ingstimulateddecayoflabilecarbon(cellulose)Wehaveidentifiedtwomechanismsoforganicmatterlossoperatingatdifferenttem-poralscalesthatprovideinsighttothepotentialforlong-termcar-bonstorageassea-levelrisesOurresultsindicatethatassealevelrises initial direct effectsof salinitywill stimulatedecayof labilecarbonbutovertimeasvegetationcommunitycompositionshiftstomoresalinewetlandtypes litterdecay (decayofrefractilecar-bon)willdeclineyieldinggreaterpotential for long-termsoilcar-bonstoragethroughnetaccretion(LoomisampCraft2012)Recentstudieshavehighlightedtheimportanceofscaleinconsideringtheinfluence of sea-level rise on carbon loss fromwetland systems(Herbertetal2015Neubaueretal2013)Ourresearchprovidesanexampleofhowmultiplemechanismsofcarbon lossoperatingatdifferentscalescanyielddifferentratesandpatternsoforganicmatter decompositionwhich is relevant not only in coastalwet-landsbutalsoinotherecosystemssuchasgrasslandsshrublandsandforestswhereplant-mediatedresponsestoclimatechangewillhavesignificant impactsoncarbondynamics (JobbagyampJackson2000)

ACKNOWLEDGEMENTS

The authors acknowledge themany people who assisted in fieldsamplecollectionandsampleprocessingAdaDizEvanBergeronKelly Darnell Lindsey Hebert Samantha Humphrey JamalMathurinAlyssaMitchellLelandMossBrettPattonSaraiPiazzaCaitlinPinsonatJacyReynoldsJacksonRollingsandRachelVillaniWe thank Thomas Blanchard and Sara Gay at Louisiana StateUniversityWetlandBiogeochemistryAnalyticalServicesandUttamKumarSahaandDavidParksatUniversityofGeorgiaCooperativeExtension for performing chemical analyses We are grateful toChristopherSwarzenski forhishelp insiteselectionBrettPatton

for rectifyingelevationdatasetsLelandMoss fordevelopmentofFigure1LaurenLeonpacherfordevelopmentofTable3andmanu-scripteditingWealsothankJoshuaJonesChristopherSwarzenskiandanonymouspeersfortheirreviewofthismanuscriptThisre-searchwasfundedbytheUSGeologicalSurveyClimateResearchand Development Program US Geological Survey EcosystemsProgram the US Geological Survey LandCarbon program theScienceandEngineeringProgramofTheWaterInstituteoftheGulfwith funds fromtheLouisianaCoastalProtectionandRestorationAuthority (CPRA) and the Baton Rouge Area Foundation (BRAF)andtheRestoretheMississippiRiverDeltaCampaignviaCoalitionto Restore Coastal Louisiana (CRCL) Any use of trade firm orproductnamesisfordescriptivepurposesonlyanddoesnotimply endorsementbytheUSGovernment

AUTHORrsquoS CONTRIBUTIONS

CLSMMBandCLPconceivedtheideasanddesignedmethod-ology CLSMMB CLP TJBC andCTH collected the dataCLSMMBandCTHanalysedthedataCLSledthewritingofthe manuscript All authors contributed critically to the drafts andgavefinalapprovalforpublication

DATA ACCESSIBILITY

Allof thesupportingdatapresented in thispaperhavebeenpubli-callyarchivedatsciencebasegovhttpsdoiorg105066f7639mvk(StaggBaustianPerryCarruthersampHall2017)

ORCID

Camille L Stagg httporcidorg0000-0002-1125-7253

Melissa M Baustian httporcidorg0000-0003-2467-2533

Courtney T Hall httporcidorg0000-0003-0990-5212

REFERENCES

AertsR(1997)ClimateleaflitterchemistryandleaflitterdecompositioninterrestrialecosystemsAtriangularrelationshipOikos 79439ndash449httpsdoiorg1023073546886

AertsRampdeCaluweH(1997)Nutritionalandplant-mediatedcontrolson leaf litterdecompositionofCarex speciesEcology 78 244ndash260httpsdoiorg1018900012-9658(1997)078[0244NAPMCO]20 CO2

Alward R Detling J amp Milchunas D (1999) Grassland vegetationchangesandnocturnalglobalwarmingScience 283118ndash231httpsdoiorg101126science2835399229

AmericanPublicHealthAssociation(2005a)3030EnitricaciddigestionInADEatonampMAFranson(Eds)Standard methods for the exam-ination of water and wastewater (21stedpp325ndash326)WashingtonDCAmericanPublicHealthAssociation

AmericanPublicHealthAssociation(2005b)3120metalsbyplasmaemis-sionspectroscopy(85)InADEatonampMAFranson(Eds)Standard methods for the examination of water and wastewater(21stedpp377ndash386)WashingtonDCAmericanPublicHealthAssociation

662emsp |emsp emspenspJournal of Ecology STAGG eT Al

sea-level rise can impact soil organic matter decomposition indi-rectly through changes in internal drivers such as plant commu-nity composition and litter qualitywhich control litter decay rate(StaggSchoolmasterKraussetal2017) Inthisstudywefoundthatdespitesignificantvariationinenvironmentalconditionsalongthisgradientexternaldriversincludingporewatersalinityandflooddurationhadnosignificantpredictivecapacityforeitherabove-orbelow-ground litterdecompositionThereforeour results indicatethattheindirectpathwaymediatedthroughchangesinlitterqualityan internaldriverwasmore important incontrolling litterdecom-positionthandirecteffectsofexternaldriversalongthislandscapegradient

Previous research has documented variation in lignin contentamongdifferentwetlandplantspecies(Buth1987GuoLuTongampGuohua2008)andourmeasurementsoflitterlignincontentforfresholigohalinemesohalineandpolyhalinespeciesweresimilartothosereported in the literature (Table3) However decay constants frommixed-specieslittermaterialwilldiffersignificantlycomparedtolittercomprisedofasinglespecies(ChapmanNewmanHartSchweitzerampKoch2013)Thereforeinsitumeasurementsoflittermixturesthatrepresentthevegetationcommunityarecriticalforachievingaccurateestimates of litter decay and identifying patterns of decompositionamongdifferentwetlandtypes

Because lignincontentcandiffersignificantlybetweentwospe-cieswithin the samewetland type or salinity zone it is difficult toidentifyuniversal patternsof lignin content alonga landscape-scalesalinity gradient Furthermore changes in plant diversity along thelandscapegradientmayimpactratesofdecompositionOdum(1988)observedgreaterplantdiversity infreshmarshescomparedtomoresalinemarsheswhichmayaffecttheproportionofdifferentspeciesand overall litter quality in a litter mixture Very few studies havecompared litter quality and decomposition along a salinity gradientthatincorporateschangesincomposition(GallagherKibbyampSkirvin

1984LopesMartinsRicardoRodriguesampQuintino2011ScartonDayampRismondo2002Windham2001)andasfarasweknowthisisthefirststudytoquantifylitterdecompositiondynamicsofdiffer-entwetlandtypesacrosstheentirecoastalmarsh landscapesalinitygradientOdumrsquos (1988) comparative review of freshvs polyhalinetidal marshes reports that freshwater macrophytes from the lowerintertidal zone such asSagittaria latifolia tend to have lower lignincontenthighernitrogencontentandhigherdecayratescomparedto

F IGURE 3emspCellulosedecayamongdifferentwetlandtypesBarsrepresentmeans(n=540)anderrorbarsrepresentSEsLettersdenotestatisticalsignificanceofpost-hocmultiplecomparisonsofmeans(FisherrsquosProtectedLSDα=005)

F IGURE 4emspPrincipalComponentAnalysisbiplotsofobservationprojectionsorfactorscoresincomponentspaceforallcomparisonsbetween(a)principalcomponent(PC)1andPC2(b)PC1andPC3and(c)PC2andPC3IneachplotfactorscoresarecolouredbywetlandtypeAsubsetofhighlycorrelatedvectorsfromeachPCoverlaythefactorscores

minus6 minus4 minus2 0 2 4 6

minus6minus4

minus20

24

6

PC

2 (L

igni

n)

FreshOligohaline Mesohaline Polyhaline

minus6 minus4 minus2 0 2 4 6

minus6minus4

minus20

24

6

PC

3 (F

lood

ing)

FreshOligohaline Mesohaline Polyhaline

minus6 minus4 minus2 0 2 4 6

minus6minus4

minus20

24

6

PC2 (Lignin)

PC

3 (F

lood

ing)

FreshOligohaline Mesohaline Polyhaline

(a)

(b)

(c)

PC1 ( )

PC1 ( )

emspensp emsp | emsp663Journal of EcologySTAGG eT Al

polyhalineorsaltmarshspeciesIncontrastfreshwatermacrophytesfrom the upper intertidal such asT latifolia more closely resemble typicalsaltmarshplantsintheirligninandnitrogencontentandexhibitslowerratesofdecaythanplantsfromthelowerintertidalfreshzoneInsupportofOdumrsquosconclusionswefoundthatlitterdecompositionwas greatest in the oligohalinemarsheswhichwere dominated byS lancifolia and S americanusandalsohadthelowestlignincontentThefreshmarshdominatedbyT latifolia and P hemitomon was similar inlignincontentanddecayratetothemesohalinemarshesdominatedby S patensandthepolyhalinemarshesdominatedbyS alterniflora and J roemerianus Thus although landscape-scale salinity patternsoflignincontentandlitterdecompositionarelargelyspecies-specific

(AertsampdeCaluwe1997)itisclearthatsea-levelrisehasthepoten-tialtoalterthequalityofcarbonandindirectlyimpactdecompositionalongthisgradient

Similarlyinitiallitterqualityintheformofnitrogencontentwasthebestpredictorof above-ground litter decomposition (MarinucciHobbie amp Helfrich 1983 Taylor Parkinson amp Parsons 1989)Althoughsomeresearchidentifiestheratiooflignin-to-nitrogeninini-tiallitterqualityasanimportantpredictorofdecomposition(MelilloAberampMuratore1982Valielaetal1984)ourresultsindicatethatonlyoneofthesevariableswasasignificantpredictoreithernitrogencontent or lignin content of above- or below-ground litter decom-positionrespectivelyLikewiseastudybyMelilloNaimanAberandEshleman(1983)identifiedeitherligninaloneorincombinationwithnitrogencontentasasignificantpredictorofdecompositionThesig-nificanceofonepredictoroveranothermaybeduetointeractionsbe-tweeninitiallitterqualityandenvironmentalconditionsIngeneralifexogenousnitrogen(egfromsoilorwatercolumn)isreadilyavailabletomicrobesandorinitiallitterlignincontentishightheninitiallitternitrogencontentmayhave little impactondecayrate (Melilloetal1982 1984) Interestingly despite these differences among above-andbelow-groundlittercontentthepatternofdecayalongtheland-scapegradientwassimilarwiththegreatestdecompositionoccurringin the oligohaline marsh Although salinity is a known regulator ofplantnitrogendynamics (BradleyampMorris1991Morris1980)wedidnotobserveasimplelineardeclineinlitterqualitywithincreasingsalinityThusitisclearthatlitterdecompositioniscontrolledthroughan indirectpathwaymediatedby internaldriversbutmoreresearchisneededtoidentifytheultimatedriversthatgenerateoptimallitterqualityforenhanceddecayintheoligohalinemarsh

To characterize the direct effects of sea-level rise ondecompo-sitionwe controlled for the influenceof litter qualitybymeasuringthe decay rate of a standard carbon source cellulose across thelandscape-scalesalinitygradient(Mendelssohnetal1999)Celluloseisalabilecompoundthatrapidlydecaysduringtheinitialphasesofde-compositionandservesasareadilyavailablefuelformicrobialactivity

TABLE 2emspResultsofmultiplelinearregressionanalysisforlitterdecompositionandcellulosedecay

Response variable Predictive variable Parameter estimate SE t- value p- value Model R2

Above-groundlitter PC1 minus177E-05 565E-05 minus0314 755 121

Decomposition PC3 minus495E-05 131E-04 minus0378 707

LeafN 00003 104E-03 268 009

LeafLignin minus736E-05 218E-04 minus0338 736

Below-groundlitter PC1 431E-05 338E-05 127 207 395

Decomposition PC3 507E-05 676E-05 0750 456

RootN 353E-04 313E-04 113 265

RootLignin minus217E-04 363E-05 minus598 110E-07

Above-ground PC1 0042 0032 130 198 027

Cellulose decay PC3 minus0021 0075 minus0280 780

Below-ground PC1 0039 0012 324 002 195

Cellulose decay PC3 0066 0028 236 021

plt05plt01plt001

F IGURE 5emspVariationinsoilredoxpotentialalonglandscapegradientBoxplotboundariesclosesttozerorepresentthe25thpercentilethelinewithintheboxesindicatesthemedianandboundariesfarthestfromzerorepresentthe75thpercentile(n=90)Whiskersindicatethe90thand10thpercentilesBlackdotsrepresentoutlyingpointsLettersdenotestatisticalsignificanceofpost-hocmultiplecomparisonsofmeans(FisherrsquosProtectedLSDα=005)

664emsp |emsp emspenspJournal of Ecology STAGG eT Al

(Hodson Chrsitian amp Maccubbin 1984) Therefore in addition toidentifyingexternalcontrolsondecaywewerealsoabletocharac-terizethedecompositiondynamicsoftheisolatedlabilecarbonpool

We found that below-ground cellulosedecay increasedwith in-creasingsalinitywhichsupportsfindingsfromrecentstudiesthatsoilrespirationisstimulatedbysalinityinshort-termexposuresthatdonotincorporatechangesincarbonsource(Chambersetal2011Westonetal2006)Whileincreasingsalinitycanhavedirectimpactsonor-ganicmattermineralizationthroughalteringthesoilchemicalcompo-sitionandreleasingpreviouslysoil-boundorganiccarbon(DouPingGuoampJorgenson2005)thepatternofcellulosedecayalongthissa-linitygradientwaslikelyinfluencedbydifferencesamongthemicrobialcommunities(Chambersetal2013)Assalinitycontinuestoincreasesulphate reducersout-competemethanogensandoverall anaerobicmetabolism isgreaterwhensulphate is thedominant terminalelec-tronacceptor(Sutton-GrierKellerKochGilmourampMegonigal2011Westonetal2006)

In contrast to salinity soil nutrients were negatively correlatedwith below-ground cellulose decay Although Mendelssohn etal

(1999)foundthatsoilnutrientshadasignificantpositiveinfluenceoncellulose decay in a Phragmites australis-dominatedwetlandthistrendisnotuniversalamongallwetlandtypesForexamplethereviewbyRybczyk Garson andDay (1996) illustrates varying impacts of soilnutrientsonsoilorganicmatterdecompositionEvensoitisunlikelythathighsoilnutrientswoulddirectlyinhibitcellulosedecayRatherweproposethatthestimulatoryeffectofelevatedsalinityandgreatersulphate availability overcame the potential negative effects of lowsoilnutrientconcentrations

Floodingwasalsoa strongpredictorofdecomposition andhada direct positive effect on below-ground cellulose decay Althoughgreaterflooddurationcanleadtoanaerobicsoilconditions(GambrellampPatrick1978Ponnamperuma1984)whichcan limit the rateofdecomposition (Day amp Megonigal 1993 McKee amp Seneca 1982WhiteampTrapani1982)decompositionisnotalwaysslowerunderan-aerobicconditionsForexampleKirwanLangleyGuntenspergenand

F IGURE 6emspLinearregressionof(a)above-groundlitterdecompositionandlitternitrogencontentand(b)below-groundlitterdecompositionandlitterlignincontentFresholigohalinemesohalineandpolyhalinewetlandtypesdifferentiatedbyshapesymbols

F IGURE 7emspLinearregressionofbelow-groundcellulosedecayand(a)PC1(Physico-chemical)and(b)PC3(Flooding)Fresholigohalinemesohalineandpolyhalinewetlandtypesdifferentiatedbyshapesymbols

(a)

(b)

emspensp emsp | emsp665Journal of EcologySTAGG eT Al

Megonigal(2013)observedastimulatoryfloodingeffectonrootandrhizomedecompositionFurthermore soil redoxpotentialalong thislandscapegradientwasmorecloselycorrelatedwithporewatersalin-itythanwithfloodduration(PwECR2=minus62plt0001AnnFloodR2=minus39p=01) indicating that the stimulating effect of floodingwasnotstronglyassociatedwithanaerobicconditionsThereforeweproposethatthepositiverelationshipbetweenfloodingandcellulosedecayillustratesthewell-documentedobservationthatmicrobialac-tivity ismoisture-limited (FrascoampGoode 1982HalupaampHowes

1995NewellArsuffiampPalm1996ReiceampStiven1983)at leastinthetop20cmofthesoilprofileAtfurtherdepthscellulosedecaybelowthesoilsurfacewaslikelyoxygenlimitedassoilsbecomemorereduced with increasing depth (Maltby 1988 Schipper amp Reddy1995)Similartoourfindingslowoxygenavailabilityhasbeeniden-tifiedasaprimaryinhibitorofcellulosedecayatdepthsbelow22cm(Mendelssohnetal1999)

Surprisingly none of the environmental drivers we measuredhad predictive capacity for above-ground cellulose decayAlthough

TABLE 3emspLitterlignincontentofdominantspeciesfromeachwetlandtypeLitterconditionidentifiedaslive(L)dead(D)orunknown(U)

Species Litter type Lignin content () Study

Typha latifolia Leaves 122L Currentstudy

58L MoranampHodson(1989)

395ndash427D WelschampYavitt(2003)

182D PoideNeiffNeiffampCasco (2006)

Roots 73L Currentstudy

Panicum hemitomon Leaves 64ndash73L Currentstudy

64L MoranampHodson(1989)

59D OsborneInglettampReddy(2007)

Roots 88ndash122L Currentstudy

Sagittaria lancifolia Leaves 77ndash77L Currentstudy

18U Laursen(2004)

Roots 54ndash74L Currentstudy

26U Laursen(2004)

Schoenoplectus americanus Leaves 79ndash89L Currentstudy

205D BallampDrake(1997)

Roots 47ndash75L Currentstudy

1443ndash2650L SaundersMegonigalampReynolds(2006)

Spartina patens Leaves 78ndash92L Currentstudy

142D BallampDrake(1997)

Roots 148ndash186L Currentstudy

2695ndash3041L Saundersetal(2006)

Juncus roemerianus Leaves 99L Currentstudy

6U Benneretal(1987)

Roots 137L Currentstudy

51U Benneretal(1987)

Spartina alterniflora Leaves 55ndash57L Currentstudy

151L MaccubbinampHodson(1980)

131ndash168L Hodsonetal(1984)

117U Wilson(1985)

11ndash12D WilsonBuchsbaumValielaampSwain(1986)

43ndash61U Benneretal(1987)

Roots 137ndash152L Currentstudy

122ndash193U Hodsonetal(1984)

93U Benneretal(1987)

666emsp |emsp emspenspJournal of Ecology STAGG eT Al

above-groundcellulosedecayvariedsignificantlyacrossthelandscape-scalesalinitygradient(higherinpolyhalinemarsh)salinitypersewasnotasignificantpredictorofdecay(sensuMendelssohnetal1999)Several recentstudieshave illustratedthatsalinitycan indirectlyaf-fect soil respiration throughchanges inmicrobial function resultinginhigherratesofcarbonmineralizationathighersalinities(Chambersetal2011Neubauer2013Sutton-Grieretal2011Westonetal2006)Thuswehypothesizethatanunidentifiedmediatingfactorthatvarieswithwetlandtypesuchasmicrobialfunction(CaponeampKiene1988) is regulating cellulose decayWe suggest that future studiesinclude focusedmeasuresofmicrobial structure and function alongthislandscape-scalegradienttoconfirmthemechanismofindirectef-fectsofsalinityoncellulosedecay(HopfenspergerBurginSchoepferampHelton2014Morriseyetal2014NeubauerGivlerValentineampMegonigal2005)

Insummaryourstudyshowedthat the indirecteffectofsea-levelriseonlitterdecompositionwasmediatedthroughchangesinplantcommunitycompositionand litterqualitywhich resulted indecliningratesoflitterdecompositionalongthegradientfromoli-gohalinetopolyhalinemarshesHoweverwhenwecontrolledforchangesinlitterqualitywefoundthatincreasingsalinityandflood-ingstimulateddecayoflabilecarbon(cellulose)Wehaveidentifiedtwomechanismsoforganicmatterlossoperatingatdifferenttem-poralscalesthatprovideinsighttothepotentialforlong-termcar-bonstorageassea-levelrisesOurresultsindicatethatassealevelrises initial direct effectsof salinitywill stimulatedecayof labilecarbonbutovertimeasvegetationcommunitycompositionshiftstomoresalinewetlandtypes litterdecay (decayofrefractilecar-bon)willdeclineyieldinggreaterpotential for long-termsoilcar-bonstoragethroughnetaccretion(LoomisampCraft2012)Recentstudieshavehighlightedtheimportanceofscaleinconsideringtheinfluence of sea-level rise on carbon loss fromwetland systems(Herbertetal2015Neubaueretal2013)Ourresearchprovidesanexampleofhowmultiplemechanismsofcarbon lossoperatingatdifferentscalescanyielddifferentratesandpatternsoforganicmatter decompositionwhich is relevant not only in coastalwet-landsbutalsoinotherecosystemssuchasgrasslandsshrublandsandforestswhereplant-mediatedresponsestoclimatechangewillhavesignificant impactsoncarbondynamics (JobbagyampJackson2000)

ACKNOWLEDGEMENTS

The authors acknowledge themany people who assisted in fieldsamplecollectionandsampleprocessingAdaDizEvanBergeronKelly Darnell Lindsey Hebert Samantha Humphrey JamalMathurinAlyssaMitchellLelandMossBrettPattonSaraiPiazzaCaitlinPinsonatJacyReynoldsJacksonRollingsandRachelVillaniWe thank Thomas Blanchard and Sara Gay at Louisiana StateUniversityWetlandBiogeochemistryAnalyticalServicesandUttamKumarSahaandDavidParksatUniversityofGeorgiaCooperativeExtension for performing chemical analyses We are grateful toChristopherSwarzenski forhishelp insiteselectionBrettPatton

for rectifyingelevationdatasetsLelandMoss fordevelopmentofFigure1LaurenLeonpacherfordevelopmentofTable3andmanu-scripteditingWealsothankJoshuaJonesChristopherSwarzenskiandanonymouspeersfortheirreviewofthismanuscriptThisre-searchwasfundedbytheUSGeologicalSurveyClimateResearchand Development Program US Geological Survey EcosystemsProgram the US Geological Survey LandCarbon program theScienceandEngineeringProgramofTheWaterInstituteoftheGulfwith funds fromtheLouisianaCoastalProtectionandRestorationAuthority (CPRA) and the Baton Rouge Area Foundation (BRAF)andtheRestoretheMississippiRiverDeltaCampaignviaCoalitionto Restore Coastal Louisiana (CRCL) Any use of trade firm orproductnamesisfordescriptivepurposesonlyanddoesnotimply endorsementbytheUSGovernment

AUTHORrsquoS CONTRIBUTIONS

CLSMMBandCLPconceivedtheideasanddesignedmethod-ology CLSMMB CLP TJBC andCTH collected the dataCLSMMBandCTHanalysedthedataCLSledthewritingofthe manuscript All authors contributed critically to the drafts andgavefinalapprovalforpublication

DATA ACCESSIBILITY

Allof thesupportingdatapresented in thispaperhavebeenpubli-callyarchivedatsciencebasegovhttpsdoiorg105066f7639mvk(StaggBaustianPerryCarruthersampHall2017)

ORCID

Camille L Stagg httporcidorg0000-0002-1125-7253

Melissa M Baustian httporcidorg0000-0003-2467-2533

Courtney T Hall httporcidorg0000-0003-0990-5212

REFERENCES

AertsR(1997)ClimateleaflitterchemistryandleaflitterdecompositioninterrestrialecosystemsAtriangularrelationshipOikos 79439ndash449httpsdoiorg1023073546886

AertsRampdeCaluweH(1997)Nutritionalandplant-mediatedcontrolson leaf litterdecompositionofCarex speciesEcology 78 244ndash260httpsdoiorg1018900012-9658(1997)078[0244NAPMCO]20 CO2

Alward R Detling J amp Milchunas D (1999) Grassland vegetationchangesandnocturnalglobalwarmingScience 283118ndash231httpsdoiorg101126science2835399229

AmericanPublicHealthAssociation(2005a)3030EnitricaciddigestionInADEatonampMAFranson(Eds)Standard methods for the exam-ination of water and wastewater (21stedpp325ndash326)WashingtonDCAmericanPublicHealthAssociation

AmericanPublicHealthAssociation(2005b)3120metalsbyplasmaemis-sionspectroscopy(85)InADEatonampMAFranson(Eds)Standard methods for the examination of water and wastewater(21stedpp377ndash386)WashingtonDCAmericanPublicHealthAssociation

emspensp emsp | emsp663Journal of EcologySTAGG eT Al

polyhalineorsaltmarshspeciesIncontrastfreshwatermacrophytesfrom the upper intertidal such asT latifolia more closely resemble typicalsaltmarshplantsintheirligninandnitrogencontentandexhibitslowerratesofdecaythanplantsfromthelowerintertidalfreshzoneInsupportofOdumrsquosconclusionswefoundthatlitterdecompositionwas greatest in the oligohalinemarsheswhichwere dominated byS lancifolia and S americanusandalsohadthelowestlignincontentThefreshmarshdominatedbyT latifolia and P hemitomon was similar inlignincontentanddecayratetothemesohalinemarshesdominatedby S patensandthepolyhalinemarshesdominatedbyS alterniflora and J roemerianus Thus although landscape-scale salinity patternsoflignincontentandlitterdecompositionarelargelyspecies-specific

(AertsampdeCaluwe1997)itisclearthatsea-levelrisehasthepoten-tialtoalterthequalityofcarbonandindirectlyimpactdecompositionalongthisgradient

Similarlyinitiallitterqualityintheformofnitrogencontentwasthebestpredictorof above-ground litter decomposition (MarinucciHobbie amp Helfrich 1983 Taylor Parkinson amp Parsons 1989)Althoughsomeresearchidentifiestheratiooflignin-to-nitrogeninini-tiallitterqualityasanimportantpredictorofdecomposition(MelilloAberampMuratore1982Valielaetal1984)ourresultsindicatethatonlyoneofthesevariableswasasignificantpredictoreithernitrogencontent or lignin content of above- or below-ground litter decom-positionrespectivelyLikewiseastudybyMelilloNaimanAberandEshleman(1983)identifiedeitherligninaloneorincombinationwithnitrogencontentasasignificantpredictorofdecompositionThesig-nificanceofonepredictoroveranothermaybeduetointeractionsbe-tweeninitiallitterqualityandenvironmentalconditionsIngeneralifexogenousnitrogen(egfromsoilorwatercolumn)isreadilyavailabletomicrobesandorinitiallitterlignincontentishightheninitiallitternitrogencontentmayhave little impactondecayrate (Melilloetal1982 1984) Interestingly despite these differences among above-andbelow-groundlittercontentthepatternofdecayalongtheland-scapegradientwassimilarwiththegreatestdecompositionoccurringin the oligohaline marsh Although salinity is a known regulator ofplantnitrogendynamics (BradleyampMorris1991Morris1980)wedidnotobserveasimplelineardeclineinlitterqualitywithincreasingsalinityThusitisclearthatlitterdecompositioniscontrolledthroughan indirectpathwaymediatedby internaldriversbutmoreresearchisneededtoidentifytheultimatedriversthatgenerateoptimallitterqualityforenhanceddecayintheoligohalinemarsh

To characterize the direct effects of sea-level rise ondecompo-sitionwe controlled for the influenceof litter qualitybymeasuringthe decay rate of a standard carbon source cellulose across thelandscape-scalesalinitygradient(Mendelssohnetal1999)Celluloseisalabilecompoundthatrapidlydecaysduringtheinitialphasesofde-compositionandservesasareadilyavailablefuelformicrobialactivity

TABLE 2emspResultsofmultiplelinearregressionanalysisforlitterdecompositionandcellulosedecay

Response variable Predictive variable Parameter estimate SE t- value p- value Model R2

Above-groundlitter PC1 minus177E-05 565E-05 minus0314 755 121

Decomposition PC3 minus495E-05 131E-04 minus0378 707

LeafN 00003 104E-03 268 009

LeafLignin minus736E-05 218E-04 minus0338 736

Below-groundlitter PC1 431E-05 338E-05 127 207 395

Decomposition PC3 507E-05 676E-05 0750 456

RootN 353E-04 313E-04 113 265

RootLignin minus217E-04 363E-05 minus598 110E-07

Above-ground PC1 0042 0032 130 198 027

Cellulose decay PC3 minus0021 0075 minus0280 780

Below-ground PC1 0039 0012 324 002 195

Cellulose decay PC3 0066 0028 236 021

plt05plt01plt001

F IGURE 5emspVariationinsoilredoxpotentialalonglandscapegradientBoxplotboundariesclosesttozerorepresentthe25thpercentilethelinewithintheboxesindicatesthemedianandboundariesfarthestfromzerorepresentthe75thpercentile(n=90)Whiskersindicatethe90thand10thpercentilesBlackdotsrepresentoutlyingpointsLettersdenotestatisticalsignificanceofpost-hocmultiplecomparisonsofmeans(FisherrsquosProtectedLSDα=005)

664emsp |emsp emspenspJournal of Ecology STAGG eT Al

(Hodson Chrsitian amp Maccubbin 1984) Therefore in addition toidentifyingexternalcontrolsondecaywewerealsoabletocharac-terizethedecompositiondynamicsoftheisolatedlabilecarbonpool

We found that below-ground cellulosedecay increasedwith in-creasingsalinitywhichsupportsfindingsfromrecentstudiesthatsoilrespirationisstimulatedbysalinityinshort-termexposuresthatdonotincorporatechangesincarbonsource(Chambersetal2011Westonetal2006)Whileincreasingsalinitycanhavedirectimpactsonor-ganicmattermineralizationthroughalteringthesoilchemicalcompo-sitionandreleasingpreviouslysoil-boundorganiccarbon(DouPingGuoampJorgenson2005)thepatternofcellulosedecayalongthissa-linitygradientwaslikelyinfluencedbydifferencesamongthemicrobialcommunities(Chambersetal2013)Assalinitycontinuestoincreasesulphate reducersout-competemethanogensandoverall anaerobicmetabolism isgreaterwhensulphate is thedominant terminalelec-tronacceptor(Sutton-GrierKellerKochGilmourampMegonigal2011Westonetal2006)

In contrast to salinity soil nutrients were negatively correlatedwith below-ground cellulose decay Although Mendelssohn etal

(1999)foundthatsoilnutrientshadasignificantpositiveinfluenceoncellulose decay in a Phragmites australis-dominatedwetlandthistrendisnotuniversalamongallwetlandtypesForexamplethereviewbyRybczyk Garson andDay (1996) illustrates varying impacts of soilnutrientsonsoilorganicmatterdecompositionEvensoitisunlikelythathighsoilnutrientswoulddirectlyinhibitcellulosedecayRatherweproposethatthestimulatoryeffectofelevatedsalinityandgreatersulphate availability overcame the potential negative effects of lowsoilnutrientconcentrations

Floodingwasalsoa strongpredictorofdecomposition andhada direct positive effect on below-ground cellulose decay Althoughgreaterflooddurationcanleadtoanaerobicsoilconditions(GambrellampPatrick1978Ponnamperuma1984)whichcan limit the rateofdecomposition (Day amp Megonigal 1993 McKee amp Seneca 1982WhiteampTrapani1982)decompositionisnotalwaysslowerunderan-aerobicconditionsForexampleKirwanLangleyGuntenspergenand

F IGURE 6emspLinearregressionof(a)above-groundlitterdecompositionandlitternitrogencontentand(b)below-groundlitterdecompositionandlitterlignincontentFresholigohalinemesohalineandpolyhalinewetlandtypesdifferentiatedbyshapesymbols

F IGURE 7emspLinearregressionofbelow-groundcellulosedecayand(a)PC1(Physico-chemical)and(b)PC3(Flooding)Fresholigohalinemesohalineandpolyhalinewetlandtypesdifferentiatedbyshapesymbols

(a)

(b)

emspensp emsp | emsp665Journal of EcologySTAGG eT Al

Megonigal(2013)observedastimulatoryfloodingeffectonrootandrhizomedecompositionFurthermore soil redoxpotentialalong thislandscapegradientwasmorecloselycorrelatedwithporewatersalin-itythanwithfloodduration(PwECR2=minus62plt0001AnnFloodR2=minus39p=01) indicating that the stimulating effect of floodingwasnotstronglyassociatedwithanaerobicconditionsThereforeweproposethatthepositiverelationshipbetweenfloodingandcellulosedecayillustratesthewell-documentedobservationthatmicrobialac-tivity ismoisture-limited (FrascoampGoode 1982HalupaampHowes

1995NewellArsuffiampPalm1996ReiceampStiven1983)at leastinthetop20cmofthesoilprofileAtfurtherdepthscellulosedecaybelowthesoilsurfacewaslikelyoxygenlimitedassoilsbecomemorereduced with increasing depth (Maltby 1988 Schipper amp Reddy1995)Similartoourfindingslowoxygenavailabilityhasbeeniden-tifiedasaprimaryinhibitorofcellulosedecayatdepthsbelow22cm(Mendelssohnetal1999)

Surprisingly none of the environmental drivers we measuredhad predictive capacity for above-ground cellulose decayAlthough

TABLE 3emspLitterlignincontentofdominantspeciesfromeachwetlandtypeLitterconditionidentifiedaslive(L)dead(D)orunknown(U)

Species Litter type Lignin content () Study

Typha latifolia Leaves 122L Currentstudy

58L MoranampHodson(1989)

395ndash427D WelschampYavitt(2003)

182D PoideNeiffNeiffampCasco (2006)

Roots 73L Currentstudy

Panicum hemitomon Leaves 64ndash73L Currentstudy

64L MoranampHodson(1989)

59D OsborneInglettampReddy(2007)

Roots 88ndash122L Currentstudy

Sagittaria lancifolia Leaves 77ndash77L Currentstudy

18U Laursen(2004)

Roots 54ndash74L Currentstudy

26U Laursen(2004)

Schoenoplectus americanus Leaves 79ndash89L Currentstudy

205D BallampDrake(1997)

Roots 47ndash75L Currentstudy

1443ndash2650L SaundersMegonigalampReynolds(2006)

Spartina patens Leaves 78ndash92L Currentstudy

142D BallampDrake(1997)

Roots 148ndash186L Currentstudy

2695ndash3041L Saundersetal(2006)

Juncus roemerianus Leaves 99L Currentstudy

6U Benneretal(1987)

Roots 137L Currentstudy

51U Benneretal(1987)

Spartina alterniflora Leaves 55ndash57L Currentstudy

151L MaccubbinampHodson(1980)

131ndash168L Hodsonetal(1984)

117U Wilson(1985)

11ndash12D WilsonBuchsbaumValielaampSwain(1986)

43ndash61U Benneretal(1987)

Roots 137ndash152L Currentstudy

122ndash193U Hodsonetal(1984)

93U Benneretal(1987)

666emsp |emsp emspenspJournal of Ecology STAGG eT Al

above-groundcellulosedecayvariedsignificantlyacrossthelandscape-scalesalinitygradient(higherinpolyhalinemarsh)salinitypersewasnotasignificantpredictorofdecay(sensuMendelssohnetal1999)Several recentstudieshave illustratedthatsalinitycan indirectlyaf-fect soil respiration throughchanges inmicrobial function resultinginhigherratesofcarbonmineralizationathighersalinities(Chambersetal2011Neubauer2013Sutton-Grieretal2011Westonetal2006)Thuswehypothesizethatanunidentifiedmediatingfactorthatvarieswithwetlandtypesuchasmicrobialfunction(CaponeampKiene1988) is regulating cellulose decayWe suggest that future studiesinclude focusedmeasuresofmicrobial structure and function alongthislandscape-scalegradienttoconfirmthemechanismofindirectef-fectsofsalinityoncellulosedecay(HopfenspergerBurginSchoepferampHelton2014Morriseyetal2014NeubauerGivlerValentineampMegonigal2005)

Insummaryourstudyshowedthat the indirecteffectofsea-levelriseonlitterdecompositionwasmediatedthroughchangesinplantcommunitycompositionand litterqualitywhich resulted indecliningratesoflitterdecompositionalongthegradientfromoli-gohalinetopolyhalinemarshesHoweverwhenwecontrolledforchangesinlitterqualitywefoundthatincreasingsalinityandflood-ingstimulateddecayoflabilecarbon(cellulose)Wehaveidentifiedtwomechanismsoforganicmatterlossoperatingatdifferenttem-poralscalesthatprovideinsighttothepotentialforlong-termcar-bonstorageassea-levelrisesOurresultsindicatethatassealevelrises initial direct effectsof salinitywill stimulatedecayof labilecarbonbutovertimeasvegetationcommunitycompositionshiftstomoresalinewetlandtypes litterdecay (decayofrefractilecar-bon)willdeclineyieldinggreaterpotential for long-termsoilcar-bonstoragethroughnetaccretion(LoomisampCraft2012)Recentstudieshavehighlightedtheimportanceofscaleinconsideringtheinfluence of sea-level rise on carbon loss fromwetland systems(Herbertetal2015Neubaueretal2013)Ourresearchprovidesanexampleofhowmultiplemechanismsofcarbon lossoperatingatdifferentscalescanyielddifferentratesandpatternsoforganicmatter decompositionwhich is relevant not only in coastalwet-landsbutalsoinotherecosystemssuchasgrasslandsshrublandsandforestswhereplant-mediatedresponsestoclimatechangewillhavesignificant impactsoncarbondynamics (JobbagyampJackson2000)

ACKNOWLEDGEMENTS

The authors acknowledge themany people who assisted in fieldsamplecollectionandsampleprocessingAdaDizEvanBergeronKelly Darnell Lindsey Hebert Samantha Humphrey JamalMathurinAlyssaMitchellLelandMossBrettPattonSaraiPiazzaCaitlinPinsonatJacyReynoldsJacksonRollingsandRachelVillaniWe thank Thomas Blanchard and Sara Gay at Louisiana StateUniversityWetlandBiogeochemistryAnalyticalServicesandUttamKumarSahaandDavidParksatUniversityofGeorgiaCooperativeExtension for performing chemical analyses We are grateful toChristopherSwarzenski forhishelp insiteselectionBrettPatton

for rectifyingelevationdatasetsLelandMoss fordevelopmentofFigure1LaurenLeonpacherfordevelopmentofTable3andmanu-scripteditingWealsothankJoshuaJonesChristopherSwarzenskiandanonymouspeersfortheirreviewofthismanuscriptThisre-searchwasfundedbytheUSGeologicalSurveyClimateResearchand Development Program US Geological Survey EcosystemsProgram the US Geological Survey LandCarbon program theScienceandEngineeringProgramofTheWaterInstituteoftheGulfwith funds fromtheLouisianaCoastalProtectionandRestorationAuthority (CPRA) and the Baton Rouge Area Foundation (BRAF)andtheRestoretheMississippiRiverDeltaCampaignviaCoalitionto Restore Coastal Louisiana (CRCL) Any use of trade firm orproductnamesisfordescriptivepurposesonlyanddoesnotimply endorsementbytheUSGovernment

AUTHORrsquoS CONTRIBUTIONS

CLSMMBandCLPconceivedtheideasanddesignedmethod-ology CLSMMB CLP TJBC andCTH collected the dataCLSMMBandCTHanalysedthedataCLSledthewritingofthe manuscript All authors contributed critically to the drafts andgavefinalapprovalforpublication

DATA ACCESSIBILITY

Allof thesupportingdatapresented in thispaperhavebeenpubli-callyarchivedatsciencebasegovhttpsdoiorg105066f7639mvk(StaggBaustianPerryCarruthersampHall2017)

ORCID

Camille L Stagg httporcidorg0000-0002-1125-7253

Melissa M Baustian httporcidorg0000-0003-2467-2533

Courtney T Hall httporcidorg0000-0003-0990-5212

REFERENCES

AertsR(1997)ClimateleaflitterchemistryandleaflitterdecompositioninterrestrialecosystemsAtriangularrelationshipOikos 79439ndash449httpsdoiorg1023073546886

AertsRampdeCaluweH(1997)Nutritionalandplant-mediatedcontrolson leaf litterdecompositionofCarex speciesEcology 78 244ndash260httpsdoiorg1018900012-9658(1997)078[0244NAPMCO]20 CO2

Alward R Detling J amp Milchunas D (1999) Grassland vegetationchangesandnocturnalglobalwarmingScience 283118ndash231httpsdoiorg101126science2835399229

AmericanPublicHealthAssociation(2005a)3030EnitricaciddigestionInADEatonampMAFranson(Eds)Standard methods for the exam-ination of water and wastewater (21stedpp325ndash326)WashingtonDCAmericanPublicHealthAssociation

AmericanPublicHealthAssociation(2005b)3120metalsbyplasmaemis-sionspectroscopy(85)InADEatonampMAFranson(Eds)Standard methods for the examination of water and wastewater(21stedpp377ndash386)WashingtonDCAmericanPublicHealthAssociation

664emsp |emsp emspenspJournal of Ecology STAGG eT Al

(Hodson Chrsitian amp Maccubbin 1984) Therefore in addition toidentifyingexternalcontrolsondecaywewerealsoabletocharac-terizethedecompositiondynamicsoftheisolatedlabilecarbonpool

We found that below-ground cellulosedecay increasedwith in-creasingsalinitywhichsupportsfindingsfromrecentstudiesthatsoilrespirationisstimulatedbysalinityinshort-termexposuresthatdonotincorporatechangesincarbonsource(Chambersetal2011Westonetal2006)Whileincreasingsalinitycanhavedirectimpactsonor-ganicmattermineralizationthroughalteringthesoilchemicalcompo-sitionandreleasingpreviouslysoil-boundorganiccarbon(DouPingGuoampJorgenson2005)thepatternofcellulosedecayalongthissa-linitygradientwaslikelyinfluencedbydifferencesamongthemicrobialcommunities(Chambersetal2013)Assalinitycontinuestoincreasesulphate reducersout-competemethanogensandoverall anaerobicmetabolism isgreaterwhensulphate is thedominant terminalelec-tronacceptor(Sutton-GrierKellerKochGilmourampMegonigal2011Westonetal2006)

In contrast to salinity soil nutrients were negatively correlatedwith below-ground cellulose decay Although Mendelssohn etal

(1999)foundthatsoilnutrientshadasignificantpositiveinfluenceoncellulose decay in a Phragmites australis-dominatedwetlandthistrendisnotuniversalamongallwetlandtypesForexamplethereviewbyRybczyk Garson andDay (1996) illustrates varying impacts of soilnutrientsonsoilorganicmatterdecompositionEvensoitisunlikelythathighsoilnutrientswoulddirectlyinhibitcellulosedecayRatherweproposethatthestimulatoryeffectofelevatedsalinityandgreatersulphate availability overcame the potential negative effects of lowsoilnutrientconcentrations

Floodingwasalsoa strongpredictorofdecomposition andhada direct positive effect on below-ground cellulose decay Althoughgreaterflooddurationcanleadtoanaerobicsoilconditions(GambrellampPatrick1978Ponnamperuma1984)whichcan limit the rateofdecomposition (Day amp Megonigal 1993 McKee amp Seneca 1982WhiteampTrapani1982)decompositionisnotalwaysslowerunderan-aerobicconditionsForexampleKirwanLangleyGuntenspergenand

F IGURE 6emspLinearregressionof(a)above-groundlitterdecompositionandlitternitrogencontentand(b)below-groundlitterdecompositionandlitterlignincontentFresholigohalinemesohalineandpolyhalinewetlandtypesdifferentiatedbyshapesymbols

F IGURE 7emspLinearregressionofbelow-groundcellulosedecayand(a)PC1(Physico-chemical)and(b)PC3(Flooding)Fresholigohalinemesohalineandpolyhalinewetlandtypesdifferentiatedbyshapesymbols

(a)

(b)

emspensp emsp | emsp665Journal of EcologySTAGG eT Al

Megonigal(2013)observedastimulatoryfloodingeffectonrootandrhizomedecompositionFurthermore soil redoxpotentialalong thislandscapegradientwasmorecloselycorrelatedwithporewatersalin-itythanwithfloodduration(PwECR2=minus62plt0001AnnFloodR2=minus39p=01) indicating that the stimulating effect of floodingwasnotstronglyassociatedwithanaerobicconditionsThereforeweproposethatthepositiverelationshipbetweenfloodingandcellulosedecayillustratesthewell-documentedobservationthatmicrobialac-tivity ismoisture-limited (FrascoampGoode 1982HalupaampHowes

1995NewellArsuffiampPalm1996ReiceampStiven1983)at leastinthetop20cmofthesoilprofileAtfurtherdepthscellulosedecaybelowthesoilsurfacewaslikelyoxygenlimitedassoilsbecomemorereduced with increasing depth (Maltby 1988 Schipper amp Reddy1995)Similartoourfindingslowoxygenavailabilityhasbeeniden-tifiedasaprimaryinhibitorofcellulosedecayatdepthsbelow22cm(Mendelssohnetal1999)

Surprisingly none of the environmental drivers we measuredhad predictive capacity for above-ground cellulose decayAlthough

TABLE 3emspLitterlignincontentofdominantspeciesfromeachwetlandtypeLitterconditionidentifiedaslive(L)dead(D)orunknown(U)

Species Litter type Lignin content () Study

Typha latifolia Leaves 122L Currentstudy

58L MoranampHodson(1989)

395ndash427D WelschampYavitt(2003)

182D PoideNeiffNeiffampCasco (2006)

Roots 73L Currentstudy

Panicum hemitomon Leaves 64ndash73L Currentstudy

64L MoranampHodson(1989)

59D OsborneInglettampReddy(2007)

Roots 88ndash122L Currentstudy

Sagittaria lancifolia Leaves 77ndash77L Currentstudy

18U Laursen(2004)

Roots 54ndash74L Currentstudy

26U Laursen(2004)

Schoenoplectus americanus Leaves 79ndash89L Currentstudy

205D BallampDrake(1997)

Roots 47ndash75L Currentstudy

1443ndash2650L SaundersMegonigalampReynolds(2006)

Spartina patens Leaves 78ndash92L Currentstudy

142D BallampDrake(1997)

Roots 148ndash186L Currentstudy

2695ndash3041L Saundersetal(2006)

Juncus roemerianus Leaves 99L Currentstudy

6U Benneretal(1987)

Roots 137L Currentstudy

51U Benneretal(1987)

Spartina alterniflora Leaves 55ndash57L Currentstudy

151L MaccubbinampHodson(1980)

131ndash168L Hodsonetal(1984)

117U Wilson(1985)

11ndash12D WilsonBuchsbaumValielaampSwain(1986)

43ndash61U Benneretal(1987)

Roots 137ndash152L Currentstudy

122ndash193U Hodsonetal(1984)

93U Benneretal(1987)

666emsp |emsp emspenspJournal of Ecology STAGG eT Al

above-groundcellulosedecayvariedsignificantlyacrossthelandscape-scalesalinitygradient(higherinpolyhalinemarsh)salinitypersewasnotasignificantpredictorofdecay(sensuMendelssohnetal1999)Several recentstudieshave illustratedthatsalinitycan indirectlyaf-fect soil respiration throughchanges inmicrobial function resultinginhigherratesofcarbonmineralizationathighersalinities(Chambersetal2011Neubauer2013Sutton-Grieretal2011Westonetal2006)Thuswehypothesizethatanunidentifiedmediatingfactorthatvarieswithwetlandtypesuchasmicrobialfunction(CaponeampKiene1988) is regulating cellulose decayWe suggest that future studiesinclude focusedmeasuresofmicrobial structure and function alongthislandscape-scalegradienttoconfirmthemechanismofindirectef-fectsofsalinityoncellulosedecay(HopfenspergerBurginSchoepferampHelton2014Morriseyetal2014NeubauerGivlerValentineampMegonigal2005)

Insummaryourstudyshowedthat the indirecteffectofsea-levelriseonlitterdecompositionwasmediatedthroughchangesinplantcommunitycompositionand litterqualitywhich resulted indecliningratesoflitterdecompositionalongthegradientfromoli-gohalinetopolyhalinemarshesHoweverwhenwecontrolledforchangesinlitterqualitywefoundthatincreasingsalinityandflood-ingstimulateddecayoflabilecarbon(cellulose)Wehaveidentifiedtwomechanismsoforganicmatterlossoperatingatdifferenttem-poralscalesthatprovideinsighttothepotentialforlong-termcar-bonstorageassea-levelrisesOurresultsindicatethatassealevelrises initial direct effectsof salinitywill stimulatedecayof labilecarbonbutovertimeasvegetationcommunitycompositionshiftstomoresalinewetlandtypes litterdecay (decayofrefractilecar-bon)willdeclineyieldinggreaterpotential for long-termsoilcar-bonstoragethroughnetaccretion(LoomisampCraft2012)Recentstudieshavehighlightedtheimportanceofscaleinconsideringtheinfluence of sea-level rise on carbon loss fromwetland systems(Herbertetal2015Neubaueretal2013)Ourresearchprovidesanexampleofhowmultiplemechanismsofcarbon lossoperatingatdifferentscalescanyielddifferentratesandpatternsoforganicmatter decompositionwhich is relevant not only in coastalwet-landsbutalsoinotherecosystemssuchasgrasslandsshrublandsandforestswhereplant-mediatedresponsestoclimatechangewillhavesignificant impactsoncarbondynamics (JobbagyampJackson2000)

ACKNOWLEDGEMENTS

The authors acknowledge themany people who assisted in fieldsamplecollectionandsampleprocessingAdaDizEvanBergeronKelly Darnell Lindsey Hebert Samantha Humphrey JamalMathurinAlyssaMitchellLelandMossBrettPattonSaraiPiazzaCaitlinPinsonatJacyReynoldsJacksonRollingsandRachelVillaniWe thank Thomas Blanchard and Sara Gay at Louisiana StateUniversityWetlandBiogeochemistryAnalyticalServicesandUttamKumarSahaandDavidParksatUniversityofGeorgiaCooperativeExtension for performing chemical analyses We are grateful toChristopherSwarzenski forhishelp insiteselectionBrettPatton

for rectifyingelevationdatasetsLelandMoss fordevelopmentofFigure1LaurenLeonpacherfordevelopmentofTable3andmanu-scripteditingWealsothankJoshuaJonesChristopherSwarzenskiandanonymouspeersfortheirreviewofthismanuscriptThisre-searchwasfundedbytheUSGeologicalSurveyClimateResearchand Development Program US Geological Survey EcosystemsProgram the US Geological Survey LandCarbon program theScienceandEngineeringProgramofTheWaterInstituteoftheGulfwith funds fromtheLouisianaCoastalProtectionandRestorationAuthority (CPRA) and the Baton Rouge Area Foundation (BRAF)andtheRestoretheMississippiRiverDeltaCampaignviaCoalitionto Restore Coastal Louisiana (CRCL) Any use of trade firm orproductnamesisfordescriptivepurposesonlyanddoesnotimply endorsementbytheUSGovernment

AUTHORrsquoS CONTRIBUTIONS

CLSMMBandCLPconceivedtheideasanddesignedmethod-ology CLSMMB CLP TJBC andCTH collected the dataCLSMMBandCTHanalysedthedataCLSledthewritingofthe manuscript All authors contributed critically to the drafts andgavefinalapprovalforpublication

DATA ACCESSIBILITY

Allof thesupportingdatapresented in thispaperhavebeenpubli-callyarchivedatsciencebasegovhttpsdoiorg105066f7639mvk(StaggBaustianPerryCarruthersampHall2017)

ORCID

Camille L Stagg httporcidorg0000-0002-1125-7253

Melissa M Baustian httporcidorg0000-0003-2467-2533

Courtney T Hall httporcidorg0000-0003-0990-5212

REFERENCES

AertsR(1997)ClimateleaflitterchemistryandleaflitterdecompositioninterrestrialecosystemsAtriangularrelationshipOikos 79439ndash449httpsdoiorg1023073546886

AertsRampdeCaluweH(1997)Nutritionalandplant-mediatedcontrolson leaf litterdecompositionofCarex speciesEcology 78 244ndash260httpsdoiorg1018900012-9658(1997)078[0244NAPMCO]20 CO2

Alward R Detling J amp Milchunas D (1999) Grassland vegetationchangesandnocturnalglobalwarmingScience 283118ndash231httpsdoiorg101126science2835399229

AmericanPublicHealthAssociation(2005a)3030EnitricaciddigestionInADEatonampMAFranson(Eds)Standard methods for the exam-ination of water and wastewater (21stedpp325ndash326)WashingtonDCAmericanPublicHealthAssociation

AmericanPublicHealthAssociation(2005b)3120metalsbyplasmaemis-sionspectroscopy(85)InADEatonampMAFranson(Eds)Standard methods for the examination of water and wastewater(21stedpp377ndash386)WashingtonDCAmericanPublicHealthAssociation

emspensp emsp | emsp665Journal of EcologySTAGG eT Al

Megonigal(2013)observedastimulatoryfloodingeffectonrootandrhizomedecompositionFurthermore soil redoxpotentialalong thislandscapegradientwasmorecloselycorrelatedwithporewatersalin-itythanwithfloodduration(PwECR2=minus62plt0001AnnFloodR2=minus39p=01) indicating that the stimulating effect of floodingwasnotstronglyassociatedwithanaerobicconditionsThereforeweproposethatthepositiverelationshipbetweenfloodingandcellulosedecayillustratesthewell-documentedobservationthatmicrobialac-tivity ismoisture-limited (FrascoampGoode 1982HalupaampHowes

1995NewellArsuffiampPalm1996ReiceampStiven1983)at leastinthetop20cmofthesoilprofileAtfurtherdepthscellulosedecaybelowthesoilsurfacewaslikelyoxygenlimitedassoilsbecomemorereduced with increasing depth (Maltby 1988 Schipper amp Reddy1995)Similartoourfindingslowoxygenavailabilityhasbeeniden-tifiedasaprimaryinhibitorofcellulosedecayatdepthsbelow22cm(Mendelssohnetal1999)

Surprisingly none of the environmental drivers we measuredhad predictive capacity for above-ground cellulose decayAlthough

TABLE 3emspLitterlignincontentofdominantspeciesfromeachwetlandtypeLitterconditionidentifiedaslive(L)dead(D)orunknown(U)

Species Litter type Lignin content () Study

Typha latifolia Leaves 122L Currentstudy

58L MoranampHodson(1989)

395ndash427D WelschampYavitt(2003)

182D PoideNeiffNeiffampCasco (2006)

Roots 73L Currentstudy

Panicum hemitomon Leaves 64ndash73L Currentstudy

64L MoranampHodson(1989)

59D OsborneInglettampReddy(2007)

Roots 88ndash122L Currentstudy

Sagittaria lancifolia Leaves 77ndash77L Currentstudy

18U Laursen(2004)

Roots 54ndash74L Currentstudy

26U Laursen(2004)

Schoenoplectus americanus Leaves 79ndash89L Currentstudy

205D BallampDrake(1997)

Roots 47ndash75L Currentstudy

1443ndash2650L SaundersMegonigalampReynolds(2006)

Spartina patens Leaves 78ndash92L Currentstudy

142D BallampDrake(1997)

Roots 148ndash186L Currentstudy

2695ndash3041L Saundersetal(2006)

Juncus roemerianus Leaves 99L Currentstudy

6U Benneretal(1987)

Roots 137L Currentstudy

51U Benneretal(1987)

Spartina alterniflora Leaves 55ndash57L Currentstudy

151L MaccubbinampHodson(1980)

131ndash168L Hodsonetal(1984)

117U Wilson(1985)

11ndash12D WilsonBuchsbaumValielaampSwain(1986)

43ndash61U Benneretal(1987)

Roots 137ndash152L Currentstudy

122ndash193U Hodsonetal(1984)

93U Benneretal(1987)

666emsp |emsp emspenspJournal of Ecology STAGG eT Al

above-groundcellulosedecayvariedsignificantlyacrossthelandscape-scalesalinitygradient(higherinpolyhalinemarsh)salinitypersewasnotasignificantpredictorofdecay(sensuMendelssohnetal1999)Several recentstudieshave illustratedthatsalinitycan indirectlyaf-fect soil respiration throughchanges inmicrobial function resultinginhigherratesofcarbonmineralizationathighersalinities(Chambersetal2011Neubauer2013Sutton-Grieretal2011Westonetal2006)Thuswehypothesizethatanunidentifiedmediatingfactorthatvarieswithwetlandtypesuchasmicrobialfunction(CaponeampKiene1988) is regulating cellulose decayWe suggest that future studiesinclude focusedmeasuresofmicrobial structure and function alongthislandscape-scalegradienttoconfirmthemechanismofindirectef-fectsofsalinityoncellulosedecay(HopfenspergerBurginSchoepferampHelton2014Morriseyetal2014NeubauerGivlerValentineampMegonigal2005)

Insummaryourstudyshowedthat the indirecteffectofsea-levelriseonlitterdecompositionwasmediatedthroughchangesinplantcommunitycompositionand litterqualitywhich resulted indecliningratesoflitterdecompositionalongthegradientfromoli-gohalinetopolyhalinemarshesHoweverwhenwecontrolledforchangesinlitterqualitywefoundthatincreasingsalinityandflood-ingstimulateddecayoflabilecarbon(cellulose)Wehaveidentifiedtwomechanismsoforganicmatterlossoperatingatdifferenttem-poralscalesthatprovideinsighttothepotentialforlong-termcar-bonstorageassea-levelrisesOurresultsindicatethatassealevelrises initial direct effectsof salinitywill stimulatedecayof labilecarbonbutovertimeasvegetationcommunitycompositionshiftstomoresalinewetlandtypes litterdecay (decayofrefractilecar-bon)willdeclineyieldinggreaterpotential for long-termsoilcar-bonstoragethroughnetaccretion(LoomisampCraft2012)Recentstudieshavehighlightedtheimportanceofscaleinconsideringtheinfluence of sea-level rise on carbon loss fromwetland systems(Herbertetal2015Neubaueretal2013)Ourresearchprovidesanexampleofhowmultiplemechanismsofcarbon lossoperatingatdifferentscalescanyielddifferentratesandpatternsoforganicmatter decompositionwhich is relevant not only in coastalwet-landsbutalsoinotherecosystemssuchasgrasslandsshrublandsandforestswhereplant-mediatedresponsestoclimatechangewillhavesignificant impactsoncarbondynamics (JobbagyampJackson2000)

ACKNOWLEDGEMENTS

The authors acknowledge themany people who assisted in fieldsamplecollectionandsampleprocessingAdaDizEvanBergeronKelly Darnell Lindsey Hebert Samantha Humphrey JamalMathurinAlyssaMitchellLelandMossBrettPattonSaraiPiazzaCaitlinPinsonatJacyReynoldsJacksonRollingsandRachelVillaniWe thank Thomas Blanchard and Sara Gay at Louisiana StateUniversityWetlandBiogeochemistryAnalyticalServicesandUttamKumarSahaandDavidParksatUniversityofGeorgiaCooperativeExtension for performing chemical analyses We are grateful toChristopherSwarzenski forhishelp insiteselectionBrettPatton

for rectifyingelevationdatasetsLelandMoss fordevelopmentofFigure1LaurenLeonpacherfordevelopmentofTable3andmanu-scripteditingWealsothankJoshuaJonesChristopherSwarzenskiandanonymouspeersfortheirreviewofthismanuscriptThisre-searchwasfundedbytheUSGeologicalSurveyClimateResearchand Development Program US Geological Survey EcosystemsProgram the US Geological Survey LandCarbon program theScienceandEngineeringProgramofTheWaterInstituteoftheGulfwith funds fromtheLouisianaCoastalProtectionandRestorationAuthority (CPRA) and the Baton Rouge Area Foundation (BRAF)andtheRestoretheMississippiRiverDeltaCampaignviaCoalitionto Restore Coastal Louisiana (CRCL) Any use of trade firm orproductnamesisfordescriptivepurposesonlyanddoesnotimply endorsementbytheUSGovernment

AUTHORrsquoS CONTRIBUTIONS

CLSMMBandCLPconceivedtheideasanddesignedmethod-ology CLSMMB CLP TJBC andCTH collected the dataCLSMMBandCTHanalysedthedataCLSledthewritingofthe manuscript All authors contributed critically to the drafts andgavefinalapprovalforpublication

DATA ACCESSIBILITY

Allof thesupportingdatapresented in thispaperhavebeenpubli-callyarchivedatsciencebasegovhttpsdoiorg105066f7639mvk(StaggBaustianPerryCarruthersampHall2017)

ORCID

Camille L Stagg httporcidorg0000-0002-1125-7253

Melissa M Baustian httporcidorg0000-0003-2467-2533

Courtney T Hall httporcidorg0000-0003-0990-5212

REFERENCES

AertsR(1997)ClimateleaflitterchemistryandleaflitterdecompositioninterrestrialecosystemsAtriangularrelationshipOikos 79439ndash449httpsdoiorg1023073546886

AertsRampdeCaluweH(1997)Nutritionalandplant-mediatedcontrolson leaf litterdecompositionofCarex speciesEcology 78 244ndash260httpsdoiorg1018900012-9658(1997)078[0244NAPMCO]20 CO2

Alward R Detling J amp Milchunas D (1999) Grassland vegetationchangesandnocturnalglobalwarmingScience 283118ndash231httpsdoiorg101126science2835399229

AmericanPublicHealthAssociation(2005a)3030EnitricaciddigestionInADEatonampMAFranson(Eds)Standard methods for the exam-ination of water and wastewater (21stedpp325ndash326)WashingtonDCAmericanPublicHealthAssociation

AmericanPublicHealthAssociation(2005b)3120metalsbyplasmaemis-sionspectroscopy(85)InADEatonampMAFranson(Eds)Standard methods for the examination of water and wastewater(21stedpp377ndash386)WashingtonDCAmericanPublicHealthAssociation

666emsp |emsp emspenspJournal of Ecology STAGG eT Al

above-groundcellulosedecayvariedsignificantlyacrossthelandscape-scalesalinitygradient(higherinpolyhalinemarsh)salinitypersewasnotasignificantpredictorofdecay(sensuMendelssohnetal1999)Several recentstudieshave illustratedthatsalinitycan indirectlyaf-fect soil respiration throughchanges inmicrobial function resultinginhigherratesofcarbonmineralizationathighersalinities(Chambersetal2011Neubauer2013Sutton-Grieretal2011Westonetal2006)Thuswehypothesizethatanunidentifiedmediatingfactorthatvarieswithwetlandtypesuchasmicrobialfunction(CaponeampKiene1988) is regulating cellulose decayWe suggest that future studiesinclude focusedmeasuresofmicrobial structure and function alongthislandscape-scalegradienttoconfirmthemechanismofindirectef-fectsofsalinityoncellulosedecay(HopfenspergerBurginSchoepferampHelton2014Morriseyetal2014NeubauerGivlerValentineampMegonigal2005)

Insummaryourstudyshowedthat the indirecteffectofsea-levelriseonlitterdecompositionwasmediatedthroughchangesinplantcommunitycompositionand litterqualitywhich resulted indecliningratesoflitterdecompositionalongthegradientfromoli-gohalinetopolyhalinemarshesHoweverwhenwecontrolledforchangesinlitterqualitywefoundthatincreasingsalinityandflood-ingstimulateddecayoflabilecarbon(cellulose)Wehaveidentifiedtwomechanismsoforganicmatterlossoperatingatdifferenttem-poralscalesthatprovideinsighttothepotentialforlong-termcar-bonstorageassea-levelrisesOurresultsindicatethatassealevelrises initial direct effectsof salinitywill stimulatedecayof labilecarbonbutovertimeasvegetationcommunitycompositionshiftstomoresalinewetlandtypes litterdecay (decayofrefractilecar-bon)willdeclineyieldinggreaterpotential for long-termsoilcar-bonstoragethroughnetaccretion(LoomisampCraft2012)Recentstudieshavehighlightedtheimportanceofscaleinconsideringtheinfluence of sea-level rise on carbon loss fromwetland systems(Herbertetal2015Neubaueretal2013)Ourresearchprovidesanexampleofhowmultiplemechanismsofcarbon lossoperatingatdifferentscalescanyielddifferentratesandpatternsoforganicmatter decompositionwhich is relevant not only in coastalwet-landsbutalsoinotherecosystemssuchasgrasslandsshrublandsandforestswhereplant-mediatedresponsestoclimatechangewillhavesignificant impactsoncarbondynamics (JobbagyampJackson2000)

ACKNOWLEDGEMENTS

The authors acknowledge themany people who assisted in fieldsamplecollectionandsampleprocessingAdaDizEvanBergeronKelly Darnell Lindsey Hebert Samantha Humphrey JamalMathurinAlyssaMitchellLelandMossBrettPattonSaraiPiazzaCaitlinPinsonatJacyReynoldsJacksonRollingsandRachelVillaniWe thank Thomas Blanchard and Sara Gay at Louisiana StateUniversityWetlandBiogeochemistryAnalyticalServicesandUttamKumarSahaandDavidParksatUniversityofGeorgiaCooperativeExtension for performing chemical analyses We are grateful toChristopherSwarzenski forhishelp insiteselectionBrettPatton

for rectifyingelevationdatasetsLelandMoss fordevelopmentofFigure1LaurenLeonpacherfordevelopmentofTable3andmanu-scripteditingWealsothankJoshuaJonesChristopherSwarzenskiandanonymouspeersfortheirreviewofthismanuscriptThisre-searchwasfundedbytheUSGeologicalSurveyClimateResearchand Development Program US Geological Survey EcosystemsProgram the US Geological Survey LandCarbon program theScienceandEngineeringProgramofTheWaterInstituteoftheGulfwith funds fromtheLouisianaCoastalProtectionandRestorationAuthority (CPRA) and the Baton Rouge Area Foundation (BRAF)andtheRestoretheMississippiRiverDeltaCampaignviaCoalitionto Restore Coastal Louisiana (CRCL) Any use of trade firm orproductnamesisfordescriptivepurposesonlyanddoesnotimply endorsementbytheUSGovernment

AUTHORrsquoS CONTRIBUTIONS

CLSMMBandCLPconceivedtheideasanddesignedmethod-ology CLSMMB CLP TJBC andCTH collected the dataCLSMMBandCTHanalysedthedataCLSledthewritingofthe manuscript All authors contributed critically to the drafts andgavefinalapprovalforpublication

DATA ACCESSIBILITY

Allof thesupportingdatapresented in thispaperhavebeenpubli-callyarchivedatsciencebasegovhttpsdoiorg105066f7639mvk(StaggBaustianPerryCarruthersampHall2017)

ORCID

Camille L Stagg httporcidorg0000-0002-1125-7253

Melissa M Baustian httporcidorg0000-0003-2467-2533

Courtney T Hall httporcidorg0000-0003-0990-5212

REFERENCES

AertsR(1997)ClimateleaflitterchemistryandleaflitterdecompositioninterrestrialecosystemsAtriangularrelationshipOikos 79439ndash449httpsdoiorg1023073546886

AertsRampdeCaluweH(1997)Nutritionalandplant-mediatedcontrolson leaf litterdecompositionofCarex speciesEcology 78 244ndash260httpsdoiorg1018900012-9658(1997)078[0244NAPMCO]20 CO2

Alward R Detling J amp Milchunas D (1999) Grassland vegetationchangesandnocturnalglobalwarmingScience 283118ndash231httpsdoiorg101126science2835399229

AmericanPublicHealthAssociation(2005a)3030EnitricaciddigestionInADEatonampMAFranson(Eds)Standard methods for the exam-ination of water and wastewater (21stedpp325ndash326)WashingtonDCAmericanPublicHealthAssociation

AmericanPublicHealthAssociation(2005b)3120metalsbyplasmaemis-sionspectroscopy(85)InADEatonampMAFranson(Eds)Standard methods for the examination of water and wastewater(21stedpp377ndash386)WashingtonDCAmericanPublicHealthAssociation

emspensp emsp | emsp667Journal of EcologySTAGG eT Al

AspilaKAgemianHampChauS(1976)Asemi-automatedmethodfordeterminationofinorganicorganicandtotalphosphateinsedimentsAnalyst 101187ndash197httpsdoiorg101039an9760100187

Ball A amp Drake B (1997) Short-term decomposition of litter pro-duced by plants grown in ambient and elevated atmosphericCO2 concentrations Global Change Biology 3 29ndash35 httpsdoiorg101046j1365-2486199700091x

BarbierEHackerSKennedyCKochEWStierACSillimanBR(2011)ThevalueofestuarineandcoastalecosystemservicesEcological Monographs 81169ndash193httpsdoiorg10189010-15101

BaustianMMStaggCLPerryCLMossLCCarrutherTJBampAllisonM(2017)Relationshipsbetweensalinityandshort-termsoilcarbonaccumulationratesfrommarshtypesacrossalandscapeintheMississippiRiverDeltaWetlands 37313ndash324

BennerRFogelMSpragueEampHodsonR (1987)Depletionof13C inligninanditsimplicationsforstablecarbonisotopestudiesNature 329708ndash710httpsdoiorg101038329708a0

Bianchi T Allison M Zhao J Li X Comeaux R S Feagin R AWasanthaKulawardhanaR(2013)Historicalreconstructionofman-grove expansion in theGulf ofMexico Linking climate changewithcarbonsequestration incoastalwetlandsEstuarine Coastal and Shelf Science 1197ndash16httpsdoiorg101016jecss201212007

BlakeG(1965)Methods of soil analysisInCBlackDEvansLEnsmingerJWhiteampFClark(Eds)MadisonWIAmericanSocietyofAgronomy

BradleyPampMorrisJ(1991)TheinfluenceofsalinityonthekineticsofNH4

+ uptake inSpartina alterniflora Oecologia 85 375ndash380httpsdoiorg101007BF00320613

Burdick D ampMendelssohn I (1987)Waterlogging responses in duneswaleandmarshpopulationsofSpartina patensunderfieldconditionsOecologia 74321ndash329httpsdoiorg101007BF00378924

Buth G (1987) Decomposition of roots of three plant communitiesin a Dutch salt marsh Aquatic Botany 29 123ndash138 httpsdoiorg1010160304-3770(87)90091-X

CaponeDampKieneR(1988)Comparisonofmicrobialdynamicsinma-rineandfreshwatersedimentsContrastsinanaerobiccarboncatabo-lism Limnology and Oceanography 33725ndash749

Chambers LOsborneTampReddyK (2013) Effectof salinity-alteringpulsingeventsonsoilorganiccarbonlossalonganintertidalwetlandgradient A laboratory experiment Biogeochemistry 115 363ndash383 httpsdoiorg101007s10533-013-9841-5

Chambers L Reddy K amp Osborne T (2011) Short-term response ofcarboncyclingtosalinitypulses ina freshwaterwetlandSoil Science Society of America Journal 75 2000ndash2007 httpsdoiorg102136sssaj20110026

ChapmanSNewmanGHartSSchweitzerJAKochGW(2013)Leaflittermixtures altermicrobial community developmentMechanismsfornon-additiveeffectsinlitterdecompositionPLoS ONE 81ndash9

ChenXAllisonTCaoWFergusonKGruumlnigSampGomezVhellipTablotN(2011)TrimbleRTXaninnovativenewapproachfornetworkRTKInProceedings of the 24th international technical meeting of the satellite division of the institute of navigation (ION GNSS 2011)pp2214ndash2219PortlandOR

ChmuraGAnisfeldSCahoonDampLynchJ(2003)Globalcarbonse-questrationintidalsalinewetlandsoilsGlobal Biogeochemical Cycles 17 1ndash12

CowardinLMCarterVGoletFCampLaRoeET(1979)ClassificationofwetlandsanddeepwaterhabitatsoftheUnitedStatesFWSOBS-7931USFishandWildlifeServiceWashingtonDC

DayJChristianRBoeschDYaacutentildeez-ArancibiaAMorrisJTwilleyRRhellip StevensonC (2008)Consequencesof climate changeon theecogeomorphologyofcoastalwetlandsEstuaries and Coasts 31477ndash491httpsdoiorg101007s12237-008-9047-6

DayFampMegonigalJ (1993)Therelationshipbetweenvariablehydro-period production allocation and belowground organic turnover in

forestedwetlandsWetlands 13 115ndash121 httpsdoiorg101007BF03160871

DeLaune R PatrickW Jr amp Pezeshki S (1987) Foreseeable floodinganddeathofcoastalwetlandforestsEnvironmental Conservation 14 129ndash133httpsdoiorg101017S0376892900011486

DrsquoElia C F Steudler P A amp Nathaniel C (1977) Determinationof total nitrogen in aqueous samples using persulfate digestionLimnology and Oceanography 22 760ndash764 httpsdoiorg104319lo19772240760

DouF PingCGuo L JorgensonT (2005) Estimating the impactofseawateron theproductionof soilwater-extractableorganiccarbonduring coastal erosion Journal of Environmental Quality 37 2368ndash2374httpsdoiorg102134jeq20070403

Ebina J Tsutsui T amp Shirai T (1983) Simultaneous determinationof total nitrogen and total phosphorus in water using peroxodi-sulfate oxidation Water Research 17 1721ndash1726 httpsdoiorg1010160043-1354(83)90192-6

EnwrightNGriffithKampOslandM (2016)Barriers to andopportu-nities for landwardmigrationof coastalwetlandswith sea-level riseFrontiers in Ecology and the Environment 14 307ndash3016 httpsdoiorg101002fee1282

FrascoBampGoodeR(1982)DecompositiondynamicsofSpartina alterni-flora and Spartina patensinaNewJerseysaltmarshAmerican Journal of Botany 69402ndash406httpsdoiorg1023072443145

Gallagher J Kibby H amp Skirvin K (1984) Community respi-ration of decomposing plants in Oregon estuarine marshesEstuarine Coastal and Shelf Science 18 421ndash431 httpsdoiorg1010160272-7714(84)90081-7

GambrellRampPatrickW (1978)ChemicalandbiologicalpropertiesofanaerobicsoilsandsedimentsInDHookampMCrawford(Eds)Plant life in anaerobic environments(pp375ndash423)AnnArborMIAnnArborScience

GaoYAbdel-SalamMChenKampWojciechowskiA(2005)Pointre-al-timekinematicpositioningInFSanso(Ed)A window on the future of geodesy vol 128 International Association of Geodesy Symposia (pp77ndash82)BerlinGermanySpringer

Godshalk G amp Wetzel R (1978) Decomposition of aquatic an-giosperms III Zostera marina L and a conceptual model ofdecomposition Aquatic Botany 5 329ndash354 httpsdoiorg1010160304-3770(78)90075-X

GuoXLuXTongSampGuohuaD(2008)Influenceofenvironmentandsubstrate quality on the decompositionofwetlandplant root in theSanjiangPlainNortheastChinaJournal of Environmental Science 20 1445ndash1452httpsdoiorg101016S1001-0742(08)62547-4

HackneyCampde laCruzA (1980) In situdecompositionof rootsandrhizomesoftwotidalmarshplantsEcology 61226ndash231httpsdoiorg1023071935178

HalupaPampHowesB(1995)EffectsoftidallymediatedlittermoisturecontentondecompositionofSpartina alterniflora and S patens Marine Biology 123379ndash391httpsdoiorg101007BF00353629

HarteJampShawR (1995)Shiftingdominancewithinamontanevege-tationcommunitymdashResultsofaclimatewarmingexperimentScience 267876ndash880httpsdoiorg101126science2675199876

HemmingaMAKokCJampdeMunckW (1988)DecompositionofSpartina anglicarootsandrhizomesinasaltmarshoftheWesterscheldeEstuary Marine Ecology Progress Series 48 175ndash184 httpsdoiorg103354meps048175

HerbertERBoonPBurginAJNeubauerSCFranklinRBArdonM hellip Gell P (2015) A global perspective on wetland salinizationEcologicalconsequencesofagrowingthreattofreshwaterwetlandsEcosphere 61ndash43httpdxdoiorg101890ES14-005341

HesterMMendelssohnIampMcKeeK (2001)Speciesandpopulationvariation to salinity stress in Panicum hemitomon Spartina patens and Spartina alternifloraMorphologicalandphysiologicalconstraints

668emsp |emsp emspenspJournal of Ecology STAGG eT Al

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HodsonRChrsitianRampMaccubbinA (1984)Lignocelluloseandlig-nin in the saltmarshgrassSpartina alterniflora Initial concentrationsand short-term post-depositional changes in detritalmatterMarine Biology 81 1ndash7

HopfenspergerKBurginASchoepferVampHeltonA (2014) Impactsofsaltwater incursiononplantcommunitiesanaerobicmicrobialme-tabolismandresultingrelationshipsinarestoredfreshwaterwetlandEcosystems 17792ndash807httpsdoiorg101007s10021-014-9760-x

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670emsp |emsp emspenspJournal of Ecology STAGG eT Al

WebsterJRampBenfieldEF(1986)Vascularplantbreakdowninfresh-waterecosystemsAnnual Review of Ecology Evolution and Systematics 17567ndash594httpsdoiorg101146annureves17110186003031

WeiderRampLangG(1982)AcritiqueoftheanalyticalmethodsusedinexaminingdecompositiondataobtainedfromlitterbagsEcology 63 1636ndash1642httpsdoiorg1023071940104

WelschMampYavittJ(2003)EarlystagesofdecayinLythrum salicariaLand Typha latifoliaL inastanding-deadpositionAquatic Botany 75 45ndash57httpsdoiorg101016S0304-3770(02)00164-X

WestonNDixonRampJoyeS(2006)Ramificationsofincreasedsalin-ity in tidal freshwater sediments Geochemistry andmicrobial path-waysoforganicmattermineralizationJournal of Geophysical Research Biogeosciencs 1111ndash14httpsdoiorg1010292005JG000071

WestonNNeubauerSCVelinskyDJampVileMA(2014)Neteco-system carbon exchange and the greenhouse gas balance of tidalmarshesalonganestuarysalinitygradientBiogeochemistry 120 163ndash189httpsdoiorg101007s10533-014-9989-7

WestonNVileMNeubauerSampVelinskyD(2011)Acceleratedmi-crobialorganicmattermineralizationfollowingsalt-waterintrusionintotidal freshwatermarsh soilsBiogeochemistry 102 135ndash151httpsdoiorg101007s10533-010-9427-4

White D amp Trapani J (1982) Factors influencing disappearance ofSpartina alterniflora fromlitterbagsEcology 63242ndash245httpsdoiorg1023071937047

Wilson J (1985) Decomposition of [14C]lignocelluloses of Spartina al-terniflora and a comparison with field experiments Applied and Environmental Microbiology 49478ndash484

Wilson J Buchsbaum RValiela I amp SwainT (1986)Decompositionin salt marsh ecosystems Phenolic dynamics during decay of litter

ofSpartina alterniflora Marine Ecology - Progress Series 29 177ndash187 httpsdoiorg103354meps029177

WindhamL (2001)Comparisonofbiomassproductionanddecomposi-tionbetweenPhragmites australis (commonreed)andSpartina patens (salthaygrass)inbrackishtidalmarshesofNewJerseyUSAWetlands 21 179ndash188 httpsdoiorg1016720277-5212(2001)021[0179COBPAD]20CO2

WoltersVSilverWBignellDColemanPvanderPuttenWdeRuiterPhellipvanVeenJ(2000)Effectsofglobalchangesonabove-andbelow-groundbiodiversityinterrestrialecosystemsImplicationsforecosystemfunctioningBioScience 501089ndash1098httpsdoiorg1016410006-3568(2000)050[1089EOGCOA]20CO2

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How to cite this articleStaggCLBaustianMMPerryCLCarruthersTJBHallCTDirectandindirectcontrolsonorganicmatterdecompositioninfourcoastalwetlandcommunitiesalongalandscapesalinitygradientJ Ecol 2018106655ndash670 httpsdoiorg1011111365-274512901