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ORGANICMATTERTURNOVERALONGANUTRIENTGRADIENT INTHEEVERGLADES By WILLIAMF.DEBUSK ADISSERTATIONPRESENTEDTOTHEGRADUATESCHOOL OFTHEUNIVERSITYOFFLORIDAINPARTIALFULFILLMENT OFTHEREQUIREMENTSFORTHEDEGREEOF DOCTOROFPHILOSOPHY UNIVERSITYOFFLORIDA 1996 UNIVERSITYOFFLORIDALIBRARIES ACKNOWLEDGEMENTS ThisresearchwassupportedinpartbytheU.S.DepartmentofAgriculture NationalResearchInitiativeCompetitiveGrantsprogram(GrantNo.92-37102-7542).The grantwasawardedtoLouisianaStateUniversityandUniversityofFlorida.Graduate FellowshipswerefundedinpartbytheUSDANationalNeedsFellowshipProgramfor studiesinthefieldofwaterscience.Ithankthosewhoselectedmeasarecipientofthis award.Thanksalsogotoallwhodonatedtheirassistance,supportandexpertise, especiallyDr.K.R.ReddyandMs.YuWang.Mostimportantly,thanksgotomywife Pattyforhermoral(and,ofcourse,financial)support. ii TABLEOFCONTENTS page ACKNOWLEDGEMENTS ii ABSTRACT v CHAPTERS 1 INTRODUCTION 1 Background 1 WetlandCarbonCycle 2 MicrobialEcology 8 FactorsAffectingDecompositionRate 9 ModelingDecompositionofHeterogeneousSubstrates 13 EcosystemModels 17 EvergladesStudySite 19 ObjectivesandScopeofResearch 21 2 ORGANICCMINERALIZATIONASAFUNCTIONOF NUTRIENTENRICHMENTANDHYDROLOGY 23 Introduction 23 MaterialsandMethods 26 Results 35 Discussion 51 SummaryandConclusions 61 3 TURNOVEROFORGANICCARBONPOOLSALONG THEWCA-2ANUTRIENTGRADIENT 62 Introduction 62 MaterialsandMethods 67 Results 74 Discussion 88 SummaryandConclusions 98 4 REGULATORSOFORGANICMATTERDECOMPOSITION ALONGTHEWCA-2ANUTRIENTGRADIENT 101 Introduction 101 MaterialsandMethods 105 Results 112 Discussion 134 SummaryandConclusions 141 iii 5 DETRTTALCARBONMODEL 143 Introduction 143 MaterialsandMethods 143 ResultsandDiscussion 155 Conclusions 161 6 SUMMARYANDCONCLUSIONS 163 LISTOFREFERENCES 167 BIOGRAPHICALSKETCH 176 iv AbstractofDissertationPresentedtotheGraduateSchool oftheUniversityofFloridainPartialFulfillmentofthe RequirementsfortheDegreeofDoctorofPhilosophy ORGANICMATTERTURNOVERALONGANUTRIENTGRADIENT INTHEEVERGLADES By WilliamF.DeBusk August,1996 Chairman: Dr.K.R.Reddy Co-chairman:Dr.J.W.Jones MajorDepartment: SoilandWaterScience Organicmatteraccumulationinwetlandsrepresentsapotentiallong-termsinkand sourcefororganiccarbon(C)andassociatednutrientsandcontaminants.Turnoverof organicCwasmeasuredinanutrient-impactedsawgrassandcattailmarshinEverglades WaterConservationArea2A(WCA-2A).Controlledlaboratoryincubationsanda microcosmstudywereconductedtodeterminepotentialratesofCmineralizationinplant litterandpeatalongagradientofphosphorus(P)enrichment.Fieldincubationsat10sites alongthenutrientgradientmeasuredinsituorganicmatterdecompositionratethroughout thefloodwater,litterandpeatprofile. OrganicCmineralizationinwetlandmicrocosmswassignificantlyenhancedby. interactiveeffectsofincreasedPavailabilityanddecreasingwatertable.Approximately 90%ofthevariabilityinpotentialorganicCmineralizationinpeatandplantlitter,measured underaerobicandanaerobicconditions,wasexplainedbytotalPandlignocellulosecontent oftheorganicsubstrate.Anaerobicmineralizationrateswere32%oftheratesmeasured underaerobicconditions.Insituorganicmatterdecompositionratewashigherinnutrient- enrichedareasofWCA-2Athaninthelow-nutrientinteriormarsh.Decompositionrate typicallywasatamaximuminthefloodwaterandlitterlayeranddecreasedwithdepthin thepeatprofile.Fieldstudiesprovidedevidencethatmicrobialdecomposersobtain nutrients,especiallyP,fromthesurroundingfloodwaterandsoilporewateraswellasfrom theorganicsubstrate. ResultsoflaboratoryandfieldstudiesindicatethatorganicCturnoverinWCA-2A isstronglyaffectedbyPavailability,although02availabilityisthemajorcontrollingfactor. AvailabilityofC(substratequality)andnitrogen(N)maylimitturnoverrateunderP- enrichedconditions. Experimentalfindingsfromthesestudiesprovideinsightintothe effectsofacceleratednutrientloadingonCcyclingandnetaccumulationoforganicmatter andnutrientsinwetlands. vi CHAPTER1 INTRODUCTION Background Organiccarbon(C)accumulationinwetlandsisthemassbalancebetweennet primaryproduction(Cfixation)andheterotrophicmetabolism(Cmineralization).Organic Cinplantlitter,peatorsoilorganicmatter(SOM)servesasthesourceofenergytodrive thedetritalfoodchaininwetlands.Mostoftheorganicmatterproducedinwetlandsis depositeddirectlyinthedetritalpool(Moranetal.,1989;Wetzel,1992),thusmicrobial decomposersplaythemajorroleinCcyclingandenergyflowinwetlands. Burialoforganicmatteraspeatprovidesameansforlong-termstorageofelements associatedwithorganicC,suchasnutrientsandheavymetals(Clymo,1983). Allochthonouscompoundsmaybeincorporatedintopeatandsoilorganicmatterthrough plantuptakeandsenescence,immobilizationwithinthesoilorganicmatrixby physical/chemicalprocessessuchasadsorption,occlusionandprecipitation,orthrough uptakebymicrobialdecomposers,withstorageeitherwithinlivingcellsormetabolicby- products.Onamuchbroaderscale,storageoforganicCinwetlandsoilisanimportant componentoftheglobalcarboncycleandthusmayimpactlarge-scale processessuchas globalwarmingandozonedepletion(HappellandChanton,1993;Whiting,1994). Underfavorableconditionsfororganicmatterdecomposition,storednutrientsor contaminantsmaybereleasedthroughmineralizationandthenrecycledintheecosystemor exportedfromthesystem(Ponnamperuma,1972;ReddyandD'Angelo,1994).Therateof netorganicmatteraccumulationisacriticaldeterminantofhowawetlandfunctionsasan ecologicalunitwithinthelandscape.Thestoragefunctionisequallyimportantfornatural 1 2 wetlands,especiallythosewhichrepresentanecotonebetweenterrestrialandaquatic ecosystems,andcreatedwetlands,whichmaybeusedfortreatmentofwastewateror runoff(Howard-Williams,1985). WetlandCarbonCycle FromaconceptualstandpointthecontinuumoforganicCtransformationsand flowswhichconstitutethewetlandCcyclemayberepresentedasacollectionofdiscrete storageunits,orcompartments,withsimultaneoustransferofmassamongthe compartments(Figure1-1).Thevegetationcomponent(includingmacrophyticandalgal species)representstransformersofinorganicC(C02)toorganicC(primaryproduction) throughtheprocessofphotosynthesis.Theheterotrophicmicrofaunarepresent transformersoforganicCbacktoinorganicCthroughcellularrespiration.OrganicCis storedinthesysteminliving(vegetationandmicrobialbiomass)andnon-living(deadplant tissue,plantlitter,peatorSOM)components.Non-livingstorageoforganicCis proportionallylargeinwetlandsinrelationtootherecosystems;thisstorageprovidesa substantialenergyreservetotheecosystemwhichisslowlyreleasedthroughthedetrital foodweb(Wetzel,1992). Peat Peatistheresultofbiological,chemicalandphysicalchangesimposedonplant remainsoveranextendedtimeperiod.Extentofdecomposition,orhumification,is qualitativelyassessedbytheextenttowhichplantstructureispreserved(Givenand Dickinson,1975;Clymo,1983).Numerousorganicconstituentshavebeenisolatedfrom peat,andmanyhavebeenusedinassessingthedegreeofdecomposition,althoughthese aregenerallyclassesandsubclassesoforganiccompoundsratherthandiscretecompounds. Forexample,peatmaterialsolubleinnon-polarsolventsisoftentermed"wax,"but includesnumerouscompoundsotherthanwaxes(estersoffattyacidswithalcoholsother 3 Figure1-1. Conceptualdiagramoftheorganiccarboncycleinawetlandecosystem. 4 thanglycerol)(Clymo.1983).Althoughasmallportionoftotalpeatmass,thisfractionis ofinterestwhendeterminingoriginsofpeat,sincefattyacidsoflipidscanbetraceddirectly orindirectlytotheoriginalplanttype(GivenandDickinson,1975;Borgaetal.,1994). Varioustypesofacidoralkalihydrolysisprocedureshavebeenusedtoisolatefractions roughlyequivalenttocellulose,hemicelluloseandlignin.Proportionsofcelluloseand hemicelluloseinplantlitterandpeattendtodecreasewithage(i.e.decompositionor "humification"),whilelignincontentincreaseswithage; thesethreestructuralgroupshave allbeenusedtocharacterizethedegreeofpeatdecomposition(Clymo,1983;Brownetal., 1988;Bohlinetal.,1989).Analysisofpeathasalsorevealedalargevarietyofphenolic compounds,manyofwhichmaybeextractedinthe"lignin"fraction.Concentrationof celluloseandligninismuchhigherinthefibrousfraction(remnantplantparts)ofpeat, whilehumicacidsaremuchmoreprevalentinthehumusfraction,althoughfulvicacids werefoundinsomewhatgreateramountsinthefibrousfraction(GivenandDickinson, 1975).Humicacidcontentoftemperatepeatsshowedatendencytoincreasewithageand depth,butpeatsintropicalandsubtropicalregions,includingtheEverglades,didnot reflectthistrend.Interpretationofchemicalanalyisisofpeatmaybecloudedbyuncertainty abouttheextenttowhichpresentcharacteristicsrepresenthistoricaldifferencesin vegetationversusvariabilityofdecompositionprocesses. Thedegreeofdecompositiongenerallyincreaseswithdepthofthepeat,therefore, theorganicmatterbecomesincreasinglyhumifiedatgreaterdepth(Clymo,1983). Assuminganhistoricallyconstantqualityandquantityofsubstrateadditiontothesoil,the rateofdecompositionisgreatestintheupperregionsoftheprofile,decreasingwithdepth. Thisisduenotonlytoantecedentdecompositionofsubstrateinthesoilprofile,buttothe verticalgradientofsoilenvironmentalparameters.Thelattercategorymayinclude dissolved concentration(insaturatedsoil),soilmoisture(unsaturatedsoil),andnutrient availability.Allofthesevariables,particularlythefirsttwo,arefunctionsofhydrologic conditions.Dissolved02concentrationisaffectedbothbytherateofdiffusionfromthe