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Effects of Fire Frequency and Soil Temperature on Soil CO2 Efflux Rates in Old-Field Pine-Grassland Forests

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Title:
Effects of Fire Frequency and Soil Temperature on Soil CO2 Efflux Rates in Old-Field Pine-Grassland Forests
Series Title:
Godwin, D.R.; Kobziar, L.N.; Robertson, K.M. Effects of Fire Frequency and Soil Temperature on Soil CO2 Efflux Rates in Old-Field Pine-Grassland Forests. Forests 2017, 8, 274.
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Godwin, David
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MDPI
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English
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Journal Article

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Soil CO2 efflux (Rs) is a significant source of carbon dioxide from soils to the atmosphere and is a critical component of total ecosystem carbon budgets. Prescribed fire is one of the most prevalent forest management tools employed in the southeastern USA. This study investigated the influence of prescribed fire on Rs rates in old-field pine-grassland forests in north Florida, USA, that had been managed with prescribed fire annually and biennially for over 40 years, or left unburned for approximately the same period. Monthly measurements were taken of Rs, soil temperature (Ts), and soil moisture from August 2009 to May 2011. Results showed that sites managed with annual and biennial dormant season prescribed fire had significantly lower monthly mean Rs rates and estimated annual soil carbon fluxes than sites where fire had been excluded. While Ts explained a significant amount of the temporal variations in Rs, it did not explain the differences in Rs among prescribed fire treatments. Our results provide new insight into the effects of prescribed fire and fire exclusion on soil carbon fluxes, and suggest that future methods to model ecosystem carbon budgets should incorporate not only current vegetative conditions, but also prescribed fire management activities.
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Collected for University of Florida's Institutional Repository by the UFIR Self-Submittal tool. Submitted by David Godwin.

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Copyright Creator/Rights holder. Permission granted to University of Florida to digitize and display this item for non-profit research and educational purposes. Any reuse of this item in excess of fair use or other copyright exemptions requires permission of the copyright holder.

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Article EffectsofFireFrequencyandSoilTemperature onSoilCO 2 EfuxRatesinOld-Field Pine-GrasslandForests DavidR.Godwin 1, *,LedaN.Kobziar 2 andKevinM.Robertson 3 ID 1 SchoolofForestResourcesandConservation,UniversityofFlorida,Gainesville,FL32611,USA 2CollegeofNaturalResources,NaturalResourcesandSociety,UniversityofIdaho,Moscow,ID83844,USA;lkobziar@uidaho.edu 3 TallTimbersResearchStationandLandConservancy,LeonCounty,FL32308,USA;krobertson@ttrs.org Correspondence:drg2814@u.edu;Tel.:+1-850-893-4153 AcademicEditors:RobertHarrisonandTimothyA.Martin Received:28June2017;Accepted:28July2017;Published:30July2017 Abstract:SoilCO2efuxRsisasignicantsourceofcarbondioxidefromsoilstotheatmosphereandisacriticalcomponentoftotalecosystemcarbonbudgets.PrescribedreisoneofthemostprevalentforestmanagementtoolsemployedinthesoutheasternUSA.ThisstudyinvestigatedtheinuenceofprescribedreonRsratesinold-eldpine-grasslandforestsinnorthFlorida,USA,thathadbeenmanagedwithprescribedreannuallyandbienniallyforover40years,orleftunburnedforapproximatelythesameperiod.MonthlymeasurementsweretakenofRs,soiltemperatureTs,andsoilmoisturefromAugust2009toMay2011.ResultsshowedthatsitesmanagedwithannualandbiennialdormantseasonprescribedrehadsignicantlylowermonthlymeanRsratesandestimatedannualsoilcarbonuxesthansiteswhererehadbeenexcluded.WhileTsexplainedasignicantamountofthetemporalvariationsinRs,itdidnotexplainthedifferencesinRsamongprescribedretreatments.Ourresultsprovidenewinsightintotheeffectsofprescribedreandreexclusiononsoilcarbonuxes,andsuggestthatfuturemethodstomodelecosystemcarbonbudgetsshouldincorporatenotonlycurrentvegetativeconditions,butalsoprescribedremanagementactivities. Keywords: soilrespiration;prescribedre;soilCO 2 efux;soiltemperature;forests 1.IntroductionTheinuenceoflong-termforestmanagementpracticesonsoilCO2efuxRsratesplaysakeyroleindeterminingtotalecosystemcarbonbudgets[13].IthasbeenestimatedthatsoilCO2efuxrepresentsoneofthelargestglobalterrestrialuxesofcarbontotheatmosphere,withthetotalannualRsuxPgCyear)]TJ/F223 7.5716 Tf 6.228 0 Td [(1anorderofmagnitudegreaterthancurrentannualanthropogeniccarbonCemissionsfromfossilfuelcombustionPgCyear)]TJ/F223 7.5716 Tf 6.228 0 Td [(1[4].SoilCO2efuxisafunctionofvariousinterrelatedbiogeochemicalfactorsthatgoverntheproductionofautotrophicsoilCO2efuxbyplantrootsandassociatedmycorrhizalfungiRaandheterotrophicsoilCO2efuxbysoilmicroandmacrobiotaR h [36].FirecaninuenceRsratesbydifferentiallyimpactingtheRhandRasourcesofCO2[4,7].Forexample,short-termautotrophicproductionofCO2canbereducedbyreduetobothabovegroundandbelowgroundplantmortalityandinjury.Whilethelong-termimpactsofreonRaarevariable,Rahasbeenshowninsomecasestoincreasewithtimesincereasvegetationrecoversfollowingdisturbance[8].Inmanycases,theburningofvegetationandsurfacefuelsreallocatesnutrientresourcesviaincompletecombustionandsubsequentdepositionofash,char,andotherresidues[8,9].Intheshort-termperiodfollowingthedepositionofthoseresidues,bothplantsandsoilmicrobesmayForests 2017 8 ,274;doi:10.3390/f8080274www.mdpi.com/journal/forests

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Forests 2017 8 ,274 2of14respondpositivelytotheavailabilityofsuchresources,subsequentlyincreasingRsrates.Long-termprescribedremanagementregimesmayalsoimpactRhsourcesofCO2,byinuencingvegetationcomposition,structureandassociatedlitterandduffquality,production,andaccumulationrates[10]whichinturnhavebeenshowntoinuencesoilmicrobialpopulationsandmetabolicactivity[11].FirecanalsoreduceRhbykillingsoilmicrobesthroughheatingoflitteranddufflayersanduppersoilhorizons[4].PreviousstudiesinmultipleecosystemshaveshownthatbothreandforestmanagementcaninuenceRsrates,soilcarbonpools,andvariouscoupledbiogeochemicalprocesses[1216].Forexample,inastudyofamixedconiferforestinCalifornia,USA,Ryuetal.[17]foundthatprescribedrereducedRsrateswhilesimultaneouslyalteringsoilconditionsthatwouldotherwisebeassociatedwithincreasedRsrates.However,twostudiesthatinvestigatedtheinuenceofprescribedreinadifferentmixedconiferforestinCaliforniaandanuploadoakQuercusspp.LforestinMissouri,USA,foundthatwhileprescribedburningsignicantlyalteredforestoorconditions,therewasnocleareffectonRsrates[18].ThecontrastingresultsreportedbythesestudiesdemonstratethecomplexnatureofpredictingtheinuenceofprescribedreonRs.WhilemanyfactorsinuencingRsrateshavebeenidentied,muchremainstobedeterminedregardingtheeffectsofspecicforestandlandmanagementpracticesonoverallRsrates[24],especiallywheremanagementhasbeenlong-termratherthanexperimental.Frequentprescribedreisoneofthedominanttoolsforforestmanagementinthepine-grasslandforestsofthesoutheasternUSA[19,20].Manyofthecurrentpine-grasslandforestsintheregionareold-eldforestassemblagesthatexistonformeragriculturallandsestimatedtocoverupto21millionhaacrossthesoutheasternUSA[21].Acrossmanyoftheseforests,frequentyearreturnintervalprescribedreisusedtoperpetuatenativespeciesassemblages,reducetheriskofdestructivewildres,andpromoteandrestorewildlifehabitat[22].Althoughlessfrequentlycited,theimpactsofprescribedreregimesonsoilcarbonsequestrationarelikelytoincreaseinimportanceasimportantcommoditytradepartnersadoptcarboncreditexchangese.g.,Canada.Thelongtermconsequencesofvaryingrereturnintervalsforcarboncyclinginpinegrasslandscaninformremanagementdecisionsandprojectionsofcarbonsequestrationcapacity.Thisstudysoughttoinvestigatetheimpactsofreregime,specicallyannualburning,biennialburning,andprolongedreexclusion,onRsratesinold-eldpine-grasslandforests.Inaddition,thisstudysoughttointerpretthepotentialresponseofRstobioticandabioticfactors,includingsoiltemperature,soilmoisture,foreststandcharacteristics,andsoilphysicalpropertiesandchemistry.Theintentofthisresearchistobuilduponourunderstandingoftheeffectsoffrequentreoncarbondynamicsandsequestrationinpine-grasslandsandsimilarwoodlandandsavannacommunitiesworldwide.Studiessuchasthisalsoprovideinsightintotheresponseofecosystemcarbondynamicstoforecastedchangesintemperatureandmoistureregimesduetoglobalclimatechange. 2.MaterialsandMethods 2.1.StudySiteThestudywasconductedonTallTimbersResearchStationTTRSinLeonCounty,FL,USA,approximately30kmfromthecitiesofTallahassee,FloridatothesouthandThomasville,Georgiatothenorth390N,84120W;Figure1.Thestudyutilizedresearchplotsestablishedonoldeldlandforlong-termstudyofreregimes.TheStoddardFirePlots,whichare0.2haplotsestablishedin1960,haveeachbeenmanagedwithaconstantrereturnintervaluntilthepresent[10,23,24].Thestudyalsoutilizeda9.2haplotnamedNB66thathadbeenre-excludedsince1966[24].Priortoestablishmentoftheplots,theareashadbeenburnedat1yearintervalssinceagriculturalabandonmentwhichoccurredfromthelate1800stothe1920s.Soilswithinthesiteswereheavilycultivatedforgrowingcornandcottonfromthe1820suntilabandonmentwithsubsequentunderstoryandoverstoryvegetationassemblageshighlyinuencedbypastagriculturalpractices[23]butlargelyrepresentingasubsetofspeciesoccurringinnativepinecommunitiesoftheregion[10,25].Soilswere

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Forests 2017 8 ,274 3of14 classiedasne-loamy,kaolinitic,thermicTypicKandiudultsoftheOrangeburgandFacevilleseries NaturalResourceConservationServiceNRCSoilSurveyGeographicDatabaseSSURGO. Figure1.Theresearchsite,TallTimbersResearchStation,waslocatedinLeonCounty,FL,USA.Thesiteisapproximately30kmnorthofthecityofTallahassee,FL,USA.Forthisstudy,samplingtookplacewithinthreeannuallyburnedYR,threebienniallyburnedYR,andtwore-excludedUBStoddardFirePlots,andanadditional0.2hare-excludedstudyareaUBwasestablishedwithintheTallTimbersNB66studysite,makingthreereplicatesofeachreregime[10,23].Replicatesetsofplotseachcontainingthethreereregimesweregroupedwithinsoilunits,suchthattwosetswerewithinFacevilleFine,kaolinitic,thermicTypicKandiudultsoilunitsandonewasinanOrangeburgFine-loamy,kaolinitic,thermicTypicKandiudultsoilunit.Thestudysiteswerelocatedapproximately60ma.s.l.Averageannualprecipitationwas137cmwiththemajorityfallingduringthesummermonthsofJune,JulyandAugustNationalClimateDataCenter2009,Thomasville,GA,USA.MeanmaximumandminimumtemperaturesforJanuaryandJulyfortheareafromlong-termrecordsare16.8Cand4.6CforJanuaryand33Cand21.8CforJulyNationalClimateDataCenter2009,Thomasville,GA,USA.Theoverstoryofthe1YRand2YRburnedplotsconsistedofamixtureofnaturallyregeneratedshortleafpinePinusechinataP.Mill,loblollypineP.taedaL.,and,toalesserextent,longleafpineP.palustrisP.Mill.Theunderstorywascomposedofamixtureofgrasses,forbs,andbroadleafwoodyplantswhicharetypicallytopkilledbyprescribedreandthenresprout[10,19,23,26].Theunburnedplots,duetotheprolongedreexclusion,containedaclosedmidstoryandoverstorycanopyofbroadleafdeciduoustreesofspeciesincludingbutnotlimitedto:wateroakQ.nigraL.,laureloakQ.laurifoliaMich.,sweetgumLiquidambarstyraciuaL.,blackcherryPrunusserotinaEhrh.,andoweringdogwood Cornusorida L.,inadditiontomaturepinetrees. 2.2.SamplingTheStoddardFirePlotscombinedwithNB66representingeachofthreeprescribedreregimesYR,2YR,UBweregroupedintoblocksreplicatedthreetimesforatotalofnineplots.SamplingforthisstudytookplacewithinsubplotsrandomlylocatedwithineachoftheStoddardFirePlots

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Forests 2017 8 ,274 4of14andNB66.Toaccountforspatialvariabilitywithinthesubplots,measurementsweretakenatninelocationswithininasquaregridarrangementwith5mseparation,followingKobziar[16].RsmolCO2m)]TJ/F223 7.5716 Tf 6.228 0 Td [(2 s)]TJ/F223 7.5716 Tf 6.227 0 Td [(1wassampledateachsubplotusingaLI-CORBiosciencesLI-8100automatedsoilCO2samplinginstrumentwitha20cmdiametersurveychamberLI-CORBiosciencesInc.,Lincoln,NE,USA.Thesurveychambertonto20cmdiameter10cmheightPVCcollarsinserted8cmintotheground.SamplingofRsforallplotsoccurredmonthly.Duringeachmonthlyvisit,allmeasurementsweretakenwithintwodays.Overthecourseofstudy,anyvegetativegrowthwithinthesamplecollarswasclippedandremovedpriortoRsmeasurement.ThePVCcollarswereinstalledinJuneandJuly2009andsamplingbeganinAugust.ThePVCcollarsandsoilsurfacescontainedwithinthemwereprotectedfromreduringburnsinMarch2010YRplotsandMarch2011YRand2YRplotsbyplacingslightlylargerdiametersheet-metalcylindersaroundthem.Assuch,themeasurementswerenotexpectedtorespondtodirectimpactsofprescribedre,e.g.,changestosoilchemistryorinjuryofmicroorganismsnearthesoilsurface,butrathergeneralenvironmentalconditionscorrespondingtotherespectivereregime.Samplingconsistedofa120smeasurementinitiatedbya15sdead-bandduringwhichgaseswereallowedtimetomixwithinthechamber.DuringRsmeasurements,adjacenttoeachPVCcollar,soiltemperatureTsCwasmeasuredat10cmdepthusinganOmega8831typeET-Handletemperatureprobe,andsoilmoisturecontentMsm3/m3wasmeasuredat5cmdepthusingaDecagonSystemsEC-5soilmoistureprobemountedontheLI-8100OmegaInc.,Stamford,CT,USA;DecagonSystemsInc.,Pullman,WA,USA.FromAugustof2009untilFebruary2010,eachmonthlysamplingperiodinvolvedmeasurementofRs,Ts,andMseighttimesperdayinordertoassessfordiurnalvariability.Ananalysisofthepreliminaryresultsfoundthatfewerdailymeasurementswouldbesufcienttocapturethevariability,sodailymeasurementswerescaledbacktothreetimesperdaymorning,mid-dayandlateafternoon-earlyeveningfromMarch2010untiltheendofthestudyinMay2011.EquipmentproblemsresultedinnoeldmeasurementsbeingtakenduringthemonthofOctober2010,butotherwisetherewereonlyfewinterruptionsduetoequipmentproblemsorseverethunderstorms.Theresultingdatasetfortheentiretwenty-one-monthstudytotaled7566Rsmeasurements.SomestrongoutliersinRs,Ts,andMswereattributedtomeasurementorequipmenterrorandwereisolatedandexcludedfromtheanalyses.Withina15mradiuscircularplot.07hacenteredonthemiddlePVCcollarineachsubplot,basalareaBAm2 ha)]TJ/F223 7.5716 Tf 6.227 0 Td [(1,pinebasalareaPBAm2 ha)]TJ/F223 7.5716 Tf 6.227 0 Td [(1,hardwoodbroadleaftreebasalareaHWBAm2 ha)]TJ/F223 7.5716 Tf 6.227 0 Td [(1,andstanddensityTPHtreesha)]TJ/F223 7.5716 Tf 6.227 0 Td [(1weremeasuredonlycountingtreesdiameteratbreastheight>10cm.Foreststandcharacteristicswereassessedoncepersubplotinthewinterorspringof2011.SoiltotalcarbonandtotalnitrogenweremeasuredinJanuary2013Table1.SubplotlevelmeanlitterdepthLittercm,duffdepthDuffcmandtotallitterandduffdepthDLcmwererecordedastheaverageofthreemeasurementstakenfromrandomlocationswith30cmofeachPVCcollar.Soilwassampledusinga2cmdiametersoilcorerto10cmdepthat30locationsspreadthroughouteachplotthencombined,homogenized,andsplittoobtainsubsamplesforanalysis.SampleswereanalyzedatAuburnUniversitytodeterminepercenttotalcarbonandnitrogenwithaTruSpecCNanalyzerLecoCorp.,St.Joseph,MI,USAusingthedrycombustionmethod,andmg)]TJ/F223 7.5716 Tf 6.227 0 Td [(1 kg)]TJ/F223 7.5716 Tf 6.228 0 Td [(1ofphosphorus,calcium,magnesium,andpotassiumweredeterminedwithaniCAPanalyzerThermoFisherScienticInc.,Waltham,MA,USAusingMehlich3extraction.Bulkdensityto10cmdepthwassampledatvelocationswithineachplotusinga4.5cmdiametersoilsamplerEijkelkampCorp.,Giesbeek,TheNetherlands.Bulkdensitymeasurementswereusedtoconvertsoilchemistrymeasurementstomassperunitareakg )]TJ/F223 7.5716 Tf 6.228 0 Td [(1 orMg )]TJ/F223 7.5716 Tf 6.228 0 Td [(1 per ha )]TJ/F223 7.5716 Tf 6.227 0 Td [(1 .

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Forests 2017 8 ,274 5of14 Table1.Environmentalvariablesmeasuredineachresearchplot,withabbreviation,unitofmeasurement,andmeasurementregimeprovided.MonthlymeasurementsweremadefromAugust2009toApril2011. PlotVariableAbbreviation/UnitsMeasured SoiltemperatureT s CMonthly Soilmoisturecontent M s m 3 /m 3 Monthly Basalarea BAm 2 ha )]TJ/F223 6.9738 Tf 5.736 0 Td [(1 Winter2011 Pinebasalarea PBAm 2 ha )]TJ/F223 6.9738 Tf 5.736 0 Td [(1 Winter2011 Hardwoodbasalarea HWBAm 2 ha )]TJ/F223 6.9738 Tf 5.736 0 Td [(1 Winter2011 Standdensity TPHtreesha )]TJ/F223 6.9738 Tf 5.736 0 Td [(1 Winter2011 DuffdepthDuffcmSpring2011 LitterdepthLittercmSpring2011 Soilbulkdensity0cm BDg cm )]TJ/F223 6.9738 Tf 5.736 0 Td [(3 Winter2013 Soiltotalcarbon0cm CMg ha )]TJ/F223 6.9738 Tf 5.736 0 Td [(1 Winter2013 Soiltotalnitrogen0cm NMg ha )]TJ/F223 6.9738 Tf 5.736 0 Td [(1 Winter2013 Soilphosphorus0cm Pkg ha )]TJ/F223 6.9738 Tf 5.736 0 Td [(1 Winter2013 Soilcalcium0cm Cakg ha )]TJ/F223 6.9738 Tf 5.736 0 Td [(1 Winter2013 Soilmagnesium0cm Mgkg ha )]TJ/F223 6.9738 Tf 5.736 0 Td [(1 Winter2013 Soilpotassium0cm Kkg ha )]TJ/F223 6.9738 Tf 5.736 0 Td [(1 Winter2013 SoilpH0cmpHWinter2013 2.3.AnalysisAnalysessoughttotestforsignicantdifferencesamongthethreereregimeswithregardtoRs,Ts,Msandforestenvironmentalconditionsmeasuredintheeld.Foreachmonth,dailymeasurementsperPVCcollarwereaveraged,andtheninePVCcollarmeanswereaveragedtoproduceasubplot-levelmeanvalueforeachmonth.RepeatedmeasuresanalysisofvarianceANOVAwasusedtotestfordifferencesinthesemonthlymeansamongthethreereregimesn=3perreregimeforRs,Ts,andMsoverthetwenty-onemonthlysamplingperiodsbetweenAugust2009andMay2011.Signicanttreatmenteffectswereidentiedatp-value<0.05.Toassessfordifferencesinforestenvironmentalconditionsamongthethreereregimes,one-wayANOVAtestswereappliedtoeachvariableseparately.Wheresignicantdifferenceswereidentied,differenceswerefurtherinvestigatedusingTukey'sHSDtest.Additionally,linearandnonlinearregressionmodelsEquationsandweredevelopedtopredictthemonthlyresponseofRstoTsaswellasRstoMsinseparateequationswithineachreregimeandwithineachseason.Non-linearmodelsoftherelationshipsbetweenRsratesandTsperreregimewereexploredusinganexponentialequationEquationfrequentlyusedtodescribetheresponseofRsratestosoiltemperature[15,18,27,28].The 1estimatesdevelopedusingEquationwereusedtoestimateQ10Equation,whichdescribestheresponseofRstoa10Cchangeinsoiltemperature[4,15,27].AllstatisticalanalyseswereperformedusingJMP9.0SASInstitute,Cary,NC,USA. R s = 0 + 1 parameter R s = 0 e 1 T s orR s = 0 e 1 MTemp Q 10 = e 10 1 whereMTempwasairtemperature, 0,and 1werecoefcientsestimatedthroughregressionanalysis.TotalmonthlyandannualsoilcarbonemissionsperreregimewereestimatedusingthenonlinearregressionmodelsofRsresponsesEquationtochangesinambienttemperatureMTempfollowingSamuelsonetal.[28].Twenty-fourhour2melevationambienttemperaturemeasurementsrecordedhourlyfrom1August2009July2010attheQuincy,FLAutomatedWeatherNetworksitelocatedapproximately30kmfromthestudysiteswereusedastheMTempinputtoestimatehourlyRsrates.TheestimatedRsratesmolCO2m)]TJ/F223 7.5716 Tf 6.228 0 Td [(2 s)]TJ/F223 7.5716 Tf 6.228 0 Td [(1werethenconvertedtohourlysoilcarbonuxesgCm)]TJ/F223 7.5716 Tf 6.228 0 Td [(2 h)]TJ/F223 7.5716 Tf 6.227 0 Td [(1whichwerethensummedtoestimatemonthlyandannualsoilcarbonuxesMgCha )]TJ/F223 7.5716 Tf 6.227 0 Td [(1 year )]TJ/F223 7.5716 Tf 6.228 0 Td [(1 .

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Forests 2017 8 ,274 6of14 3.Results 3.1.DifferencesinR s ,M s ,T s ,andVegetationamongFireRegimesMeanRsratesvariedsignicantlybetweenreregimesp=0.0007,withthehighestRsratestypicallyintheUBtreatmentandthelowesttypicallyinthe1YRtreatmentFigure2;Tables2and3.ThetreatmenteffectofreregimeonR s ratesvariedmonthlytreatment time p <0.0001,withthegreatestdifferencebetweenreregimesobservedduringthesummermonthsandtheleastduringthewintermonthsTable3.Acrossalltreatments,Rsrangedfrom0.98molCO2m)]TJ/F223 7.5716 Tf 6.228 0 Td [(2 s)]TJ/F223 7.5716 Tf 6.228 0 Td [(1duringthestudyperiod,withthehighestRsratesduringthewarmermonthsandthelowestratesduringthecoolermonthsFigure2. Table2.ResultsoftherepeatedmeasuresANOVAshowingeffectsofprescribedreregimeFR;andtimemonthlymeansofmeasurementsonsoilCO2efuxRs,soiltemperatureTsandsoilmoisturecontentM s n =9threereplicateswithinthreetreatments. Analysis Period Source R s T s M s dfFp-ValuedfFp-ValuedfFp-Value Entire Study Month20105.19<0.000118306.76<0.00011997.51<0.0001FR Month403.25<0.0001364.04<0.0001381.680.0190 FR220.720.000723.090.100722.950.1130 Fall Month5114.16<0.00015241.78<0.0001566.73<0.0001FR Month104.120.0012101.310.2684102.060.0611 FR217.650.003122.700.145724.260.0705 Winter Month525.16<0.0001361.75<0.0001525.45<0.0001FR Month101.960.076160.400.8650101.960.0753 FR213.400.0061210.480.003626.300.0335 Spring Month525.61<0.00015243.07<0.0001575.96<0.0001FR Month102.730.0168105.500.0001101.030.4454 FR211.240.005424.110.064621.410.3003 Summer Month216.700.0003238.05<0.00011139.44<0.0001FR Month43.780.032744.700.016323.020.1235 FR216.030.003929.910.012621.320.3340 p -Valueswereconsideredsignicantat =0.05, df =degreesoffreedom. Table3.MeanseasonalandtotalstudyperiodsoilCO2efuxratesmolCO2m)]TJ/F223 6.9738 Tf 5.736 0 Td [(2 s)]TJ/F223 6.9738 Tf 5.736 0 Td [(1Rsperprescribedreregime. FRFallR s WinterR s SpringR s SummerR s StudyMeanR s 1YR3.36.71b1.24.87b2.56.21b4.93.95b2.67.87b 2YR3.75.02b1.60.24b3.09.39b5.49.11b3.09.12b UB4.93.36a2.61.49a3.98.94a7.59.25a4.22.52a Valuesareseasonalmeanswithstandarddeviationsinparentheses.LettersshowsignicantdifferencesbetweenreregimesTukey'sHSDtest. p -Valueswereconsideredsignicantat =0.05.

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Forests 2017 8 ,274 7of14 Figure2.MonthlymeansoilCO2efuxratesRsmolCO2m)]TJ/F223 6.9738 Tf 5.736 0 Td [(2 s)]TJ/F223 6.9738 Tf 5.736 0 Td [(1forthreeprescribedreregimesattheTallTimbersResearchStationnearTallahassee,FL,USA.Datawerefrom27samplepointsperreregime,measuredthree-timesdailyoncepermonth.MeasurementswerenottakeninOctoberof2010.Althoughsoiltemperaturedidnotdiffersignicantlyamongthereregimesp=0.1007,thelowesttemperatureswereintheUBsitesandthehighestinthe1YRsitesFigure3.AdistinctseasonalTstrendamongthereregimeswasobserved,withthe1YRsitesrecordingthehighestmeanTsinthespringthroughfallseasonsandtheUBsitesrecordingthehighestmeanTsinthewinterseasons.SoilmoistureMsdidnotdiffersignicantlyamongthereregimesp=0.11,althoughthelowestsoilmoisturecontentswereobservedintheUBsitesandthehighestobservedinthe1YRsitesFigure4;Table2. Figure3.MonthlymeansoiltemperatureTsCforthreeprescribedreregimesattheTallTimbersResearchStationnearTallahassee,FL,USA.Meanmonthly2mairtemperaturealsoshownforcomparison.Datawerefrom27samplepointsperreregime,measuredthree-timesdailyoncepermonth.EquipmentproblemseliminatedmeasurementstakeninJanuary2009andFebruary2011.MeasurementswerenottakeninOctoberof2010.

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Forests 2017 8 ,274 8of14 Figure4.MonthlymeansoilmoisturecontentMsm3/m3forthreeprescribedreregimesattheTallTimbersResearchStationnearTallahassee,FL,USA.MechanicaldifcultyresultedinerroneousdatacollectedinAugust2009.MeasurementswerenottakeninOctoberof2010.ForeststandcharacteristicsandlitterandduffdepthsvariedsignicantlyamongreregimesTable4A.Patternswererelatedtohighertreestockingandduffandlitteraccumulationwithloweroccurrenceofre.DuffdepthwasoveranorderofmagnitudehigherintheUBsitesthanthe1YRsitesTable4A.Soilbulkdensitywassignicantlyhigherinthe1YRand2YRreturnintervaltreatmentsandpercenttotalcarbonwashigherinthe2YRtreatmentthanintheunburnedtreatmentTable4B.Valuesfornitrogen,phosphorus,calcium,magnesium,potassium,andpHallshowedadecreasingtrendfromthe1YRtounburnedtreatments,althoughresultsweresignicantonlyformagnesiumTable4B. Table4.MeanforestcharacteristicsperprescribedretreatmentFR.SeeTable1forabbreviationsandunits.Datainparenthesesarestandarddeviation.LetterspercolumnshowsignicantdifferencesbetweenrereturnintervalsTukey'sHSDtest; p -Valueswereconsideredsignicantat =0.05. A FRTPHHWBAPBABADuffLitter 1YR282.9.83a3.87.28a7.92.68a11.79.22a0.08.06a1.77.91a 2YR400.81.87a6.30.28ab9.16.14a15.45.15a0.46.41b2.17.79b UB1716.41.42b15.73.59b 21.99.22 a 37.72.36b1.58.55c2.81.58c B FRBDCNPCaMgKpH 1YR1.24.04a26.53.49ab1.23.1833.46.18853.1.4185.5.8a38.08.805.48.37 2YR1.26.07a30.33.16a1.18.7311.69.58737.8.3141.5.6ab27.54.125.29.18 UB1.07.04b21.56.38b1.05.1611.16.78301.8.466.3.4b25.21.684.86.31 3.2.DriversofR s withinTreatmentLinearregressionsindicatedmonthlymeanTssignicantlypredictedmonthlymeanRswithineachreregimeR2>0.62;Figure5.Incontrast,MsdidnotpredictRsR2<0.14[29].NonlinearexponentialmodelsalsousedtopredictRsfromTsandMsbyreregimeshowedsimilarresultsasthelinearmodels,inwhichcaseMswasalsonotsignicantandnotreportedTable5.Modelcoefcients 0and 1fromthenonlinearmodelsweresimilartothosereportedbySamuelsonetal.[28]andKobziarandStephens[15].Q10valuesrangedfrom1.65to2.16anddifferedamongreregimes

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Forests 2017 8 ,274 9of14Table5.TheUBsiteshadthegreatestRssensitivitytochangesinsoiltemperatureQ10=2.16,whilethe1YRsitesweretheleastsensitivetosoiltemperaturechangesQ 10 =1.65. Figure5.LinearregressionoftherelationshipsbetweenmonthlymeansoilCO2efuxratesRsmolCO2m)]TJ/F223 6.9738 Tf 5.736 0 Td [(2 s)]TJ/F223 6.9738 Tf 5.736 0 Td [(1andmonthlymeansoiltemperatureTsCforthreeprescribedreintervalsattheTallTimbersResearchStationnearTallahassee,FL,USA.Eachpointrepresentsmonthlymeanvaluespersampleplot. p -Valueswereconsideredsignicantat =0.05. Table5.LinearandnonlinearregressionrelationshipsbetweensoilCO2efuxratesRsandsoiltemperatureT s byrereturnintervalFR. FRModelTypeModelandEstimatesQ 10 R 2 p-Value 1YRLinear R s = )]TJ/F223 8.9664 Tf 7.374 0 Td [(0.5101 + 0.16029 T s 0.62<0.001 1YRNonlinear R s = 0.9727 e 0.0502 T s 1.650.60<0.001 2YRLinear R s = )]TJ/F223 8.9664 Tf 7.375 0 Td [(1.1736 + 0.2218 T s 0.78<0.001 2YRNonlinear R s = 0.7973 e 0.0673 T s 1.960.80<0.001 UBLinear R s = )]TJ/F223 8.9664 Tf 7.375 0 Td [(2.0303 + 0.3337 T s 0.65<0.001 UBNonlinear R s = 0.9712 e 0.0770 T s 2.160.76<0.001 ModeldatausemeanmonthlymeasurementsofRsandTs. 3.3.SeasonalVariationinR s andT s RelationshipsInthesimplelinearmodelsandnon-linearmodelspredictingmeanmonthlyRsfromTswithineachreregimeshowedthattherelationshipwasstrongestduringthefallandwinterR2=0.69.90andweakestduringthespringandsummerR2=0.11.63inallreregimesTables6and7.Q10alsovariedseasonally,withthelowestQ10valuesobservedduringsummerandthehighestQ10valuesobservedduringthewinterinallreregimesTable7.

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Forests 2017 8 ,274 10of14 Table6.LinearregressionrelationshipbetweensoilCO2efuxratesRsandsoiltemperatureTsbyrereturnintervalFRandseason. p -Valueswereconsideredsignicantat =0.05. FR Season 1 ModelandEstimates R 2 p-Value 1YR SpringR s = 1.0261 + 0.0700 T s 0.290.021 SummerR s = 12.8311 )]TJ/F223 8.9664 Tf 9.118 0 Td [(0.2799 T s 0.400.068 FallR s = )]TJ/F223 8.9664 Tf 0.98 0 0 1 287.07 673.313 Tm [(1.4494+0.2159 T s 0.71<0.001 WinterR s = )]TJ/F223 8.9664 Tf 0.98 0 0 1 287.07 662.304 Tm [(0.4895+0.1435 T s 0.690.002 2YR SpringR s = 1.0808 + 0.0976 T s 0.480.002 SummerR s = 15.7301 )]TJ/F223 8.9664 Tf 9.118 0 Td [(0.3751 T s 0.280.144 FallR s = )]TJ/F223 8.9664 Tf 0.98 0 0 1 287.07 624.476 Tm [(3.1609+0.3197 T s 0.90<0.001 WinterR s = )]TJ/F223 8.9664 Tf 0.98 0 0 1 287.07 613.468 Tm [(0.4894+0.1661 T s 0.710.002 UB SpringR s = 1.5400 + 0.1302 T s 0.110.170 SummerR s = 49.5720 )]TJ/F223 8.9664 Tf 9.119 0 Td [(1.6300 T s 0.630.011 FallR s = )]TJ/F223 8.9664 Tf 0.98 0 0 1 287.07 575.64 Tm [(4.3665+0.4270 T s 0.71<0.001 WinterR s = )]TJ/F223 8.9664 Tf 0.98 0 0 1 287.07 564.631 Tm [(2.0244+0.3544 T s 0.90<0.001 1 SpringMarchMay,summerJuneAugust,fallSeptemberNovember,andwinterDecemberFebruary. Table7.SeasonalnonlinearmodelsofsoilCO2efuxRsratesusingsoiltemperatureTsasapredictor.ModelsareofmonthlymeanRsresponsestoTs.p-valueswereconsideredsignicantat =0.05. FRSeasonEquationQ 10 R 2 p-Value 1YR Spring R s = 1.5095e 0.0238 T s 1.270.260.031 Summer R s = 37.6590e )]TJ/F223 6.9738 Tf 5.736 0 Td [(0.0721 T s 0.490.450.049 Fall R s = 0.8564e 0.0598 T s 1.820.66<0.001 Winter R s = 0.2704e 0.1200 T s 3.320.770.000 2YR Spring R s = 0.2778e 0.1250 T s 3.490.330.012 Summer R s = 36.2629e )]TJ/F223 6.9738 Tf 5.736 0 Td [(0.0692 T s 0.500.270.149 Fall R s = 0.5967e 0.0823 T s 2.280.89<0.001 Winter R s = 0.3701e 0.1114 T s 3.050.780.001 UB Spring R s = 2.2967e 0.0292 T s 1.340.100.195 Summer R s = 259.9167e )]TJ/F223 6.9738 Tf 5.736 0 Td [(0.1388 T s 0.250.620.012 Fall R s = 0.6935e 0.0883 T s 2.420.69<0.001 Winter R s = 0.3990e 0.1383 T s 3.990.96<0.001 3.4.TotalCarbonEmissionsEstimatedtotalmonthlysoilcarbonemissionswereconsistentlyhigherintheunburnedUBsitesthanthefrequentlyburned1YRand2YRsites.Similarly,estimatedtotalannualsoilcarbonemissionsperreregimeshowedthehighestsoilcarbonefuxintheUBsites.88Mgha)]TJ/F223 7.5716 Tf 6.227 0 Td [(1 year)]TJ/F223 7.5716 Tf 6.228 0 Td [(1andthelowestinthe1YR.69Mgha )]TJ/F223 7.5716 Tf 6.228 0 Td [(1 year )]TJ/F223 7.5716 Tf 6.228 0 Td [(1 and2YR.68Mgha )]TJ/F223 7.5716 Tf 6.228 0 Td [(1 year )]TJ/F223 7.5716 Tf 6.228 0 Td [(1 sites. 4.DiscussionResultsofourstudyshowthatRswasconsistentlyhigherinlongre-excludedpine-grasslandsthaninfrequentlyburnedpine-grasslands,whiletherewaslittledifferencebetweenannuallyandbienniallyburnedcommunities.Althoughdeterminingspecicmechanismsfortheobservedpatternwasbeyondthescopeofthisstudy,thereisevidencethatRsrespondedtoenvironmentalconditionsinuencedbyreregimeeffectsonforeststructure.IntheUBsites,abovegroundlivingtreebiomass150Mgha)]TJ/F223 7.5716 Tf 6.228 0 Td [(1wasmuchgreaterthaninthe1YRand2YRsites50Mgha)]TJ/F223 7.5716 Tf 6.228 0 Td [(1and80Mgha)]TJ/F223 7.5716 Tf 6.228 0 Td [(1,respectively,suggestingthatthepresumablycorrespondinghighertreerootbiomassintheunburnedplotsmayhaveinuencedRs.OtherstudiesinvestigatingRsratesacrossstandageandbiomassgradientshavefoundmeanRsratestobehigherin

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Forests 2017 8 ,274 11of14olderstandswithgreaterabovegroundbiomass[30,31].InatrenchingandexclusionexperimentalongachronosequenceoftemperateforestsinChina,Luanetal.[32]foundthatRsratesweresignicantlycorrelatedwithsitebasalarea R 2 =0.59, p <0.05.Highersoilrespirationratesintheunburnedsitesmayhavealsobeeninuencedbytreespeciescompositionandassociatedlitterquality,specicallydominancebydeciduousbroadleaftreeswhichwereessentiallyabsentintheburnedplots.SoilCO2efuxrateshavebeenshowninotherstudiestobelowerinconiferousforeststhaninbroad-leafedforestsofthesamesoiltype[5].InastudyofRsratesinamixedconiferdeciduousforestinBelgium,Yusteetal.[33]foundthatmeanRsrateswerelowerunderconifertreecanopiesthanunderdeciduouscanopies,withtotalestimatedannualcarbonuxapproximately50%greaterinthedeciduoussites.82.2MgCha)]TJ/F223 7.5716 Tf 6.227 0 Td [(1 year)]TJ/F223 7.5716 Tf 6.228 0 Td [(1thanintheconiferoussites.80.7MgCha)]TJ/F223 7.5716 Tf 6.228 0 Td [(1 year)]TJ/F223 7.5716 Tf 6.227 0 Td [(1.Intheirreview,RaichandTufekciogul[5]suggestedthattheobserveddifferencesinRsbetweenconiferousanddeciduousforestsmayhavebeendrivenbyforestlitterproductionandquality,carbonallocation,andautotrophiccontributionstototalR s .Inourstudy,litterandduffdepthswerehighestintheUBsites,reectinghighertreestockingandlackofre,inturnpossiblycontributingtohigherRsratesintheUBsites.Otherstudieshavereportedthatfrequentregreatlyreducestheaccumulationoflitterandduff[34].ThelowersoilcarbonlevelsandlowestRsinthe1YRplotscomparedtothe2YRplotslikelyreectlimitedinputsoforganicmatterbecauseofannualburning.The2YRplotsinturnareassumedtohavelowerorganicmatterinputstothesoilthantheUBplotsbecausefrequentre,yetthetotalcarbonwashigherinthe2YRplots,suggestinglowerratesofdecompositionthantheUBplotsconsistentwiththelowerRsmeasured.Itappearsthatsomecharacteristicofthefrequentlyburnedenvironmentsuppressedsoilrespiration,whetherthroughitseffectonRaorRh.Increasedbulkdensityassociatedwithburning,foundinthisandotherstudies[3537]hasbeenattributedtoinltrationofmicroporespacebyashandchar,whichmightnegativelyinuencemicrobialactivity.Similarpatternsofhighertotalcarboninthemineralsoilofburnedthanunburnedareashavebeenseeninmanylong-termreexperimentsinthesoutheasternUSA[36,3841].TheRsratesobservedinthisstudyresultedinlowerestimatedtotalannualsoilcarbonemissionsinthe1YR%and2YR%sitesrelativetotheUBsites.TheestimatedmonthlycarbonemissionsforallsitesweresimilartothosereportedbySamuelsonetal.[28]foranunburnedloblollypineplantationinsouthwesternGeorgia,USA,whiletheestimatedtotalannualsoilcarbonemissionsreportedinourstudyweresimilartothose.10Mgha)]TJ/F223 7.5716 Tf 6.228 0 Td [(1 year)]TJ/F223 7.5716 Tf 6.228 0 Td [(1reportedbyMaierandKress[42]foranunburnedloblollypineplantation,butgreaterthanthose.78.66Mgha)]TJ/F223 7.5716 Tf 6.228 0 Td [(1 year)]TJ/F223 7.5716 Tf 6.227 0 Td [(1reportedbytheSamuelsonetal.[28]study.TemporalvariabilityinRsrateswasexplainedmorebysoiltemperaturethananyotherrecordedparameter.TheseresultswereconsistentwithotherstudiesinsoutheasternUSAforestsystemsinwhichRsrateswerefoundtohavestrongcorrelationswithsoiltemperature[28,42,43].TheRscorrelationwithTsreportedinthisstudyR2=0.62.78inlinearmodelsandR2=0.60.80innonlinearmodelswerehigherthanthosereportedbySamuelsonetal.[28]foraGeorgia,USAloblollypineplantationR2=0.38.56,muchhigherthanthosereportedbyTempletonetal.[43]acrossarangeofloblollypineplantationsin11southeasternUSAstatesR2=0.45,andsimilartothosereportedforaloblollypineplantationinNorthCarolina,USAR2=0.70[42].Cautionmustbeusedincomparingcorrelationsamongstudies,asvariationsinthemodeledRsandTsrelationshipmaybeinuencedbystatisticalmodelingtechniquesregardlessofthebiogeochemicalcouplingsobservedintheeld.WhileTsexplainedthemajorityofthetemporalvariabilityinRsrates,alackofsignicantdifferencesinTsamongtreatmentssuggeststhattemperaturewasnotthekeyfactorcontributingtothedifferencesinR s ratesamongthetreatments.Overthecourseofthisstudy'ssamplingperiod,relationshipsbetweenRsandTsvariedseasonally,withstrongerrelationshipsduringthewinterseasons.ThispatternwasevidentintheR2valuesoftheseasonallinearandnonlinearmodelsaswellasintheseasonalQ10values.DistinctseasonalvariationsinQ10valueshavebeennotedbyothers[44,45]andsuchvariationshavepotentialimplicationsfor

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Forests 2017 8 ,274 12of14modelingRsvaluesbasedontemperaturemeasurements.Yusteetal.[45]suggestedthatwhileannualQ10valuesmaysufcientlymodelannualRsrates,seasonalorshorter-termestimatesofRsshouldbebasedonseasonspecicQ10functionsthatcapturetheseasonalvariationinRstemperatureresponse,asobservedinourstudy.TheseasonalvariationintherelationshipbetweenRsandTsmaybetheresultofphenology-relatedshiftsintherelativecontributionsofRaandRhtoRs.PreviousresearchinpartitioningthesourcesofRshaveshownthatduringperiodsofabovegroundvegetativegrowth,RacontributionstoRscanincreaserelativetoRh,asplantsallocaterecentCphotosynthatebelowground,drivinghigherrootmaintenance,rootgrowth,andmycorrihizalfungalrespirationrates[6,46].Inaddition,otherstudieshaveshownthatduringperiodsofabovegroundvegetativegrowth,theTsandRsrelationshipweakensasothervariablessuchassoilmoistureandavailablephotosyntheticallyactiveradiationPARbecomeimportantingoverningbelowgroundCallocationbyplants[4749].Similartoourndings,Fennetal.[50]foundthatsoiltemperatureexplainedlessofthevariationinRsduringthesummerthanduringthespringinamulti-seasonstudyofRsratesinawoodlandinOxfordshire,UK. 5.ConclusionsInthisstudycomparinglong-termredisturbanceintervals,soiltemperatureexplainedthemajorityofthetemporalvariationinRsrates,butneithertemperaturenormoistureexplainedthedifferencesinRsobservedamongtherefrequencies.Assuch,wesuggestthatfutureeffortstomodelsoilcarbonuxesinotherwisesimilarburnedandunburnedforestsshouldaccountfortheimpactsofbothseasonalvariabilityandprescribedremanagementregimeontherelationshipsbetweenRsandTs.DifferencesinCO2efuxrates,aswellassoilchemistryandbulkdensity,reectthelong-termimpactofcontrastingredisturbanceregimes,aswellasthebiophysicalconsequencesofthoseregimes.LowertotalannualsoilCemissions,combinedwithlowerratesofsoilCefuxandhigherstoredsoilC,suggestthatevenbienniallyburnedsitesareeffectiveatsequesteringsoilcarbon.Acknowledgments:ThisresearchwassupportedbytheKobziarFireEcologyLabandthetheSchoolofForestResourcesandConservation,UniversityofFlorida,Gainesville,FL,USA.AdditionalsupportwasprovidedbytheJointFireScienceProgramGraduateResearchInnovationgrant-3-1-21.Theauthorsthanktwoanonymousreviewerswhoprovidedvaluablefeedbackonearlierversionsofthispaper.Specialthanksforprescribedburning,soilcollection,andliteratureassistancefromA.Reid,J.Patterson,andC.ArmstrongoftheFireEcologyProgramandtheParker-WilliamsLibraryattheTallTimbersResearchStationandLandConservancy.SoilwascollectedbyJamesPatterson.LogisticsandeldworkweresupportedbyA.Reidand.A.Kattan. AuthorContributions:DavidR.Godwin,LedaN.KobziarandKevinM.Robertsonconceivedanddesignedtheexperiments;DavidR.Godwinperformedtheexperimentsandanalyzedthedata;KevinM.Robertsonprovidedsoilchemicalandphysicalmeasurementsandsupervisedprescribedburns;DavidR.Godwin,LedaN.KobziarandKevinM.Robertsonwrotethepaper. ConictsofInterest: Theauthorsdeclarenoconictofinterest. References 1.Raich,J.W.;Schlesinger,W.H.Theglobalcarbondioxideuxinsoilrespirationanditsrelationshiptovegetationandclimate. TellusB 1992 44 ,81.[CrossRef] 2.Schlesinger,W.;Andrews,J.Soilrespirationandtheglobalcarboncycle.Biogeochemistry2000,48,7.[CrossRef] 3.Ryan,M.G.;Law,B.E.Interpreting,measuring,andmodelingsoilrespiration.Biogeochemistry2005,73,3.[CrossRef] 4.Luo,Y.;Zhou,X. SoilRespirationandtheEnvironment ;AcademicPress:Burlington,MA,USA,2006. 5.Raich,J.W.;Tufekciogul,A.Vegetationandsoilrespiration:Correlationsandcontrols.Biogeochemistry2000,48 ,71.[CrossRef] 6.Subke,J.;Inglima,I.;Cotrufo,M.F.TrendsandmethodologicalimpactsinsoilCO2efuxpartitioning:Ameta-analyticalreview. Glob.Chang.Biol. 2006 12 ,921.[CrossRef]

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