Carbon avail-ability and temperature control the p

Carbonavailabilityandtemperaturecontrolthepost-snowmeltdeclineinalpinesoilmicrobialbiomassDavidA.Lipson*,StevenK.Schmidt,RussellK.MonsonDepartmentofEnvironmental,PopulationandOrganismicBiology,UniversityofColorado,Boulder,CO80309-0334,USAAccepted8April1999AbstractInColoradoalpinedrymeadowsoils,microbialbiomasshasbeenobservedtoincreaseduringfallandwinterandtorapidlydeclineaftersnowmeltinthespring.IthasbeenshownthatthesemicrobialpopulationdynamicsarelinkedtoNavailabilitytoalpineplants,buttheunderlyingmechanismshavenotbeenexplained.Wehypothesizedthat:(1)freeze-thaweventsinthespringcausereductionofthemicrobialbiomass,(2)thewintermicrobialcommunityissensitivetoprolongedtemperaturesabove08C,and(3)theincreaseofbiomassinfallanditsdeclineinspringareduetochangesinCavailability.Weperformedlaboratoryexperimentstotestthee€ectoftemperatureregimeonsoilmicrobialbiomass,respirationandCavailability,andmadeseasonalmeasurementsofCpools.Soilmicrobialbiomasswasuna€ectedbyfreeze-thaweventsinwhichrealisticratesoffreezingandthawingwereused.Somesigni®cante€ectswereobservedatfasterfreezingrates.Despitethistolerancetotemperature¯uctuations,thewintermicrobialcommunityshowedsensitivitytoprolongedtemperaturesabove08C.Thise€ectmayhavebeencausedindirectlybyane€ectoftemperatureonsubstrateavailability.TwoweekincubationsatincreasedtemperaturescausedareductioninthequantityofextractableorganicCinthesoil.Thesoilconcentrationsofcelluloseandhotwater-solubleorganicCwerethelowestinthesummerandthehighestinspringandautumn,mirroringpreviouslymeasuredpatternsofmicrobialbiomass.ThissuggeststhatCfromlitterinputscouldbeastrongcontrolovermicrobialbiomass.Respirationratesinsoilscollectedbeforesnowmeltwerehighat08C,anddidnotrespondimmediatelytoadditionofglutamate.At228C,orafteratwoweekincubationat08C,respirationinthesesoilsbecamesubstrate-limited.Respirationratesinsoilscollectedduringthesummerwereverylowat08C,butrespondedimmediatelytoglutamateadditionatboth0and228C.TheseresultsshowthattheCbalanceofthesoilmicrobialbiomassundergoesacriticalshiftbetweenwinterandsummerduetoanincreaseintemperatureandacorrespondingdecreaseinCavailability.Thisshiftcouldexplainthedeclineinmicrobialbiomassaftersnowmelt.72000ElsevierScienceLtd.Allrightsreserved.Keywords:Alpinesoilmicrobialbiomass;Carbonavailabilitye€ects;Coldresistance;Post-snowmeltdecline;Temperaturee€ects;Freeze-thawevents;Substrate-limitation1.IntroductionPreviousstudiesintheColoradoalpineecosystemhaveshowntwopredictableseasonalpatternsinsoilmicrobialbiomass.Microbialbiomassincreasesinthefallandwinter(Brooksetal.,1996;Jaegeretal.,1999;Lipsonetal.,1999)anddecreasesinthespringatthestartoftheplantgrowingseason(Brooksetal.,1998;Lipsonetal.,1999).ThedeclineofmicrobialbiomassinthespringhasbeenlinkedtoapulseofNavail-abilitytoplantsinthealpinedrymeadow(Lipsonetal.,1999).However,thecausesforthesepatternshavenotbeenestablished.Ithasbeenspeculatedthatthedeclineinmicrobialbiomassinspringtimeislinkedtofreeze-thawevents(Brooksetal.,1998;Jaegeretal.,1999).Inalpinedrymeadows,thesnowpackrecedesinmid-tolate-spring,leavingsoilsexposedatatimewhennighttimeairtemperaturesareregularlybelowSoilBiology&Biochemistry32(2000)441±4480038-0717/00/$-seefrontmatter72000ElsevierScienceLtd.Allrightsreserved.PII:S0038-0717(99)00068-1www.elsevier.com/locate/soilbio*Correspondingauthor.Tel.:[phone];fax:[phone].E-mailaddress:[e-mail](D.A.Lipson).08C.Numerousstudieshaveobservedthatfreeze-thaweventscauseaburstofmicrobialactivitysuchasmin-eralizationofCorN(DeLucaetal.,1992;Ross,1972;SchimelandClein,1996;Skoglandetal.,1988).Thisburstofactivityhasgenerallybeenattributedtoincreasedlevelsoflabilesubstrateinthesoilresultingfromdamagetomicro-organismsbyfreeze-thawevents.Suchdamagetosoilmicrobeshasbeendemon-strated(Morleyetal.,1983;SoulidesandAllison,1961).However,previousworkshowedthatthesoilmicrobialbiomassinalpinedrymeadowswasnota€ectedbyasinglefreeze-thawevent(LipsonandMonson,1998).Also,asmentionedabove,microbialbiomasshasbeenobservedtoincreaseduringthefallwhensoilsarealsoexposedto¯uctuationsaroundthefreezingpoint(Lipsonetal.,1999).Thereasonthatpreviousexperimentsshowednoe€ectoffreeze-thaweventsmayhavebeenthattheexperimentalcoolingandwarmingrateswerecarefullycontrolledtosimu-latesoiltemperaturesduringthealpinespringorautumn(LipsonandMonson,1998).Ithasbeenshownthatratesoffreezingandthawinganddepthoffreezecanhavelargee€ectsonsurvivalofmicro-organisms(Mazur,1980;Meyeretal.,1975).However,thesepreviousstudieshavebeenperformedinpureculture,andhaveusedarangeofcooling/warmingrates(12±60,0008Chÿ1)thataremuchhigherthanthoseobservedinthealpineduringthespringorfall(seeSection3).Herein,wetestthehypothesisthatfreeze-thaweventsexplainthepost-snowmeltdeclineinmicrobialbiomassbysubjectingsoilstorealisticsimulationsofvernalfreeze-thawevents.Asanalternativetothishy-pothesis,weproposethattheinputofcarbon(C)fromplantlitterinthefallallowscold-adaptedmicrobestogrowuntilsnowmelt,thenadeclineinCavailabilityandprolongedtemperaturesabove08Ccauseadeclin