22Fastmicrobesregulateslowsoilfeedbacks

news&viewsBIOGEOCHEMISTRYFastmicrobesregulateslowsoilfeedbacksEarth’sfutureclimatedepends,inpart,onrapidsoilmicrobialprocessesthatmayadduptolong-termimpacts.Observationsfromageothermalgradientrevealdecadalincreasesinsoil-carbonlossduetopersistentincreasesinmicrobialactivity.Elise PendallTheglobalsoil-carbon(soilC)poolrepresentsapotentiallyverylargefeedbacktotheclimatesystem,becauseitsdecompositionincreasesexponentiallywithtemperature.ThisisconcerningbecausesoilrespirationistheprocessthattranfersthemostCO2fromlandtotheatmosphere.MostsoilCaccumulatesoveraperiodofseveraldecades,makingitchallengingtodetectchangesovershort-termstudies(lessthanadecade).WritinginNatureClimateChange,Walkeretal.tackledthisproblem1bycollectingsamplesfromacrossnaturalgeothermalgradientsinIceland,wheresoilshavebeenwarmedforyearstocenturies.Theirworkshowsthatshort-termmicrobialprocessesleadtolong-termsoilClosses,butthattheselossescanbemitigatedbystabilizingfeedbackswithinthesoilsystem(Fig.1).Limitedunderstandingofhowmicrobesrespondtochangingenvironmentalconditionshasledtomajoruncertaintiesinpredictingclimate–carbonfeedbacks.EcosystemwarmingexperimentsprovidesomeinsightsintothesensitivityofsoilCdecompositiontotemperature2,butonlyahandfulhavebeenconductedforadecadeormore3.Arecentmeta-analysisof143warmingstudiessuggeststhatthesignal-to-noiseratioisstilltoolowtodetectsignificantimpactsonsoilC(ref.4),largelybecauseofashortageoflong-termstudies.However,overthelast25years,globallyaveragedheterotrophicrespirationfromsoilhasincreasedsignificantlyby1.2%intandemwithatemperatureincreaseof0.7°C(ref.5).Theseobservationsareconsistentwithwarming-enhanceddecompositionratesbutdonotprovidestrongcausalevidence.Experimentalevidencehasbeeninconclusive,partlybecausewarmingoftenreducessoilmoisturecontentandhencesubstrateavailability.Untilnow,studiesinvestigatingthemicrobialmechanismsthatdrivesoilCresponsestowarminghavenotdistinguishedbetweensubstratedepletionandmicrobialacclimationtoincreasedtemperature.Walkeretal.1conductedtemperature-controlledlaboratoryassaysofmicrobial-processratesinsoilsthathadbeenwarmedbyupto6°Cbynaturalgeothermalsourcesforover50years,incomparisonwithnearbysoilsfromambient-temperaturezones.Microbe-scaleratesofgrowth,respirationandcarbonuptakewerefoundtobegreaterinwarmedsoils,evenafterdecadesofexposure,aswouldbeexpectedifintrinsicmicrobialtemperaturesensitivitywasmaintained(Fig.1a).Thisresultdemonstratesthatmicrobialprocessrates—whenmeasuredperunitofmicrobialbiomass—donotacclimatetowarming.Themicrobialcarbonuseefficiency(CUE)wasconservedacrossthewarminggradient,suggestinghomeostasis.Quantificationofthesedynamicswasmadepossiblebygrowingmicrobesinnativesoilwith18O-enrichedwater,andthenmeasuringtheenrichedmicrobialDNA.Thisrecentlydevelopedmethodallowsestimationofturnoverrates,CUEGrowth,respirationandCuptakeperunitofmicrobialbiomassTemperatureIntrinsicmicrobialprocessratesAmbient+6°CCUETimesincewarmingEcosystemresponsesDecreasingIncreasing0WeeksDecadesSoilrespirationSoilCabFig.1|TheGreenValleygeothermalfieldinIceland,wheresoilsampleswerecollected.a,Intrinsicratesofmicrobialprocessesrampedupwithshort-termandlong-termwarming,whileCUEremainedstable.b,Soilrespirationinitiallyincreasedwithwarming.ThiseventuallyleadtosoilClossuntilmicrobialsubstratesweredepletedandmicrobialpopulationscrashed,presumablysettingupaself-regulatingfeedback.Credit:IvanJanssensNaTureClImaTeChaNGe|–preyrelationships,inthemicrobialuseofsoilC,andthiswasalsoobservedinalong-termmanipulativewarmingexperimentinaNewEnglandforest(USA)3.However,oscillationsinsimulatedsoilCpoolscouldreflectunrealisticresponsestoperturbationsandhavenotbeenwellsupportedbyobservations9.ThebestframeworkformodellingsoilCdecompositionisstillamatterofdebate.Includingintrinsicmicrobialprocessratesmayimprovetherealismofmodelsbutrequiresparametersthatarechallengingtomeasure.Muchofthecarbonlostinresponsetowarmingisprobablyderivedfrommicrobiallyaccessiblesubstrate,butthestabilityofoldercarbonpoolsoverthescaleofcenturiesremainsuncertain.Moreover,warmingmaystimulateplantproduction,increasinginputstosoilpoolsandoffse