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TheISMEJournal:effectsofprecipitationregimeonsoilmicrobialactivityuponrewettingofaplant-soilsystemIlonkaC.Engelhardt1●AmyWelty1,5●StevenJ.Blazewicz2●DavidBru1●NadineRouard1●Marie-ChristineBreuil1●ArthurGessler3●LucíaGaliano3●JoséCarlosMiranda4●AyméSpor1●RomainL.Barnard1Received:22August2017/Revised:21December2017/Accepted:29December2017©InternationalSocietyforMicrobialEcology2018AbstractChangesinfrequencyandamplitudeofrainevents,thatis,precipitationpatterns,resultindifferentwaterconditionswithsoildepth,andlikelyaffectplantgrowthandshapeplantandsoilmicrobialactivity.Here,weused18Ostableisotopeprobing(SIP)toinvestigatebacterialandfungalcommunitiesthatactivelygrewornotuponrewetting,atthreedifferentdepthsinsoilmesocosmspreviouslysubjectedtofrequentorinfrequentwateringfor12weeks(equaltotalwaterinput).Phylogeneticmarkergenesforbacteriaandfungiweresequencedafterrewetting,andplant-soilmicrobialcouplingdocumentedbyplant13C-CO2labeling.Soildepth,ratherthanprecipitationpattern,wasmostinfluentialinshapingmicrobialresponsetorewetting,andhaddifferentialeffectsonactiveandinactivebacterialandfungalcommunities.Afterrewetting,activebacterialcommunitieswerelessrich,moreevenandphylogeneticallyrelatedthantheinactive,andreactivatedthroughoutthesoilprofile.Activefungalcommunitiesafterrewettingwerelessabundantandrichthantheinactive.Thecouplingbetweenplantsandsoilmicrobesdecreasedunderinfrequentwateringinthetopsoillayer.Wesuggestthatdifferencesinfungalandbacterialabundanceandrelativeactivitycouldresultinlargeeffectsonsubsequentsoilbiogeochemicalcycling.IntroductionWateravailabilityisakeyregulatorofecosystemfunc-tioning,directlycontrollingplantandsoilmicrobialactiv-ity.Thepredictedlargechangesinprecipitationbroughtonbyclimatechangeincludeperiodsofincreasedwaterlim-itationfollowedbylargermagnituderaineventsformanypartsoftheworld[1].Changesinthetotalamountofprecipitationconstrainecosystemfunctioning,andtheireffectshavebeendocumentedinmanywaterinputreduc-tionexperiments(e.g.,[2–8]).However,climatechangeispredictedtoaffectnotonlytheamountbutalsothetemporaldistributionofrain.Changesinfrequencyandamplitudeofrainevents,thatis,precipitationpatterns,likelyshapetheactivityofplantsandsoilmicroorganisms[9–11].Microbialreactivationuponrewettingisakeymomentinecosystemfunctioning.Substratebecomesreadilyavailabletosoilmicroorganisms,triggeringthemicrobialactivitythatdrivessoilbiogeochemicalcycles.However,notallmicrobesrespondsimilarlytorewettingevents,indicatingadaptationinlifestrategiesthatmaybephylogeneticallyconservedecologicaltraits[12,13].Furthermore,microbialcommunitieshavingbeenexposedtoahistoryoferraticTheseauthorscontributedequally:IlonkaC.Engelhardt,AmyWelty.*[e-mail]1Agroécologie,INRA,AgroSupDijon,Univ.BourgogneFranche-Comté,F-21000Dijon,France2LawrenceLivermoreNationalLaboratory,7000EastAvenue,Livermore,CA94550,USA3SwissFederalResearchInstituteWSL,Zuercherstr.111,8903Birmensdorf,Switzerland4ForestHistory,PhysiologyandGeneticsResearchGroupUniversidadPolitecnicadeMadridCiudadUniversitarias/n28040MadridSpain5Presentaddress:DepartmentofPlantPathologyandMicrobiology,IowaStateUniversity,Ames,IA50011,USAElectronicsupplementarymaterialTheonlineversionofthisarticle()containssupplementarymaterial,whichisavailabletoauthorizedusers.1234567890();,:moisturefluctuationsareadaptedtotheseconditionsandshowsmallerchangesinresponsetorewettingevents[54].Withinsoilmicrobialcommunities,bacteriaandfungidifferintheirresistancetodesiccation,aswellasintheirresponsetorewetting[14–16].Fungalpopulationshavebeenshowntobemoreresistanttowaterlimitation,likelyduetotheirabilitytoaccesswaterfromdistantmicroporeswiththeirextensivehyphalnetwork[17].Bacteriatypicallyrespondfasterthanfungitochangesinwateravailability,albeitwithawiderangeofresponsesinthebacterialcommunity[12,13].Soilmicroorganismsdrivebiogeo-chemicalcyclesinsoil,suchthatchangesintherelativecontributionofbacteriaandfungimayaffectecosystemfunctioning.Forexample,increasedfungal:bacterialratioresultingfromdry–wetcyclesimprovedsoilnutrientretention[18].Itisthereforecrucialtoconsiderbothbac-terialandfungalresponseswhenevaluatingeffectsofchangesinprecipitationonthesoilmicrobialcommunity.Dry–wetcyclesareexpectedtoaffectsoildifferentiallyatdifferentdepths,sincethetopsoillayerexperiencesmorefluctuatingwaterconditions,iswetmoreoften,anddriesoutquickerthanthedeeperlayers,likelyshapingmicrobialcommunitycompositionandfunction[19,20].Decreasedbacterialdiversityuponhydrationissupportedbyphysicalmodelingapproachesthatindicateincreasedcompetitionwhensoilrehydrationrestoresconnectivity[20–22].Moreover,asmostrootsareusuallydevelopedinthetopsoillayersandplants’rootactivityisexpectedtorespondtoprecipitationpatterns(e.g.[23]),weexpectthestrongestmicrobialresponseinthetopsoillayers.Undermesicconditions,plantsgenerallyremainalivebetweenraineventsandtheirinteractionwithsoilmicroorganismsmaydrasticallyimpacthowth