2 动员难溶性磷Bioavailability of soil inorganic P in the

PlantandSoil237:173–195,2001.©2001KluwerAcademicPublishers.PrintedintheNetherlands.173BioavailabilityofsoilinorganicPintherhizosphereasaffectedbyroot-inducedchemicalchanges:areviewPhilippeHinsingerINRA–UMRSol&Environnement,PlaceViala34060,MontpellierCedex1,FranceReceived5September2000.Acceptedinrevisedform1February2001.Keywords:bioavailability,organicacid,phosphate,pH,rhizosphere,rootAbstractInmostsoils,inorganicphosphorusoccursatfairlylowconcentrationsinthesoilsolutionwhilstalargeproportionofitismoreorlessstronglyheldbydiversesoilminerals.PhosphateionscanindeedbeadsorbedontopositivelychargedmineralssuchasFeandAloxides.Phosphate(P)ionscanalsoformarangeofmineralsincombinationwithmetalssuchasCa,FeandAl.Theseadsorption/desorptionandprecipitation/dissolutionequilibriacontroltheconcentrationofPinthesoilsolutionand,thereby,bothitschemicalmobilityandbioavailability.ApartfromtheconcentrationofPions,themajorfactorsthatdeterminethoseequilibriaaswellasthespeciationofsoilPare(i)thepH,(ii)theconcentrationsofanionsthatcompetewithPionsforligandexchangereactionsand(iii)theconcentrationsofmetals(Ca,FeandAl)thatcancoprecipitatewithPions.Thechemicalconditionsoftherhizosphereareknowntoconsiderablydifferfromthoseofthebulksoil,asaconsequenceofarangeofprocessesthatareinducedeitherdirectlybytheactivityofplantrootsorbytheactivityofrhizospheremicroflora.TheaimofthispaperistogiveanoverviewofthosechemicalprocessesthataredirectlyinducedbyplantrootsandwhichcanaffecttheconcentrationofPinthesoilsolutionand,ultimately,thebioavailabilityofsoilinorganicPtoplants.Amongstthese,theuptakeactivityofplantrootsshouldbetakenintoaccountinthefirstplace.AsecondgroupofactivitieswhichisofmajorconcernwithrespecttoPbioavailabilityarethoseprocessesthatcanaffectsoilpH,suchasproton/bicarbonaterelease(anion/cationbalance)andgaseous(O2/CO2)exchanges.Thirdly,thereleaseofrootexudatessuchasorganicligandsisanotheractivityoftherootthatcanaltertheconcentrationofPinthesoilsolution.ThesevariousprocessesandtheirrelativecontributionstothechangesinthebioavailabilityofsoilinorganicPthatcanoccurintherhizospherecanconsiderablyvarywith(i)plantspecies,(ii)plantnutritionalstatusand(iii)ambientsoilconditions,aswillbestressedinthispaper.TheirpossibleimplicationsfortheunderstandingandmanagementofPnutritionofplantswillbebrieflyaddressedanddiscussed.IntroductionComparedwiththeothermajornutrients,phosphorusisbyfartheleastmobileandavailabletoplantsinmostsoilconditions.Itisthereforefrequentlyama-jororeventheprimelimitingfactorforplantgrowth.Indeed,itisestimatedthat5.7billionsofhectaresworldwidecontaintoolittleavailablephosphorusforsustainingoptimalcropproduction(Batjes,1997,in∗FirstInternationalSymposiumonPhosphorusintheSoil-PlantContinuum,heldinBeijing(China)17-23rdSeptember2000.Tel:[phone];Fax:[phone];E-mail:[e-mail],2000).Thepoormobilityofsoilinorganicphosphorusisduetothelargereactivityofphos-phate(P)ionsrelativetonumeroussoilconstituentsandtotheconsequentstrongretentionofmostofsoilphosphorusontothose.Therefore,onlyamarginalproportionofsoilphosphorusispresentasPionsinthesoilsolution.AlthoughPionscanreachlargerconcentrationsinhighlyfertilizedsoils,inmanysoilsindeed,theirconcentrationinthesoilsolutionisinthemicromolarrange,rangingbetween0.1and10µM(Ozanne,1980;MengelandKirkby,1987;Rag-hothama,1999;Frossardetal.,2000).TheseareratherlowcomparedwiththeadequatePconcentrationsfor174optimalplantgrowth(externalPrequirement)whichcanreachvaluesofseveralµMortensofµMforthemostdemandingcropspeciessuchasbean,cot-ton,pea,potato,onion,spinachortomato(AsherandLoneragan,1967;Föhseetal.,1988).Thisisofpar-ticularconcernforsubtropicalandtropicalregionsoftheworldthatcomprehendhighlyweatheredsoils(Aeetal.,1990;Batjes,1997)andfortheregionsofthemediterraneanbasinthatarelargelydominatedbycal-careousandalkalinesoils(Mataretal.,1992).ThisislargelyduetothepeculiarmineralogyandambientgeochemistryofthesetypesofsoilsthatfavorastrongretentionofPionsontotheirsolidconstituentsandmaintainlowlevelsofPionsintheirsoilsolution.Inspiteofthelargeattentionthatphosphorushasreceivedoverdecadesofintensiveresearchinthe20thcentury,becauseofthelackofappropriatemethodsforstudyingitsspeciationandbiogeochemicalbeha-vior,themobilityofinorganicphosphorusinmostsoilsisstillratherpoorlyunderstoodandhardlypre-dictable.ThisisevenworsewhenconsideringtheproblemofthebioavailabilityofPtoplants,becauseofthenecessitytothenintegratethemanyinteractionsthatoccurintherhizosphere.Considerableamountofsometimescontroversialdatahasbeengatheredinthatrespect,though.Thispaperaimsatreview-ingthemajorchemicalprocessesthatareinducedbyplantrootsandwhichcontributesomechangesinthemobilityandbioavailabilityofPionsintherhizo-sphere,namely:root-induceddepletion/accumulationofPions,acidification/alkalinizationandexudationoforganicacids/anions.EachofthesewillbeconsideredindetailafterhavingbrieflysummarizedthemajorfactorsandprocessesgoverningthemobilityofPionsinsoils.ThepoormobilityofPinsoils:soilfactorsandprocessesthatdeterminethebioava