安师大环境土壤学讲义14土壤退化与土壤质量

PAGEPAGE12314土壤退化与土壤质量14.SoilDegradationandSoilQuality目的要求：使学生了解土壤退化的概念、种类与退化的背景、态势；掌握土壤质量的内涵与其评价指标体系与方法；了解土壤退化的类型与防治。14.1Initialconceptsandtypesofsoildegradation土壤退化的概念及分类14.1.1Initialconceptsofsoildegradation土壤退化的概念土地（壤）退化指的是数量减少和质量降低。数量减少可以表现为表土丧失，或整个土体的毁失，或土地被非农业占用。质量降低表现在土壤物理、化学、生物学方面的质量下降。（1）土壤（地）退化是一个非常复杂的问题，它是由自然因素和人为因素共同作用的结果。（2）土壤（地）资源从数量上是无再生能力的，因而具有土壤（地）数量有限性的特点，但从质量上，可以改良培肥土壤，保持“地力常新”。耕地土壤是人类赖以生存最珍贵的土壤（地）资源，是农业生产最基本的生产资料。1.Landdegradationandsoildegradation土地退化与土壤退化Landdegradation土地退化是指人类对土地的不合理开发利用而导致土地质量下降乃至荒芜的过程。包括森林的破坏及衰亡、草地退化、水资源恶化与土壤退化。土地退化的直接后果是：①直接破坏陆地生态系统的平衡及其生产力；②破坏自然景观及人类生存环境；③通过水分和能量的平衡与循环的交替演化诱发区域乃至全球的土被破坏、水系萎缩、森林衰亡和气候变化Soildegradation土壤退化在自然环境的基础上，因人类开发利用不当而加速的土壤质量和生产力下降的现象和过程。土壤退化的标志：对农业而言：土壤肥力和生产力的下降；对环境而言：土壤质量的下降14.1.2Typesofsoildegradation土壤退化的分类联合国粮农组织（联合国粮农组织（FAO)侵蚀盐碱有机废料传染性生物工业无机废料农药放射性重金属肥料洗涤剂旱涝障碍土壤养分亏缺耕地非农业占用14.2BackgroundandgeneraltrendofsoildegradationinChina我国土壤退化的背景与基本态势我国土壤退化的自然社会条件制约：1.土壤（地）资源短缺，空间分布不均（1）人均土壤资源占有率低（2）土地资源空间分布不均匀，区域开发压力大（3）生态脆弱区，范围大（4）耕地土壤质量总体较差，自维持能力弱2.人口增长与社会经济发展对土壤退化的制约3.水资源短缺对土壤退化的制约我国土壤退化的现状与态势：1.土壤退化的面积广，强度大，类型多2.土壤退化的发展快、影响深远SoilDegradationandControl土壤退化与防治Basicconcepts14.3.1.1SoildegradationSoildegradationhasbeendefinedasthedeclineinsoilqualitycausedthroughitsmisusebyhumanactivity.Morespecifically,itreferstothedeclineinsoilproductivitythroughadversechangesinnutrientstatus,organicmatter,structuralstabilityandconcentrationsofelectrolytesandtoxicchemicals(LalandStewart,1990).土壤退化——在自然环境的基础上，因人类开发利用不当而导致的土壤质量及生产力下降的现象和过程。Archaeologicalevidencesuggeststhatsoildegradationhasbeenon-goingsincethebeginningofsettledagricultureseveralthousandyearsago.Thedeclineofmanyancientcivilisations,includingtheMesopotamiansoftheTigrisandEuphratesvalleysinIraq,theHarappansoftheIndusvalleyinPakistanandtheMayansofCentralAmerica,wasdueinparttosoildegradation(Olson,1981).Morerecently,aneventofmajorsignificancewasthedustbowlwhichoccurredintheGreatPlainsoftheAmericanmidwestduringthe1930s.14.3.1.2InfluencedfactorsTheextentofsoildegradationisinfluencedbyanumberoffactorsincludingsoilcharacteristics,topography,climate,landuse,andsocio-economicandpoliticalcontrols.影响因素:自然因素、人为因素AccordingtotheGlobalAssessmentofSoilDegradationproject,about15percentofthegloballandareabetween72°Nand57°Sisstronglydegraded(Ayoub,1991),largelyasaresultofdeforestation,inappropriateofcroppedlandandovergrazing.Inrecentdecadestheglobalrateofsoildegradationhasincreaseddramatically,andislikelytoincreasefurtherasweapproachthe21thcentury。14.3.1.3Typesofsoildegradation土壤侵蚀Erosion土壤沙化Desertification土壤盐渍化Salinasation土壤碱化Sodification土壤酸化Acidification土壤潜育化GleyizationSoildegradationincorporatesanumberofenvironmentalproblems,someofwhichareinterrelated,includingerosion,compaction,waterexcessanddeficit,acidification,salinisationandsodification,andtoxicaccumulationofagriculturalchemicalsandurban/industrialpollutants.Inmanyinstances,thesehaveledtoaseriousdeclineinsoilqualityandproductivity.Theeffectsofsoildegradationarenotrestrictedtothesoilalone,buthaveanumberofoff-siteimplications.Soilerosionisoftenassociatedwithincreasedincidenceofflooding,siltationofrivers,lakesandreservoirs,anddespositionofmaterialinlow-lyingareas.Soilacidificationiscommonlylinkedwithacidificationandaluminiumcontaminationofsurfacewater.Salinisationandsodificationofsoilareoftenassociatedwithpoorqualityirrigationwater.Contaminationofsoilsbyurbanandindustrialpollutants,suchasheavymetalsandradionuclides,mayledtotoxicaccumulationinarableproduceandinherbageforgrazinganimals,thushavingimportantimplicationsforhuman14.3.1.4ManagementofsoildegradationWhetherataglobal,regionalorlocalscale,managementofsoildegradationisclearlyacomplexissueandrepresentsoneofourmostchallengingenvironmentalproblems.Emphasisshouldbeplacedonsustainableratherthanexploitativelandusepractices.Managementofsoildegradationrequiresaholistic,multidisciplinaryapproachinvolvingagronomists,soilscientists,engineers,sociologists,economists,politicians,andmostimportantly,thelandusersthemselves.Theapproachtosoilconservationhasshiftedinrecentyearsfromarathertechnocentricstandpointtoamoreecocentricposition.Centraltothisapproacharetheconceptsoflandhusbandryandsustainabledevelopment.14.3.2ErosionSoilerosionoccurswhentherateofremovalofsoilbywaterand/orwindexceedstherateofsoilformation(1cm/100-400years,=0.3-1.3t/ha•a,FAO).Itisimportanttodifferentiatebetweennaturalorbackgrounderosion,anderosionwhichhasbeenacceleratedlargelyasaresultofhumanactivity.Backgrounderosionratesareoftensimilartoratesofsoilformationat<1.0t/ha•a.Ratesofacceleratederosioncommonlyexceed10t/ha•aandsometimesexceed100t/ha•a.Thehighestsoilerosionrates:>200t/ha•a,loessplateauareaofChinaHimalayanfoothillsofNepal14.3.2.1ThecausalfactorsTheextentofsoilerosionisgovernedbyanumberoffactors,inparticulartheerosivityoftheerodingagent,erodibilityofthesoil,slopecharacteristics(steepnessandlength),landusepracticesandconservationstrategies,andsocio-economiccontrols.TheUniversalSoilLossEquation:E=R·K·L·S·C·PE:meanannualsoillossR:rainfallerosivityindexK:soilerodibilityindexL:slopelengthS:slopesteepnessC:croppingfactorP:conservationpracticefactorLanduseisthemostsignificantfactorinfluencingsoilerosion,fortwomainreasons.First,manylandusepracticesleavethesoildevoidofaprotectivevegetationcover,orwithonlyapartialcover,forsignificantperiodsoftime.Second,theyinvolvemechanicaldisturbanceofthesoil.Specificaspectsoflanduseoftenassociatedwithacceleratedsoilerosionincludeexpansionandintensificationofarablecultivation,overgrazing,deforestation,certainforestrypractices,siteclearanceinpreparationforurbanandindustrialconstruction.Anumberofsocio-economicandpoliticlefactorshavebeenassociatedwithacceleratedsoilerosion,particularlyinthedevelopingworld.Theseincludepopulationpressure,skewedlandresourcedistribution,povertyandmarginalization,increasingdemandforwoodfuel,inappropriatelandtenureandfarmpolicies,smallsizeofland-holdingsandpoorinfrastructure.14.3.2.2Theon-siteeffectsofsoilerosionAttheglobalscale,itisestimatedthatunlesssoilconservationmeasuresareintroducedonallcultivatedland,544millionhaofpotentiallyproductiverain-fedcroplandwillbelost,andagriculturalproductionwilldecreasebyalmost20percent,bytheyear2010(FAO).14.3.2.3Theoff-siteeffectsofsoilerosionTheoff-siteeffectsofsoilerosionarejustasseriousastheon-siteeffects,andareoftenmoredramatic.Theseincludedisruptionoftransportandcommunicationsystems,flooding,siltationofwatersuppliesanddamagetoproperty.14.3.2.4ThemanagementofsoilerosionThreemainstrategies：Agronomicpracticesaimtoprotectthesoilthroughsensiblecroppingprogrammesandarebasedontheencouragementofadensevegetationcoverandplantrootnetwork.Soilmanagementtechniquesaimtoincreasetheresistanceofsoiltoerosion,andfocusmainlyontheimprovementandmaintenanceofsoilstructure.Mechanicaltechniquesaimtoreducetheenergyoftheerodingagentandofteninvolvethemodificationofsurfacetopography.14.3.3AcidificationAcidificationhasanumberofnaturalandanthropogeniccauses.Themainnaturalcausesarelong-termleaching,microbialrespiration,plantnutrientuptakeandnitrification.Themainanthropogeniccausesincludeexcessiveuseofinorganicnitrogenfertilizers,needleleafafforestationandaciddeposition.14.3.3.1ThemainnaturalcausesTheacidsfoundinrainwater(carbonicacid)andindecomposingorganicmaterial(humicandfulvicacids)canstimulateleachingbydissociatingintoH+ionsandtheircomponentanionswhichthendisplaceorattractbasecationsfromthesoilexchangecomplex.Microbialrespirationleadstosoilacidificationthroughtheproductionofcarbondioxide,whichisdissolvedinsoilwatertoformcarbonicacid.Duringplantgrowth,nutrientbasecationsareobtainedthroughrootsystemsinexchangeforH+ions,thusleadingtoincreasedsoilacidity.Nitrificationisanoxidativeprocessoforganicdecomposition,withH+ionsasaby-product:NH4++1.5O2→NO3-+4H+H+ionsarethenavailablefordisplacingandattractingbasecationsfromthesoilexchangecomplex,thusleadingtosoilacidification.14.3.3.2ThemainanthropogeniccausesExcessiveuseofinorganicnitrogenfertilizersPartlythroughtheprocessofnitrification.IflevelsofNO3-ionsinsoilareinexcessofplantrequirements,theywillbehaveasmobileanions,thusencouragingtheleachingprocess.NeedleleafafforestationNeedleleaftreesproducelitterwhichisveryacidicincomparisonwithmostbroadleafspecies.Becauseoftheirhighcanopysurfacearea,needleleaftreesareableto‘scavenge’acidpollutantsfromtheatmosphere,laterreleasingthemintothesoilviathroughfallandstemflow.Duotomodificationsofsurfaceandsoilhydrologybydrainagechannelsandshallowrootnetworks,watertransferisrapidandisconcentratedeitheratsurfaceorintheuppermostlayersofsoil.Thus,thecontributionsofweatheringandionexchangereactionstothebufferingprocessarelimited.14.3.3.3TheconsequencesofsoilacidificationTheacidificationofsoilshasanimpactonanumberofsoilcharacteristics.Morenutrientlossesthroughleaching.Themobilityofaluminiumandheavymetals(suchaslead,zincandcadmium)insoilsincreasesastheyacidify,especiallywhensoilpHfallsbelowabout5.0.Acidificationofsoilsiscommonlyassociatedwithsurfacewateracidification,aluminiumcontaminationanddamagetofreshwaterecosystems.14.3.3.4SoilbufferingoracidneutralizingcapacityThebufferingcapacityofasoilisdefinedastheamountofacidthatneedstobeaddedtocauseareductioninpHofoneunit(Trugill,1988).Soilswhichcontainsignificantquantitiesofbase-rich,weatherablemineralshaveahighbufferingcapacity,whereasthosewhicharedominatedbyquartzandsimilarlyresistantmineralshavealowbufferingcapacity.14.3.3.5ManagementapproachestosoilacidificationThereareanumberofapproachestothemanagementandremediationofsoilandsurfacewateracidity,includingliming,sensibleforestrymanagementandreductionofacidemissionintotheatmosphere,andassessmentofcriticalpollutionloadsarealsoimportantinthisrespect.ThelimerequirementofasoilvariesdependingonitsbufferingcapacityandisusuallyexpressedastheamountofCaCO3(t/ha)requiredtoraisethepHofthetop15cmofsoiltothedesiredvalue.14.3.4SalinisationandsodificationSalinisationandsodificationdescribetheaccumulationinsoiloftoxicsaltsandNa+ionsrespectively.Salinisationdescribestheaccumulationofsaltsinsoil,whereassodificationoralkalisationreferstothedominanceofthesoilexchangecomplexbyNa+ions.14.3.4.1MechanismsofSalinisationandsodificationThesalinisationandsodificationprocessesoccurmostcommonlyinsemi-aridandaridenvironmentswhereevaporationexceedsprecipitation,andwheresoilparentmaterialsandgroundwatersarerichinsodiumsalts.Indryenvironments,soilwatermovementisdrivenlargelybyevaporationandoccursinanupwarddirectionbycapillaryaction.Aswaterevaporates,saltsprecipitateouttoformsalinesoils.SlightdissolutionofthesesaltsandtheconsequentreleaseoftheirconstituentionsintosolutionmayleadtodominanceofthesoilexchangecomplexbyNa+ions,resultingintheformationofsodicsoils.14.3.4.2CharacteristicsofsalineandsodicsoilsThesoilsalinityismeasuredintermsoftheelectricalconductivityofasaturatedextract(ECe),whereassoilsodicityisestablishedfromtheexchangeablesodiunpercentage(ESP)orsodiumadsorptionratio(SAR).Soilsareclassifiedassaline,iftheECevalueis4.0mmhocm-1ormore,pHis8.5orlessandESPislessthan15percent.Sodicsoilsarenon-saline,haveapHofmorethan8.5(usuallylessthan10.0)andanESPvalueof15percentorabove.Soilsareclassifiedassaline-sodiciftheyarebothsalineandhaveanESPvalueof15percentorabove.14.3.4.3ThemaincausesofsalinisationandsodificationAlthoughsalinisationandsodificationoccurnaturallyinsemi-aridandaridenvironments,theyareoftenexacerbated(恶化)asaresultofhumanactivity.Themaincausesarethereplacementofdeep-rootedtreesbyshallow-rootedcrops,andinappropriateirrigationpractices.Overwatering,andtheuseofwaterwhichishighinNa+ionsandlowintotalsalts,mayleadtodestabilisationandbreakdownofsoilstructure.14.3.4.4TheconsequencesofsalinisationandsodificationSoilsalinityandsodicityhaveadetrimental(有害的)effectonbothchemicalandphysicalaspectsofsoilquality.Inachemicalcontext,highsalinityandsodicityareassociatedwithelevatedsoilpH,andunderthesecon