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The influence of land use on desertification processes

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The influence of land use on desertification processes
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  J.RangeManage.57:320 324May2004  heinfluenceofl nduseondesertific tionprocesses MARlSELAPANDO-MORENO ENRIQUEJURADO MARIOMANZAN02,ANDEDUARDOESTRAD.~ iprofessorsoftheSchoolofForestScience UniversidadAutonomadeNuevoLeon Linares N.L.67700Mexico e mail:mpando55@ ... 2AssistantProfessor CenterforEnvironmentalQuality TecnologicodeMonterrey Monterrey N.L.Mexico 64849, e mail:mario.manzano@i;e: -::. Abstract Sitedegradationoccursmainlythroughdeteriorationofthesoil'scapacitytocaptureandstorewater,aswellasthelossoforganicmatterortheaccumulationofsaltsorothertoxicsub.stancesinthesoil.Thisdegradationprocess,leadingtothereductionofthebioticpotentialofthesite,isknownasdesertifi.cation.Inthisstudy,changesinbulkdensity,organicmatter,andelectricalconductivityareusedasindicatorsofdesertificationinnortheastMexico.Thehypothesesputforwardherearethatdegradationprocessesareaffectingextensiveareasoftheregion,andthatthetypeofprocessesandtheirmagnitudedifferaccord-ingtospecificlanduses.Thirty-onesitesunderdifferentlandusesystems(agriculture,rangeland,inducedgrassland,andapro-tectedsite)weresampledforbulkdensity,organicmatter,andsalinity.Soilsamplesforbulkdensityestimationwerecollectedin1996,1997,and1999,whilethosefororganicmatterandsalin-ityweretakenin1993,1997,and1999.Soilbulkdensityandorganicmattershowedsignificantchangesacrosstimeinrange-landsites.Noneofthesitesshowedsignificantchangesinsalinity.Organicmatterwassimilarinagriculture,rangeland,andgrass-landsitesacrossdates.Soilbulkdensitywassimilaringrass.landsandrangelandsandlowerinagriculturesites.Valuesoforganicmatterwerelowerandthoseofsoilbulkdensitywerehigherwhencomparedtoaprotectednativevegetationsite. KeyWords:soilbulkdensity,organicmatter,landuse,NortheastMexico,landdegradationDesertificationisaglobalphenomenonwithserioushumanandenvironmentalrepercussionsaffectingthelivelihoodoftoday'shumanpopulationandthatofgenerationstocome(Grainger1992,Amalds2000).Inspiteofthecontroversialhistoryoftheterm'desertification'(Medellfn-LealI978,Landaetal.1997),thereisgeneralagreementthatitincludesaseriesofprocessesleadingtotheimpoverishmentofsoilandvegetation,suchassoilerosion,lossofsoilfertility,anddeclineofproductivity(Medellfn-Leal1978,Dregne1983,Grainger1992,Mainguet1994,CONAZA-SEDESOL1994,Toulmin1995,Pandoetal.1996).Landdegradationoccursmainlythroughdeteriorationofthesoil'scapacitytocaptureandstorewater,thelossofnutrientsortheaccumulationofsaltsorothertoxicsubstancesinthesoil J.L.PalacioandJ.L6pez-Blancomadeusefulcontributionstoearlierversionsofthismanuscript.ThisstudywaspartiallysupportedbyCONACyT(960055),ANUIES(B96-3743)andPADEP(010339),O.AguirregaveexpertstatisticaladviceandNickMosshelpedwithgrammar.Manuscriptaccepted22Aug.03. 320 Resumen Ladegradaciondeunsitioocurreprincipalmenteab'rCi'deteriorodelacapacidaddelsueloparacaptaryale. agua,laperdidadenutrientes0laacumulaciondesales. = _ sustanciastoxicasenelsuelo.Esteprocesodedegra~..,_ conducealareducciondelpotencialbioticodelsitio, sen.=.... comodesertificacion.Enesteestudio,ladensidadapaJ e8Sr :. suelo,contenidodemateriaorganicayvaloresde~utilizadoscomoindicadoresdedesertificacionenei-. - Mexico.Lashipotesisplanteadassonque105pr::u: degradacionestanafectandoextensasareasenlarepia   tipodeprocesos,asicomolamagnituddeest8S. L dependiendodelusodelsuelo.SemuestrearontniI::_bajodiferentesusosdesuelo(agricultura,ag~ -_ unsitioprotegido)paradeterminarlade~_,,:,contenidodemateriaorganicaysalinidad.Las~_' :..._ evaluardensidaddelsuelofueroncolectadasez:: . ~~ 1999;mientrasquelasdemateriaorganicaye= ~, electricasetomaronen1993,1997y1999.La~._delsueloylamateriaorganicamostraroncambias;-;::_:_atravesdeltiempoenlossitiosdeagostadero.~- sitiosevaluadosmostrocambiossignificativosez::s .~ materiaorganicafuesimilarenlossitiosde~ agostaderoypastizal.Ladensidadaparenter. :'agostaderoypastizalesymenorensitiosdeape._'valoresdemateriaorganicafueronmenores ySasL.._ aparentedelsuelomayoresquelosdelsitiodeT~ - protegido. (Smith1989,Friedel1991,NationalResearch C~__ Thisdegradationprocess,leadingtothereducti~potentialofasite,isknownasdesertification.Soilsmaylosetheircapacitytocaptureand~;..compactionofthesoil,lossoforganicmatterQ( depth.Inaddition,increasesinsalinityreduce\J,a;.;,-__forplants.Soilcompactionduetouseofhea,,:=--agriculturalland(Hartge1988)andlivestoc_(ManzanoandNavar2000,Yatesetal.2000)U5l-.undesirablechangesinthehydrologicalprocesse'suchasreducedwaterinfiltrationcapacity(Br_2002).Lossoforganicmatterinturnisassociated  _ paction,declineinfertilityandageneraldeterior_qualitythroughdestabilizationofaggregates and ~_ cationicexchangecapacity(Bohnetal.1993,FassbeL.Salinityreducestheabilityofplantstoextract  _ soil;ithasbeenrecognizedasasourceoflanddegn.-irrigationpractices(ThomasandMiddleton1993..._thefactorsleadingtothefalloftheMesopotamia : = _ JOURNALOFRANGEMANAGEMEW ~.::;  yearsago(JacobsenandAdams1958).Ithasalsobeendocumentedthatreplacinganaturalvegetationcommunitywithcropsorgrasslandforgrazingcanbesignificantforsalinization(Bettenay1986).LanduseinnortheastMexicoischarac-terizedbypracticesthatarecommonlyassociatedwithdesertification,suchasextensivelandclearing(MaldonadoandPando-Moreno1994,Juradoetal.2001),pooragriculturalpractices,andovergraz-ing(Manzanoetal.2000,Melladoetal.2003).Itispossiblethatlanddegradationisoccurringinthisareaeventhoughitmaybeatanearlystagethatisonlyrevealedbylaboratorytests(Pandoetal.1996).Inthisstudyweevaluateddesertifi-cationbasedonchangesinthephysical,chemicalandbiologicalconditionsofthesoil,which,tosomeextent,determineitsbioticpotential.Changesoverthelastdecadeinsoilcompaction,organicmattercontentandsalinitywereana1yzedinagri-culture,rangelands,andinducedgrass-lands(seededafterclearingnativevegeta-tion)ofnortheastMexico.Thehypothesesputforwardarethatdegradationprocessesareaffectingextensiveareasoftheregion,andthatthetypeofprocessesandtheirmagnitudedifferaccordingtospecificlanduses. MaterialsandMethods ResearchsiteThestudywasconductedintheareasurroundingLinares,NuevoLeon(24047'N,99032'W),innortheastMexico.Soilsintheareaaresiltclay,clayorsiltclayloam,mostlyVerti~olsandRegosols,fol-lowingFAO'sclassification(1998).Themostcommonlandformsareplainsandgentlyundulatingslopesinterspersedwithhillsupto50mabovetheterrain.Ongen-dehillsandupperslopes,outcropsofUpperCretaceousshaleoccur,oftenover-lainbysiltyclayloams.ScatteredPlioceneandQuaternaryterracesofconglomerate,oftenconsolidatedinalimestonematrixIcaliche)arealsopresentthroughouttheregion(Ruiz1990).Climateoftheareahasbeenclassifiedas,A)C(WO)(Garcia1981),semi-warm,sub-ilumid,withasummerrainfallregime.\verageannualrainfallis761.2mmandmeanannualtemperature22.40CwithiIll11II1eremperaturesoftenreaching400C.ThevegetationisaTamaulipanthorn--crubplantcommunity(Muller1947),mownlocallyas matorral .Thisnativeegetationisadiverse,spiny,sometimesJensescrubdominatedbywoodyplants(Heiseke1986)withsome80speciesofshrubsandtreesranginginheightfrom1to15meters(Reidetal.1990).Thornyshrubs(e.g. Acaciaberlandieri Benth., Acaciafarnesiana (L.)Willd., Acaciagreggii Gray, Acaciarigidula Benth., Havardiapallens (Benth.)Britton&Rose)andtrees(e.g. Ebenopsisebano (Berl.)Barneby, Prosopislaevigata (Humb.&Bonpl.exWilld.)M.C.Johnst., Prosopisglandulosa Torr.var. torreyana M.C.Johnst.)dominatethenaturalvegeta-tion,butgrasses(e.g. Eragrostismexicana (Horn.)Link., Boutelouacurtipendula (Michx.)Torr., Boutelouabarbata Lag.),forbs(e.g. Crotonciliato glandulosus Ort., Lantanacamara L., Verbesinaence lioides (Cav.)A.Gray, Waltheriaindica L.)andsucculents(e.g. Opuntiaengel mannii Salm-Dyck,O. leptocaulis DC., Opuntiaphaeacantha Engelm.)arealsoprominent(Gonzalez-Medrano1972).TheareahasbeengrazedbyintroducedlivestocksincetheSpanishconquestinthelatterpartofthe16thcentury(Rojas-Mendoza1965,delHoyo1979).Selectiveloggingandcuttingfortimberandfire-woodhasoccurredintheareaforcen-turies(Reidetal.1990),whileextensivelandclearingforagricultureorcattlegraz-ingininducedgrasslandsbecameexten-siveinthe1970's(USFWS1983,VazquezandGonzalez1999).Nativeveg-etationisstillbeingconvertedtonon-irri-gatedmaizeandsorghumandmoreoftentoinducedgrasslandsofbuffelgrass  Pennisetumciliare (L.)Link)forcattlegrazing.MethodsSiteswereselectedashomogeneousareasoflanduse,rangingfrom20to100ha,inthesameclimate.Distancebetweensitesrangedfrom5kmto50km.Atthecentreofeachsiteacircularplotof100mdiameterwaslocatedinwhichsamplesweretakenforsoilbulkdensity,salinityandorganicmatter.Samplingwasdoneinthefallowperiodduringthedryseason(lateJuneandearlyJuly).Sampleshavebeencollectedaspartofalong-termresearchondesertificationattheSchoolofForestScience,inLinares,NuevoLeon,Mexico.Allsampleswerecollectedandanalyzedusingthesameprocedures(describedbelow).Earlyresultshavebeenpresentedaspartofunpublisheddissertations(MaldonadoandPando1994,Gutierrez1997,Reyes2000,andPando2002).Atotalof33siteson4landuses(agri-culture,rangelands,inducedgrasslands,andareasprotectedfromgrazing)were ..oURNALOFRANGEMANAGEMENT57 3 May2004 sampledfororganicmatterandsalinityin1993,1997,and1999.Samplingforsoilbulkdensitywascarriedoutin1996,1997and1999.Twooutof3protectedsiteswereclearedandthusnotincludedinlaterevaluations.Thus,statisticaltestswereonlydoneforlanduseswithsufficientnumberofreplicates:17rangelandsites,10agriculturalsites,and3grasslandsites.Theremainingprotectedsitewasusedonlyforvisualcomparisons.SamplingsiteswerepreciselylocatedusingGPS.Samplesfororganicmatterweretakenfromthetop30cmoncethecoveringlitterwasremoved.Organicmat-terwasindirectlyestimatedthroughmulti-plicationoforganiccarbonconcentrationby1.724.Carbonconcentrationwasdeter-minedusingthewetcombustionWalkley-Blackmethod(NelsonandSommers1982).Salinitywasdeterminedbyelectri-calconductivityofthesolutionextractedfroma1:5soil-watermixture,afterhalfanhourofshaking(Rhoades1982).Soilbulkdensitywasestimatedusingthecoremethodtoasoildepthof10cm(3.5cmindiameter)(BlakeandHartge1986).Fivesamplesfromeachof10randomlyselectedsiteswerecollectedandusedforpre-samplinganalysisforsoilbulkdensi-ty,organicmatterandelectricalconduc-tivity.Thenumberofplotsrequiredfor95 confidencewasdeterminedusingBonham'sequation(1980):whereNisthecomputedsamplesize,tisthetabulatedvalueforthespecifiedconfi-dencelevelandthedegreesoffreedomofthesample,S2isthevarianceoftheinitialsample,kistheacceptederror(0.1)andXisthemean.Resultsfromthepre-samplingtestindi-catedthattheminimalnumberofsamplesrequiredforex~0.05were:5samplespersiteforbulkdensity,4samplesfororganicmatter,and2samplesforelectricalcon-ductivity.Hence,forpracticalreasons,5samplesweretakenforalltheparameters.Samplesweretakenatthecenterofthesiteand50metersapartintheNorth,South,East,andWest.Allsampleswereanalyzedseparately,thusanaverageresultof5sampleswasobtainedforeachsite.Pairedt-testswereruntoevaluatechangesthroughtheperiods1993-1997,1996-1997,1997-1999,and1993-1999fortheentirearea,usingtheaveragevalueofthesites.PairedHest,knownasthe beforeandafter design,isthekindofanalysisrecommendedwhenthesame 32  ind.:.-jdua;ismeasuredmorethanonce;ihedataarecontinuousvariables,andha -e.atleastapproximately,anormaldis-uibution(Dythman1999).At-testwasalsoappliedtocomparebulkdensity,electricalconductivityandorganicmatterlandusevaluesfromthefirstsampling(1996forsoilbulkdensity,and1993fortheother2)tothelastone(1999).Thesameanalysiswasalsousedtoevaluatechangesateachindividualsite. ResultsandDiscussion Significantchanges(P~0.05)onsoilbulkdensityandorganicmatterwereonlydetectedacrossthelongestperiod.About67 ofthesiteshaddetrimentalchangeseitherinsoilbulkdensityororganicmat-terand27 ofthesiteshaddetrimentalchangesinbothparameters.Noneofthesitesshowedsignificantlevelsofsalinity«200 IlS cm orchangesinthisparame-terduringthe6-yearevaluationperiod.SoilbulkdensitySpatialdistributionofsoilbulkdensityintheupper5cmwasveryhomogeneouswithineachsite.Thehighestvariation coefficientwas8.7 (n = 5)forarange- landsite.ThisfigureissimilartothoseofMartfnezandZinck(1994)whoreporteda variationcoefficientof9 (n = 15)forthe ~15cmlayerininducedgrasslandsintheAmazon,and13 foratropicalforest.Soilbulkdensityhadatendencytoincreasewithtime(Fig1)implyingthatsoilsintheareaareinaprocessofcom-1.451.3 Table1.Soilbulkdensity,organicmattervaluesandlandusebysite,ateachevaluationdate. rangelandrangelandrangelandrangelandrangelandrangelandrangelandrangelandrangelandrangelandrangelandrangelandrangelandrangelandrangelandrangelandrangelandagricultureagricultureagricultureagricultureagricultureagricultureagricultureagriculture agricultureagriculturegrasslandgrasslandgrassland protectedsite Soilbulkdensity199619971999 (gcm,3)_________________ 1.141.211.311.311.441.471.101.241.421.281.301.221.331.471.561.371.411.401.411.451.451.401.321.421.411.431.401.401.451.431.361.391.421.241.311.371.351.421.481.291.291.431.321.491.471.461.541.541.321.351.361.201.241.381.181.181.321.271.261.221.211.241.211.271.291.181.081.111.071.281.301.291.271.161.171.281.271.351.251.261.481.401.421.421.251.341.271.441.471.451.071.091.05 paction.Soilbulkdensityvaluessignifi-cantlyincreasedfrom1996to1999(P = 0.020),butnotbetween1996and1997(P = 0.073)norbetween1997and1999(P = 0.255).From1996to1999soilbulkden-sityincreasedfor13sites(P~0.05),withonly1sitedecreasinginsoilbulkdensity Organicmatter199319971999 ( )----------------- 3.711.962.573.141.981.085.423.844.213.574.112.452.610.960.722.182.442.613.921.41.772.072.072.212.582.742.541.212.261.722.283.933.763.092.493.392.751.701.723.941.732.252.31.581.432.092.841.403.82.153.172.043.002.92I.991.341.832.412.721.654.474.234.103.434.483.683.162.972.802.622.041.693.592.282.282.622.422.472.802.951.922.972.613.312.323.002.361.901.942.0516.4312.8915.32 and16remainingconstant.Bulkdensityincreasedin10outofthe30sitesforbothperiods(Table1).Theaverageincreaseinsoilbulkdensi-ty,forthosesitesthatshowedasignificantdifferencefrom1996to1999,was0.163gcm-3(12.8 ).ThispercentageissimilartothatfoundbyVanHaveren(1983)forfinetexturedsoils,of13.4 forheavilygrazedpasturescomparedtothoselightlygrazed.Asimilarsoilbulkdensityincrease(0.2gcm-3)wasfoundafteranexperimentalheavyovergrazingeventinnortheastMexicobyManzanoandNavar(2000),Soilbulkdensitywaslower(P<0.05)foragriculturalsitesthanforrangelandandgrasslandsites(Table2).Becausenoheavymachineryisusedonagriculturalsoilandthelatterisploughedatleastonceayearinthisregion,lowersoilbulkdensi-tyvalueswereexpected.Evidenceforsoilcompactionwasfoundforrangelandsites,assoilbulkdensitywashigher(P<0.05)in1997and1999thanin1996.Soilbulkdensityvalueswereconsistentlyloweracrosstimefortheprotectedsiteusedasreference(Table2).1997200099619981999)leerrag.1.Averagesoilbulkdensityvalues(n = 30)forthestudyarea.Errorbarsareforconfi- denceintervals(a = 0.05). 322 JOURNALOFRANGEMANAGEMENT57(3) May 4 - 1.4 . E uCD i 1.35 OIl cIU C1.3¥ 5.cl SIII1.251.21995  Table2.Averagesoilbulkdensity(gcm'3)forthe3evaluationperiods,groupedbylanduse. YearRangeland(17)Agriculture(10)Grassland(3)Protectedsite(I) (gcm.3)____________________________________________ 19961.323'AI1.228bA1.361'A1.07319971.384'B1.232bA1.407'A1.0919991.421'B1.266bA1.381'A1.053 'Meanswithinrowswithdifferentlowercaselettersdiffer(P~0.05).Meanswithincolumnswithdifferentuppercaselettersdiffer(P~0.05).Numbersinparenthesisindicatenumberofsites.Duetolossofreplicatesforprotectedsites,dieremainingvalueisshownforcomparativepurposes.Table3.Averageorganicmattervalues(%)forthe3evaluationperiods,groupedbylanduse. YearRangeland(17)Agriculture(10)Grassland(3)Protectedsite(1)---(%)-----------19932.98'AI2.91'A2.40'A16.4319972.36'B2.84'A2.51'A12.8919992.29'B2.53'A2.57'A15.32 'Differentlowercaseletters(inlines)areforsignificantdifferences(P~0.05)betweenlanduses.Differentuppercase ,ners(incolumns)areforsignificantdifferences(PS0.05)betweenevaluationperiods.NumbersinparenthesisareDumberofsitesfortheanalysis.Duetolossofreplicatesforprotectedsites,theremainingvalueisshownforcompara-livepurposes. OrganicmatterOrganicmattercoefficientofvariation,n = 5)averagedforthe30siteswas17.7%andonlyIsiteshowedaveryhighcoefficientofvariation(53%).Thisvaria-tionwaslessthanthatreportedbyMartinezandZinck(1994)whofoundacoefficientofvariation(n = 15)of24.5% fortheupper10cminatropicalforestand24%ininducedgrasslands.Organicmattersignificantlydiminishedfrom1993to1999(P = 0.016)whileno changeswereregisteredfrom1993to1997(P = 0.06),norfrom1997to1999(P = 0.275).Thetrend(Fig.2)calculatedfor4theaveragevalues,indicatesareductioninsoilfertilityinthearea.Duringthe199~-1999period,15outof30sitesshowedasignificant(P~0.05)declineintheirorganicmattercontent,whiletheother15sitesremainedconstant(TableI).Organicmatterwassimilarforrange-land,agriculturalandgrasslandsites(Table3).Organicmatterfortheprotectedsiteusedasreferencewasabout4timeshigherthanthoseofotherlanduses.Rangelandsiteshadlower(P<0.05)organicmatterin1997and1999whencomparedto1993(Table3),whichiscon-sistentwithincreasedsoilbulkdensity I ?-~9.........-.--- t 54 f 40 2. ~ = 0.9959 32119911995199119992001993 year 1g.2.Averageorganicmattervalues(n = 30)forthestudyarea.Errorbarsareforconfi- denceintervals(a = 0.05). OURNALOFRANGEMANAGEMENT57 3 May 2004 valuesforthesesites.Thedecreaseinorganicmatterinrange-landsfrom1993to1999maywellbetheresultofalackofmanagement,combinedwithuseexceedingcarryingcapacity(ManzanoetaI.2000,Narjisse2000).Therewerenochangesintheorganicmat-tercontentofinducedgrasslands;howeverlowvalueswerepresumablyaresultofremovalofnativevegetation,ashasbeenfoundelsewhere(Nair1984).Agriculturalsitesdidnotshowahighercontentoforganicmatterthanotherlanduses.Thisisinlinewithexpectations,asinthesesitescropresiduesareusedasfodderandnotincorporatedintothesoil.Theprotect-edsitehadthehighestcontentoforganicmatter(Table3),whichisinagreementwithfindingsofBravo(1999),forsimilarecosystemsinnortheastMexico,inwhichareaswithnativevegetationandno-usehadthehighestlevelsoforganicmatter.Evidenceofdesertificationwasfoundforrangelands,whichshowedasignificantincreaseinsoilbulkdensityfrom1996to1999andasignificantdecreaseinorganicmatterfrom1993to1999,whilegrasslandandagriculturalsitesdidnotdifferthroughtime.Ithasbeenshownthatremovalofnativevegetationresultsinthelossoforganicmatterandsoilcompaction(Young1990,KingandCampbell1994).However,inthisstudywehavefoundthatovergrazingwithoutclearingthelandalsoresultsinsoildegradation.Whileorganicmatterandsoilbulkden-sityforinducedgrasslandsandagricultur-alsitesdidnotchangewithinthisstudy,valuesforbothparameterswerepoorerwhencomparedtoaprotectedsite.Thus,itispossiblethatsoildegradationinthesesitesoccurredpriortoourfirstevaluationdates.Resultspresentedhereshouldnotbeusedoutofcontexttopromotelandusechangesleadingtotheclearingofthorn-scrubwithoutspecificstudies. 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