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Phytoliths and phytolith carbon occlusion in aboveground vegetation of sandy grasslands in eastern Inner Mongolia, China PDF

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ScienceoftheTotalEnvironment625(2018)1283–1289 ContentslistsavailableatScienceDirect Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv Phytoliths and phytolith carbon occlusion in aboveground vegetation of sandy grasslands in eastern Inner Mongolia, China NingRua,b,1,XiaominYanga,1,ZhaoliangSonga,⁎,HongyanLiuc,QianHaoa,XuLiuc,XiuchenWud aInstituteoftheSurface-EarthSystemScienceResearch,TianjinUniversity,Tianjin300072,China bSchoolofEnvironmentalandResourceSciences,ZhejiangAgricultureandForestryUniversity,Lin′an,Zhejiang311300,China cCollegeofUrbanandEnvironmentalSciences,PekingUniversity,Peking100871,China dCollegeofResourcesScienceandTechnology,BeijingNormalUniversity,Beijing100085,China H I G H L I G H T S G R A P H I C A L A B S T R A C T • Grasslands play a crucial role in the long-term carbon sequestration of phytoliths. • Desertificationhasoccurredextensively insandygrasslandsofnorthernChina. • Phytolith production flux in sandy grasslands was 8.94–47.8kgha−1 year−1. • PhytOCproductionfluxinsandygrass- landswas0.06–0.48kgha−1year−1. • Desertificationsignificantlydecreases thephytolithandPhytOCproduction fluxes. a r t i c l e i n f o a b s t r a c t Articlehistory: Grasslandsplayacrucialroleinthecoupledbiogeochemicalcyclesofcarbon(C)andsilicon(Si)becausethey Received10November2017 havealargebiogenicSipool(i.e.phytoliths).Inrecentdecades,desertificationhasoccurredextensivelyin Receivedinrevisedform3January2018 sandygrasslandsduetohumanactivitiesandtoincreasedaridityasaconsequenceofclimatechange.Thepres- Accepted7January2018 entstudydeterminedthecontentsofphytolithsandCocclusionwithinphytoliths(PhytOC)insandygrassland Availableonline12January2018 withdifferentvegetationcoveragefromeasternInnerMongolia,Chinaandpreliminarilyassessedtheeffectsof desertificationonphytolithsandPhytOCproduction.OurresultsshowedthatthephytolithandPhytOCcontents Editor:JayGan amongdifferentplantspeciesvariedfrom0.68to9.23%and0.03to1.13‰,respectively.However,thecommu- Keywords: nity-weightedmeansofthephytolithandPhytOCcontentsforthetotalabovegroundvegetationwereonly1.13– Phytoliths 3.61%and0.09–0.35‰,respectively,andtheirrespectiveproductionfluxesrangedfrom8.94to47.8kgha−1 PhytOC year−1andfrom0.06to0.48kgha−1year−1,respectively.Asdesertificationprogressed,thetotalcontentsof Long-termcarbonsequestration phytolithsandPhytOCinabovegroundvegetationdidnotchangesignificantly,whereastheproductionfluxes InnerMongoliasteppe ofphytolithsandPhytOCweremarkedlyreduced.Thisstudyindicatesthatgrasslanddesertificationdecreases Grasslanddesertification therangeofthetotalcontentsofphytolithandPhytOCbyreducingspeciesrichness,anddecreasestheproduc- Grasslandrestoration tionfluxesofphytolithsandPhytOCbyreducingabovegroundbiomass.Grasslandrestorationcantheoretically enhancetheproductionfluxesofphytolithsandPhytOC~five-fold. ©2018ElsevierB.V.Allrightsreserved. ⁎ Correspondingauthorat:InstituteoftheSurface-EarthSystemScienceResearch,TianjinUniversity,No.92WeijinRoadNankaiDistrict,Tianjin300072,China. E-mailaddress:[email protected](Z.Song). 1 Theauthorscontributeequallytothemanuscript. https://doi.org/10.1016/j.scitotenv.2018.01.055 0048-9697/©2018ElsevierB.V.Allrightsreserved. 1284 N.Ruetal./ScienceoftheTotalEnvironment625(2018)1283–1289 1.Introduction hasbeenrecentlychallengedduetothelowerCcontentinphytoliths extractedbyrapidH SO /H O digestion(SantosandAlexandre,2017). 2 4 2 2 GrasslandsaccountforN20%ofglobalterrestrialarea,andtheyplaya Forgrasslandecosystems,whilephytolithsandPhytOChavebeen crucialroleinlivestockfarmingandglobalcarbon(C)cycles(Bleckeret wellstudied,(Bleckeretal.,2006;Songetal.,2012;Qietal.,2017; al.,2006;Jiangetal.,2006;Songetal.,2012).Duetoclimatedryingand Panetal.,2017),changesinphytolithandPhytOCcontents,andinthe humanactivities(e.g.,overgrazing,croplandmisuse,excessiveexploita- productionfluxesoftheseplantcomponentsinsandygrasslands,as tionoffuel,andunregulatedcollectionofmedicalplants),halfofthe wellastheirrelationstograsslanddesertificationgradients,arenot world'sgrasslandareahasbeendegraded(ZhaoandZhao,1993;Zhu wellknown,especiallyinthearidandsemi-aridgrasslandareasof andChen,1994;Jiangetal.,2006).Desertification,oneofthemostse- China.Therefore,theaimsofthisstudyweretodeterminethecontents veretypesofgrasslanddegradation,ismainlycharacterizedbyaeolian ofphytolithsandPhytOCinsandygrasslandofeasternInnerMongolia; (wind-driven)soilerosionofsandygrasslandsinaridandsemi-aridre- toassessthephytolithCsequestrationpotentialofsandygrasslands, gions(Wang,2000;Yangetal.,2005;Jiangetal.,2006).InChina,there andtoexploretheeffectsofdesertificationongrasslandphytolithCse- areN6.64×106halandssubjectedtodesertificationinarid,semi-arid questration.Thisstudycouldenhanceourknowledgeofchangesin anddrysub-humidregions,accountingfor~7%oftheterritory(Zhang phytolithCsequestrationresultingfromchangeingrasslands;present etal.,2016).Suchalarge-scaledesertificationnotonlyseriouslyexacer- somesuggestionsonhowtoincreasephytolithCsequestration;and batesthedegradationofthealreadypoorenvironmentalquality,but contributetostudiesofthecoupledbiogeochemicalcyclesofCandSi. alsoincreasestheproblemofpovertyinpartsofnorthChina(Jianget al.,2006). Silicon(Si),thesecondmost-abundantelementintheearth'scrust, 2.Materialsandmethods canprotectplantsfromvariousabioticandbioticstresses,afteruptake bytheplantrootsintheformofdissolvedmonosilicicacid(Epstein, 2.1.Experimentalsiteandfieldinvestigation 1994;MaandYamaji,2006).Phytolithsarethemaindepositsofsilica (SiO )inmanyplanttissues,especiallyattheendpointsoftranspiration, ThisstudywasconductedinthewesternpartoftheNortheastChina 2 thoughthemechanismofSiO depositionisstillunclear(Hodsonetal., Transect,XilingolLeague,InnerMongolia,China(Fig.1).Thearealiesat 2 2005;Schalleretal.,2013).Ingeneral,thephytolithcontentranges 42–44°Nand115–118°E,withanelevationof840–1496m.TheNorth- fromb0.5%inmostdicotyledonsto~3%intypicaldrylandgrasses, eastChinaTransecthasbeenusedtostudytheresponsesofterrestrial andmayevenreachupto10–15%intheCyperaceaeandinwetland ecosystemstoglobalclimatechange(Zhouetal.,2002).Thewestern speciesofthePoaceae(Epstein,1994;Parretal.,2010).Duringthefor- areaoftheNortheastChinaTransectismainlycomprisedofmeadow mationofphytolithsinplanttissues,0.1%–6%organicC,originating anddesertsteppes,dominatedbyatemperatecontinentalclimate. largelyfromphotosynthesis,canbeincorporated(Parretal.,2010; Themeanannualtemperatureandprecipitationinthisareaare0–3°C ParrandSullivan,2011;ZuoandLü,2011;Lietal.,2013;Zuoetal., and295mm,respectively.Grasslandswithsandysoilsubstratesareex- 2017),referredtoasCocclusionwithinphytoliths(PhytOC).Since tensivelydistributedthroughoutthisregion. phytolithsareresistanttodecomposition,hightemperatureandoxida- WecarriedoutfieldinvestigationandsamplinginJulyandAugustof tion,thePhytOCcanoccupy82%ofthetotalorganicCinsomesoilsthat 2014,whenplantgrowthisatitsmostmaturestage.Accordingtoour areN1000-yearsold(ParrandSullivan,2005).Thus,PhytOChasbeen fieldinvestigation,thestudyareahassufferedfromdifferentdegrees consideredtobeoneofthemostpromisingmechanismsofterrestrial ofdesertification.Oneoftheobviousdesertificationfeaturesisthere- biogeochemicalCsequestrationatacentennial-millennialscale(Parr gional variation of vegetation coverage. However, it is possible for andSullivan,2005;Zuoetal.,2014;Songetal.,2012,2013;Dasetal., somewherewithoutdesertificationthatthevegetationcoveragemay 2016),althoughthesignificanceofPhytOCforglobalCsequestration besparse.Therefore,wefirstlysystematicallysetup39sitesbasedon Fig.1.DistributionofmaingrasslandtypesinChina.FilledtrianglesindicatesamplingsitesfromeasternInnerMongolia.ThedatasetisprovidedbyDataCenterforResourcesand EnvironmentalSciences,ChineseAcademyofSciences(RESDC,http://www.resdc.cn). N.Ruetal./ScienceoftheTotalEnvironment625(2018)1283–1289 1285 thevariationofvegetationcoverage,inordertoselectsitessuffering Community-weightedmean(CWM)isbroadlyusedtocalculatethe fromdesertification.Ateachsite,werandomlysetupthreereplicate communityfunctionaltraitsbymanyresearchers(RicottaandMoretti, 1m×1m quadrats. The coverage, abundance, and height of each 2011;Qiuetal.,2016).Toassessthecontentsofameasuredindex plantspecieswererecordedineachquadrat(TableS1).Aftercompre- (e.g.,SiO ,phytolith,andPhytOCcontents)ineachplotmorecompre- 2 hensivelyanalyzingthevegetationtype,speciescompositionandvege- hensively,wemodifiedthecommunity-weightedmeanasfollows: tationcoverageofdifferentsites,12sitesofthe39siteswereselectedto represent the desertification sites (Table 1). In the field study, the P abovegroundbiomassofeachquadratwasharvestedtoestimatethe CWMX¼ si¼1IVi(cid:2)Xi ð2Þ abovegroundnetprimaryproductivity(ANPP)(Table1).Meanwhile, IVts theabove-groundparts(approximately150g)oftherepresentative speciesineachquadratwereharvestedtoassessthecontentsandpro- inwhichXisthemeasuredfunctionalindex,CWM_Xisthecommunity- ductionfluxesofbothphytolithsandPhytOC,usingthemethodofthe weightedmeancontentofXineachsite,IV istheimportancevalueof community-weightedmean. i sampledspeciesiandcanbecalculatedfromformula(1),X isthecon- i tentofXinsampledspeciesi,Sisthetotalnumberofsampledspecies, 2.2.Sampleanalysis andIV isthetotalIVofallsampledspecies. ts Ineachplot,theproductionfluxes(PF)ofdifferentmeasuredfunc- Thesamplesofabovegroundbiomassandoftherepresentativespe- tionalindicesintheabovegroundvegetationcouldbeestimatedusing cieswererinsedwithtapwatertoremoveattacheddust,rinsedthree thefollowingequation(Songetal.,2012,2013): timeswithultrapurewater,anddriedat65°Cfor48htoaconstant weight.Thedriedabovegroundbiomasssamplesweredirectlyweighed andtherepresentativespeciessamplesweredividedintotwoparts: PFX¼ANPP(cid:2)CWMX ð3Þ onepartwaspowderedtoanalyzetheSicontent,andtheotherpart wascuttob2mmtoanalyzephytolithcontent.Thepowderedvegeta- tionsampleswerefusedwithLi-metaborateandthendissolvedindi- inwhichPF_Xistheproductionfluxofthemeasuredfunctionalindex lute nitric acid (Li et al., 2013). The Si content of the solution was (X),ANPPistheabovegroundnetprimaryproductivityofeachsite analyzedbythemolybdenumbluecolorimetricmethod,usingultravio- (Table1),andCWM_Xiscalculatedfromformula(2). let–visiblespectrophotometer(Jietal.,2017).Inordertomorereliably quantifythetotalcontentofPhytOC,phytolithswereextractedbythe 2.4.Datastatistics modifiedmicrowavedigestionmethod;purityofthephytolithswas testedbyWalkley-Blacktypedigestion(WalkleyandBlack,1934;Liet Excel(2010)andSPSS(18.0)wereusedtostatisticallyanalyzethe al., 2013; Parr and Sullivan, 2014). The extracted phytoliths were data.One-wayanalysisofvariance(ANOVA)andtheleastsignificant driedat65°Cfor24hincentrifugetubes,cooled,andthenweighed. difference(LSD)testwereappliedtoexaminethedifferencesbetween Aftertheweighedphytolithsweredissolvedin1molL−1hydrofluoric thedatagroups.FiguresweremadewithOrigin(8.0). acid(HF)solutionat50°Cfor1htoreleaseCfromthephytoliths,the PhytOCconcentrationofthesolutionwasmeasuredbythetraditional 3.Results potassiumdichromatemethod(Lietal.,2013).TheorganicCdatawas monitoredwithstandardsoilsamplesofGBW07405.Theprecisionis 3.1.Variationinthemeasuredindicesinsampledspecies betterthan7%. SiO contentofallsampledspeciesvariedfrom0.94to7.88%,andthe 2 2.3.Datacalculations phytolithcontentrangedfrom0.68to9.23%(TableS2).Calamagrostis epigeiosgrowinginsite2hadthehighestabilitytooccludeC(2.33%) Importancevalue(IV)usesplantspeciesheight,cover,andabun- duringthephytolithformation(TableS2).ThelowestPhytOCcontent dance to weight each species when constructing a comprehensive (0.03‰)wasfoundinSalsolacollinaandPotentillalongifolia,growing indexofplantcommunities(Liuetal.,2008).Ineachquadrat,theIV insites1and8,respectively(TableS2).Intheabovegroundpartofdif- ofeachspecieswascalculatedby: ferentspecies,therewasasignificantpositivecorrelation(R2=0.847, Pb.01)betweenSiO contentandphytolithcontent(Fig.2a).However, 2 IV¼ðRHþRCþRAÞ=3 ð1Þ therewasnosignificantcorrelationbetweenthephytolithcontentand Ccontentofphytoliths(Fig.2b).Asignificantpositivecorrelation(R2= inwhichRHreferstotherelativeheight,RCreferstotherelativecover, 0.558,Pb.01)alsoexistedbetweenphytolithcontentandPhytOCcon- andRAreferstotherelativeabundanceoftheparticularspecies. tentintheabovegroundpartofallsamples(Fig.2c). Table1 Informationofsamplingsites. Sitesa Altitude(m) Vegetationcoverage(%) ANPP(tha−1year−1) Representativeplantspecies 1 1496 80 1.33 Psammochloavillosa,Salsolacollina,Artemisiadesertorum,Leymuschinensis 2 1335 75 1.32 Agropyrondesertorum,Calamagrostisepigeios,Scutellariascordifolia,Artemisiafrigida 3 1350 75 1.84 Bromusircutensis,Potentillalongifolia,Leymuschinensis 4 1357 70 1.46 Artemisiadesertorum,Potentillaacaulis 5 1275 65 1.62 Psammochloavillosa,Carpesiumabrotanoides,Cleistogenessquarrosa 6 1360 65 2.08 Agropyrondesertorum,Silenejenisseensis,Calamagrostisepigeios,Bassiadasyphylla 7 1400 60 1.38 Agropyrondesertorum,Calamagrostisepigeios,Bromusircutensis,Thymusmongolicus 8 1350 60 1.04 Agropyrondesertorum,Carpesiumabrotanoides,Potentillalongifolia 9 1345 50 0.66 Agropyrondesertorum,Potentillanivea,Artemisiadesertorum 10 1350 40 0.68 Artemisiadesertorum 11 1354 35 0.63 Potentillalongifolia,Chamaerhodoserecta 12 1238 15 0.50 Psammochloavillosa,Chenopodiumacuminatum,Thymusmongolicus a Sequenceofthesamplingsitesbasedonvegetationcoverage(hightolow). 1286 N.Ruetal./ScienceoftheTotalEnvironment625(2018)1283–1289 3.2.Communityweightedmeanofmeasuredindexindifferentsites S2).Forexample,thephytolithcontentofArtemisiadesertorumgrowing insite1,4,9and10is1.23%,0.68%,3.46%and1.88%,respectively(Table Accordingtocommunity-weightedmeansofdifferentsampledspe- S2).Asthesamplingsitesaresystematicallyselectedbasedonthevar- cies,thecorrelationsbetweenSiO contentandphytolithcontent,be- iationofvegetationcoverage,theenvironmentalconditionsamong 2 tweenphytolithcontentandCcontentinthephytoliths,andbetween someofthesesitesaredifferent(e.g.,thedesertificationstatusofsite phytolithcontentandPhytOCcontentintotalabovegroundbiomass 9andsite10ismoreseriousthanthatofsite1andsite2).Inaddition, ofallsamplingsites(Fig.S1)weresimilartothosecorrelationsineach compared with previous studies, the phytolith content of Leymus individualplantspecies(Fig.2).ThecontentofSiO ,phytoliths,Cin chinensisgrowinginsite1(1.89%;TableS2)islowerthanthatgrowing 2 phytoliths,andPhytOCinallsitesrangedfrom1.30%to3.65%,from inmeadowsteppes(2.44%;Songetal.,2012)andtemperateforests 1.13%to3.61%,from0.57%to1.55%,andfrom0.093‰to0.349‰,re- (2.53%;Yangetal.,2015).Thesedifferencesareprobablycausedbyen- spectively (Table 2). The highest contents of SiO and phytoliths vironmentalfactorssuchastheweatheringstageoftheunderlying 2 (1.30%and1.13%,respectively)werefoundinsite2,whilethehighest rocks, latitude, temperature, topography, land-use change, and the C in phytoliths (1.55%) and the highest PhytOC content (0.349‰) availabilityofwaterandSiindifferentsoiltypes(Henrietetal.,2008; were found in site 7. The average values for the content of SiO , Parretal.,2010;ParrandSullivan,2011;Zhangetal.,2012;Yanget 2 phytoliths,Cinphytolith,andPhytOCofallspeciesweresignificantly al.,2016;Yingetal.,2016). positivelycorrelatedwiththeircommunity-weightedmeans(Fig.3). Duringtheformationofphytolithsinsandygrasslands,thepresent Furthermore,thephytolithproductionfluxinallsitesrangedfrom studyindicatesthat0.37–2.33%Ccouldbeoccluded(TableS2).Al- 8.94to47.8kgha−1year−1,andthePhytOCproductionfluxranged thoughthecontentofCoccludedbyphytolithsisnotcorrelatedwith from0.062to0.482kgha−1year−1(Table2). thequantityofthephytoliths(Fig.2b),thepercentageofPhytOCona dryweightbasisinsampledplantsshowsasignificantlypositivecorre- lationwiththephytolithcontent(Fig.2c).Thisismainlyattributedto 4.Discussion thephenomenonthatplantsproducinglotsofphytolithsaremorelikely to have more PhytOC in biomass because of the sheer numbers of 4.1. Factors of controlling phytoliths and PhytOC contents in sandy phytolithsinthatbiomass,eveniftheplantsdonotencapsulatenearly grasslands asmuchPhytOCperphytolith.ThestudyofParretal.(2010)suggested thatitwasthenatureofsilicadepositionandtheefficiencyofencapsu- GrasslandrepresentsaparticularlylargeandactivebiogenicSipool latingCbysilicawithinthecellwallsofphytolithsratherthanthequan- interrestrialecosystems(Bleckeretal.,2006;Songetal.,2012;Haynes, tityofphytolithsthatdeterminedtherelativePhytOCyield(Parretal., 2017).Theconstructivespeciesofundisturbedgrassland(e.g.,members 2010).However,itonlyfocusedonbamboospecieswhichallproduce ofthePoaceaeandCyperaceae)cangenerallyaccumulateSitoconcen- largequantitiesofphytoliths.Onthecontrary,presentstudyincluded trationsof1–3%,thoughsomeaccompanyingspecies(e.g.,members manyplanttaxawithawiderangeofphytolithproduction.Therefore, of the Fabaceae) are Si excluders (Epstein, 1994; Conley, 2002; ourresultsimplythatthePhytOCcontentofbiomassismorelikelycon- Schalleretal.,2016;Table2).Manymonocotsareconsideredtoaccu- trolledbyphytolithquantityinsandygrasslands,thoughthemecha- mulatemoreSithannon-monocotplantspecies(Epstein,1994).How- nisms underlying the encapsulation of C by silica during the plant ever,ahighSicontentisnotageneralfeatureofmonocots(Hodsonet growthanditsefficiencyarestillunknown. al.,2005).Forexample,manyindicatorspeciesofdegradedgrassland (e.g.Artemisiadesertorum)alsoaccumulatealargeamountofSiduring 4.2.AssessingmeasuresofphytolithandPhytOCproductioninsandygrass- theirgrowth(TableS2).AsphytolithsarethemaindepositsofSiin landecosystems planttissues,phytolithcontentissignificantlypositivelycorrelated withSiO content(Fig.2a).Thisfindingissupportedbythestudiesof Thecompositionofcommunitiesplaysacrucialrolewhenassessing 2 Parretal.(2010),Songetal.(2012,2013),Huangetal.(2015)and community functional traits, especially for grassland ecosystems Yangetal.(2016). (RicottaandMoretti,2011;Qiuetal.,2016).Forestimatingtheproduc- Insandygrasslands,thephytolithcontentinplantsrangesfrom tionfluxesofphytolithsandPhytOCingrasslandecosystems,theim- 0.68%to9.23%(Table2).Hodsonetal.(2005)andYangetal.(2015) portanceofspeciesweightingisoftenoverlooked(Songetal.,2012; studiedtheeffectsofphylogeneticvariationsontheSicontentofplants, Qietal.,2017).Aftercommunityweighting,therewasasignificantly suggestingthatSicontent,andthusphytolithproduction,wasmore positivecorrelationbetweencommunityweightedmeansandtheaver- likelyinfluencedbythehigher-levelphylogeneticpositionofaplant agevaluesofallmeasuredindicesindifferentsamplingsites,buttheav- species.Inpresentstudy,thephytolithcontentsofthesomeplantspe- eragevaluesweregenerallyhigherthantheweightedmeans,exceptfor ciesgrowingindifferentsamplingsitesaremarkedlydifferent(Table thecontentofCinphytoliths(Fig.3).Thus,usingtheaveragevaluesto Fig.2.Relationship(a)betweenSiO2contentandphytolithcontent,(b)betweenphytolithcontentandCcontentinphytoliths,and(c)betweenphytolithcontentandPhytOCcontentin differentspecies. N.Ruetal./ScienceoftheTotalEnvironment625(2018)1283–1289 1287 Table2 ContentsofSiO2,phytoliths,CinphytolithsandPhytOC,andtheproductionfluxesofphytolithandPhytOCindifferentsamplingsites. Sites SiO2(%) Phytoliths(%) Cinphytoliths(%) PhytOC(‰) Phytolithproductionflux(kgha−1year−1) PhytOCproductionflux(kgha−1year−1) 1 1.60(0.06) 1.25(0.05) 0.79(0.06) 0.093(0.004) 16.6(0.73) 0.123(0.005) 2 3.65(0.07) 3.61(0.14) 1.16(0.05) 0.324(0.022) 47.8(1.91) 0.430(0.029) 3 1.30(0.04) 1.13(0.03) 1.43(0.03) 0.159(0.000) 20.8(0.50) 0.293(0.000) 4 2.38(0.13) 2.44(0.04) 1.09(0.10) 0.170(0.013) 35.6(0.64) 0.249(0.018) 5 1.57(0.04) 1.19(0.01) 1.16(0.13) 0.134(0.016) 19.4(0.08) 0.217(0.025) 6 1.79(0.03) 1.47(0.11) 1.26(0.03) 0.144(0.011) 30.6(2.20) 0.300(0.022) 7 3.24(0.07) 2.35(0.13) 1.55(0.13) 0.349(0.033) 32.5(1.81) 0.482(0.045) 8 1.79(0.02) 1.60(0.13) 0.80(0.08) 0.153(0.008) 16.7(1.35) 0.159(0.009) 9 2.34(0.13) 2.43(0.09) 0.81(0.06) 0.155(0.008) 16.0(0.58) 0.102(0.005) 10 2.24(0.08) 1.88(0.36) 0.86(0.16) 0.156(0.000) 12.8(2.46) 0.106(0.000) 11 2.38(0.04) 2.02(0.06) 0.57(0.03) 0.146(0.007) 12.8(0.37) 0.093(0.004) 12 2.15(0.06) 1.79(0.07) 0.83(0.02) 0.123(0.004) 8.94(0.34) 0.062(0.002) Datapresentascommunity-weightedmeanswithstandarddeviation(inbrackets). assessthecontentsofSiO ,phytoliths,andPhytOCinsandygrasslands fieldinvestigation,allofoursamplingsiteshavesufferedfromdifferent 2 mayoverestimatetheresults.Consideringthatthecorrelationsbe- degreesofdesertification.Ingeneral,desertificationcanleadtode- tweenSiO contentandphytolithcontent,betweenphytolithcontent creasesinplantbiodiversity,abovegroundbiomass,andANPPofthe 2 andCcontentinthephytoliths,andbetweenphytolithcontentand grasslands(Yangetal.,2005).Inthepresentstudy,thesignificantpos- PhytOCcontentinallsamplingsitesafterweighting(Fig.S1)weresim- itivecorrelationbetweenvegetationcoverageandANPPindifferent ilartothosecorrelationsineachindividualplantspecies(Fig.2),the sitesimpliesthat,thelowerthevegetationcoverage,themoresevere presentstudysuggestedthatusingtheweightedmeantoassessthe thedesertification(Fig.4a).Toassesstheeffectsofdesertificationpro- productionofSiO ,phytoliths,andPhytOCinsandygrasslandswas cessonthedistributionandproductionofphytolithsandPhytOC,all 2 morereliablethanusingthearithmeticmean. thedesertificationsitesarecategorizedintothreegroups,basedon thevariationrangeofcurrentvegetationcoverage,namely≥70%,70– 4.3.EffectsofdesertificationonphytolithandPhytOCproduction 50%,and≤50%,respectively(Fig.5).Althoughthesethreegroupsdo notrepresenttheactualdegreeofdesertification,theycanimplythein- Desertification,oneofthemostseveretypesofgrasslanddegrada- creasingtrendofdesertificationtosomeextent. tion,mainlyoccursinsandygrasslandsinaridandsemi-aridregions Itisreportedthatthestorageofsoilnutrientsdecreasessignificantly (Wang,2000;Yangetal.,2005;Jiangetal.,2006).Accordingtoour asgrasslanddesertificationprogresses(Larneyetal.,1998;Duanetal., Fig.3.Relationshipbetweenaveragevaluesandcommunityweightedmeansofmeasuredindicesforallsampledspeciesindifferentsamplingsites.(a)SiO2,(b)phytolith,(c)Cin phytolith,and(d)PhytOC. 1288 N.Ruetal./ScienceoftheTotalEnvironment625(2018)1283–1289 Fig.4.Relationship(a)betweenvegetationcoverageandANPP,(b)betweenANPPandphytolithproductionflux,and(c)betweenANPPandPhytOCproductionfluxindifferentsampling sites. 2001;Fengetal.,2002;Zhouetal.,2008;KukalandBawa,2014).How- etal.,2015;TableS2).Correspondingly,theexacerbationofdesertifica- ever,thecontentsofphytolithsandPhytOCinabovegroundvegetation tionwillinfluencethedistributionpatternsofphytolithandPhytOC exhibitnosignificantvariationamongsitesofdifferentdesertification content(Fig.5a,b).Furthermore,theproductionfluxesofphytoliths status(Fig.5a,b).Zhouetal.(2008)hasconfirmedthat,aswellasphy- andPhytOCaresignificantlycorrelatedwiththeANPPofaboveground tolithandPhytOC contents,theconcentration ofCandnitrogenin vegetationinterrestrialecosystems(Bleckeretal.,2006;Songetal., plantsandplantlitterdonotsignificantlydecreasewiththedesertifica- 2012;Fig.4b,c),whichresultsinadeclineinboththevariationranges tionprocess.Thesefindingsindicatethattheeffectsofdesertification andthevaluesofphytolithproductionflux,aswellasPhytOCproduc- processonthetotalcontentsofphytolithsandPhytOCintheabove- tionflux,asdesertificationproceedsonaregionalscale(Fig.5c,d). groundbiomassareweak. Generally,aslongasthedesertificationstatushasnotexceededthe Inourstudyareas,asthestageofdesertificationincreased,thecom- capacity for self-recovery, grasslands generally restore themselves plexplantcommunitygraduallydegeneratesintosimplecommunity whengrazingisstopped(Liuetal.,2002).Forexample,boththevege- comprisedmainlyofdrought-resistantsandyspecies(e.g.Artemisia tation coverage and species richness in extremely degraded sandy desertorum,Psammochloavillosa,Agropyrondesertorum).Thedecrease grasslandincreaseovertimefollowingexclosureandgrazingprohibi- inplantbiodiversitymeansthatthevariationintherangeofphytolith tioninanattempttoachievenaturalrevegetation(Zhangetal.,2016). andPhytOCcontentsinabovegroundvegetationdecreasedbecause Inaddition,appropriatenitrogenadditiontodegradedgrasslandsis thephytolithandPhytOCcontentsvariedamongthedifferentplant alsoaneffectivewaytoenhancetheabovegroundbiomass(Xuetal., species(Hodsonetal.,2005;Parretal.,2010;ZuoandLü,2011;Yang 2015)andthephytolithandPhytOCcontentsinherbs(Zhaoetal., Fig.5.Boxplotsshowingvariationsin(a)phytolithcontent,(b)PhytOCcontent,(c)phytolithproductionflux,and(d)PhytOCproductionfluxunderdifferentdesertificationstatus. DifferentlowercaselettersindicatesignificantdifferencesbetweengroupsatPb.05levelbasedontheleastsignificantdifference(LSD)test.Thetop,midandbottomlineofthebox representtheupperquartiles,medianandlowerquartiles,respectively.Thetopandbottombarrepresentthemaximumandminimum,respectively. N.Ruetal./ScienceoftheTotalEnvironment625(2018)1283–1289 1289 2016).Therefore,iftheseveredesertificationgrasslandcouldrecoverto Liu,M.,Jiang,G.,Li,Y.,Gao,L.,Yu,S.,Niu,S.,Li,L.,2002.Anexperimentalanddemonstra- itsinitiallevel,theproductionfluxesofphytolithsandPhytOCwould tEicoonla.lSisntu.2d3y,o2n71r9e–st2o7r2a7ti.onofdegradedecosystemsinHunshandakSandland.Acta markedlyincreaseatleastfive-fold.Inpresentstudy,onlytheabove- Liu,H.,Yin,Y.,Tian,Y.,Ren,J.,Wang,H.,2008.Climaticandanthropogeniccontrolsoftop- groundvegetationwasconsidered.Furtherstudiesinsandygrasslands soilfeaturesinthesemi-aridEastAsiansteppe.Geophys.Res.Lett.35,L04401. arerequiredtoassessmorecomprehensivelytheeffectsofdesertifica- Ma,J.F.,Yamaji,N.,2006.Siliconuptakeandaccumulationinhigherplants.TrendsPlant Sci.11,392–397. tiononphytolithCsequestration,aswellasthecoupledcyclesofC Pan,W.,Song,Z.,Liu,H.,VanZwieten,L.,Li,Y.,Yang,X.,Han,Y.,Liu,X.,Zhang,X.,Xu,Z., andSi. 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