CalciumCalcium ChlorideChloride ExtractableExtractable CadmiumCadmium asas anan estimateestimate ofof CadmiumCadmium UptakeUptake byby SubterraneanSubterranean CloverClover

M.M. G.G. WhittenWhitten andand G.G. S.S. I?P. RitchieRitchie

AbstractAbstract

CadmiumCadmium (Cd)(Cd) maymay accumulateaccumulate inin soilssoils whichwhich havehave beenbeen regularlyregularly fertilizedfertilized withwith phosphatephosphate fertilizersfertilizers whichwhich containcontain CdCd originatingoriginating inin rockrock phosphate.phosphate. SoilSoil waswas takentaken fromfrom threethree sitessites inin thethe wheatbeltwheatbelt ofof WesternWestern AustraliaAustralia whichwhich werewere estimatedestimated toto havehave receivedreceived differentdifferent amountsamounts ofof phosphatephosphate fertilizerfertilizer overover thethe pastpast decade.decade. TheThe pHpH waswas adjustedadjusted withwith dilutedilute HClHCl oror CaC03.CaC03. NoNo CdCd waswas addedadded experimentally.experimentally. TrifoliurnTheThe concentrationconcentration ofof CdCd inin thethe wholewhole topstops ofof Trifolium

in a 0-2-0.8subterraneumsubterraneum cvcv MtMt BarkerBarker growngrown ina glasshouseglasshouse potpot experimentexperiment increasedincreased fromfrom 0·2-0·8 pgpg 6 -6-6.9 (1: 0 - 01 CaC12) pg g-l 4.1-4 - 2. TheThe highesthighestg-lg-l drydry wtwt atat pHpH 6·6-6·9 (l : 55 0·01 MM CaCI2) toto 2-42-4 pgg-l atat pHpH 4·1-4·2.

concentrationconcentration ofof CdCd inin thethe plantplant topstops atat anyany particularparticular pHpH occurredoccurred onon thethe soilsoil whichwhich hadhad thethe highesthighest concentrationconcentration ofof PP inin thethe CaCl2 extract.extract. ThereThere waswas aa linearlinear relationshiprelationship betweenbetween thethe concentrationconcentration ofof CdCd inin thethe wholewhole topstops ofof sub-cloversub-clover andand thethe concentrationconcentration ofof CdCd inin thethe

CaC12

CaCl2 extractsextracts whichwhich waswas independentindependent ofof site.site. TheThe concentrationconcentration ofof CdCd inin thethe CaCl2 extractsextractsCaC12 CaC12 CaC12waswas aa functionfunction ofof pHpH andand concentrationconcentration ofof PP inin thethe CaCl2 extract.extract.

IntroductionIntroduction

CadmiumCadmium (Cd)(Cd) cancan enterenter thethe agriculturalagricultural foodfood chainchain viavia thethe disposaldisposal ofof sewagesewage sludgesliidge onon agriculturalagricultural iand, accumuiationland, accumulation inin soilsoil fromfrom industrialindustrial pollutionpollution (Cartwright(Cartwright etet al.al. 1976;1976; MerryMerry andand TillerTiller 1978)1978) oror applicationapplication ofof phosphatephosphate fertilizersfertilizers (Williams(Williams andand DavidDavid 1973).1973). SuperphosphateSuperphosphate inin AustraliaAustralia maymay containcontain upup toto 5050 pgJ1g g-lg-l cadmiumcadmium (Williams(Williams andand DavidDavid 1973),1973), approximatelyapproximately 50%50% ofof whichwhich isis water-solublewater-soluble (Mann(Mann 1989).1989). TheThe CdCd contentcontent ofof pasturepasture speciesspecies hashas beenbeen shownshown toto increaseincrease withwith thethe totaltotal amountamount ofof superphosphatesuperphosphate appliedapplied (Williams(Williams andand DavidDavid 1973)1973) oror whenwhen soilsoil pHpH waswas decreaseddecreased (Williams(Williams 1977;1977; TillerTiller 1988).1988). TheThe concentrationconcentration ofof CdCd inin plantsplants growngrown onon soilssoils inin whichwhich thethe pHpH rangedranged fromfrom stronglystrongly acidacid toto slightlyslightly acidacid oror alkalinealkaline waswas poorlypoorly correlatedcorrelated withwith soilsoil CdCd extractedextracted byby acidacid oror EDTAEDTA (Symeonides(Symeonides andand McRaeMcRae 1977)1977) oror DTPADTPA (Kuo(Kuo 1990).1990).

TheThe objectivesobjectives ofof thisthis workwork werewere toto examineexamine thethe effecteffect ofof pHpH onon thethe concentrationconcentration ofof CdCd inin subterraneansubterranean cloverclover growngrown onon threethree acidacid soilssoils fromfrom

CaC12thethe WesternWestern AustralianAustralian wheatbeltwheatbelt andand toto determinedetermine whetherwhether CdCd inin CaCh soilsoil extractsextracts waswas aa goodgood measuremeasure ofof plant-availableplant-available Cd.Cd.

MaterialsMaterials andand MethodsMethods TheThe concentrationconcentration ofof CdCd waswas measuredmeasured inin Trifolium subterraneumsubterraneum L.L. cvcv MtMt BarkerBarker growngrown inin

soilsoil fromfrom threethree sitessites whichwhich hadhad beenbeen acidifiedacidified oror limedlimed toto givegive aa pHpH rangerange ofof approximatelyapproximately 4-74-7 (l(1 :: 55 0·010.01 MM CaCI2).

Trifoliurn

CaC12).

ExperimentalExperimental ProcedureProcedure

SitesSites onon threethree farmsfarms inin thethe WesternWestern AustralianAustralian wheatbeltwheatbelt (Table(Table 1)1) werewere sampledsampled toto aa depthdepth ofof approximatelyapproximately 1010 cmcm afterafter clearingclearing thethe surfacesurface ofof plantplant debris.debris. TheThe soilssoils werewere air-driedair-dried

(12 TheThe sitessites differeddiffered inin recentrecent mangementmangementandand sievedsieved (:$ 2 mm)mm) immediatelyimmediately afterafter collection.collection. histories.histories. SiteSite 11 waswas inin thethe pasturepasture phasephase ofof aa longlong rotation,rotation, sitesite 22 hadhad beenbeen continuouslycontinuously croppedcropped forfor thethe pastpast 1010 yearsyears andand sitesite 33 waswas inin thethe cropcrop phasephase ofof aa shortshort crop:crop: pasturepasture rotation.rotation.

TableTable 1.1. SummarySummary ofof soilssoils TheThe vicinitiesvicinities ofof thethe soilsoil sitessites were:were: sitesite 1,1, Meckering;Meckering; sitesite 2,2, Goomalling;Goomalling; sitesite 3,3, MerredinMerredin

SiteSite Classifica-Classifica­tionAtionA

p ~ * PC

(pg g-'1

pHB Bic-Bic­pC

(f1g g-l)

PRIDPRID e ~ t r a c t . ~ g-'1

A1 (%I

Ox.Ox. extract.E Org.Org. (( f1g g-l) CFCF

Al FeFe (%)

ParticleParticle sizesize (%)(%)

SandSand SiltSilt ClayClay

1 2 3

Dy 5·51 Dy 5·51 Gn 2·21

4·4 4·4 4·8

15·8 23·5 20·6

1·43 0·75 4·43

178 175 285

261 155 687

1·0 1· 0 1·3

90 88 80

4 4 3

6 7

17

AA NorthcoteNorthcote (1984).(1984). 0 - 0 1 CaC12.B11 :: 55 0·01 MM CaCl2.

C BicarbonateBicarbonate extractableextractable PP (Colwell(Colwell 1963).1963). DPhosphatePhosphate retentionretention index:index: PP adsorbedadsorbed byby 11 gg soilsoil inin 1010 mLmL 0·01 MM KCI initiallyinitially containingcontaining0 -01 KC1

pg m ~ - l1010 f1g mL-1 PP asas KH2P04KH2P04 divideddivided byby thethe finalfinal concentrationconcentration ofof P.P. E OxalateOxalate extractableextractable P;P; 33 gg soilsoil 100100 mL-1 acidacid ammoniumammonium oxalateoxalate (Searle(Searle andand DalyDaly 1977h FYeomansYeomans andand BremnerBremner (1988).(1988).

m ~ - ' 1977).

AlthoughAlthough preciseprecise fertilizerfertilizer historieshistories areare notnot known,known, sitesite 22 isis estimatedestimated toto havehave receivedreceived moremore phosphatephosphate fertilizerfertilizer thanthan thethe otherother twotwo sitessites forfor thethe pastpast 1010 yearsyears atat least.least. ThereThere waswas nono knownknown environmentalenvironmental oror industrialindustrial sourcesource ofof CdCd nearnear thethe collectioncollection sitessites oror thethe buildingsbuildings wherewhere thethe experimentexperiment tooktook place.place.

TheThe glasshouseglasshouse experimentexperiment consistedconsisted ofof subterraneansubterranean cloverclover growngrown inin potspots inin root-coolingroot-cooling tankstanks inin aa factorialfactorial arrayarray ofof 33 soils,soils, 88 pHpH values,values, 22 sourcessources ofof nitrogennitrogen (N2(N2 fixationfixation andand NH4N03)NH4N03) withwith threethree replicatesreplicates andand hashas beenbeen describeddescribed inin detaildetail elsewhereelsewhere (Whitten(Whitten andand RitchieRitchie 1991).1991). TheThe pHpH treatmentstreatments werewere dilutedilute HClHCl (range:(range: 0·06-0·8 cmolcmol H+H+ kg-I), nilnil (deionized(deionized0-06-0.8 kg-'), water)water) oror precipitatedprecipitated CaC03CaC03 (1·25 andand 2·5 gg perper 33 kgkg pot)pot) followedfollowed byby incubationincubation atat fieldfield capacitycapacity forfor 88 daysdays atat 40°C.40°C. kg-'

(1 -25 2 .5 NutrientNutrient solutionssolutions supplyingsupplying (mg(mg kg-1 soil)soil) Na2Mo04.2H20Na2Mo04.2H20

(0·67), H3B03H3B03 (0·83), CuS04.5H20CuS04.5H20 (5),(5), ZnS04.7H20ZnS04.7H20 (10), MnS04.H20MnS04.H20 (15), CoS04.7H20CoS04.7H20 (0·4),(0 - 671, (0 - 83), (1O), (15), (0 - 4), K2S04K2S04 (145), andand KH2P04KH2P04 [11 0 (soils(soils 11 andand 2)2) andand 176176 (soil(soil 3)]3)] werewere appliedapplied asas solutionssolutions toto thethe surfacesurface ofof thethe soilsoil whichwhich waswas thenthen drieddried andand mixed.mixed. TheThe N-fertilizedN-fertilized treatmentstreatments receivedreceived 2424 mgmg NN kgkg soilsoil asas NH4N03NH4N03 beforebefore sowingsowing andand atat 1010 dayday intervalsintervals thereafter.thereafter. PlantsPlants werewere harvestedharvested atat 55 weeksweeks andand werewere washedwashed inin deionizeddeionized waterwater toto removeremove anyany adheringadhering soilsoil particles.particles. NoNo CdCd waswas addedadded experimentallyexperimentally andand allall reagentsreagents forfor pHpH adjustmentsadjustments andand nutrientsnutrients werewere analyticalanalytical grade.grade.

(1454, [I10

CadmiumCadmium AnalysisAnalysis ofof PlantsPlants andand SoilsSoils

AllAll labwarelabware forfor CdCd analysisanalysis waswas cleanedcleaned inin 11 MM HClHCl andand 0 . 10·1 MM Na2EDTA.Na2EDTA. TheThe undilutedundiluted nitric/perchloricnitricjperchloric acidacid digestdigest ofof thethe wholewhole topstops ofof thethe subterraneansubterranean cloverclover waswas analysedanalysed forfor CdCd byby flameflame atomicatomic absorptionabsorption spectrophotometryspectrophotometry (FMS) onon aa Perkin-ElmerPerkin-Elmer 50005000 atomicatomic absorptionabsorption spectrophotometerspectrophotometer usingusing backgroundbackground correctioncorrection toto eliminateeliminate interferenceinterference fromfrom non-atomicnon-atomic absorption.absorption. TheThe rangerange ofof measuredmeasured CdCd inin referencereference plantplant materialmaterial (kale;(kale; BowenBowen

(FAAS)

0.64+0- 12 pg g-'.1975)1975) waswas 0·64±0·12 f1g g-l. BowenBowen (1975)(1975) reportedreported thethe CdCd concentrationconcentration ofof thisthis referencereference 1-00+0- pg g-I whenwhen analysedanalysed byby MS, butbut suggestedsuggested thatthat thisthis waswas ananmaterialmaterial toto bebe 1· OO±O· 1919 f1g g-l AAS,

overestimationoverestimation byby 32%32% becausebecause ofof non-atomicnon-atomic absorption.absorption. StandardsStandards fromfrom aa stockstock solutionsolution HClofof CdCd metalmetal dissolveddissolved inin HCI werewere mademade upup inin perchloricperchloric acid.acid.

SubsamplesSubsamples ofof soilsoil whichwhich hadhad notnot receivedreceived nutrientsnutrients werewere shakenshaken end-over-endend-over-end forfor 1616 hh atat 1 : 0 - 0 1 CaC12, mademade upup fromfrom aa stockstock solutionsolution whichwhich hadhad beenbeen purifiedpurified20°C20°C inin 1: 55 0·01 MM CaCI2,

withwith dithizonedithizone extractedextracted intointo chloroformchloroform toto removeremove anyany tracetrace amountsamounts ofof Cd.Cd. TheThe extractsextracts werewere filteredfiltered throughthrough 0·40 . 4 pm/lm polycarbonatepolycarbonate membranesmembranes (Nuclepore(Nuclepore Corporation,Corporation, U.S.A.).U.S.A.). CadmiumCadmium inin thethe filtratefiltrate waswas complexedcomplexed withwith ammoniumammonium pyrrolidinepyrrolidine dithiocarbamatedithiocarbamate (APDC)(APDC) andand extractedextracted intointo 4-methylpentan-2-one4-methylpentan-2-one (syn.(syn. methylmethyl isobutylisobutyl ketone,ketone, MIBK)MIBK) accordingaccording toto thethe methodmethod ofof DudasDudas (1974)(1974) inin orderorder toto eliminateeliminate potentialpotential matrixmatrix effectseffects duringduring analysis.analysis. TheThe MIBKMIBK extractextract waswas thenthen analysedanalysed forfor CdCd flamelessbyby flameless AAAA usingusing pyrolyticallypyrolytically coatedcoated graphitegraphite tubestubes inin aa Perkin-ElmerPerkin-Elmer HGAHGA 500500 graphitegraphite furnacefurnace coupledcoupled toto aa PEPE 50005000 withwith backgroundbackground correction.correction. StandardsStandards werewere mademade upup inin double-deionizeddouble-deionized waterwater immediatelyimmediately beforebefore complexingcomplexing withwith APDCAPDC andand extractingextracting intointo MIBK.MIBK.

CaC12 extractsextracts werewere anan OrionOrion EAEATheThe pHpH andand concentrationconcentration ofof PP inin thethe CaCb measuredmeasured withwith 940940 pHpH metermeter andand byby thethe methodmethod ofof JohnJohn (1970),(1970), respectively.respectively.

4

It .. 3 . ~

CIl

CIl .=; ., Q.

.2 2 ... \~~'0 ¥:.:c £ 'tl () \

C ts A

0

764 5

pH

Fig.Fig. 1.1. (pgConcentrationConcentration ofof CdCd (/lg g-l)g-l) inin thethe wholewhole topstops ofof subterraneansubterranean cloverclover plottedplotted againstagainst 0 .01 CaC12. (A) (n)pHpH inin 11 :: 55 extractextract inin 0·01 MM CaCl2. SymbolsSymbols (.6.) sitesite 1,1, (+)(+) sitesite 22 andand (0) sitesite 3.3. ErrorError barsbars

indicateindicate standardstandard errors.errors.

5

4 ~'" ...

C ~~ 3

CIl 2:

x ~ ~ ... 2 C £ C 'tl () I

A C !0 li

0

6 74

pH

(pg L-I) 1 : 0 - 0 1 CaC12.Fig.Fig. 2.2. ConcentrationConcentration ofof CdCd (/lg L-l) plottedplotted againstagainst pHpH inin 1: 55 extractextract inin 0·01 MM CaCho (A) (a)SymbolsSymbols (.6.) sitesite 1,1, (+)(+) sitesite 22 andand (0) sitesite 3.3. ErrorError barsbars indicateindicate standardstandard errors.errors.

4

3 I';'

C>

C> ~ III Co

2.9 Cll

"0 .r;;

.= ~

'C u

i 0

00 1 22 33 44 5

CdCd inin extractextract (1l9 L·1 )(pg L*' )

Fig.Fig. 3.3. (pgConcentrationConcentration ofof CdCd (Jig g-l)g-l) inin thethe wholewhole topstops ofof subterraneansubterranean cloverclover plottedplotted againstagainst (pg L - ~ ) 1 : 0.01 CaC12. SymbolsSymbols (.6.) sitesite 1,1, (+)(+) sitesite 22 andand (0) sitesite 3.3.CdCd (Jig L-l) inin 1: 55 extractextract inin 0·01 MM CaClz. (A) (o)

ErrorError barsbars indicateindicate standardstandard errors.errors.

ResultsResults

TheThe concentrationconcentration ofof CdCd inin thethe wholewhole topstops ofof thethe subterraneansubterranean cloverclover 0 - 2-0 8 pg g-l 6 -6-6 - 9 pg g-I atat pHpHincreasedincreased fromfrom 0·2-0 -8 llg g-1 drydry wtwt atat pHpH 6·6-6·9 toto 2-42-4 llg g-1

4·1-4·24.1-4-2 (Fig.(Fig. 1).1). InIn thethe soilsoil extracts,extracts, therethere waswas aa correspondingcorresponding increaseincrease inin 0 - 3-0.5 pg L-I toto 3·0-4·33 0-4 llg L-1 (Fig.(Fig. 2).2). TheThethethe concentrationconcentration ofof CdCd fromfrom 0·3-0· 5 llg L-1 - 3 pg L-l

relationshiprelationship betweenbetween thethe concentrationconcentration ofof CdCd inin thethe wholewhole topstops ofof N-fixingN-fixing subterraneansubterranean cloverclover andand CdCd inin thethe CaClz extractsextracts waswasCaC12

Cd (pg g-l) = -0 301 + 0.775 Cd (pg L-l), (1)(1)

01, yr2=0·91) andand waswas independentindependent ofof sitesite (Fig.(Fig. 3).3).(P << 00 -01, adj.adj. Z = 0.91) TheThe highesthighest concentrationconcentration ofof CdCd inin thethe plantplant topstops atat anyany particularparticular

pHpH occurredoccurred onon soilsoil 22 whichwhich hadhad thethe highesthighest concentrationconcentration ofof phosphatephosphate inin

(P

bicarbonatebicarbonate andand CaCh extracts.extracts. AlthoughAlthough therethere waswas aa significantsignificant (P(P << 00 ·01) linearlinear CaC12 extracts,extracts,

CaC12 - 01) relationshiprelationship betweenbetween thethe concentrationsconcentrations ofof CdCd andand PP inin thethe CaCh thethe concentrationconcentration ofof PP accountedaccounted forfor onlyonly 24-5% ofof thethe variationvariation inin thethe24·5% concentrationconcentration ofof Cd.Cd. TheThe concentrationconcentration ofof CdCd inin thethe CaC12CaCh extractsextracts waswas bestbest predictedpredicted byby multiplemultiple linearlinear regressionregression againstagainst bothboth thethe pHpH andand thethe concentrationconcentration Of PP inin thethe CaCh extract.extract. TheThe regressionregression equationequation waswasof CaCI2

CdCd (llg L-1) = 7·7348 -1·146 pHpH + 3·541 PP (mg(mg L-1).(pg L-l) = 7-7348 - 1- 146 + 3 - 5 4 1 L-l). (2)(2)

BothBoth pHpH andand PP werewere 0·0001) andand thethe adjustedadjusted Z waswas 0·903. PlantsPlants fertilizedfertilized withwith NH4N03NH4N03 containedcontained slightlyslightly moremore CdCd thanthan thosethose whichwhich

reliedrelied onon symbioticallysymbiotically fixedfixed N.N. TheThe relationshiprelationship waswas

significantsignificant (P(P == 0.0001) yr2 0-903.

CdCd (/lg g-1) N-fertilizedN-fertilized = 00 ·27 ++ 1·132 CdCd (/lg g-1) N-fixing.N-fixing. (3)(3)(pg g-l) = 27 1 (pg g-l)

(P=0-0001, r2 =0.92).=0-92) .(P=O·OOOl, adj.adj. y2

DiscussionDiscussion

CalciumCalcium chloridechloride extractableextractable CdCd waswas aa goodgood indicatorindicator ofof thethe CdCd contentcontent ofof subterraneansubterranean clover.clover. TheThe concentrationconcentration ofof CdCd inin thethe plantplant materialmaterial andand thethe CaCl2CaCl2 extractsextracts increasedincreased inin aa similarsimilar mannermanner asas thethe pHpH ofof thethe soilsoil decreased.decreased.

CadmiumCadmium inin Sub-cloverSub-clover

TheThe concentrationconcentration ofof CdCd inin subterraneansubterranean cloverclover nearlynearly doubleddoubled inin eacheach ofof soils 4 . 5thethe W.A.W.A. sojls asas thethe pHpH decreaseddecreased fromfrom aboutabout 4·5 toto 44 andand waswas approximatelyapproximately

asas pHpH fromfrom aboutabout 4 - 5 toto aboutabout 5 - 5 .halvedhalved thethe increasedincreased 4·5 5·5. TheThe highesthighest concentrationconcentration ofof CdCd inin thethe wholewhole topstops ofof subterraneansubterranean cloverclover occurredoccurred inin soilsoil fromfrom sitesite 22 whichwhich isis estimatedestimated toto havehave receivedreceived thethe mostmost phosphatephosphate fertilizerfertilizer duringduring thethe lastlast 1010 years.years. InIn WesternWestern Australia,Australia, therefore,therefore, acidificationacidification ofof soilssoils usedused forfor pasturepasture productionproduction couldcould increaseincrease thethe uptakeuptake ofof CdCd byby plantsplants andand hencehence grazinggrazing stock.stock. InIn addition,addition, thethe continuedcontinued useuse ofof phosphatephosphate fertilizersfertilizers manufacturedmanufactured fromfrom rockrock phosphatesphosphates containingcontaining CdCd maymay increaseincrease thethe amountamount ofof CdCd availableavailable toto plantsplants andand animals.animals.

VariationVariation inin concentrationconcentration ofof CdCd inin plantsplants couldcould alsoalso bebe duedue toto differencesdifferences betweenbetween soilsoil type.type. TillerTiller (1988)(1988) reportedreported twofoldtwofold differencesdifferences inin thethe concentrationconcentration ofof CdCd inin sub-cloversub-clover growinggrowing inin aa redred podzolicpodzolic andand aa solodizedsolodized solonetzsolonetz overover aa pHpH rangerange spanningspanning 5·5-8·5.5-8 - 55 (l(1 :: 55 water).water). TheseThese differencesdifferences decreaseddecreased withwith thethe additionaddition ofof Cd.Cd.

TheThe slightlyslightly higherhigher concentrationconcentration ofof CdCd inin thethe plantplant materialmaterial fromfrom thethe N-fertilizedN-fertilized treatmenttreatment comparedcompared withwith thethe N-fixingN-fixing treatmenttreatment maymay bebe duedue toto

NH+4displacementdisplacement ofof exchangeableexchangeable CdCd byby NH+4 oror aa slightlyslightly lowerlower soilsoil pHpH inin thethe N-fertilizedN-fertilized treatment.treatment. However,However, thethe finalfinal pHpH ofof thethe N-fertilizedN-fertilized treatmentstreatments waswas estimatesestimates toto bebe onlyonly marginallymarginally lowerlower thanthan inin thethe N-fixingN-fixing treatments.treatments. ThisThis estimateestimate waswas basedbased onon thethe differencesdifferences inin finalfinal soilsoil pHpH underunder cloverclover withwith differentdifferent sourcessources ofof NN suppliedsupplied atat 350350 mgmg atat NN perper 33 kgkg potpot underunder conditionsconditions

(Jarvissimilarsimilar toto ourour experimentexperiment andand withwith aa soilsoil ofof similarsimilar pHpH bufferingbuffering capacitycapacity Uarvis andand Robson,Robson, 1983).1983). ItIt waswas calculatedcalculated thatthat productionproduction ofof 11 gg shootshoot drydry mattermatter inin ourour experimentexperiment wouldwould generategenerate anan excessexcess ofof H+H+ inin thethe N-fertilizedN-fertilized treatmenttreatment

2x10-~ L - ~ inin thethecomparedcompared withwith thethe N-fixingN-fixing treatmenttreatment ofof approximatelyapproximately 2x10-6 molmol L-1

CaC12 0 .04CaCl2 extract.extract. ThisThis wouldwould bebe equivalentequivalent toto aa differencedifference ofof approximatelyapproximately 0·04 4 -pHpH unitsunits ifif thethe finalfinal pHpH waswas 4· 55 inin thethe N-fixingN-fixing treatment.treatment.

CadmiumCadmium andand PhosphorusPhosphorus inin thethe SoilSoil ExtractsExtracts

TheThe concentrationconcentration ofof CdCd inin thethe extractsextracts waswas aa functionfunction ofof pHpH andand concentrationconcentration ofof PP inin thethe extract.extract. TheThe correlationcorrelation betweenbetween thethe concentrationsconcentrations ofof CdCd andand PP

CaC12inin thethe CaClz extractextract waswas poorpoor inin comparisoncomparison withwith thatthat forfor CdCd extractedextracted withwith HC1 0 - 5 r2H2SO4 r2

statedstated therein),therein), oror betweenbetween CdCd extractedextracted withwith 0 -O· 11 MM EDTAEDTA atat pHpH 66 andand PP inin dilutedilute H2S04H2S04 (r2(r2 = 0.55;

11 MM HCI andand PP extractedextracted withwith 0·5 MM H;S04 (Williams(Williams andand DavidDavid 1973;1973; notnot

= 0·55; MerryMerry 1988).1988). InIn thisthis work,work, thethe concentrationconcentration ofof PP inin thethe CaC12CaC12 extractsextracts differeddiffered markedlymarkedly betweenbetween soils,soils, butbut withinwithin eacheach soilsoil thethe effecteffect ofof pHpH waswas muchmuch lessless pronouncedpronounced thanthan itsits effecteffect onon Cd.Cd.

InIn thethe CaC12CaC12 extracts,extracts, exchangeableexchangeable formsforms ofof CdCd wouldwould gogo intointo solution,solution, whereaswhereas specificallyspecifically adsorbedadsorbed CdCd wouldwould notnot bebe readilyreadily displaceddisplaced byby CaCa (Tiller(Tiller etet al.al. 1979).1979). TheThe relationshiprelationship betweenbetween thethe concentrationconcentration ofof CdCd andand pHpH inin thethe

extractsextracts reflectsreflects increasedincreased adsorptionadsorption ofof CdCd withwith increasedincreased pHpH (Tiller(Tiller etet al.al. 1979).1979). TheThe amountamount ofof adsorbedadsorbed CdCd wouldwould alsoalso bebe influencedinfluenced toto somesome extentextent byby thethe amountamount ofof adsorbedadsorbed P.P. OverOver thethe pHpH rangerange 4-74-7 (which(which correspondscorresponds withwith thethe pHpH rangerange ofof thethe experimentexperiment describeddescribed inin thisthis paper),paper), adsorptionadsorption ofof CdCd byby ferricferric hydroushydrous oxidesoxides hashas beenbeen shownshown toto increaseincrease asas thethe amountamount ofof adsorbedadsorbed PP increasedincreased (Kuo(Kuo andand McNealMcNeal 1984).1984). a 1 M HC1AtAt thethe lowlow pHpH ofof aIM HCI extract,extract, relativelyrelatively littlelittle CdCd wouldwould remainremain adsorbed,adsorbed, regardlessregardless ofof thethe amountamount ofof adsorbedadsorbed PP oror thethe pHpH ofof thethe soil.soil.

CorrelationCorrelation betweenbetween CdCd inin SoilSoil ExtractsExtracts andand PlantPlant TopsTops

InIn acidacid soils,soils, thethe concentrationconcentration ofof CdCd inin thethe 1 : CaC12 extractsextracts atat thethe1: 55 CaCl2 ionicionic strengthstrength ofof thethe soilsoil solutionsolution maymay bebe aa moremore appropriateappropriate measuremeasure ofof CdCd uptakeuptake byby plantsplants thanthan thatthat inin acidicacidic oror bufferedbuffered extractsextracts becausebecause thethe pHpH ofof thethe 1 : AlterationAlteration ofof thethe pHpH wouldwould1: 55 extractextract isis similarsimilar toto thatthat inin thethe soil.soil. affectaffect thethe solubilitysolubility andand adsorptionadsorption ofof Cd.Cd. SymeonidesSymeonides andand McRaeMcRae (1977)(1977) foundfound thatthat thethe concentrationconcentration ofof CdCd inin radishradish topstops growngrown onon soilssoils toto whichwhich

4 8-7.9; 2 . 5CdCd hadhad beenbeen addedadded (pH(pH 4·8-7·9; 11 :: 2·5 water)water) waswas betterbetter correlatedcorrelated withwith thethe M M HC1concentrationconcentration ofof soilsoil CdCd extractedextracted inin unbufferedunbuffered 11 M N H 4 N 0 3NH4N03 thanthan inin 11 MHCI

0.00125 Moror 0·00125 M EDTA.EDTA. KuoKuo (1990)(1990) foundfound thatthat thethe concentrationconcentration ofof CdCd inin wholewhole 5-5- 5; 1 :topstops ofof BetaBeta vulgarisvulgaris L.L. growngrown onon aa rangerange ofof soilssoils (pH(pH 5-5·5; 1: 22 water)water) toto

whichwhich differentdifferent ratesrates ofof sewagesewage sludgesludge hadhad beenbeen addedadded waswas onlyonly moderatelymoderately (r2= 0.53)wellwell correlatedcorrelated (r2 = 0·53) withwith DTPADTPA extractableextractable Cd,Cd, andand thatthat plantplant CdCd couldcould

increaseincrease asas thethe pHpH decreaseddecreased whilewhile DTPADTPA extractableextractable CdCd remainedremained constant.constant.

ConclusionsConclusions

TheThe CdCd extractedextracted inin CaC12 extractsextracts waswas ableable toto predictpredict CdCd uptakeuptake bybyCaCh subterraneansubterranean cloverclover growngrown inin agriculturalagricultural soilsoil fromfrom thethe threethree sitessites inin whichwhich

0 - 0 1M CaC12.thethe pHpH hadhad beenbeen alteredaltered toto spanspan thethe pHpH rangerange ofof 4-74-7 inin 11 :: 55 0·01 M CaCho IfIf similarsimilar resultsresults cancan bebe achievedachieved overover aa similarsimilar soilsoil pHpH rangerange withwith aa widerwider rangerange ofof soilsoil typestypes andand plantplant species,species, thisthis testtest maymay assistassist inin identifyingidentifying sitessites atat whichwhich CdCd contentcontent ofof plantsplants couldcould bebe ofof concern.concern. SoilSoil acidityacidity maymay bebe aa contributingcontributing factorfactor inin thethe accumulationaccumulation ofof CdCd byby grazinggrazing animalsanimals onon soilssoils toto whichwhich largelarge amountsamounts ofof superphosphatesuperphosphate containingcontaining tracetrace amountsamounts ofof CdCd havehave beenbeen applied.applied. However,However, soilsoil ingestioningestion couldcould alsoalso bebe anan importantimportant sourcesource ofof dietarydietary CdCd inin grazinggrazing animals.animals.

AcknowledgmentsAcknowledgments

ThisThis workwork waswas basedbased onon aa projectproject onon soilsoil acidityacidity fundedfunded byby thethe AustralianAustralian WoolWool Corporation.Corporation. WeWe areare gratefulgrateful toto DrDr M.M. McLaughlinMcLaughlin forfor commentscomments mademade onon anan earlierearlier draftdraft ofof thethe paper.paper.

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