Accumulation of heavy metals in rice seeds as influenced by metal speciation and soil properties

The contents and speciation of heavy metals(Cd,Cr,Pb,Cu and Zn) in soils from the Chengdu Plain were analyzed and the contents of heavy metals in rice seeds from the same sampling sites were also determined.Statistical analyses including spatial interpolation and stepwise multiple linear regressions were used to determine the spatial distribution of these heavy metals and their speciation in the soils and to evaluate the relationship between heavy metal speciation in soil and bioaccumulation in rice seeds.The results indicated that the soil pollution is not significant in these sites since the heavy metal contents were lower than the level II criteria of the national environmental quality standard for soils except for Cd.The content of exchangeable fractions was low for all heavy metals studied.Heavy metals Cr,Cu,Zn and Pb were mainly found in the residual fractions,and Cd was mainly bound with Fe-Mn oxides.The accumulation sequence of heavy metals in rice seeds was: CdZnCuPbCr.The labile fractions(exchangeable,carbonate bound,Fe-Mn oxides bound,organic matter bound) in soils which had good correlations with the heavy metal accumulation in rice seeds were carbonate bound Cd,exchangeable Cr,organic matter bound Pb,and carbonate bound Cu.There was no good correlation between the accumulation of Zn in rice seeds and any labile fractions of Zn in soils.The contents and bioavailability of different labile heavy metals fractions had various dependencies on the soil physical-chemical properties.The contents of labile Cd were mainly correlated to soil organic matters,pH and bulk density significantly(P0.01) while the contents of labile Cr were also related to CEC besides these factors.The contents of labile Pb were related to soil organic matter content,bulk density,medium fine silt and sand content.The contents of labile Cu were affected by clay and organic matter contents significantly while the contents of labile Zn were affected by the pH,organic matter,sand and bulk density.In summary,the major common factors that had apparent effects on the labile contents of Cd,Cr,Pb,Cu and Zn in soils were organic matter,pH and bulk density while the properties related to soil adsorption such as particle size distribution and CEC have little effects on the contents of labile heavy metal fractions in these paddy soils.