Residual effects of biosolids and farm manure on speciation and plant uptake of heavy metals in a calcareous soil

Application of biosolids to agricultural land can improve soil fertility and physical properties. However, the bioavailability of heavy metals in soil may increase after cessation of biosolids application. In this study, chemical speciation and plant availability of Cu, Ni, Pb, and Zn were investigated 3 years after application of 0 (control), 25, 50, and 100 Mg ha(-1) of biosolids (BS) and farm manure (FM) to a calcareous soil. Results showed that Pb and Ni concentrations in the grains of wheat grown in the BS and FM treatments were significantly lower than those grown in the control. Meanwhile, the concentrations of Zn and Cu were greater in the grains of wheat grown in the BS- and FM-amended soils compared to those grown in the control soil. The concentrations of DTPA-extractable Zn, Cu, Pb, and Ni in the biosolid-amended soils were significantly higher than those in the control soil. The results of sequential extraction showed that majority of Cu, Zn, and Ni were in residual fraction, while Pb bound mainly with Fe-Mn oxides. From the environmental point of view, the residual fraction of heavy metals is less bioavailable under the conditions normally found in calcareous soils. On the basis of the heavy metal speciation, the mobility factor values did not change 3 years after application of the amendments.