Cadmium, Zinc and Nickel Bioavailabilities to Celery (Apium graveolens) Grown in Contaminated Soils from the Arid Oasis, Northwest China

A pot experiment was conducted to examine cadmium (Cd), zinc (Zn) and nickel (Ni) bioavailabilities in arid oasis agricultural soils. Celery (Apium graveolens) was selected as the tested plant. The chemical forms of the metals in soils were characterized by sequential extraction analysis technique. The results showed that in uncontaminated arid soils, Cd, Zn and Ni existed predominantly in the less available forms, associated to Fe-Mn oxides and residuals forms. Heavy metals addition in arid celery-cultivated soils significantly increased the concentration of the exchangeable and bound-to carbonate Cd forms, however the Fe-Mn oxides form of Cd decreased. Meanwhile, for Zn and Ni, the increase occurred in the bound-to-carbonate and bound-to Fe-Mn oxides forms, and the residuals forms of Zn and Ni decreased. When these metals were determined in plants, both shoots and roots, a greater concentration of Cd, Zn and Ni was found in celery, most of the metals being in roots. The contents of heavy metals in celery at low to high level metals treatments were higher than the tolerance limits of national standard of China, which would threaten human health. The relationship between the metals contents in plants and the different forms of metals in soils was evaluated through multiple linear regressions to estimate the heavy metals bioavailabilities. Different results were obtained, independent variables positively contributing to Cd, Zn and Ni bioavailabilities in soils were the bound-to-carbonate forms for celery roots. The bound-to Fe-Mn oxides, bound to organic matter and the exchangeable forms would contribute positively to the contents of Cd, Zn and Ni celery shoots, respectively.