Fractions and Bioavailability of Cadmium and Nickel to Carrot Crops in Oasis Soi

The Hexi Corridor is the most important area for desert oasis farming in northwestern China. Due to persistent drought and water shortage, sewage irrigation is widely used in this area. Heavy metal pollutants contained in the sewage could remain in the surface layer of agricultural soil and accumulate in plants. Our research used pot experiments to evaluate carrot crop (Daucus carota L.) production, heavy metal uptake, and bioavailability under single cadmium (Cd) or nickel (Ni) contamination and compound (Cd-Ni) contaminations in irrigated desert oasis soil. The results show that Cd existed in the Fe-Mn oxide bound fraction and Ni presented in the residual fraction mainly in original (control) soils. Low concentrations of Cd could promote the growth of carrots, while high concentrations of Cd significantly restrain the growth of the crops. However, Ni had a poisonous effect on the carrots even at the lowest concentrations. There was an antagonistic effect between Cd and Ni in the compound contaminated oasis soils. The bio-concentration factors (BCF) of Cd in carrots were higher than those of Ni, and the BCF of Cd and Ni in single-contaminated soils were higher than those in compound-contaminated soils. Cd and Ni contents in different parts of the carrots were correlated with the exchangeable fraction in contaminated oasis soils, which would cause potential risk to human health through the food chain.