Effects of biochar dose on cadmium accumulation in spinach and its fractionation in a calcareous soil

Biochars have attracted attention because of their capability to improve soil fertility, soil carbon content as well as crop yield and productivity. Also, biochars may act as a sorbing material that is being used for the immobilization of different types of contaminants such as potentially toxic metals and make them less bioavailable. In this paper, we investigated the Cd fractionation and spinach uptake of Cd, Fe, Cu, Mn, and Zn in a highly Cd-contaminated soil (0, 5, 10, 25, 50, 100 mg Cd kg −1 soil) amended with rice straw biochar (0, 0.5, 1, 2, 5, 10% (w/w)). Cd application enhanced the Cd accumulation in spinach plant, whereas a decrease of dry matter yield and concentrations of Fe, Cu, Zn, and Mn in spinach biomass is observed. Biochar addition to soil increased spinach dry matter yield and Zn and Fe concentrations but decreased Cd and Cu concentrations in the spinach. Biochar altered the easily exchangeable Cd fractions to the less available ones. Application of biochar decreased the Cd fractions of exchangeable and carbonate-bound, whereas Fe–Mn oxide-bound, organic-bound, and residual fractions increased. Our results showed that the application of biochar can decrease the availability and toxicity of Cd. The low Cd concentration in spinach shoots driven by increasing biochar dose indicates that the application of rice straw biochar is a good approach to reduce Cd transfer risks to human beings and the surrounding environment of metal-contaminated soils. Therefore, for soil remediation purposes, the application of 10% rice straw biochar can be suggested as an optimal application dose.