Combined Application of Biochar and Calcium Superphosphate Can Effectively Immobilize Cadmium and Reduce Its Uptake by Cabbage

Biochar and phosphate fertilizer are commonly employed for the mitigation of soil cadmium (Cd) contamination. Nevertheless, there is a dearth of research regarding the mechanism behind their joint implementation. In this study, a combination of corn straw biochar (0 (C0), 5 (C5), and 10 (C10) g kg−1) and calcium superphosphate (0 (P0), 0.1 (P1), 0.2 (P2), 0.5 (P5), and 1.0 (P10) g kg−1) was applied in pot experiments, and the effects of the combined application on Cd bioavailability and its uptake by cabbage were investigated in Cd-contaminated soils. The results demonstrated that the combined treatment of applying biochar and Ca(H2PO2)2 yielded a significant decrease in the uptake of Cd by cabbage in alkaline soil, in contrast to the individual treatments of biochar or Ca(H2PO2)2. Compared to the CK treatment (C0P0), the Cd content in the shoots decreased by 46.26% and in the roots decreased by 24.81%, while the biomass of the cabbage demonstrated a noteworthy increase in C5P10 treatment. Compared to the CK treatment, the content of available phosphate (AP) in the soil increased by 17.57 mg kg−1, residual Cd increased by 22.02%, the exchangeable Cd decreased by 45.86%, and carbonate-bound Cd decreased by 20.55% in the C5P10 treatment. Therefore, it is advisable to use a combination of 5 g kg−1 biochar and 1 g kg−1 Ca(H2PO2)2 for the restoration of soil contaminated with Cd.