S-Rich Biochar Enhances Cd Immobilization by Boosting Fe Transformation Under Decreasing pe + pH Conditions

Biochar application can effectively immobilize Cadmium (Cd) in soil. However, it is largely unknown how the biogeochemical processes of sulfur (S) in biochar affect Cd fixation under conditions of decreasing pe + pH. Using two field-contaminated paddy soils with different Cd concentrations (Shangyu (SY) 0.56 mg kg−1 and Tongling (TL) 2.32 mg kg−1), and rape straw biochars with low S (LB) and high S (HB) contents, we investigated how S-rich biochar regulates Cd solubility in paddy soils that were incubated anaerobically for 40 d. The soluble and extractable Cd content decreased as pe + pH decreased with flooding, and showed a steady trend by day 20. However, Cd was immobilized through different mechanisms in TL and SY soil. The rapid decrease in pe + pH in TL soil enhanced the involvement of S in Cd immobilization and Fe transformation. In SY soil, the delayed reduction in SO42− promoted Cd adsorption onto amorphous Fe oxides. During this process, the liming effect of biochar is critical for Cd immobilization in soil. Furthermore, biochar might promote the biogeochemical processes of S and Fe transformation, thereby enhancing Cd fixation in soils.