Effect of peanut shell and wheat straw biochar on the availability of Cd and Pb in a soil–rice (Oryza sativa L.) system

Soil amendments, such as biochar, have been used to enhance the immobilization of heavy metals in contaminated soil. A pot experiment was conducted to immobilize the available cadmium (Cd) and lead (Pb) in soil using peanut shell biochar (PBC) and wheat straw biochar (WBC), and to observe the accumulation of these heavy metals in rice (Oryza sativa L.). The application of PBC and WBC led to significantly higher pH, soil organic carbon (SOC), and cation exchange capacity (CEC) in paddy soil, while the content of MgCl₂-extractable Cd and Pb was lower than that of untreated soil. MgCl₂-extractable Cd and Pb showed significant negative correlations with pH, SOC, and CEC (p < 0.01). The application of 5% biochar to contaminated paddy soil led to reductions of 40.4–45.7 and 68.6–79.0%, respectively, in the content of MgCl₂-extractable Cd and Pb. PBC more effectively immobilized Cd and Pb than WBC. Sequential chemical extractions revealed that biochar induced the transformation of the acid-soluble fraction of Cd to oxidizable and residual fractions, and the acid-soluble fraction of Pb to reducible and residual fractions. PBC and WBC clearly inhibited the uptake and accumulation of Cd and Pb in rice plants. Specially, when compared to the corresponding concentrations in rice grown in control soils, 5% PBC addition lowered Cd and Pb concentrations in grains by 22.9 and 12.2%, respectively, while WBC addition lowered them by 29.1 and 15.0%, respectively. Compared to Pb content, Cd content was reduced to a greater extent in grain by PBC and WBC. These results suggest that biochar application is effective for immobilizing Cd and Pb in contaminated paddy soil, and reduces their bioavailability in rice. Biochar could be used as a soil amendment for the remediation of soils contaminated with heavy metals.