Effects of rice husk biochar and its modified forms on Cd2+ immobilization, humus dynamics, and soil fertility in Cd-contaminated paddy soils

With the widespread Cd 2+ contamination of paddy soils threatening agricultural sustainability and food safety, this study explored the remediation potential of raw rice husk biochar (BC) and chemically modified BCs (BC-HCl, BC-NaOH, and BC-KMnO 4 ) for Cd-contaminated paddy soils. A 120-day incubation experiment was conducted across four Cd 2+ gradients (0, 10, 20, and 40 mg/kg), and analytical techniques were used to characterize BC microstructure, Cd 2+ immobilization efficiency, humus (HS) dynamics, and soil fertility parameters. Chemical modifications improved BC properties: BC-KMnO 4 had the highest –COOH content and degree of aromatization, while BC-NaOH showed strong alkalinity. At Cd 2+ concentrations ≤20 mg/kg, BC-KMnO 4 performed best in Cd 2+ immobilization and HS enhancement, whereas BC-NaOH was superior in stabilizing soil properties under severe Cd 2+ contamination (40 mg/kg). This work clarified the synergistic mechanisms of BC in Cd passivation and soil fertility improvement and provided theoretical support for safe rice production in Cd-contaminated agricultural systems.