Calcium–Silicon–Magnesium Synergistic Amendment Enhances Cadmium Mitigation in Oryza sativa L. via Soil Immobilization and Nutrient Regulation Dynamics

Soil passivation conditioners effectively reduce cadmium (Cd) bioavailability and limit its accumulation in rice, though their efficacy and stability vary considerably among different types. A three-year paddy field study in southern China evaluated a calcium–silicon–magnesium composite (CSM) applied at 1500 and 3000 kg/ha (CSM1 and CSM2), with a no-CSM control (CK), on Cd behavior, soil properties, and functional groups. Results demonstrated a clear dose–response relationship, with CSM reducing brown rice Cd by 35−74% across sites (2021−2023). High-dose treatments achieved grain safety standards (0.183 mg/kg, p < 0.05). Soil pH increased annually by 0.2−0.37 units, while DTPA-extractable Cd decreased by 2.6−27% over three years. CSM application significantly transformed Cd speciation, reducing exchangeable Cd by 3% while increasing the iron–manganese oxide-bound fraction by 5%. Soil base saturation increased from 42.6% to 73.2% (HS) and 71% to 97.3% (XY). FTIR analysis revealed enhanced silicate polymerization, increased hydroxyl group abundance, and Si-O-Mg/Fe vibrations indicating a significant increase in Cd complexation in treated soil. The CSM passivator immobilizes Cd by elevating soil pH to promote its transformation into stable Fe-Mn-bound forms, enhancing hydroxyl and siloxane complexation with Cd, and synergizing with silicon–calcium ionic antagonism, collectively reducing Cd bioavailability while improving soil fertility through base saturation regulation.