[Variation Characteristics of Exogenous Cadmium with Different Contents in Red Soil]

To investigate the change characteristics of exogenous cadmium (Cd) into red soil over time, different concentrations of Cd (0, 0.3, 2.0, 5.0, 10, 30, 60, and 100 mg·kg^-1) were added to farmland soil, and samples were taken at the 3^rd, 7^th, 14^th, 28^th, 42^th, 56^th, 70^th, and 84^th day, respectively. The occurrence of Cd in farmland soil was analyzed using the BCR hierarchical extraction method, and the stabilization and distribution characteristics of Cd in farmland soil were simulated by three dynamical models. The results showed that:① After exogenous Cd entered the soil, the soil Cd form was redistributed, tending to the original soil Cd form distribution characteristics, the binding strength of Cd and soil increased as time increased, and the total redistribution coefficient gradually tended to 1. ② Stabilization of exogenous Cd in red soil was a long-term process. The extractable state of weak acid gradually transformed to the other three forms and finally to the residue state. Within 84 days, the oxidation state at 0.3 mg·kg^-1 gradually decreased with time, and the other concentrations showed the opposite. Within 84 days, when the reducible state was below 30 mg·kg^-1, it gradually decreased with time, and the other concentrations showed the opposite. ③ The stabilization process of exogenous Cd in red soil was mainly a diffusion process controlled by multiple reaction mechanisms. The higher the concentration of exogenous Cd, the higher the change rate of the residual state. These research results can provide theoretical basis for the related research based on exogenous cadmium and the risk control and remediation of soil contaminated with heavy metals.