Smooth Vetch (Vicia villosa var.) Coupled with Ball-Milled Composite Mineral Derived from Shell Powder and Phosphate Rock for Remediation of Cadmium-Polluted Farmland: Insights into Synergetic Mechanisms

Herein, a ball-milled composite mineral derived from shell powder and phosphate rock (BSPR) was innovatively coupled with smooth vetch (SV) planting to remediate Cd-polluted soil. The results showed that BSPR coupled with SV could reduce the bioavailability of Cd in soil by 68.1%, which was 2.2 and 3.4 times those of BSPR (31.1%) and SV (20.0%) alone, respectively. The outstanding passivation performance was confirmed to be due to the synergistic effect of BSPR and SV. Specifically, the employment of BSPR not only increased the biomass of SV but also promoted its decomposition, which directly led to the conversion of SV into more humic acid (HA). Under the action of HA, more calcium and phosphorus were released by BSPR and combined with Cd to form stable compounds (e.g., Cd3(PO4)2, Cd5(PO4)3Cl, and CdnCa5–n(PO4)3OH). Amazingly, due to the excellent performance of synergetic BSPR and SV on Cd passivation and soil improvement, the Cd content in pepper fruit decreased by 38.2% (6.3 times that of SV), and the fresh weight and Ca content of pepper fruit were enhanced by 28.2% and 43.4% (6.7 and 22.8 times that of SV, respectively). Overall, this study provided a novel strategy, i.e., green manure coupled with mineral, for simultaneous remediation of Cd-contaminated farmland and enhancement of pepper's yield and quality, thus achieving "killing three birds with one stone".