Dual Role of Functional Bacterial Agent Based on Bacillus subtilis Coupled With Smooth Vetch (Vicia villosa var.) on Cadmium-contaminated Soil: Synergistic Passivation and Nutrient Supply

Herein, an innovative coupling remediation system integrating a functionalized bacterial agent (BBC-BS) derived from bone char (BC) and Bacillus subtilis (BS) combined with smooth vetch ( Vicia villosa var.) (SV) was developed and applied to actual Cd-contaminated farmland soil. Results revealed that mechanical ball milling significantly enhanced the S BET of BBC-BS, refined its particle size, and modified its crystalline structure, enabling effective BS immobilization while providing a stable microenvironment. This optimization elevated the phosphate solubilization capacity, representing a 26.9% increase compared to free BS. The coupled BBC-BS+SV system exhibited dual synergistic effects during plant growth: (1) BBC-BS promoted SV growth, while (2) markedly suppressing Cd accumulation in SV. Furthermore, after 28 d of SV pressed into soil, the DTPA-extractable Cd content treated by BBC-BS+SV decreased by 53.1% compared to the control (CK), surpassing the combined remediation efficiencies of individual BBC-BS application and SV incorporation alone (37.3%). The application of BBC-BS not only stimulated SV biomass accumulation but also accelerated SV residue decomposition via functional microbiota activation, generating more low-molecular-weight organic acids (LMWOAs) and humic acid (HA). Reciprocally, these products enhanced active sites on the BBC-BS surface, promoted Ca and P release from BBC-BS, and further strengthened Cd passivation. The coupled system ensured compliance with Cd residue standards in pepper while significantly improving fruit nutritional quality, achieving simultaneous soil remediation and safe crop production. This strategy provided a practical solution for Cd-contaminated farmland rehabilitation and sustainable enhancement of subsequent crop yield and quality.