Effects of a microbial composite amendment and ryegrass on multi-metal(loid) stabilization and nutrient cycling functional potential in smelting-contaminated soil.

Multi-metal(loid) contamination from nonferrous smelting threatens soil safety and weakens microbial nutrient cycling, yet field remediation is assessed mainly by metal immobilization. Whether microbial amendments and vegetation can simultaneously reduce metal risk and improve the cycling functional potential of C, N, P, and S remains poorly understood under field conditions. A 300-day in situ experiment at a Pb-Zn smelting site evaluated four treatments: control (CK), microbial amendment (M), ryegrass (P), and combined (MP). MP treatment reduced Pb, Cd, and Zn bioavailability by 42-52% and shifted metals to stable fractions, while As leaching reached 0.017 mg L⁻¹ with 9-15% increases in Fe-Mn oxide-bound As. MP treatment increased microbial biomass carbon by 145% and alkaline phosphatase 10-fold. Among 63 functional genes, MP enriched C cycling (aclB, korA, glx, mnp; up to 3.2-fold), N cycling (narG, napA, nirS3; up to 2.9-fold), P mineralization (phoD, phoX; up to 4.0-fold), and S oxidation (soxY; 2.2-fold) genes. These responses correlated positively with MBC and enzymes but negatively with available nutrients, indicating associations between functional gene profiles and soil biochemical properties. The combined system achieves concurrent metal stabilization and shifts in nutrient cycling functional genes, providing field-scale evidence for linking risk control with soil functional assessment.