Synergistic drivers of microbial carbon, nitrogen fixation and phosphorus mobilization in mining soils by integrating machine learning with causal analysis

Microbial carbon (C), nitrogen (N) fixation, and phosphorus (P) mobilization are key processes in ecosystem nutrient cycling. However, the key environmental factors driving these microbial functions in heavy metal-contaminated soils of mining areas remain poorly understood. In this study, we employed machine learning and causal analysis models to systematically identify the primary environmental drivers governing the relative abundances of key functional genes associated with C, N fixation and P mobilization in 48 metal mining areas across China. The results revealed that Fe, C/N ratio, and soil pH exert dominant controls on microbial functional genes. Among them, Fe was identified as a shared environmental driver across all functional groups, while C/N balance and soil pH exhibited consistent direct or indirect regulatory effects on microbial functional potential. Furthermore, machine learning-based threshold analysis revealed distinct ecological regulation windows in which microbial functional potential remained at relatively high levels, including Fe contents of 9-30%, C/N ratios of 20.47-50.62, and soil pH values of 5.8-6.30. This study elucidates the key environmental factors shaping microbial functions in mining-affected ecosystems and provides quantitative insights to support precision regulation in ecological restoration of contaminated soils.