Land Use Conversion Regulates Soil Nutrient–Metal Coupling and Bacterial Diversity Through Ecological Mechanisms in Mine Reclaimed Ecosystems

Land use conversion profoundly affects the ecological functioning of reclaimed soils. However, the coupling mechanisms among soil nutrient accumulation, heavy metal dynamics, and microbial diversity under different land use types remain unclear. This study investigated reclaimed mining soils under five land use types to evaluate changes in soil properties, heavy metal distribution, and bacterial diversity, and to elucidate their regulatory mechanisms. Wheat fields and Ligustrum plantations showed higher carbon, nitrogen, and phosphorus contents, with increased lead and arsenic concentrations, whereas abandoned grasslands exhibited relatively lower nutrient availability. Land use explained 26.4% of bacterial community variation (R2 = 0.264, p = 0.001). Structural equation modeling revealed that nutrient enrichment significantly promoted heavy metal accumulation (β = 0.606, p < 0.001). Nutrients negatively affected bacterial richness (β = −0.401, p = 0.035). In contrast, moderate metal stress may promote microbial richness (β = 0.460, p = 0.015) through micronutrient effects or hormesis (low-dose toxic stimulatory response). These findings indicate that land use conversion regulates microbial diversity through coupled changes in nutrient availability and metal dynamics. Reclamation strategies should integrate nutrient management, vegetation selection, and heavy metal risk assessment to achieve long-term restoration of soil functions.