Four-Year Continuous Rhizobium–Legume Green Manure Synergy Improves Reclaimed Mine Soil Fertility by Regulating the Soil Biochemical Processes

Reclaimed mine soils in northern China suffer from extreme nutrient deficiency, low organic matter content, and impaired microbial activity, which severely challenge ecological restoration. Leguminous green manures inoculated with high-efficiency rhizobia offer a promising bioremediation strategy. However, long-term field evidence and understandings of microbial mechanisms remain insufficient. A 4-year consecutive field experiment was conducted with three legumes (Vicia villosa Roth, Vicia sativa L. and Glycine max (L.) Merr.) inoculated with host-specific rhizobia. Plant traits, soil properties, enzyme activities, and microbial communities were analysed using redundancy analysis (RDA), co-occurrence network analysis, and structural equation modelling (SEM). Compared with the corresponding non-inoculated green manure controls, rhizobium inoculation increased dry matter yield by 96.61–185.40% and improved soil nutrient contents and enzyme activities (p < 0.05) across legume species. The rhizobium–legume green manure treatments markedly reshaped the soil bacterial and fungal community structure relative to the bare-soil control, enriching putatively beneficial genera and suppressing a pathogenic genus (p < 0.05); because the non-inoculated green manure plots were not sequenced, these shifts cannot be attributed to inoculation alone. RDA identified soil organic matter and invertases as core drivers of bacterial communities, whereas total nitrogen dominated fungal community structuring. Co-occurrence networks of inoculated treatments displayed markedly higher complexity and tighter connectivity, with soil organic matter, bacterial Shannon diversity, and urease identified as core regulatory hubs. SEM revealed a robust positive cascading pathway, whereby rhizobium inoculation promoted green manure growth, enhancing soil fertility, which in turn stimulated soil enzyme activities. The standardised effect of inoculation on green manure growth was 0.885, and its indirect effects transmitted through growth reached 0.614 on soil fertility and 0.276 on soil enzyme activity. Planting green manure per se also exerted a strong effect on soil fertility (β = 0.470, p < 0.001), with an indirect effect of 0.208 on soil enzyme activity, indicating that crop establishment accounted for a substantial part of the observed improvement. Rhizobium inoculation enhanced remediation efficiency mainly by amplifying this plant-growth-mediated cascade rather than by directly altering soil biochemical properties. The Vicia villosa Roth + Rhizobium leguminosarum M16 system was optimal, improving soil fertility via a microbial enzyme-mediated positive cascade. This study provides a promising model for mine soil restoration.