Six decades of afforestation reshape deep soil microbial networks on the Loess Plateau: Implications for environmental assessment and ecological restoration policy in the Yellow River Basin

Large-scale afforestation programs, including the ‘Grain for Green’ program implemented in the Yellow River Basin, are central to combating land degradation, yet their long-term belowground effects remain difficult to assess. This study evaluated the 60-year effects of Caragana korshinskii afforestation on the Loess Plateau using 0-10 m soil profiles, soil physicochemical analyses, 16S rRNA gene sequencing, and molecular ecological network analysis. The deep-profile design allowed us to identify where restoration signals were strongest and where afforestation effects attenuated along loess deposits. Long-term afforestation was associated with lower deep-layer soil water content, a decline in soil pH from 9.08 to 8.72, and a 23% increase in surface dissolved organic carbon. Bacterial diversity followed a U-shaped trajectory, recovering to exceed cropland levels after six decades. Microbial co-occurrence networks were more connected and showed higher topological robustness under targeted node removal than the cropland network (0.655 ± 0.011 vs. 0.563 ± 0.006). Soil pH was the environmental variable most strongly associated with this network shift; here, it is interpreted as an integrative proxy for vegetation-mediated changes in root activity, carbon inputs, nitrate redistribution, and hydrology rather than as an independent causal driver. These findings indicate that the gradual moderation of alkalinity co-varied with subsurface microbial reorganization. Together with soil water, DOC, and nitrate, pH provides a practical indicator set for assessing belowground restoration progress in the alkaline loess-derived soils examined.