Effects of Restoration Duration and Vegetation Type on Rhizosphere Soil Microbial Carbon and Nitrogen Cycling in an Open-Pit Coal Mine Spoil Heap in Western Inner Mongolia

Low-fertility reconstructed soils in spoil heaps cause slow and fluctuating soil carbon (C) and nitrogen (N) accumulation over time; identifying key drivers is vital. We used metagenomics to examine restoration age and vegetation-type effects on rhizosphere nutrients, aggregate structure, core microbiota, and C–N-cycling genes in western Inner Mongolia spoil heaps. Key findings: (1) A. fruticosa increased mineral-associated organic carbon (MAOC) by 26.03% on average across 28 years compared to other vegetation (p < 0.001); M. sativa boosted transient dissolved organic carbon (DOC) by 41.94% in early restoration (6 years) versus one-year alfalfa (p < 0.001). (2) The 28-year M. sativa treatment enhanced organic carbon degradation gene abundance by 134.98% over 28-year A. fruticosa (p < 0.05), while A. fruticosa increased the nitrification-related nirK gene by 326.42% versus M. sativa (p < 0.05). (3) Core microbiota (e.g., Nocardioides) regulated iron-oxide-bound organic carbon (58.34% variance explained), and microbial C–N-cycling gene abundance correlated significantly with it (r=0.665, p < 0.001). These findings provide a theoretical basis for soil ecological function recovery in open-pit coal mine spoil heaps within the temperate semi-arid steppe–desert steppe ecotone and a scientific foundation for soil reconstruction technologies to accelerate rehabilitation in western Inner Mongolia.