The soil micro-food web, a complex biotic network governing belowground ecological processes, plays a pivotal role in maintaining critical ecosystem functions through nutrient cycling and energy flow. Despite increasing recognition of anthropogenic impacts on soil micro-food webs, the structural reorganization of soil micro-food webs and its cascading effects on biogeochemical cycling following the conversion of subalpine natural forests to monoculture plantations remain poorly understood. Here, we investigated how forest plantations affect the soil micro-food web (including soil microbes and nematodes) and soil carbon (C) and nitrogen (N) mineralization in a subalpine region of southwestern China. Our study found that forest plantations substantially altered the soil micro-food web composition and structure, manifesting as reduced microbial biomass (−10 %), nematode abundance (−41 %, P < 0.01) and, more importantly, the deceased stability of the soil micro-food web (−57 %, P < 0.05). Additionally, compared with the natural forest, the soil C mineralization rates had significantly increased (P < 0.05) by approximately 133 % in the spruce plantation, potentially explaining the observed depletion of soil organic carbon stocks. In contrast, N mineralization rates showed no significant differences. The path modelling further demonstrated that the soil micro-food web significantly mediated the effects of forest plantations on the soil C mineralization. Overall, these results emphasized the importance of the soil micro-food web in understanding the ecological consequences of forest plantations and providing insights for the sustainable management of plantations.