Organic–Inorganic Fertilization Enhances Soil Carbon but Weakens Its Stability via Reduced Microbial Necromass in Reclaimed Mining Land

ABSTRACT Large‐scale coal mining regions hold substantial potential for soil carbon (C) sequestration through reclamation, but the long‐term influence of fertilization on the soil organic carbon (SOC) dynamics of these ecosystems remains largely unclear. A 14‐year field study was conducted at a representative coal mining reclamation site to examine how different fertilization treatments—unfertilized control (CK), inorganic fertilizer (F), and low/high manure combined with F (LMF/HMF)—affect microbial necromass carbon (MNC) and SOC composition, alongside stabilization pathways. Compared with the CK treatment, the application of organic manure significantly increased the total SOC content by 30.8%–50.7% ( p < 0.05), mainly by increasing the proportion of particulate organic carbon (POC) but decreasing mineral‐associated organic carbon (MAOC). Fertilization also had an essential effect on soil aggregation, with large macroaggregates (> 2 mm) increasing by 129%–149% and small macroaggregates (0.25–2 mm) decreasing by 39%–45% ( p < 0.05). Enzyme activity assessments revealed that prolonged organic fertilization markedly elevated β‐glucosidase (BG) and N‐acetylglucosaminidase (NAG) activities by 112%–178% and 51%–169%, respectively ( p < 0.05). In contrast, inorganic fertilization dramatically reduced BG activity by 29.0% ( p < 0.05), but it had no significant effect on NAG activity ( p > 0.05). The microbial C:N stoichiometry shifted under the organic treatments, with increased C demand but decreased nitrogen demand. Notably, although organic fertilization increased MNC by 58.8%–69.7%, its relative contribution to SOC declined (16.7% in HMF vs. 21.4% in CK). Random forest analysis identified BG activity and total N as key predictors of MNC, with microbial properties explaining 17.8% of the MNC variation compared with just 5.2% explained by soil nutrients. As suggested by partial least squares path modeling, microbial metabolism served as a primary driving factor for MNC dynamics, with soil aggregates exerting indirect effects. These findings indicate that although organic fertilization increases the SOC content in reclaimed soils, it also reduces stability by lowering microbial necromass contributions. The results highlight the importance of microbial activity and the lack of mineral protection mechanisms as key factors regulating necromass persistence in anthropogenically modified soils. These results suggest the need for balanced reclamation approaches that integrate organic amendments with strategies aimed at stabilizing microbially derived carbon to achieve sustainable soil restoration.