Six-year nitrogen addition alters microbial-derived carbon accumulation in association with soil stoichiometry and microbial community reorganization in an alpine coniferous forest

Background Atmospheric nitrogen (N) deposition can strongly alter soil organic carbon (SOC) stabilization by regulating microbial-derived carbon (C) formation. Microbial necromass carbon (MNC) and glomalin-related soil protein (GRSP) are important contributors to SOC persistence, yet how long-term N enrichment affects these microbial-derived C pools and their microbial drivers across soil depths remains unclear in alpine forest ecosystems. Methods We investigated the effects of six-year N addition on bacterial necromass C (BNC), fungal necromass C (FNC), MNC, total GRSP (T-GRSP), their SOC-normalized proportions, and associated soil and microbial mechanisms in an alpine coniferous forest on the southeastern Xizang Plateau. Four N addition treatments—control, low N, medium N, and high N addition—were established, and soils were collected from the 0–20 and 20–40 cm layers. Random forest analysis, bacterial–fungal co-occurrence networks, and microbial module analysis were used to identify variables associated with microbial-derived C accumulation. Results Low and medium N addition increased BNC, FNC, MNC, and T-GRSP, whereas these positive effects were weakened under high N addition. Although microbial-derived C contents were consistently higher in surface soil, the SOC-normalized contribution of microbial necromass responded more strongly to N addition in the 20–40 cm layer, while T-GRSP:SOC remained relatively stable across treatments and depths. Random forest analysis suggested that soil pH, nutrient availability, and C:N:P stoichiometry were closely associated with MNC and T-GRSP variation, with additional associations involving enzyme activities, microbial diversity, and community composition, particularly in deeper soil. Exploratory depth-specific bacterial–fungal co-occurrence networks indicated fewer associations and a higher proportion of negative links in deeper soil. Module analysis further identified specific bacterial and fungal modules associated with SOC fractions, microbial necromass, and GRSP-related indicators. Conclusion Moderate N enrichment was associated with greater microbial-derived C accumulation, whereas high ammonium-sulfate N addition showed weaker positive effects accompanied by nutrient imbalance, soil acidification, and altered microbial attributes. These findings highlight the importance of integrating microbial residues, GRSP, soil nutrient stoichiometry, and microbial co-occurrence patterns when assessing SOC stabilization under increasing N deposition.