Contrasting elevational patterns in microbial necromass carbon content and contribution across soil depths of a subtropical alpine ecosystem

Abstract Background Microbial necromass carbon (MNC) is a critical component of persistent soil organic carbon (SOC). However, the elevational patterns of MNC accumulation and its contribution to SOC across different soil layers remain poorly understood in climate-sensitive alpine ecosystems. We investigated these dynamics across topsoil and subsoil layers along a subtropical elevational gradient (1700–3500 m) in southwest China, which encompasses five vegetation types from evergreen broadleaved forests to alpine meadows. Results The results showed that MNC content increased linearly with elevation, with a more pronounced accumulation in topsoil. This pattern resulted from a clear depth-dependent shift in primary regulators, where fine root biomass and mean annual temperature dominated topsoil MNC accumulation while total phosphorus dominated subsoil accumulation. Notably, the contribution of MNC to SOC remained consistent across soil depths (averaging 29.9 ± 3.3%) and exhibited a quadratic elevational trend, peaking in high-elevation meadows. Structural equation modeling revealed that iron oxides and total phosphorus exerted significant direct effects on the MNC contribution to SOC, while fine root biomass had an indirect effect by enhancing phosphorus availability. Conclusions These findings highlight alpine meadows as key reservoirs of MNC and reveal distinct depth-dependent mechanisms governing its accumulation. This study provides a mechanistic framework for predicting SOC dynamics under climate change in vulnerable alpine ecosystems.