Contribution of microbial necromass to soil organic carbon and its influencing factors under diverse ecosystems in Southwest China

Abstract Background and aims Against the backdrop of accelerating climate change and land degradation, enhancing soil carbon (C) stocks through optimized land use has garnered global attention. Conservation tillage and planted forests represent two key ecological restoration strategies that can promote long-term soil organic C (SOC) sequestration. Methods We quantified microbial necromass C (MNC) and a suite of soil properties to assess the contribution of MNC to SOC and its primary controls in conservation tillage systems(combining no-tillage and cover cropping) and planted forests, covering elevations from 1280 to 3269 m above sea level in Southwest China. Results Unexpectedly, SOC and MNC exhibited no clear elevational trends, yet both were significantly affected by land-use type and soil depth. Furthermore, we found SOC was significantly higher under conservation tillage than in planted forests. This difference may be partially explained by the enhanced MNC accumulation. While bacterial necromass was more abundant, its contribution to SOC became proportionally smaller as SOC increased, suggesting limited effectiveness in long-term stabilization. Accordingly, the stronger correlation between fungal necromass and SOC, despite its lower abundance, may be attributed to its higher inherent stability, which enhances its contribution to long-term C sequestration. The key edaphic determinants governing SOC variability were associated with nutrient profiles, with nitrogenemerging as the predominant regulatory factor for both SOC and MNC accumulation across conservation tillage and planted forest ecosystems. Conclusion These findings offering critical insights into the microbial necromass pathways driving SOC sequestration in conservation tillage and planted forests ecosystems.