Stand age reduces microbial necromass contribution to soil organic carbon in plantation forests of a karst rocky desertification region

Afforestation is an effective strategy for restoring the ecological functions of karst rocky desertification areas. Soil microbial necromass carbon (MNC) constitutes an essential component of stable soil organic carbon (SOC) pools in terrestrial ecosystems. However, MNC contribution to SOC varies with stand age and the underlying mechanism remain poorly understood in karst areas. We investigated MNC accumulation dynamics and its contribution to SOC in Delavaya toxocarpa forests of different stand ages (4, 11, and 18 years old) located in a karst desertification area in Southwestern China. The results showed that MNC accumulation in the 18-year-old plantation was 59.2% and 47.1% lower than that in the 11- and 4-year-old plantations, respectively. MNC accumulation decreased with increasing soil pH. The MNC contributed 36.7% to SOC, and the MNC/SOC ratio decreased with increasing stand age. Key factors regulating MNC's contribution to SOC included soil carbon-to‑nitrogen (C/N) ratio, exchangeable magnesium content, total phosphorus (TP) content, bacterial and fungal network complexity, and the relative abundance of Agaricales and Betaproteobacteriales. Specifically, soil C/N ratio directly modulated MNC contribution, while TP limitation indirectly suppressed MNC accumulation by enhancing microbial network complexity. This finding suggests that newly established artificial forests enhance microbial-mediated SOC sequestration during early forest development, highlighting the need for improved management practices, such as applying magnesium-containing fertilizers in the middle age. This study enhances our understanding of microbial-mediated carbon sequestration in degraded karst landscapes and provides guidance for effective ecological restoration to increase soil carbon storage.