Wildfire severity reshapes soil organic carbon fractions and stabilization pathways in a subalpine forest

Introduction Wildfire disturbance is a major driver of soil carbon cycling in subalpine forests, yet differential responses of soil organic carbon fractions to fire severity remain poorly understood. Methods We investigated a subalpine forest in northwestern Sichuan, China, 6 years after wildfire. Four fire-severity classes were established: unburned control (UB), low-severity fire (LSF), moderate-severity fire (MSF), and high-severity fire (HSF). Soil samples were collected from 0–10 and 10–20 cm depths to quantify total soil organic carbon (SOC), labile fractions (DOC, MBC, ROC), intermediate-turnover fractions (POC, LFOC), and persistent fractions (MOC, HFOC). The carbon pool management index (CPMI) assessed carbon-pool stability redundancy analysis and structural equation modeling explored potential drivers of carbon-fraction dynamics. Results Fire severity exerted nonlinear effects on SOC and its component fractions. Both LSF and HSF increased the content of SOC and intermediate-turnover and persistent carbon fractions, whereas HSF additionally increased MBC by 90.22%–93.96% relative to UB. By contrast, MSF reduced SOC and most intermediate-turnover and persistent fractions, while significantly increasing ROC. Upper-layer carbon fractions were generally more responsive to fire severity. However, the relative SOC increase under HSF was greater at 10–20 cm (514.02%) than at 0–10 cm (234.28%). Fire severity also differentially affected carbon-pool stability. MSF showed the highest carbon-pool activity and CPMI but lower carbon persistence, whereas LSF and HSF carbon pools were dominated by persistent fractions and exhibited higher carbon pool indices. Soil physicochemical properties were associated with carbon-fraction patterns: available potassium explained 63.5% of upper-layer variation, and total porosity (POR) explained 48.5% in the deeper layer. Structural equation modelling showed POC exerted the strongest direct positive association with SOC, which may suggest that fresh organic matter inputs during post-fire vegetation recovery may contribute to SOC increase. Discussion These findings show a nonlinear response of subalpine forest soil organic carbon and its components to fire severity and provide insights into post-fire soil carbon accumulation and stabilization. These findings can offer data support for post-fire ecosystem management and regional carbon sink assessments.