Contrasting land use systems regulate active and passive soil carbon pools and the carbon management index across...

Understanding how soil carbon pools respond to contrasting land use systems is essential for evaluating soil functioning and land sustainability. Here, we examined the response of total organic carbon (TOC), oxidizable carbon fractions, microbial biomass carbon (MBC), soil organic carbon (SOC) stocks, and the carbon management index (CMI) across four contrasting land use types (forest, orchard, cropland, and abandoned land) at two soil depths (0-10 and 10-20 cm) in a sub-humid watershed. Oxidizable carbon fractions were grouped into active pools (very labile + labile) and passive pools (less labile + non-labile). Forest soils showed the highest TOC and MBC, whereas conversion to cropland and abandoned land reduced surface TOC by 36.0% and 47.4%, respectively, and MBC declined markedly under non-forest uses. Surface SOC stocks also decreased by 30.8% in cropland and 41.3% in abandoned land relative to forest. Active carbon pools declined substantially in the 0-10 cm layer, with reductions of 50.0% in cropland and 45.2% in abandoned land, while passive fractions accounted for a greater proportion of total SOC under these land uses, indicating a shift toward more stable carbon forms. CMI values further highlighted relative differences in SOC status among land uses: at 0-10 cm, CMI was 85.05 in orchard, 56.18 in abandoned land, and 44.16 in cropland, while at 10-20 cm the corresponding values were 77.25, 40.28, and 63.99, respectively. Integrating SOC fractionation with CMI provides a useful comparative framework for detecting relative changes in soil carbon status across contrasting land use systems.