Abstract: Soil organic carbon (SOC) plays a critical role in sequestering atmospheric carbon dioxide and mitigating global climate change in forests. However, how forest succession affects soil organic carbon fractions and their stability in subtropical forests remains unclear. To address this knowledge gap, we examined the effects of forest succession on soil organic carbon stock and stability and their regulatory factors along the successional gradient of coniferous forest, coniferous and broad-leaved mixed forest, evergreen and deciduous broad-leaved mixed forest, and evergreen broad-leaved forest in Matoushan, Jiangxi Province. Soil samples were collected from the humus layer, 0-10 cm soil layer, and 10-30 cm soil layer. The variations of soil organic carbon, particulate organic carbon (POC), mineral-associated organic carbon (MAOC) contents, and soil physical and chemical properties were analyzed. The results showed that SOC stock increased along forest succession stages. Broad-leaved mixed forests (5065 g C·m^-2) and evergreen broad-leaved forests (5940 g C·m^-2) had significantly greater SOC stock than coniferous forests (2900 g C·m^-2) and coniferous and broad-leaved mixed forests (3132 g C·m^-2). As forest succession proceeded, POC concentration increased across all soil layers, whereas MAOC concentration gradually increased in the humus and 0-10 cm soil layers but initially declined and then increased in the 10-30 cm soil layer. The proportion of POC to SOC generally increased with forest succession, but the proportion of MAOC to SOC declined, resulting in elevated POC∶MAOC ratio. Compared to coniferous forests, the average POC∶MAOC ratio increased by 87%, 245%, and 360% in the coniferous and broad-leaved mixed forests, broad-leaved mixed forests, and evergreen broad-leaved forests, respectively. In the humus layer, soil organic C and POC concentrations were primarily influenced by available phosphorus, while MAOC concentration was influenced by fine root biomass. In the 0-10 cm soil layer, SOC and POC concentrations were mainly affected by soil total nitrogen (N) concentration and C∶N ratio, whereas MAOC concentration was mainly regulated by pH. In the 10-30 cm soil layer, organic C and its fractions were predominantly influenced by total N concentration and C∶N ratio. These findings suggest that community succession enhances SOC accumulation but reduces SOC stability by increasing the proportion of POC fraction in subtropical forests. These change trends are primarily modulated by fine root biomass and soil nutrient (i.e., N and phosphorus) availability during forest succession.

Key words: particulate organic carbon, mineral-associated organic carbon, forest succession, soil organic carbon fraction, subtropical forest