Forest succession accelerates soil carbon accumulation by increasing recalcitrant carbon stock in subtropical forest topsoils

Soil carbon (C) stocks are known to increase as a consequence of the forest succession of a pioneer pine forest (PF) to a pine and broad-leaved mixed forest (PBMF), and then to a monsoon evergreen broadleaf forest (MEBF). However, the underlying mechanisms of soil C stock accumulation along the succession of these subtropical forests remain unknown. Here, we tested the hypothesis that the accumulation of soil C stock depends on the increase in soil recalcitrant C (RC), especially in the topsoil, resulting from an increased input of litter and fine roots and an increased association between soil organic C (SOC) and minerals. We measured RC and labile C including microbial biomass carbon (MBC), dissolved organic carbon (DOC), particulate organic carbon (POC), and readily oxidizable organic carbon (ROC) down to 60 cm soil depth in three successional stages. Succession of PF to PBMF and then to MEBF increased soil C stocks (relative to PF) by 30.9% and 53.7%, respectively, and increased RC stock by 81.8% and 135.5%, respectively, across all soil layers (0–60 cm), which was more pronouced in the topsoil (0–30 cm) than in the subsoil (30–60 cm). The increase of RC stocks accounted for 76% and 77% of the increase in bulk soil C stocks in the topsoil with succession of PF to PBMF and then to MEBF, respectively. The increase in the concentration of organic-bound iron oxides (Fep) was greater in the topsoil than in the subsoil (45.1% vs 6.4% for PBMF and 126.0% vs 57.6% for MEBF, respectively), which contributed to the increase in topsoil RC and SOC stock. The SOC accumulation was also associated with an increase in POC and ROC, which was resulted from the increase in fine root biomass and annual litterfall. We concluded that soil C stock increased, especially in the topsoil, along forest succession. The increase was associated with a direct increase in the binding of SOC with minerals and an indirect increase in the new C input from litter and roots. These results help explain why soil C continues to accumulate in mature even old growth forests along forest succession.