Recalcitrant organic carbon plays a key role in soil carbon sequestration along a long-term vegetation succession on the Loess Plateau

Vegetation restoration effectively promotes soil quality and enhances soil organic carbon (SOC) sequestration. However, the dynamics and driving factors of SOC fractions during long-term vegetation succession remain unclear. In this study, a complete ∼ 160 years successional chronosequence from farmland to climax forest was used to study the dynamics and driving factors of SOC fractions in topsoil (0–20 cm) and subsoil (20–40 cm). The results showed that vegetation succession age significantly affected the SOC and its fractions (p < 0.05). The content of SOC fractions increased with succession age, especially recalcitrant organic carbon (ROC), which accounted for 62%–85% of the total SOC. Long-term vegetation succession enhanced the stability of SOC pools, reduced the proportion of active C, and facilitated the fixation of C. ROC was the best indicator of SOC accumulation within the entire profile. When vegetation succession reached the pioneer forest stage (∼110 years), the SOC content and fractions increased significantly (p < 0.05) owing to continuous plant biomass inputs. Moreover, soil C sequestration was controlled by total nitrogen content in the topsoil and by belowground biomass in the subsoil. This study indicates that long-term vegetation succession can effectively improve SOC accumulation and SOC pool quality, emphasizing the need to focus on SOC pool stabilization mechanisms under future climate change.