Long-term vegetation restoration enhances soil carbon sequestration along a 170-year chronosequence

Long-term agricultural cultivation accelerates soil erosion and exacerbates water and soil organic matter loss. Vegetation restoration is widely recognized as an effective strategy for improving soil carbon (C) sequestration. However, the effects of vegetation restoration following farmland abandonment on soil organic carbon stock (SOCS) and its fractions across soil profiles remain poorly understood. Here, we investigated the dynamics of SOCS, particulate organic carbon stock (POCS), mineral-associated organic carbon stock (MOCS) and their driving factors across a 0-100 cm soil profile over a 170-year chronosequence following farmland abandonment on the Loess Plateau of China. The results showed that 170 years of vegetation restoration increased SOCS and POCS by 41.5% and 81.2%, respectively. In the 20-100 cm layer, SOCS and POCS initially declined and began to increase after approximately 30 years of vegetation restoration. MOCS remained unchanged in the 0-60 cm layer, whereas increased in the 60-100 cm layer after vegetation restoration. The 40-60 cm layer served as a "transitional zone", marking a shift in dominant sequestration mechanisms from POCS-dominated accumulation in the topsoil (0-40 cm) to MOCS-dominated stabilization in the subsoil (60-100 cm). SOCS and POCS were driven by plant-derived inputs, while MOCS was mainly controlled by soil physical properties and microbial activity. Our findings demonstrate that long-term vegetation restoration effectively enhances soil organic carbon and particulate organic carbon accumulation in surface soils while promoting the stabilization of mineral-associated organic carbon in subsoil, highlighting the importance of vegetation restoration for sustainable soil C management.