SOIL CARBON SATURATION CONSTRAINS LONG-TERM SEQUESTRATION UNDER REVEGETATION ON CHINA'S LOESS PLATEAU

Revegetation can enhance soil organic carbon (SOC) sequestration and contribute to climate change mitigation, yet the mechanisms controlling long-term sequestration remain insufficiently resolved, particularly with respect to particulate organic carbon (POC), mineral-associated organic carbon (MAOC), and carbon saturation deficit (CSD). Here, we systematically investigated SOC fractions, CSD, and their biotic and abiotic drivers across five representative geomorphic regions of China's Loess Plateau under long-term revegetation. Results showed that MAOC accounted for 49%-70% of total SOC and was the dominant fraction contributing to long-term carbon stabilization, while the co-accumulation of POC and MAOC promoted SOC sequestration. Reforestation and grassland restoration reduced CSD by 72% and 42%, respectively, indicating that prolonged revegetation progressively diminished the residual carbon sequestration potential (CSP) of restored ecosystems. Estimated CSP was 56.47, 36.52, 28.54 Mg ha-1 in farmland, grassland, and forest, respectively, with below 20 cm exhibiting 1.3-1.8 times greater sequestration than 0–20 cm soils. Modeling further confirmed revegetation altered soil physicochemical properties by increasing plant carbon inputs and restructuring microbial communities, thereby promoting POC accumulation (R² = 0.79) and MAOC stabilization (R² = 0.88). Together, these results demonstrate that mineral-associated stabilization, rather than carbon input alone, is the dominant control on soil carbon saturation. This study provides a quantitative and mechanistic framework for predicting sequestration limits in restored ecosystems and offers important implications for restoration planning, regional carbon management, and climate mitigation in fragile dryland environments.