Grassland degradation disrupts microbial nutrient cycling, yet the role of nitrogen (N) limitation in regulating soil organic carbon (SOC) dynamics during restoration remains poorly understood. Here, 10 years of active (sowing of seeds of native plants) and passive restoration (sand barrier protection) in degraded grasslands on the Qinghai–Tibetan Plateau are compared. Restoration impacts are assessed by integrating microbial metabolic traits such as stoichiometry-based nutrient limitation and C use efficiency (CUEST) with SOC fractionation, which considers both POC and MAOC). Active restoration reduces microbial N limitation by 44–71%, driving a 291–467% increase in SOC stocks, from 0.81 to 3.15 kg m−2 in topsoil and 0.54 to 3.08 kg m−2 in subsoil. It also reduces CUEST by 54% in topsoil and 34% in subsoil, boosting POC by 483–557% and MAOC by 621–1,071%. MAOC dominates SOC accumulation, exceeding POC by 2.3–7.2 times. The CUEST reduction aids POC transformation into MAOC, stabilizing SOC storage. In contrast, passive restoration slightly reduces N limitation by 36–39% and CUEST by 10–23%, but failed to enhance C fractions or SOC stocks due to persistent nutrient constraints. The findings demonstrate that alleviating microbial N limitation by active restoration is critical for stabilizing SOC through MAOC accumulation.
