Background and aims <p>Forest restoration would significantly influence terrestrial carbon sequestration. However, the stage-specific dynamics and compositional changes of soil organic carbon (SOC) during forest restoration remain poorly understood.</p> Methods <p>Four forest restoration stages with distinct age gradients on the eastern Qinghai-Tibetan Plateau were selected. Lignin phenols and amino sugars were employed as specific biomarkers to investigate the accumulation and contributions of plant detritus carbon (PDC) and microbial necromass carbon (MNC) to SOC.</p> Results <p>The results showed that forest restoration consistently increased MNC and SOC contents by 391.2–444.8% and 143.6–218.9%, respectively, in both soil layers, indicating the critical role of microbial proliferation-mortality processes in SOC accumulation. The MNC/SOC ratio was higher than the PDC/SOC ratio and exhibited a significant increasing trend with forest restoration, whereas the PDC/SOC ratio decreased markedly. This was primarily attributed to the increased soil nutrient levels accelerated microbial oxidation of PDC and MNC accumulation. Additionally, PDC, MNC and SOC contents in the subsoil were significantly lower than those in the topsoil. The PDC/SOC ratio increased significantly with soil depth, while the MNC/SOC ratio showed an opposite trend. Such vertical divergence may be related to suppressed microbial decomposition processing in deeper soil layers due to reduced BG enzyme activity and lower TN content.</p> Conclusions <p>Our findings highlight the importance of forest restoration in promoting soil C sequestration. Thus, conserving native climax species and facilitating natural regeneration represent an effective strategy for enhancing the terrestrial C sink in fragile subalpine forest ecosystems.</p>