Background and aims <p>The incorporation of crop residues into cultivated soils is widely adopted to improve soil quality of degraded lands. However, the understanding of how soil organic carbon (SOC) mineralization responds to consecutive residue return and the underlying microbial mechanisms remains limited.</p> Methods <p>Peanut shell or its derived biochar was annually applied at low and high rates to degraded upland red soils under a sweet potato system over two years, to assess changes in SOC mineralization, soil biochemical characteristics, microbial activities, and crop yield.</p> Results <p>The second-year application of high biochar generated a more pronounced increase in SOC mineralization compared to peanut shell application. This result was associated with lower soil nitrogen (N) availability in biochar-amended soils, which increased microbial demand for N from soils. In addition, the first-year addition of biochar improved stable microbial biomass C in soils, leading to microbes living in a resource-limited environment. The second-year addition of biochar could then activate the starving community to mine SOC by providing sufficient resources. In contrast, the first-year addition of peanut shell increased labile dissolved organic C in soils, leading to less resource restriction. The second-year addition of peanut shell thus had small stimulation on labile SOC mineralization. Moreover, biochar applications improved bacterial abundance which drove strong SOC mineralization. Peanut shell applications increased fungal abundance which dominated SOC mineralization.</p> Conclusion <p>The annual high amounts of biochar application may lead to greater soil C release than raw crop residues within two years, providing new insights for optimizing agricultural management.</p>