【Objective】Soil organic carbon (SOC) is one of the key factors influencing crop yield in the Shajiang black soil region, and straw returning is an effective method for continuously improving SOC. However, the accumulation characteristics of plant- and microbial-derived carbon and their relative contribution to SOC under different straw returning methods (no-tillage with straw returning, NTS; rotary tillage with straw returning, RTS; deep ploughing with straw returning, DPS) remain poorly understood. Therefore, this study aims to explore these characteristics and contributions of plant- and microbial-derived carbon in 0~10 cm, 10~20 cm and 20~40 cm soil layers of Shajiang black soil under different straw returning methods. 【Method】A seven-year field experiment was conducted using lignin phenols and amino sugars as biomarkers. Mixed soil samples were collected from depths of 0~10, 10~20, and 20~40 cm. The content of plant- and microbial-derived carbon and their contributions to SOC were calculated based on the biomarkers content. 【Results】The results revealed that, RTS and DPS significantly increased the SOC content of each soil layer, with an increase of 113% (P < 0.05). In contrast, the effects of NTS on SOC were mainly concentrated in the 0~10 cm layer after seven years, showing a phenomenon of surface accumulation in SOC. There was no significant difference in lignin phenol content between NTS and RTS (P > 0.05), however, in the 10~20 cm and 20~40 cm layers under DPS treatment, lignin phenol content increased by 57.3% and 36.3%, respectively (P < 0.05), despite a marked decrease in the 0~10 cm layer (P > 0.05). Additionally, the relative contents of Vanillyl (V) and Syringyl (S) phenols under DPS were significantly increased (P < 0.05) and the degree of oxidative degradation of lignin in the 10~40 cm layer was lower than that under NTS and RTS. Furthermore, amino sugar content showed no significant difference between NTS and RTS in each soil layer. However, under DPS, amino sugar content in the 10~20 cm and 20~40 cm layers increased by 45.6% and 35.8% in comparison with RTS, respectively (P < 0.05). The variation trend of Glucosamine (GluN) and Galactosamine (GalN) with soil depth was similar to amino sugar content, but in the 0~20 cm layer, Muramic acid (MurN) content under NTS and DPS was lower than that of RTS, with a highest decrease of 47.2%. Interestingly, DPS promoted the transformation of microbial community towards fungi, with the carbon ratio of fungal necromass to bacterial necromass in the 10~20 cm and 20~40 cm layers increasing by 177% and 58.0%, respectively, compared to RTS (P < 0.05). 【Conclusion】The substantial increase in SOC content primarily results from a significant rise in crop residue content observed in the topsoil (0~10 cm) under NTS and RTS, as well as in the deeper soil (10~40 cm) following DPS. Our findings suggest that DPS promotes the accumulation of plant- and microbial-derived carbon in deeper soil layers, increases the contribution of microbial carbon to SOC and enhances the stability of carbon pool, which is crucial for the efficient utilization of straw resources and improvement of soil quality in Shajiang black soil region.