【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 at 0-10, 10-20 and 20-40 cm soil depths 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 depth, with an increase of 113% (P < 0.05). In contrast, the effects of NTS on SOC were mainly concentrated at the 0-10 cm depth 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, at the 10-20 cm and 20-40 cm depths under DPS treatment, lignin phenol content increased by 57.3% and 36.3%, respectively (P < 0.05), despite a marked decrease at the 0-10 cm depth (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 at the 10-40 cm depth was lower than that under NTS and RTS. Furthermore, amino sugar content showed no significant difference between NTS and RTS at each soil depth. However, under DPS, amino sugar content at the 10-20 and 20-40 cm depths 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 at the 0-20 cm depth, 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 at the 10-20 and 20-40 cm depths 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 at deeper soil depths, 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.