To investigate the effects of different fertilization regimes on organic carbon content and the contribution characteristics of carbon from different sources in Shajiang black soil, a long-term fertilization experiment was conducted over nine consecutive years. Treatments of five fertilization strategies were established: no fertilization (CK), chemical fertilization alone (NPK), chemical fertilization with straw return (NPK+S), commercial organic fertilizer with straw return (COF+S), and combined application of commercial organic fertilizer and nitrogen fertilization with straw return (N+COF+S). The study analyzed the distribution of soil organic carbon (SOC), particulate organic carbon (POC), and mineral-associated organic carbon (MAOC) in the 0-10 cm and 10-20 cm soil layers, as well as the contributions of carbon from different sources. The results showed that in the 0-10 cm layer, compared with CK, the SOC content increased by 25.2%, 100.0%, and 72.1% (P＜0.05) under NPK+S, COF+S, and N+COF+S treatments, respectively. In the 10-20 cm layer, only the COF+S treatment significantly increased SOC content by 38.3% (P＜0.05). In both soil layers, the COF+S treatment increased the mass percentage of POC by 53.6% and 30.4% (P＜0.05), respectively. All fertilization treatments enhanced the contribution of POC to SOC (P＜0.05) while reducing the contribution of MAOC (P＜0.05). In the 0-10 cm layer, compared with CK, the four fertilization treatments increased the C content of POC by 115.9%, 155.6%, 266.2%, and 197.4% (P＜0.05), respectively. For the MAOC fraction, NPK+S, COF+S, and N+COF+S treatments increased the C content by 14.9%, 56.2%, and 39.0% (P＜0.05), respectively. In the 10~20 cm layer, NPK+S, COF+S, and N+COF+S treatments significantly increased the C content of POC by 148.3%, 275.8%, and 194.5% (P＜0.05), respectively, but only the COF+S treatment increased the C content of MAOC by 27.4% (P＜0.05). Across all treatments in the 0~20 cm soil layer, fungal-derived carbon consistently represented the primary source contributing most significantly to SOC. In the 0-10 cm layer, compared with CK, all four treatments reduced the contribution of bacterial-derived carbon to SOC by 5.1%, 4.3%, 24.3%, and 18.3% (P＜0.05). Compared to CK, all four fertilization treatments increased the contribution of plant-derived to SOC in the 0~20 cm layer, with COF+S showing the highest increase 30.2% (P＜0.05) in the 0-10 cm layer and 10.2% (P＜0.05) in the 10-20 cm layer. In both soil layers, SOC fractions and the contributions of plant-derived and microbial-derived carbon were significantly positively correlated with SOC (P＜0.05). Correlation analysis revealed that across both soil layers and all treatments, SOC fractions exhibited statistically significant positive correlations with plant-derived carbon, and significant negative correlations with fungal- and bacterial-derived carbon (P＜0.05). The study demonstrated the combined application of commercial organic fertilizer with straw return enhanced the contribution of plant-derived carbon to SOC in both soil layers of Shajiang black soil. This enhancement reached statistical significance (P＜0.05) when the commercial organic fertilizer was applied at the adequate rate of kg/(hm²·a). Therefore, this combined practice effectively enhanced the carbon pool level in Shajiang black soil. This was achieved by enriching the soil with organic materials, which suppressed bacterial transformation of exogenous carbon, thereby preserving more plant residues in the form of POC within the soil.