Building soil organic carbon (SOC) is influenced by both carbon input from C3 and C4 plants and their decomposition rates, which are linked to soil properties and environmental conditions. However, limited understanding of the mechanisms behind C3/C4-derived SOC accumulation poses challenges for enhancing SOC levels in farmlands. The significant difference in δ13C values between C3 and C4 plants provides a basis for distinguishing their contributions to SOC content. This study aims to disentangle the contribution of soil properties (such as soil pH, texture, SOC, and total nitrogen) as well as mean annual precipitation (MAP) and mean annual temperature (MAT) to C3- vs. C4-derived SOC using 68 soil samples collected from 32 Chinese farmland sites. The results showed that an increase in SOC content was accompanied by a significant rise in C3-SOC, whereas C4-SOC began to decline once SOC exceeded 27-35 g kg-1. SOC content was directly influenced by MAT (path coefficient -0.53), C/N (0.43), MAP (0.34), and sand content (-0.19). For δ13C, the direct influences were MAP (-0.52), MAT (0.44), and pH (0.28). Additionally, C3-SOC was directly affected by MAT (-0.61), MAP (0.42), pH (-0.29), and C/N (0.22), while C4-SOC was only directly affected by C/N (0.57) and pH (0.38). In conclusion, as soil C/N increased, SOC contents increased, including both C3- and C4-derived SOC fractions. Meanwhile, with increasing pH, C3-derived SOC declined, while C4-derived SOC increased. These findings indicate that C/N ratio and soil pH are key regulators of plant-derived SOC accumulation, suggesting that pH regulation may shift the balance between C3- and C4-derived SOC, which has important implications for long-term carbon storage strategies in agricultural soils.