Spatial distribution patterns and influencing factors of soil carbon, phosphorus, and C:P ratio on farmlands in southeastern China

Soil organic carbon (SOC), total phosphorus (STP), and the carbon-to-phosphorus ratio (C:P) are important indicators of soil fertility. Knowledge regarding the spatial patterns of SOC, STP, and C:P and their influencing factors are of great significance for precise fertilization, soil management, food security, environmental protection, and global climate change. A total of 16,582 topsoil samples (0–20 cm depth) were collected from farmland in Jiangxi Province, southeastern China. The spatial variability and driving factors of SOC, STP, and C:P were characterized using classical statistics, geostatistical analysis, and random forest (RF) methods. The results showed that the mean values of SOC, STP, and C:P were 17.90 g kg−1, 0.52 g kg−1, and 38.29, and their coefficients of variation were 30.98%, 33.49%, and 45.08%, respectively, indicating a moderate degree of variability. The RF model contained 47 explanatory variables related to parent materials, soil types, soil physicochemical properties, environmental variables, climate, farmland use types, straw returning methods, and fertilizer application rates, which accounted for 72.70%, 60.89%, and 63.71% of the spatial variability in SOC, STP, and C:P, respectively. Soil physicochemical properties had the most significant influence on SOC, STP, and C:P, with contributions of 45.83%, 50.93%, and 51.06%, respectively. Of all the factors, soil total nitrogen was identified as mostly driving the spatial distributions of SOC, STP, and C:P (variable importance of 29.39%, 11.72%, and 23.95%, respectively), followed by the nitrogen application rate (variable importance of 20.90%, 11.37%, 12.79%, respectively). Therefore, conservation tillage, crop rotation, straw return, combined application of nitrogen and phosphorus, and green manure application are the most effective measures for improving carbon sequestration and phosphorus availability. According to the ecological stoichiometry characteristics of carbon, nitrogen, and phosphorus and their temporal variation trends, the farmland soils in Jiangxi Province are typical carbon sink systems that are likely to be limited by phosphorus. The results of this study provide important insights for assessing soil carbon and phosphorus pools and sustainable agricultural development.