Concentrations and Stoichiometric Characteristics of C, N, and P in Purple Soil of Agricultural Land in the Three Gorges Reservoir Region, China

Soil stoichiometry is an essential tool for understanding soil nutrient balance and cycling. Previous studies have recognized that some relationships were observed between particle size and carbon and nitrogen parameters. This study attempted to evaluate nutrient element concentrations and their stoichiometric ratios of surface soil (0–10 cm) under different land use types (forest, sloping arable land, paddy fields, and orchards). and different particle sizes (<32 µm, <63 µm, and <125 µm) from a small typical hilly catchment (0.35 km 2 ) in the Three Gorges Reservoir Region of China. The contents of soil organic carbon (SOC), total nitrogen (TN). and total phosphorus (TP) were measured, and the ratios of C:N, C:P, N:P were calculated. The results indicated that land use type and soil particle size have diverse impacts on the studied indexes (SOC, TN, TP, C:N, C:P, and N:P). Six indexes were significantly affected by land use type ( p < 0.01), while only C:N ratio was statistically influenced by soil particle size ( p < 0.05). Furthermore, several significant differences of studied parameters of four land use types grouped within three particle sizes were found. The concentrations of SOC (12.34~13.46 g kg −1 ), TN (1.27~1.59 g kg −1 ), and TP (0.71~0.92 g kg −1 ) in the study site were lower than the national average values of China. Moreover, the productivity in the study area was mainly limited by TN concentration. Additionally, the concentration of TP decreased obviously with the increase in particle size. Furthermore, various coupling relationships were validated by linear and nonlinear fitting among different indexes. At the small catchment scale, take forest as a reference, human activities have significant impact on C-N-P stoichiometry ( p < 0.05). Especially, tillage may reduce SOC and TN contents, leading to a decline in soil quality. Overall, our findings can provide a basis for rational utilization and sustainable development of land resources.