Distribution characteristics of soil carbon, nitrogen, and phosphorus stocks in the plain-hilly transition zone of the middle-lower Yangtze River: a case study of five soil types in Jiayu County.

Changes in land use in the plain-hilly transition zone in the middle and lower reaches of the Yangtze River have altered soil nutrient cycling, but the contribution of subsoil layers remains insufficiently understood. This study focused on Jiayu County and used 422 topsoil samples and 0-1.20 m soil-profile data to quantify carbon, nitrogen, and phosphorus stocks and stoichiometric characteristics across five soil types. The results showed that the 0.20-1.20 m subsoil layers stored more than 60% of the regional nutrient inventory, indicating that ignoring subsoil layers can substantially underestimate regional nutrient stocks. Soil nutrient patterns reflected the combined effects of land use, parent material, and geomorphic setting: paddy soil had high carbon and nitrogen stocks, alluvial soil was enriched in phosphorus, and red soil represented a low-nutrient background pool. After conversion to molar units, the average topsoil C:N:P ratio was approximately 37.4:3.5:1, suggesting stoichiometric imbalance rather than direct evidence of phosphorus saturation. Available phosphorus and pH were key factors associated with spatial heterogeneity. These findings indicate that differentiated management, including phosphorus control in paddy and alluvial soils and carbon enhancement in red soil, may support nutrient balance and sustainable agricultural management in plain-hilly transition zones.