Role of environmental factors on concentrations and ratios of subsoil C–N–P in subtropical paddy fields

Purpose Environmental controls of soil organic carbon (SOC), total nitrogen (TN), and total phosphorus (TP) in the subsoil are important for understanding nutrient cycles; however, these are poorly understood in subtropical paddy soils, especially lacking systematic regional soil profile investigations in the subsoil. The study aims to evaluate the role of environmental factors on concentrations and ratios of SOC, TN, and TP in subsoil (20–100 cm) vs. topsoil (0–20 cm). Methods We sampled 166 soil cores (1-m depth) from paddy fields across 10 typical counties in 4 provinces of subtropical China. Soil properties of SOC, TN, TP, pH, bulk density, and clay were determined in each soil layer. Spearman correlation analysis and random forest model were performed to correlate environmental factors with soil nutrients and to rank their relative importance throughout the soil profile. Results SOC was strongly coupled with TN and both decreased with soil depth. However, TP was high in topsoil and did not change in the subsoil. Thus, the soil C to N ratio (12.88–14.93) exhibited minimal variation, whereas N to P (30.93–68.99) and C to P (2.52–5.59) ratios decreased markedly with the soil depth. As compared with topsoil, subsoil revealed similar dominant negative effects of climate (temperature, precipitation) and bulk density on the concentrations and ratios of SOC, TN, and TP. The soil clay had no effect on SOC and TN, but was the dominant control on TP throughout the soil profile. The influences of straw incorporation and chemical fertilization on SOC and TN were primarily observed in topsoil. Conclusions There were dominant climate controls (temperature and precipitation) of soil C–N–P concentrations and ratios in the subsoil, although different soil factors control SOC and TN vs. TP. These findings highlight the importance of managing subsoil nutrients in subtropical paddy soils, since they are consistently sensitive to climate as topsoil nutrients.