Characteristics of runoff and nitrogen, phosphorus, and carbon loss in sloping cultivated lands in the typical hilly mountainous region of the Upper Yangtze River Basin

Continuous rainfall on the slope may yield surface runoff and subsurface flow. Surface runoff can transfer a large amount of nutrients in the topsoil (usually 0-20 mm thick) from the soil to the receiving environment through dissolution, erosion, and transportation. The change of rainfall intensity and slope can affect the intensity of slope runoff, thus changing the concentration and flux process of total nitrogen (TN), total phosphorus (TP), dissolved organic carbon (DOC) and other nutrients in runoff. A rainfall experiment considering three typical rainfall intensities (40, 60, 90 mm/h) and slopes (6°, 12°, 18°) was conducted, and the characteristics of runoff and the loss of TN, TP, and DOC on purple soil slopes in hilly areas under different conditions were explored. The results showed that: (1) The critical slope gradient for runoff was not a fixed value. When the rainfall intensity was 60 mm/h or less, the critical slope gradient ranged from 6° to 18°. However, no distinct critical slope was observed when the rainfall intensity reached 90 mm/h. (2) Under the same rainfall intensity, the concentration of TN and DOC in runoff was positively correlated with the slope gradient (18°>12°>6°). However, at a rainfall intensity of 90 mm/h, the relationship between TP concentration in runoff and slope was 6°>12°>18°, whereas under other rainfall intensities, it was 12°>18° >6°. With the exception of the 12° slope, the loss of DOC increased as rainfall intensity increased. The maximum loss of TP occurred on the slope of 6° under a rainfall intensity of 90 mm/h, reaching 0.91 mg/m², belonging to the source limitation type. In contrast, TP losses on other slopes belonged to the transport limitation type. (3) The runoff and sediment yield on the slope were primarily influenced by rainfall intensity. Significant correlations existed between runoff and rainfall intensity, as well as between runoff and the losses of TP and DOC. The correlation between the slope gradient and the losses of carbon, nitrogen, and phosphorus was not significant. However, a significant correlation was observed between the loss of TN in runoff and the losses of TP and DOC. Comprehensive research showed that under the combination of high-intensity rainfall and a small slope gradient, the runoff, TP and DOC loss were significantly higher than the other combinations of rainfall intensity and slope. In contrast, in the combination of heavy rainfall intensity and a steep slope gradient, TN loss reached its peak. Therefore, special attention needs to be paid to the loss of TP and DOC in purple soil on small slopes under heavy rainfall and the loss of TN on steep slopes under heavy rainfall.