Effects of gravel contents on the soil nitrogen and phosphorus loss of sloping farmland in Three Gorges Reservoir of China

Sloping farmland is one of the most main sources of soil and water loss in the Three Gorges Reservoir area. A large number of soil nutrients are lost with soil erosion, especially nitrogen and phosphorus in the soil. Water eutrophication and secondary river pollution have posed a serious threat to the ecological environment and sustainable reservoirs. The sloping farmland soil in the Three Gorges Reservoir is also characterized by the outstanding shallow and gravelly. However, it is still lacking the soil erosion and nutrient loss of gravel-containing soil on sloping farmland. Taking the gravel-containing soil on sloping farmland as the research object, this study aims to clarify the effects of gravel content on the soil nitrogen and phosphorus loss of sloping farmland. Artificial simulated rainfall experiments were conducted at three rainfall intensities (60, 90, and 120mm/h) and four gravel contents with different mass proportions (0, 10%, 20%, and 30%). The results indicated that the gravel promoted the runoff and sediment production by altering soil structure, thus increasing the nitrogen and phosphorus loss in soil. But there was also an outstanding effect on sediment yield. The runoff and sediment yield played an important intermediary role in the rainfall-induced soil nitrogen and phosphorus loss in the gravel-containing sloping farmland, although the primary pathway remained lost with the sediment. The gravel content was dominated by the low impact on the transform in the concentrations of nitrogen and phosphorus loss. The loss concentration under different gravel contents showed a trend of linear decline in a short period, and finally fluctuated within a small range. The higher coefficient of variation was found in the sediment yield under different gravel contents. There was also a more significant effect of gravel content on the nitrogen and phosphorus loss of sediment, compared with the runoff. Nitrogen and phosphorus were mainly lost with the sediment in the form of total phosphorus and nitrogen. The active components were accounted for a relatively small portion (less than 15%). Relatively speaking, the loss of the active ingredients with the runoff accounted for a higher proportion of the total. The amount of nitrogen loss in the runoff was about 10 times that of phosphorus. A large amount of nitric nitrogen accounted for about 70% of the available nitrogen loss. The cumulative sediment yield and the loss of nitrogen and phosphorus elements with the erosion reached the maximum at 20% gravel content, whereas, the loss was relatively small at 10% gravel content. Some suggestions were given to reduce the erosion sediment in the control of the nitrogen and phosphorus loss. Much more attention must be paid to water monitoring and fertilization control during heavy rainfall seasons. In addition, the gravel in the soil can be removed to keep the level less than 10%. There was a significant effect of the gravel on soil erosion and the nitrogen and phosphorus loss on the sloping farmland. The finding can provide the scientific reference to control the soil and water loss, as well as the nitrogen and phosphorus loss of gravel-containing soil on sloping farmland in the Three Gorges Reservoir.