Sprinkler-based rainfall simulation experiments to assess nitrogen and phosphorus losses from a hillslope cropland of purple soil in China

Currently, water eutrophication has become a serious environmental pollution problem in China due to large amounts of nutrients input into Chinese rivers every year. In order to better understand the mechanisms of nitrogen and phosphorus transport induced by agricultural non-point pollution, we conducted simulated rainfall experiment on a hilly cropland of purple soil under different levels of rainfall intensity and cropland gradient. The dimensions of the experimental plot are: length × width × depth = 4.5 m × 1.5 m × 0.6 m, wherein cropland gradient can be adjusted. The results showed that the discharge process of surface and subsurface runoff was significantly enhanced with the elevated cropland gradient and simulated rainfall intensity. The total phosphorus (TP) transported by runoff was significantly increased with the elevated cropland gradient and simulated rainfall intensity, and accompanied by irregular fluctuation. The discrepancy of TP transport caused by rainfall intensity and cropland gradient was smaller by subsurface runoff than by surface runoff. The transport concentration of total nitrogen (TN) was higher for the subsurface runoff than for surface runoff. Due to different rainfall intensity and cropland gradient, the discrepancy on TN transport was larger by subsurface runoff than by surface runoff. In current study, TN transport by surface runoff was observed between 3.5 and 5.5 mg/L, but the concentration of TP is close to 100 g P/L under rainstorm events, which is the threshold for P eutrophication. The large nutrients losses for local farmers would be an important non-point pollution source to the Three Gorges Reservoir area that receives runoff from the watershed.