Study on the Mechanism of Rainfall-Runoff Induced Nitrogen and Phosphorus Loss in Hilly Slopes of Black Soil Area, China

In order to identify the effects of the slope and precipitation intensity on the soil runoff depth and runoff rate, different tillage patterns (slope-ridge direction, horizontal slope-ridge direction, no-ridge farming) and different slopes (3° and 5°) were set up, and five typical rainfalls from June to September 2021 were selected, to dynamically monitor the soil-erosion dynamics of the test plots under different rainfall intensities. The results show that cross-slope-ridge cropping has a retention effect on runoff, which effectively inhibits the ineffective loss of rainfall confluence. Among these results, the variation range in the soil runoff depth under cross-slope-ridge treatment conditions was 0.11~0.94 mm, while that under the slope-ridge treatment and no-ridge treatment conditions was increased to 1.44~12.49 mm and 3.45~14.96 mm, respectively. It found that the loss of soil nutrients was significantly higher in the slope-ridge direction and in the no-ridge farming condition than in the horizontal slope-ridge direction. It is worth noting that, as the slope of the cultivated land increases, the erosive capacity of the precipitation runoff for the soil phosphorus increases, while the carrying capacity of the soil nitrogen decreases, and the correlation analysis results confirm that the corresponding relationship between the free diffusion capacity of the soil ammonium nitrogen and soil erosion is weaker than that between the nitrate nitrogen and soil erosion. The effects of single factors, such as the slope, ridge direction, and precipitation intensity of the cultivated land, have a significant impact on the soil water- and fertilizer-loss process, while the influence effect of the multi-factor coupling process on soil erosion is weakened. It was confirmed that the erosion process of rainfall runoff on soil nitrogen and phosphorus loss in slope cultivated land is the result of multi-factor action, and the artificial modification of the tillage mode can effectively regulate the effect of farmland water and fertilizer loss.