Background Soil erosion has been the most serious environmental problem around the world, which leads to the losses of soil nitrogen (N) and phosphorus (P) in sloping farmland, resulting in dramatically non-point source pollution. Purple soil is widely distributed in southwestern China and is easily eroded owing to its relatively fast weathering rate of purplish parent materials. Sloping farmland is an important grain planting strategy in purple soil area, which usually experiences cross ridge and combined application of organic fertilizer and other agricultural measures to control soil erosion. This work is to determine the effects of different farming treatments on N and P losses in purple sloping farmland.

Methods Three rainfall intensities (60, 90 and 120 mm/h) were adopted to conduct the rainfall simulations on runoff plots (length 8 m×width 4 m) located in a 15 ° purple sloping farmland.The changed processes of runoff and sediment yield and the characteristics of nitrogen and phosphorus losses were analyzed with three farming treatments including longitudinal ridge with single application of chemical fertilizer (T1, control), longitudinal ridge with chemical fertilizer and organic fertilizer (T2), and cross ridge with single application of chemical fertilizer (T3). One-way ANOVA and regression analysis were used to analyze the relationship between indicators.

Results 1) The losses of N tended to be first increase and then maintained a certain volatility after reaching the peak, while the losses of P increased first and then declined after reaching the peak. 2) The different farming treatments showed statistically significant effects on runoff coefficient, sediment yield, and N and P losses under most conditions (P < 0.05), except for the runoff coefficient (P>0.05) under the 90 mm/h intensity. 3) T2 and T3 significantly reduced the losses of N and P in comparison to T1 under different rainfall intensities, especially in lower rainfall intensity. The N and P reduction ratio were 19.36%-62.19% and 20.03%-58.01% for T2, and were 47.09%-78.45% and 50.97%-82.27% for T3, respectively. 4) Both N and P losses showed significantly positive power correlation with runoff coefficient and sediment yield (R² is 0.787-0.893).

Conculsions The longitudinal ridge with chemical fertilizer and organic fertilizer on the cross ridge with single application of chemical fertilizer can effectively reduce soil erosion and N and P losses in purple sloping farmland. These measures have significant effects on the prevention and control of non-point source pollution in purple sloping farming, and effectiveness of the cross ridge with single application of chemical fertilizer is more stable than the longitudinal ridge with chemical fertilizer and organic fertilizer.