Straw returning on sloping farmland reduces the soil and water loss via surface flow but increases the nitrogen loss via interflow

The eutrophication caused by nitrogen loss from sloping farmland is a serious concern, especially in the context of increasing frequency of extreme rainfall events. The relative importance that the surface runoff and interflow processes govern the nitrogen loss under extreme rainfall events, however, is ambiguous. Moreover, this ambiguity could be further enhanced by conservation practices on sloping farmland, such as straw returning and contour tillage. To better understand these ambiguities, five simulated rainfall experiments at the intensity of 100 mm h−1 under four treatments including downslope tillage (DT), cross-slope tillage (CT), cross-slope tillage with whole straw returning (CT + WR), cross-slope tillage with crushed straw returning (CT + CR), were conducted in 2 years' maize season on a typical purple sloping farmland in the hilly area of Sichuan, China. Results showed: 1) Compared with DT, straw returning and cross-slope tillage significantly reduced surface runoff and nitrogen load of surface runoff. However the concomitant potential increase of the interflow runoff would result in the increase of nitrogen loss via interflow, offsetting the benefits of conservation practices; 2) The nitrogen loss through interflow is 6.38 ± 0.21 kg ha−1, accounting for 77.9 % of the total nitrogen loss and suggesting that interflow is the dominant process; 3) Dissolved organic nitrogen is one of the main nitrogen loss forms, accounted for 41.06 % (24.31–57.69 %) of the total nitrogen loss in surface runoff and 32.02 % (12.28–47.81 %) for interflow, should not been ignored; 4) The results of the three prediction models showed that the nitrogen loss caused by interflow drainage under extreme rainfall should not be ignored. These findings enhance our understanding of nitrogen exports induced by extreme rainfall events and provide references for nitrogen loss predictions and control in sloping farmland.