Effects of Counter Tillage and Slope Gradient on Nutrient Losses on Sloping Farmland

Abstract

Counter tillage is a typical cultivation practice on the Loess Plateau, which can influence the soil erosion process by intercepting runoff and increasing infiltration. However, few studies have investigated the mechanisms of nutrient losses associated with counter tillage. This study was conducted to reveal the effects of counter tillage and slope gradient on the soil nutrient loss mechanism on sloping farmland. In this study, the rainfall simulation was conducted with a rainfall intensity of 90 mm.h−1 and with five slope gradients (5.24%, 8.75%, 17.63%, 26.79%, 36.40%). The runoff plots involved the counter tillage (CT) and traditional plow (CK), in order to investigate the characteristics of soil erosion and available phosphorus (AP), ammonium nitrogen (NH4+-N) and nitrate nitrogen (NO3−- N) losses. The soil erosion characteristics included the time until runoff generation, RR (runoff rate), and SR (sediment rate); the nutrient loss characteristics included nutrient loss concentrations, nutrient loss and nutrient loss rate in runoff or sediment. The results indicated that the RR and SR with a slope gradient of 5.24~26.79% on CT decreased by 11.77~94.92% and 20.69~99.27%, respectively, compared with that of CK. As the slope gradient increased (36.40%), a break in the ridge occurred and the reduction in the RR and SR was weakened; this was likely to be close to that of the CK. Nutrient losses differed significantly between different slope gradients and tillage practices. Nutrient losses increased with an increasing slope gradient. The nutrient losses of AP, NH4+-N, and NO3−-N in runoff, with the slope gradient of 36.40%, increased 75.75%, 76.34%, 75.63%; meanwhile, in sediment, it increased 32.93, 30.70, 32.18 times, compared with the slope gradient of 5.24% on CT. The CT with the slope gradient of 5.24~26.79% had a good effect in controlling nutrient losses; however, for the slope gradient of 36.40%, the effects of CT in controlling nutrient losses decreased. The nutrient loss rate and RR or SR satisfied a linear positive correlation. The reduction benefits of nutrient losses on CT in runoff and sediment can reach 57.7% to 100% and 45.5% to 100%, respectively. In conclusion, CT is an effective tillage practice to control soil erosion and nutrient losses. This study can provide a reference for soil erosion and nutrient loss control on sloping farmland on the Loess Plateau.