Objectives Purple sloping cropland is characterized by shallow tillage, poor structural stability, and substantial nitrogen loss, which affects crop yields. We conducted a field experiment of optimized fertilization in conjunction with straw return to the field in a typical purple soil, to determine its effectiveness in preventing agricultural non-point source pollution, and improving crop yields on purple sloping croplands.

Methods The field experiment was conducted from 2019−2021 in a typical purple sloping cropland of Chongqing, with the cropping system of sorghum-rape rotation. Three treatments of conventional fertilization (CK), optimized fertilization (IFT) and optimized fertilization + straw return (IFT+SMT), were set up to investigate the yields of sorghum and rape, and the contents of runoff total nitrogen (TN), nitrate nitrogen (NO₃⁻-N), and ammonium nitrogen (NH₄⁺-N), and to analyze the nitrogen loss characteristics.

Results The rape yield was significantly increased by 30.70% and 26.34% in 2019 and 32.03% and 27.48% in 2020 for IFT+SMT and IFT treatments, respectively, compared with CK (P<0.05). Heavy and above class rainfall events generally resulted in higher runoff volumes and elevated concentrations and losses of TN, NO₃⁻-N, and NH₄⁺-N in runoff compared to light and moderate rainfall events. Under the same rainfall conditions, the IFT+SMT treatment exhibited lower runoff depth and runoff coefficient compared to the CK and IFT treatments. In 2019−2021, the NO₃⁻-N concentrations of the IFT+SMT treatments all decreased compared to CK, but the differences were not significant. The concentrations of TN, NO₃⁻-N, and NH₄⁺-N in runoff were not significantly different among treatments in the same year. Significant differences in NH₄⁺-N concentrations existed only for the IFT+SMT treatments under different years (P<0.01), the NH₄⁺-N concentrations in 2021 being 39.55% and 49.70% lower than those in 2019 and 2020, respectively (P<0.01). In 2019, compared to CK, the annual cumulative loss of TN and NO₃⁻-N was significantly reduced by 10.19% and 17.72% for IFT+SMT treatment, respectively (P<0.05). In 2021, compared to CK, the annual cumulative loss of TN, NO₃⁻-N, and NH₄⁺-N in the IFT+SMT treatment was significantly reduced by 21.65%, 19.12%, and 33.24%, respectively (P<0.01). However, the annual cumulative TN, NO₃⁻-N, and NH₄⁺-N losses in 2020 were significantly increased in the IFT+SMT treatment compared to CK by 39.56%, 51.10%, and 40.42%, respectively (P<0.01). The IFT+SMT treatment was effective in reducing NO₃⁻-N losses. Sorghum and rape yields were all significantly and positively correlated with runoff and annual cumulative NH₄⁺-N losses (P<0.05).

Conclusions The runoff coefficient, TN and NO₃⁻-N concentration and loss were reduced in the IFT+SMT treatment, especially the blocking and controlling effect on NO₃⁻-N loss was more prominent, in addition, the treatment significantly increased the yield of rape and sorghum, increased crop uptake and utilization of nitrogen, thus effectively reducing nitrogen loss.