Nitrate loss by runoff in response to rainfall amount category and different combinations of fertilization and cultivation in sloping croplands

Nitrate (NO3-N) loss by runoff from agricultural lands causes substantial soil fertility loss and water quality degradation in downstream aquatic systems. Rainfall, fertilization and cultivation direction are key factors influencing NO3-N loss in runoff from sloping croplands; however, these effects are still elusive. The responses of runoff NO3-N and total nitrogen (TN) losses to rainfall amount, fertilization and cultivation direction were studied under more than 100 natural rainfall events occurring between 2008 and 2020 in sloping croplands. Five treatments (three replications for each) including CK (no fertilizer + downslope cultivation), T1 (combined manure and chemical fertilizers + downslope cultivation), T2 (chemical fertilizer + downslope cultivation), T3 (1.5-fold chemical fertilizer + downslope cultivation) and T4 (chemical fertilizer + contour cultivation) were deployed on fifteen plots (8 m long × 4 m wide for each). The results showed that runoff depth was 83.9–318 % higher in the large rainstorms than in the other rainfall events, and 29.3–77.8 % higher in the unfertilized plots (i.e., CK) than in the fertilized plots (i.e., T1, T2, T3 and T4). The runoff NO3-N concentration, on average, was significantly higher in the moderate and heavy rains than in the light rain and large rainstorms but did not differ in these five treatments. Conversely, the average TN concentration was not affected by the rainfall amount category but was significantly higher in the unfertilized plots than in the fertilized plots. Significantly higher losses of NO3-N and TN were found in the large rainstorms, implying the dominant role of runoff rather than nutrient concentrations. The unfertilized plots presented 50–100 % and 66.7–150 % higher NO3-N and TN losses, respectively, than the fertilized plots. The NO3-N: TN ratio differed markedly by rainfall amount category, and was higher in the contour cultivation (i.e., T4) than in the downslope cultivation (i.e., T2). Our results have great significance for understanding the dynamics of NO3-N loss by runoff driven by changeable rainfall features and intense agricultural management practices in sloping croplands.