Control of sugarcane planting patterns on slope erosion-induced nitrogen and phosphorus loss and their export coefficients from the watershed

Globally, replanted sugarcane (Rp) on hillslopes has accelerated soil erosion and associated nutrients entering the river, causing serious water pollution. Sugarcane planting patterns (SPPs), including the area ratios of Rp and intercropping (Ic) on the hillslopes, may mitigate these negative impacts by increasing surface coverage. But the effectiveness of SPPs on these impacts has not been quantified at both hillslopes and watershed scale. We measured extreme rainfall erosion-induced nutrients losses on the upslopes with different area ratios of Rp and the deposition ratio on the downslopes with different Ic levels by using beryllium-7 (Be-7) technique. We estimated the export loads and coefficients of eroding sediment nutrients from the watersheds with different spatial configuration of Rp and Ic levels by combinative use of Be-7, compound-specific stable isotope (CSSI) and watershed monitoring. Nitrogen (N) and phosphorus (P) loss increased with area ratios of Rp, ranging from 6.9 to 21.2 kg ha(-1) event(-1) and 2.8-9.6 kg ha(-1) event(-1), respectively. Delivery ratios of eroding nutrients from the upslopes decreased with intercropping levels on the downslopes, ranging between 38.0% and 49.6%. Export loads and coefficient of eroding nutrients varied with the spatial configuration of Rp ratios and Ic levels in the watershed, ranging from 0.04 kg ha(-1) to 1.76 kg ha(-1) and 1.09-25.15%, respectively. The magnitudes of sediment nutrient losses on the upslope were positively correlated with area ratios of Rp (P < 0.05), whereas their delivery ratios on the downslope were negatively correlated with Ic levels, although they were not statistically significant (P > 0.05). The changes in ground litter cover (Glc) caused by Rp and Ic explained the variations of 53.6-55.3% in nutrient loss rate and of 74.1-74.4% in nutrient delivery ratio. Our results suggest that regulating SPPs could effectively mitigate erosion-induced nitrogen and phosphorus loss on the slopes and their export loads from the watershed.