Contribution of the ratoon sugarcane with planting years to river pollution evidenced from the four-year watershed observation

Abstract

Non-point source pollution water erosion caused by sugarcane planting on sloping land is a serious environmental problem in tropical and subtropical regions. The adjustment of sugarcane planting patterns (ratio of perennial to new planting areas) will affect slope soil erosion. However, the relationship between the contribution of sugarcane land to river pollution load and the planting years of ratoon sugarcane remains unclear. To quantitatively analyse the contribution of sugarcane fields to eroded sediment nitrogen (N) and phosphorus (P) loads into the river in different ratoon years and growing seasons, this study was conducted in the intensive sugarcane cultivation area of the southern subtropical Guangxi Nala sub watershed, using the specific compound specific stable isotope (CSSI) traceability technology and real-time monitoring in the watershed. The changes of N and P loading of sugarcane soil erosion and sediment into the river was determined at different ratoon years (RS₀-RS₃) in addition to the related influencing factors. The results showed that the influence of different ratoon years on the N and P input loads of eroded sediment from sugarcane fields was significantly greater than that of runoff. The contribution of different ratoon sugarcane planting years (including root density) to the loads of sediment N and P into the river was 67.1 % (p < 0.01), which was 2.8 times that of runoff (24.3 %) (p < 0.05). With increasing ratoon sugarcane planting years, the N and P loads of sediment into the river from the sugarcane land decreased significantly; the ratoon sugarcane planting years showed a highly significant negative correlation with the N loads of sediment into the river from the sugarcane fields (p < 0.01), and a significant negative correlation with P (p < 0.05) was observed. Newly planted sugarcane contributed the most to the N and P sediment loads into the river at the seedling stage and the least at the maturity stage. Therefore, in the intensive sugarcane planting areas, to effectively control erosion-induced non-point source pollution, perennial sugarcane planting areas should be rationally in the newly planted sugarcane fields, especially at the seedling stage, to increase land surface cover.