Effects of rainfall characteristics and sugarcane growth stage on soil and nitrogen losses

High intensity rainfall in southern China has led to soil erosion on sloping farmland, causing serious ecological and environmental problems. But how the interaction of rainfall factors and growth stages influence soil erosion and nitrogen loss on sugarcane-cultivated slope under natural rainfall have not been studied considerably. This study concentrated on the in situ runoff plot observation test. Surface runoff, soil erosion, and nitrogen loss under individual natural rainfall events during the different sugarcane growth stages (seedling stage (SS), tillering stage (TS), elongation stage (ES)) from May to September in 2019 and 2020 were recorded and measured. The effects of rainfall factors (intensity and amount) on soil erosion and nitrogen loss were quantified by path analysis. The influence of rainfall factors and sugarcane planting on soil erosion and nitrogen loss was analyzed. Surface runoff, soil erosion, and nitrogen loss on sugarcane-cultivated slope were 4354.1 m³/ha, 155.4 t/ha, and 25.87 kg/ha during 2019 to 2020, and were mainly concentrated in SS, accounting for 67.2%, 86.9%, and 81.9% of total surface runoff, soil erosion, and nitrogen loss, respectively. Nitrogen losses were mainly concentrated in surface runoff, accounting for 76.1% of total nitrogen loss, and the main form in surface runoff was nitrate nitrogen (NO₃^--N, 92.9%). Under individual rainfall events, surface runoff, soil erosion, and nitrogen loss changed with the changing of rainfall characteristics and sugarcane growth. Surface runoff and nitrogen loss were obviously affected by rainfall characteristics, while the soil erosion and nitrogen loss were affected by both rainfall characteristics and sugarcane growth stages. Path analysis indicated that maximum rainfall intensities at 15 min (I₁₅) and 60 min (I₆₀) were most significant to the production of surface runoff and soil erosion with direct path coefficients of 1.19 and 1.23, respectively. NO₃^--N and ammonium nitrogen (NH₄⁺-N) losses in surface runoff were mostly influenced by maximum rainfall intensity at 30 min (I₃₀) and I₁₅ with direct path coefficients of 0.89 and 3.08, respectively. NO₃^--N and NH₄⁺-N losses in sediment yield were mostly influenced by I₁₅ and rainfall amount, and the direct path coefficients were 1.61 and 3.39, respectively. The main stage of soil and nitrogen loss was seedling stage, while the significant factors of rainfall characteristics affecting surface runoff, soil erosion, and nitrogen loss were quite different. The results provide theoretical support for soil erosion and quantitative rainfall erosion factors of sugarcane-cultivated slope in southern China.