Effects Of Sugarcane Planting Patterns On Soil Erosion-induced Nutrient Loss Into The River In Nala Watershed

Globally,large-scale monoculture sugarcane planting on slopes,particularly replanted sugarcane（Rp）has accelerated soil erosion and associated sediment nutrients entering the river,causing serious water pollution.It is crucial to reduce the negative effects caused by erosion in sugarcane plantation,and establish non-point source pollution prevention and control measures to ensure sustainable agricultural development and good drinking water in this region.In this study,Nala watershed in Fu Sui County,Guangxi was selected as the study area.We measured extreme rainfall erosion-induced nutrients losses on the upslopes with different area ratios of Rp and the delivery ratio on the downslopes with different intercropping（Ic）ratios by employing beryllium-7（⁷Be）technique.We estimated the export loads and coefficients of eroding sediment nutrients from the watersheds with different spatial configuration of Rp and Ic ratios by combinative use of ⁷Be,compound-specific stable isotope（CSSI）and watershed monitoring,and explored the effects of sugarcane planting patterns（SPPs）on soil erosion and nutrient loss.The main results and conclusions of this research are as follows:（1）Generally,⁷Be tracer results indicated that the upper slopes are soil erosion and associated nutrients loss area,and the lower slopes are eroded soil and nutrients deposition area.（2）Soil erosion and nutrient loss rates were positively correlated with the ratios of Rp in the upper slope（P<0.05）.Soil erosion,total nitrogen（TN）and total phosphorus（TP）loss rates of L_Rpwere 5.5 t ha^-1,6.9 and 2.8 kg ha^-1,respectively.Soil erosion and nutrient loss rates of L_Rpwere 69.1%,67.5%and70.8%lower than those of H_Rp.Compared with soil erosion and nutrient loss rates of M_Rp,those of L_Rpdecreased by 49.1%,47.7%and 49.1%.（3）Intercropping（Ic）reduced soil erosion and nutrients delivery ratios by 7.7%～23.4%in the lower slope.Delivery ratios of eroding soil and nutrient decreased with the increase of Ic level,although there was no significant difference between different Ic levels（P>0.05）.（4）According to the evidence of CSSI,more than 95%of the export pollution contributions to the river are from sugarcane slope,stream channel and road in Nala watershed.Different Sub-watersheds have different sources of export contribution.In the two upper sub-watersheds,the contributions of sugarcane slope,stream channel and road ranged from 24.5%～42.8%,23.7%～28.0%and 25.1%～50.4%,respectively.In the lower sub-watershed,80.2%of the pollution came from the two upper sub-watersheds.The monitoring results showed that the total export loads of sediment and associated TN and TP were 0.01～2.69 t ha^-1,0.02～4.39 and 0.01～1.16 kg ha^-1in different rainfall events in Nala watershed,respectively.（5）Combining the use of ⁷Be,CSSI and watershed monitoring renealed that eroding sediment and associated nutrients export load from sugarcane field ranged from 0.01 to 1.07 t ha^-1、0.01 to1.76 and 0.01 to 0.49 kg ha^-1,and that export coefficients ranged from 0.01%to 25.1%,respectively.Regulating SPPs could not only effectively mitigate erosion-induced nutrient losses on the slopes,but also reduce export loads and coefficients of eroding sediment and nutrients from the watershed.Keeping suitable ratios of Rp（30%）and Ic（15%）could intercept part of soil nutrients and preserve them in sugarcane land,reduce soil and associated nutrient losses and their export by increasing ground litter cover.