Study On Source Identification And Influencing Factors Of N&P Non-point Source Pollutant Load Into River In An Intensive Sugarcane Area

Guangxi is the most important sugarcane growing region in China,and sugarcane planting has an important contribution to the local sugar industry.However,intensive agricultural production in the sugarcane region,which uses excessive fertilizer and accelerated soil water erosion,leads to a large amount of nitrogen and phosphorus loss from farmland into rivers and lakes,which has become the most important contribution to surface water pollution and causes a series of ecological and environmental problems.In particular,soil erosion in sugarcane areas causes soil and nutrients loss with water migration,which has high temporal and spatial variability and is affected by many factors such as terrain,soil,rainfall,land use and hydrological connectivity.In runoff erosion areas,farmland nitrogen and phosphorus can transport into the downstream water only under the drive of rainfall and runoff erosion,and different types of water erosion（such as surface erosion,ditch erosion,road erosion,ditch erosion,etc.）lead to great differences in the form and load of nitrogen and phosphorus pollution into the river.However,the relative contribution of water erosion and fertilization to non-point source pollution is still unclear.Therefore,it is necessary to elucidate the key processes and factors of nitrogen and phosphorus loss,transport and inflow into the river,and to quantitatively analyze the load,path and key influencing factors of downstream surface water pollution caused by anthropogenic accelerated soil erosion in the upper reaches,which is very important for selecting the best measure for the prevention and control of non-point source pollution in the basin.In this study,real-time monitoring field experiment was conducted in Nala watershed of Kelan Reservoir basin in Zuojiang Basin in the upper reaches of Xijiang River（including two sub-watersheds S2 and S3 in the upper reaches and one sub-watershed S1 in the lower reaches）.The change of sediment contribution to the river before and after the expansion of the farmland from 2017 to 2018 was monitored,and the loss of nitrogen and phosphorus to the river from the farmland in the Nala watershed from 2018 to 2020 year was monitored.The contribution of land use to instream pollution was determined by using monomer stable compound isotope tracer technique（CSSI）.Nitrogen and oxygen bistable isotope tracer technology to determine the source of nitrate nitrogen in water.Monitoring technology-CSSI to determine the particle nitrogen and phosphorus into the river source load.Monitoring technology-nitrogen and oxygen bistable isotope tracer technology to determine the source load of dissolved nitrogen into the river.Using drones and monitoring to obtain vegetation coverage,planting patterns,land use and other information.In this study,the main aims were to answer three questions: 1)The influence of different land use types on sediment contribution to river before and after land expansion;2)The key process and element of nitrogen and phosphorus loss,transport and flow into rivers;3)quantitatively analyze the load,path and the key influencing factors of downstream surface water pollution caused by anthropogenic accelerated soil erosion in the upstream.The main results are showed as follow:（1）After FSI,the eucalyptus planting area decreased,sugarcane land area increased,new roads increased,and new ditches increased,resulting in significant differences in the sediment contribution ratio of each source to river inflow from 2017 to 2018 year,in which the contribution of eucalyptus,which accounted for the main area of the watershed,decreased.Increased contribution from sugarcane fields.The study shows that although the area of ditches is relatively small,they are indeed the main source of sediment contribution into the river.（2）Based on the study of sediment particulate nitrogen and phosphorus load into river and its influencing factors,the results show that the pollution load of particulate nitrogen and phosphorus（PN and PP）into river has decreased significantly during the observation period from 2018 to 2020 year.The pollution load of PN into the river is equivalent to 2.6-13.3% of the fertilizer applied in the basin（185-285 N kg/ha）,and that of PP is equivalent to 6.5-51.3% of the fertilizer applied in the basin（19.6-27.2 P kg/ha）.Compared with 2018,the pollution load of PN into the river in 2019 and 2020 decreased by 78% and 87% respectively.The pollution load of PP into the river decreased by 83% and 91%,respectively.（3）Based on CSSI traceability,we found that sugarcane slope,river bank and road were the main sources of particulate nitrogen and phosphorus pollution in the watershed.From 2018 to 2020,the contribution ratio of sugarcane slope land,ditches and roads to river sediment ranges from 28-66.4%,23-30.1% and 5.4-35%,respectively,which is 137.1% higher than that of 2018 and 2019.Ditches and roads reduced by 17.6% and 84.6% respectively.In 2020,the contribution of sugarcane slopes and ditches to rivers increased by 102.9% and 7.5%,respectively,while roads decreased by 81.4%.（4）The results show that during the observation period from 2018 to 2020,the pollution load of dissolved total nitrogen（TDN）,total phosphorus（TDP）and nitrate nitrogen（NO₃^--N）into river has decreased significantly.The instream pollution load of TDN is equivalent to 10.6-13.8% of the fertilizer applied in the basin（185-285 N kg/ha）.Compared with 2018,the pollution load in 2019 and 2020,TDN decreased by 44.3% and 51.1% respectively.The inflow pollution load of TDP decreased by 72.2%.The pollution load of NO₃^--N into the river decreased by 43.5% and 55.3% respectively.（5）The combined application of nitrogen and oxygen bistable isotope tracer technology and monitoring technology found that,compared with 2018,the NO₃^--N loading of fertilizer,soil and organic fertilizer in 2019 and 2020 decreased by 48.2% and 57.3%,38.1% and 48.5%,44.3% and 57.52%,respectively.Fertilizer,organic fertilizer and soil nitrogen are the main sources of nitrate nitrogen,accounting for 90% of the total load of nitrate nitrogen collectivelty.（6）The results showed that rainfall runoff was the main driving force leading to the loss of particulate nitrogen and phosphorus nutrients and the pollution load into the river,and the seasonal export of sediment and particulate nutrients was mainly driven by the comprehensive utilization of fertilizers,planting patterns and accelerated soil erosion.The output of dissolved nutrients varied with the growth stage of sugarcane,and was affected by the combined effects of surface runoff,fertilization,land cover and planting pattern.（7）The results showed that the source of nitrate nitrogen in water was greatly affected by fertilization factors,mainly from fertilizer and soil nitrogen.Manmade activities of agricultural intensive production such as fertilization,sugarcane planting management mode and field expansion are the main factors that increase the nitrogen and phosphorus loss and the pollution in the river.The purpose of this study was to explore the source and the influencing factors of nitrogen and phosphorus non-point source pollutants entering rivers in the intensive sugarcane area,and to establish a quantitative identification system of agricultural non-point source pollution entering rivers in the intensive agriculture area of Guangxi through the combined use of basin monitoring technology,CSSI tracer technology and nitrogen and oxygen dual isotope technology.It is indicated that rainfall runoff is the main driving force of nitrogen and phosphorus nutrient loss and pollution load in sugarcane area.It is clear that slope,road and river bank erosion areas are the main sources of nitrogen and phosphorus pollution in sugarcane basin.Fertilizer,organic fertilizer and soil nitrogen were the main sources of nitrate nitrogen.Agricultural intensive production and other human activities are the main factors that increase the nitrogen and phosphorus loss of slope and the pollution of the river.