Study On Non-point Source Pollution Characteristics Of Shipanqiu Small Watershed In Three Gorges Reservoir Area

The eutrophication of water is the main hazard caused by non-point source pollution.According to the ecological and Environmental Monitoring Bulletin of the Three Gorges Project of the Yangtze River in 2015,the water quality of the main stream of the Yangtze River is generally good,but the degree of eutrophication in several tributaries and some backwater areas is higher than that in previous years.Agricultural non-point source pollution is the main source of water eutrophication.As a relatively independent catchment unit,agricultural small watershed in the Three Gorges Reservoir Area is the source of non-point source pollution;nitrogen and phosphorus loss caused by surface runoff and soil erosion in small watershed is an important cause of non-point source pollution,which is also the key control object of non-point source pollution.Land use,a comprehensive reflection of human activities,which is the key factor affecting non-point source pollution,the combination of land use type and structure has important influence on non-point source pollution.It is a current research trend to study non-point source pollution from the perspective of overall optimization of land use pattern and local best management measures control.In recent years,the relationship between different land use types and non-point source pollution has been studied by artificial rainfall simulation,runoff plot test,watershed observation at different scales and model simulation,however,the long-term positioning observation of small watershed is relatively rare in natural rainfall conditions.Therefore,Shi pan qiu small watershed with typical land use type,soil type and topography in the three gorges reservoir area was selected as a basic hydrological unit to study the source and flux of non-point source pollutants.The runoff at the outlet of Shi pan qiu watershed was sampled continuously from 2017/11-2018/11,and the characteristics and regularity of nitrogen and phosphorus loss in the watershed were measured during the observation period;at the same time,the runoff plots were set for different land use types(vegetable field,citrus orchard,natural forest,dry sloping field,paddy field)to measure the concentration and flux of nitrogen and phosphorus in surface runoff;and the physical and chemical properties of soil in different types of land use at the beginning and end of the rainfall concentration period(march and October,2018)were further analyzed,Including:total nitrogen,available nitrogen,nitrate nitrogen,ammonium nitrogen,total phosphorus,available phosphorus,organic matter,pH,cation exchange capacity(CEC),bulk density.The purpose of this study is to establish a relationship between soil physical and chemical properties in different land use types and the concentrations of nitrogen and phosphorus in various forms and loss flux of non-point source pollutants in surface runoff,and to provide a theoretical basis for the control of non-point source pollution,the main results are as follows:(1)The order of TN loss flux in different land use types was paddy field>citrus orchard>dry sloping field>vegetable land>Natural Forest during the observation period(March-November2018),which peaks were mainly concentrated in April and May.The loss flux and annual average concentration of total nitrogen(TN),nitrate nitrogen(NN)and ammonium nitrogen(AN)in paddy field runoff were the highest.The fluxes of nitrogen loss in paddy fields during the observation period were 9.94 kg/hm~2(TN)?2.40 kg/hm~2(NN)?2.95 kg/hm~2(AN)respectively.Nitrogen loss in paddy fields mainly occurred during the period of cultivation,fertilization and rainfall in late April and early may.The annual loss fluxes of TN(0.64 kg/hm~2),NN(0.36 kg/hm~2)and AN(0.05kg/hm~2)in natural forest was the minimal respectively,and the TN loss of paddy field is 31 times that of natural forest.The TN loss flux of citrus orchards,arid slopes and vegetable fields which were major contributors to nitrogen loss in small watershed ranged from 4.00 to 4.98 kg/hm~2.In terms of the proportion of different nitrogen forms in TN,the largest and smallest proportion of NN in TN is the natural forest(56.07%)and the dry sloping land(16.14%).The largest and smallest proportion of AN in TN is paddy field(21.42%)and citrus orchard(3.28%).There was a significant correlation between total phosphorus(TP)and particulate phosphorus(PP)in runoff in different land use types.The order of TP loss flux was vegetable field>in citrus garden>in paddy field>natural forest>dry sloping field.The vegetable field had the highest loss fluxes of TP and PP,which were 4.42 kg/hm~2 and 4.14 kg/hm~2,respectively.The loss of total phosphorus(TP)and total dissolved phosphorus(DTP)was the lowest in the dry sloping land,which were 0.19 kg/hm~2 and 0.08 kg/hm~2,respectively,and the TP loss flux in vegetable field was 23 times that in dry sloping field.In terms of the proportion of different phosphorus forms in TP,the largest proportion of DTP in TP is the in citrus garden(56.79%);the PP is the most important form of phosphorus loss in vegetable field,which accounted for 93.67%of TP.In general,in order to prevent the loss of nitrogen and phosphorus from causing non-point source pollution to water bodies,it is necessary to focus on preventing the overlap between the fertilization period and the precipitation period of paddy fields,and prevent the loss of PP in vegetable fields in heavy rainfall.(2)The order of soil organic matter content is paddy field>citrus garden>vegetable field>natural forest>dry sloping field among different land use types,the bulk density(1.18-1.28g/cm~3)of paddy soil was significantly lower than that of other land use types.The pH of natural forest and arid slope soil is alkaline,vegetable soil is acidic,citrus orchard and paddy field are neutral.The soil CEC of natural forest and dry sloping field was higher than that of other land use types,the paddy field was the lowest.The contents of total nitrogen and available nitrogen in paddy field and citrus orchard soil were higher than those in other land use types and the dry sloping field was lowest.There were significant differences in soil nitrate nitrogen content between march and October in citrus orchard,which are obviously higher than other land use types.There is not much ammonium nitrogen in citrus orchard soil due to fertilization and rainfall.In this experiment,soil nitrogen forms of different land use types,including total nitrogen and available nitrogen,were greatly affected by organic matter,pH and CEC,the more organic matter,the more total nitrogen and available nitrogen.Soil total phosphorus content is related to parent material and fertilizer application.The total phosphorus content in the surface soil of citrus orchard was the highest due to the large amount of fertilizer application.There was an extremely significant positive correlation between soil available phosphorus and soil total phosphorus(r~2=0.802,p<0.01),and there was a significant negative correlation between soil available p and soil pH(r~2=-0.446,p<0.05),as a result,citrus garden with neutral pH and high total phosphorus content and vegetable fields with acidic pH have higher available phosphorus,and the content of available phosphorus was lower in natural forest and dry sloping field with alkaline soil.(3)the months with large loss concentrations and fluxes of nitrogen(total nitrogen,nitrate nitrogen and ammonium nitrogen)are mainly concentrated at the end(December 2017,November2018)and the middle(April to June 2018)of the year when farming,fertilization and precipitation periods coincide in the surface runoff at the water outlet of Shi pan qiu small watershed.The months with the largest flux of total nitrogen and ammonium nitrogen loss are in May 2018 which reaching1.417 kg/hm~2 and 0.065 kg/hm2 respectively.NN also reached a higher level in May(0.570 kg/hm~2),but the maximum was seen in November 2018(0.997 kg/hm~2).Nitrate nitrogen is the main form of nitrogen loss in small watershed which accounting for 50.34%of the total nitrogen loss,and there is an extremely significant positive correlation between Nitrate nitrogen and total nitrogen(r~2=0.73,p<0.01).Ammonium nitrogen loss was less which accounting for only 5.06%of total nitrogen loss.The monthly flux of total phosphorus,soluble total phosphorus and positive phosphate gradually increased from April,2018,and reached the maximum in June,which were 0.187 kg/hm~2,0.165kg/hm~2 and 0.126 kg/hm~2 respectively.The fluxes of particulate phosphorus loss ranged from 0.024kg/hm~2 to 0.027 kg/hm~2 in April-June 2018,and reached the maximum monthly fluxes in December2017(0.060 kg/hm~2).The main loss form of phosphorus is total dissolved phosphorus which accounting for 79.01%of total phosphorus,the loss of particulate phosphorus is less which accounting for 21.99%of total phosphorus loss,and there was an extremely significant correlation of concentration between total phosphorus and particulate phosphorus(r~2=0.90,p<0.01).The loss flux of total nitrogen(166.32 kg/hm~2)in small watershed was 9.44 times that of total phosphorus(17.62 kg/hm~2),and the risk of non-point source pollution of nitrogen was far greater than that of phosphorus.Therefore,the key control object of non-point source pollution is nitrogen for Shi pan qiu watershed.In particular,it is necessary to prevent the loss of nitrogen in paddy fields when the periods of fertilization and precipitation coincide;at the same time,measures should be taken to prevent the loss of particulate phosphorus caused by heavy rainfall in vegetable fields.