Temporal And Spatial Distribution Characteristics Of Nitrogen And Phosphorus Pollution And Its Impact Factors In Huixian Experimental Area Of Guilin City

At present,agricultural non-point source pollution has become a global concern.It is very essential to study the change laws and impact factors of agricultural non-point source nitrogen and phosphorus pollution for pollution control.As the largest karst wetland in Lijiang River Basin,Huixian karst wetland has important environmental regulation function as well as great ecological benefits.However,due to the influences of the agricultural production activities,unreasonable utilization of water resources and other factors,the situation of nitrogen and phosphorus pollution in Huixian wetland is severe.In this paper,Huixian experimental area(376.42 km~2)of Huixian karst wetland is taken as the research object.While particular emphasis on the agricultural non-point source N/P pollution of surface water(Mudong River,Huixian River and Xiangsi River)in the test area,the condition of soil N/P pollution as well as shallow groundwater N/P pollution is also analyzed in order to systematically reveal the current status of non-point source pollution in Huixian wetland.Based on two years’field monitoring of surface water,one year’s field monitoring of soil and groundwater,the temporal and spatial differences of ammonia nitrogen,nitrate nitrogen,total nitrogen,total dissolved phosphate and total phosphorus in surface water,nitrate nitrogen,total nitrogen and total phosphorus in soil and groundwater were analyzed,and the influences of soil nitrogen and phosphorus contents on the water environmental quality of rivers and the differences of nitrogen and phosphorus pollution between surface water and groundwater were analyzed.Grey correlation degree analysis and Pearson correlation analysis were used respectively to studied the influences of impact factors(environmental-meteorological factors and underlying surface)on nitrogen and phosphorus pollution.The main conclusions are as follows:(1)According to the class III water quality standard of surface water,the nitrogen pollution of surface water in the study area was more serious than phosphorus pollution during the monitoring period(2017.10-2019.09).In time dimension,the concentrations of nitrate nitrogen,total nitrogen and total dissolved phosphate in non-irrigation period of Mudong River and Huixian River were higher than in irrigation period,while the concentrations of ammonia nitrogen and total phosphorus in irrigation period were higher when in non-irrigation period,and the concentrations of nitrogen and phosphorus in Xiangsi River were higher in non-irrigation period.In space dimension,the nitrogen and phosphorus concentrations of Mudong River decreased first and then increased,for Huixian River,its nitrogen and phosphorus concentrations increased along the river while Xiangsi River showed a contrary trend.(2)The monthly average nitrogen and phosphorus loads of Mudong River,Huixian River and Xiangsi River were significantly different in different years,and the nitrogen and phosphorus loads of year 18-19(2018.10-2019.09)were higher than year 17-18(2017.10-2018.09).In different periods,the ammonia nitrogen loads,total dissolved phosphate loads and total phosphorus loads in irrigation period were higher than that in non-irrigation period,while the loads of nitrate nitrogen and total nitrogen had the opposite behavior.The spatial distribution characteristics of nitrogen and phosphorus loads in the experimental area were high in the north and low in the south.(3)The nutrient contents of sediments in Mudong River,Huixian River and Xiangsi River were high in spring,and the contents of nitrate nitrogen in cultivated land soil and riparian soil were low in summer.Along the river,the nutrient contents of Mudong River sediments showed an inverted"V"shape trend,and the nutrient contents of Xiangsi River and Huixian River increased from upstream to downstream.Based on the single factor index method,it was found that the evaluation indexes of soil total nitrogen and total phosphorus were at a high level,indicating a high risk of water pollution.For shallow groundwater,according to the class III water quality standard of groundwater,the nitrate nitrogen pollution was relatively light.The concentrations of nitrate nitrogen and total nitrogen reached peak in April and May,while total phosphorus was in July.The nitrogen and phosphorus pollution of shallow groundwater in the study area showed the characteristics of high concentrations in the middle area(basin)of the wetland and western runoff outlet(Xiangsi River channel),and low concentrations in the north and south regions of the study area.(4)Grey correlation degree analysis was used to analyze the relationship between the environmental-meteorological factors(precipitation,pH,water temperature,and dissolved oxygen)and the concentrations of nitrogen and phosphorus pollution of surface water.It was found that the nitrogen concentrations were closely related to pH,and the phosphorus concentrations were strongly connected with precipitation.The correlation analysis between the underlying surface properties and nitrogen and phosphorus concentrations in the studied area showed that,paddy field had a positive impact on the nitrogen and phosphorus concentrations;And grassland was also positively related to the concentrations of ammonia nitrogen and phosphorus;Brush forest,dry land and rural residential area had negative correlation influences on the concentration of nitrogen and phosphorus.Pond wetland had an impact on nitrogen and phosphorus concentrations in a negative way.For different aquifer types,the limestone aquifer had a negative correlation with the nitrogen and phosphorus concentrations,while loose aquifer had positive impact on the nitrogen and phosphorus concentrations.