Nitrogen And Phosphorus Budget And Driving Factors

Non-point source pollution is the main problem affecting the water ecological environment,which will affect the safety of water quality and ecosystem function.Compared with point source pollution,non-point source pollution has the characteristics of universality,randomness,hysteresis and latency,which makes it relatively difficult to control.The process of non-point source pollution in the watershed is affected by many factors such as hydrological process,environmental background characteristics of the watershed itself and human activities.Nitrogen and phosphorus are indispensable elements for crop growth.Excessive nitrogen and phosphorus enter water body with runoff produced by rainfall,which is one of the main causes of non-point source pollution.The generation of non-point source pollution is,in the final analysis,an imbalance of nutrient transport.Therefore,a clear understanding of the main driving factors of nitrogen and phosphorus surplus in the watershed can provide effective theoretical support for nutrient management and non-point source pollution control in the watershed,and also ensure water quality safety,so as to promote sustainable development.This paper chooses hujiashan watershed,a typical watershed in Danjiangkou reservoir area,as the research object.Based on topography,soil,rainfall in the study area,land use,such as social and economic data,combined with field investigation,statistical yearbook collection,river water quality monitoring,the use of geographic information system and remote sensing information technology means,such as using the net for nitrogen and phosphorus input model and TOPMODEL model,methods such as redundancy analysis,hierarchical segmentation,reveals the watershed in 2008-2018 annual characteristics of nitrogen and phosphorus output,estimate the basic situation of nitrogen and phosphorus input and output,the main driving factors of nutrient surplus.The main results and conclusions are as follows:(1)From 2008 to 2018,the main nitrogen and phosphorus concentrations in the Hujiashan watershed were:the annual average concentration of total nitrogen ranged from1.68 to 4.80 mg/L,the annual average concentration of total phosphorus ranged from 0.02to 0.17 mg/L,the annual average concentration of nitrate nitrogen ranged from 1.02 to 2.28mg/L,and the annual average concentration of ammonia nitrogen ranged from 0.11 to 0.50mg/L.The total nitrogen concentration was highest in 2008 and lowest in 2012.The total phosphorus concentration was highest in 2013 and lowest in 2017.The pollution conditions of total nitrogen and total phosphorus were improved greatly compared with the initial monitoring period,while ammonia nitrogen and nitrate nitrogen showed no obvious trend of change.(2)The net anthropogenic nitrogen input index in the Hujiashan watershed from2008 to 2018 ranges from 8310.75 kg/km~2 to 9999.05 kg/km~2,while the net anthropogenic phosphorus input index ranges from 3413.02kg/km~2 to 4188.33kg/km~2.The annual average values of the net anthropogenic nitrogen input index and the net anthropogenic phosphorus input index are 9045.91kg/km~2 and 3566.71kg/km~2 respectively.The highest values of the net anthropogenic nitrogen input index and the net anthropogenic phosphorus input index appeared in 2013 and 2012 respectively.Fertilizer input is the main component of the net anthropogenic nitrogen input index and net anthropogenic phosphorus input index in the watershed,with contributions of 37%-53%and 69%-85%respectively.(3)The TOPMODEL model simulation results show that the annual runoff flux of Hujiashan small watershed ranges from 3.92×10~6 to 10.1×10~6 m~3,and the annual average runoff flux is 7.39×10~6 m~3,with the maximum runoff flux appearing in 2010 and the minimum flux appearing in 2018.The total nitrogen flux,total phosphorus flux,nitrate flux and ammonia flux per year in Hujiashan small watershed are within the range of 11129.24to 29219.56 kg,82.44 to 963.62 kg,4714.82 to 19297.94 kg and 917.68 to 2740.90 kg respectively.In the time range from 2008 to 2018,the average output fluxes of these four nutrients in the watershed are respectively 19,618.94 kg,518.44 kg,11,692.09 kg and1,721.26 kg.(4)Nitrogen and phosphorus on the analysis of watershed on the basis of input and output flux on the lag analysis,the results showed that total nitrogen flux and the net anthropogenic nitrogen input index present lag clockwise,and as a whole is divided into two stages,namely,from the first stage(2008-2013)with the increase of net anthropogenic nitrogen input index,watershed nitrogen flux decreases,then the second stage(2013-2018),with fewer net anthropogenic nitrogen input index,the total nitrogen in the watershed is flux increases;Watershed of the total phosphorus flux and the net anthropogenic phosphorus input index shows the tendency of anticlockwise lag,and as a whole is divided into four stages,the first stage(2008-2010)phosphorus flux with the net anthropogenic phosphorus by the rise of input index,the second stage(2010-2012)phosphorus flux with the net anthropogenic phosphorus by the rise of input index,the third stage(2012-2013)of phosphorus flux with the loss of the net anthropogenic phosphorus input index,the fourth stage(2013-2018)phosphorus flux with the loss of the net anthropogenic phosphorus input index.(5)The N surplus and livestock and poultry breeding,tobacco,wheat,rice and sesame seeds grow present positive correlation,the P surplus and livestock farming and planting tobacco were positively correlated,nitrate and rainfall in the watershed is positively correlated,sweet potato planting and tobacco planting,the ammonia nitrogen in the watershed and tobacco and sweet potato planting and rainfall were positively correlated,and the response variables and four woodland area,rape planting and negatively correlated with wheat.The main driving factors of N and P surplus in the watershed were livestock farming,which contributed 43.20%and 28.00%of the explanatory ratio,respectively.Nitrate nitrogen flux was mainly affected by rainfall,accounting for 19.88%of the explanatory ratio,and ammonia nitrogen was mainly affected by tobacco and sweet potato cultivation,accounting for 12.49%and 22.28%of the explanatory ratio,respectively.