Study On The Potential Nutrient Reduction Based On The Environmental Threshold Of Water

In the past few decades,with the rapid development of social economy and the acceleration of urbanization,agricultural production has become an important source of water pollution.The callenges of China’s food security,together with the rapidly growing economy,required more for the argricultural production system.In order to meet the crop demand,different nitrogen and phosphorus nutrient application measures have been adopted,resulting in increased output of river nutrients and more frequent eutrophication in China’s coastal areas,resulting in serious water environmental costs.In recent years,the central and local governments have taken a series of measures to comprehensively improve river water quality and to restore water ecology.Although some positive effects have been seen,there are certain water pollution problems left.The Yangtze River Basin(YRB),as one of China’s major grain-producing areas,contributes about 40 % of China’s grain output.As an important tributary of the upper Reaches of the YRB,the Wu River Basin(WRB)contributes 5% of rice,3% of eggs and 13% of milk to the YRB.It plays an important role on transporting nitrogen and phosphorus nutrients in the YRB.In order to meet the demand of animal and crop production,the nutrient input of nitrogen and phosphorus in WRB is high,which poses a serious threat to water quality.However,there are few quantitative studies on the migration characteristics and total output of nitrogen and phosphorus nutrients from land pollution sources to rivers and estuaries(the main stream of the YRB)in the WRB.Taking WRB as an example,NUFER(Nutrient Flows in Food chains,Environment and Resources Use)-MARINA(Model to Assess River Inputs of Nutrients to se As)Model was coupled to esitimate: 1.Nitrogen and phosphorus flow characteristics of planting and rearing systems in WRB.2.Characteristics of nutrient flux from land to river and Yangtze River in WRB.3.Potential of nitrogen and phosphorus reduction in the WRB.Input parameters and data were obtained by collecting and publishing literature,national Statistical Bureau,National Statistical Yearbook,county statistical bulletin and peasant household survey.This study focuses on the response of the combination of planting and raising and the improvement of sewage treatment facilities to the reduction of nitrogen and phosphorus nutrients in water in the WRB.Through scenarios designing,we evaluated the reduction potential of nitrogen and phosphorus in the water system of WRB,providing theoretical basis for the green development of agriculture in the WRB.The main results are as following:(1)NUFER model was used to evaluate the characteristics of nitrogen and phosphorus flow in planting and rearing systems of each county in the WRB,and to clarify the main input sources of nitrogen and phosphorus nutrients,spatial differences of environmental losses and nutrient utilization efficiency in the WRB.In 2018,the total input of nitrogen and phosphorus to the planting and breeding systems in the WRB was1,994 and 330 tons respectively.Fertilizer application,biological nitrogen fixation,and atmospheric nitrogen deposition were 958 Gg,114 Gg,and 349 Gg respectively.Feed import was also found as an important form of nutrient input,accounting for 25% of the total nitrogen input source of the aquaculture system,which was 473 Gg Fertilizer application in the phosphorus input source of the cropping system was 226 Gg.The feed import of phosphorus input source for aquaculture system was 104 Gg.Fertilizer application was the main input source of nitrogen and phosphorus,contributing 51% and68% of the input of nitrogen and phosphorus,respectively.The loss of nitrogen and phosphorus nutrients to the environment is high,and nitrogen loss is mainly caused by leaching,runoff and erosion.In 2018,the loss of nitrogen reached 327 Gg,accounting for 44% of the total emissions The total amount of phosphorus loss from leaching,runoff and erosion is 19 Gg.The spatial difference of nitrogen and phosphorus loss from planting and rearing system to environment was significant.On the whole,the loss areas of nitrogen and phosphorus in districts and counties of WRB are mainly in the lower reaches of WRB.Districts and counties with high loss of nitrogen and phosphorus to the environment have high overlap.The average nitrogen and phosphorus utilization efficiency of planting systems in counties of WRB were 25% and 30%,respectively.The average utilization efficiency of nitrogen and phosphorus was 9% and 3%,respectively.It is very important to identify the hot spots of nitrogen and phosphorus nutrient discharge and reduce the loss and discharge of nutrients to the environment to improve nutrient efficiency.(2)The NUFER-MARINA model was coupled to calculate the total nitrogen and phosphorus emissions from multiple land pollution sources to rivers and estuaries(the main stream of the YRB)in WRB in 2018.In 2018,the total nitrogen and phosphorus inputs to rivers from land were 566 Gg and 28 Gg,respectively.Agricultural systems,natural systems,livestock and poultry production and domestic sewage emissions accounted for 77%,15%,3% and 5% of total nitrogen input,respectively.Agricultural systems,natural systems,livestock and poultry production and domestic sewage emissions accounted for 54%,1%,23% and 22% of total phosphorus input,respectively.The emission of nitrogen and phosphorus in different forms was significantly different.The total amount of nutrients discharged to rivers in DIN form was 546 Gg,mainly from farmland system,accounting for 79%.The total amount of nutrients discharged into rivers in the form of DON is 20 Gg,and the main source of discharge is domestic sewage,accounting for 33%.The total amount of nutrients discharged to rivers in the form of DIP was 24 Gg,60% of which came from non-point source emissions from farmland systems.The total amount of nutrients discharged to rivers in the form of DOP was 4,000 Gg,of which 46% came from livestock and poultry production.The total amount of nitrogen and phosphorus from The WRB to the main stream of the YRB was 197 Gg and 9 Gg respectively.(3)Based on the Coupling of NUFER-MARINA model,three scenarios were designed to evaluate the impact of the two main measures on the total nitrogen and phosphorus emissions from land pollution sources in the WRB to rivers and estuaries(the main stream of the YRB),and to clearly reduce the potential of reducing nitrogen and phosphorus nutrient load from countries in the WRB to the WRB.Three main scenario models were: scenario 1-combination of planting and rearing,scenario 2-upgrading of sewage treatment facilities,and scenario 3-combination of planting and rearing + sewage treatment facilities.Under scenario 1,nitrogen and phosphorus nutrient inputs in WRB in 2018 were both higher than crop nutrient requirements.Nitrogen fertilizer imports were down 77 % and phosphorus 88 % compared to 2018.Based on the current situation of emissions in 2018,scenario 1-improving the integration of planting and raising capacity will reduce the total amount of nitrogen and phosphorus transported from the WRB to the YRB mainstream by 50% and 64%,respectively.After scenario 2-analysis,the total amount of nitrogen and phosphorus transported from WRB to the estuary decreased by6% and 17% respectively.In scenario 3,under the dual measures of combination of planting and raising and upgrading of sewage treatment facilities,the total amount of nitrogen and phosphorus transported to the estuary of the WRB decreases by 56% and80% respectively.Conclusions and Significance: In this study,NUFER model,MARINA model and NUFER-MARINA model were coupled to evaluate the whole process of nitrogen and phosphorus nutrient input from land to river and then from river to estuary(the main stream of YRB)in WRB at sub-watershed scale.The model was used to quantify the total amount of nitrogen and phosphorus nutrient transport in the whole process and identify the main emission sources.Based on model coupling,three scenarios were designed to evaluate the reduction potential of nitrogen and phosphorus in WRB under different measures,providing theoretical basis for green agricultural development in WRB.