Quantitative Study On Lag Effect Of Watershed Non-point Source Nitrogen Pollution

As nitrogen(N)point source pollution is gradually controlled,non-point source pollution has become the major cause of N excess in many water bodies.This problem not only seriously threatens the health of water ecosystems,but also has lead to environmental consequences such as eutrophication and hypoxia zones in downstream estuaries and coastal zones.Although many efforts have been made for non-point source pollution control in many watersheds,N concentrations in water bodies are still in high level.One of the key reasons for the embarrassing situation above is the N-leaching lag effect.However,quantitative understanding of non-point source N pollution lag effect is still limited,which hinders the processes of N pollution control.Combining watershed characteristics,meteorology,sources of pollution,land-use and long-term(1980-2018)monitoring water quality and hydrological data in Yongan River Watershed,this study explored the watershed anthropogenic N input and riverine N export dynamics.By developing new model,improving previous model as well as combining stable isotope analysis,this study explore the composition of river runoff and the hydrological lag time,as well as the N transformation characteristics during transportation.The contribution of different components N to riverine N export was preliminarily identified,such as current nitrogen source vs legacy N sources,point source vs non-point source as well as surface runoff vs subsurface runoff(lateral flow and groundwater),which could provide key scientific basis for effective control of N pollution.The main results are as follows:(1)Net anthropogenic nitrogen input(NANI)increased first and then decreased during 1980-2010,and as a whole increased by more than 70%;Riverine total nitrogen(TN)export flux continued to increase,with an overall increase of more than 90%.There was an opposite trend between NANI and riverine TN export during 2000-2010,which may be due to changes of land-use and the influence of N lag effect.(2)Combined with the modeling methods and the stable isotopes based sine-wave fitting method,the hydrological processes of Yongan River watershed and its sub-catchments were preliminarily clarified.Majority(73-91%)of the runoff in the Yongan River Watershed is derived from groundwater,while surface flow contributed less water(9-27%).The young water fraction with ages within 44-68 days in Yongan River Watershed ranged from 6%to 21%,and the mean transit time(MTT)of river water is about 15.1 years.The numbers of storms and paddy field area percentage may be the main factors influencing the spatio-temporal variability of Fyw,while the topographic factors were the main factor affecting the spatial variability of MTT.(3)The combined water quality and hydrological data,as well as the nitrate stable isotopes(?15N-NO3-and ?18O-NO3-)suggested that non-point source pollution is the main form of N pollution in the Yongan River Watershed.During the process of N transportation,nitrification has been recognized the main N transformation process,while the denitrification was relatively weak.This N conversion characteristic promoted the enrichment of excessive N in groundwater system and gradually accumulated year by year,which lead to the possibility of N lag effect.The prevailing of nitrification also promoted N2O emission in the Yongan River Watershed.The N2O emission law in Yongan River Watershed is similar to that of global riverine N2O emissions.The N2O emission fluxes in rivers were mainly determined by the available N concentration in the river system.While the N2O emission factors were significant negatively correlated to DIN concentrations and DOC:DIN ratios with power function relationship.(4)By introducing cross-correlation analysis and Koyck transformation method,this study premilarily identified the N-leaching lag times in Yongan River Watershed.Based on the estimated lag times,the Regional Nutrient Management(ReNuMa)model was improved to conduct N sources identification.This study also developed a dynamic watershed N delivery model for quantifying transient storage of legacy N and its resulting riverine TN flux.This study then combined the nitrate stable isotopes data,water quality and hydrological data,as well as two models mentioned above to initially verify the N-leaching lag effect.The lag time of nitrogen pollution in Yongan River Basin is more than 10 years;-77%of the riverine TN export derived from the legacy N pool,and only 10%derived from the current year N input.For the exported nitrate,?50%was derived from groundwater and>30%was derived from soil N,which is consistent to the results above.In summary,subsurface flow with relatively long transit time is the main component of runoff,which combined with the excessive N input and strong nitrification and weak denitrification rates indicated the significant lag effect of non-point source pollution in Yongan River Watershed.The developed models and methods have preliminarily identified the lag time between anthropogenic N input and river nitrogen output,as well as the contribution of legacy N to riverine N export.To control river N pollution effectively,more attention should be paied to groundwater remediation and soil N management in addition to the reduction of N sources(e.g.,fertilizers).