Changes Of Nitrogen Interception In River-Reservoir System In Jialing River Basin During 1988-2020

The nitrogen cycle is a very important link in the biogeochemical cycle of the basin.The nitrogen content in the water body determines the health of the water body and the stability of the water environment.Since the reform and opening up,the boom of reservoir construction has resulted in the increase of dry and tributary reservoirs year by year,which has damaged the continuity of rivers and had an important impact on the migration,transformation and removal of nitrogen in rivers.Jialing River is the second largest river system of the Yangtze River and also the largest tributary that enters the Three Gorges Reservoir.Its nitrogen load directly affects the nitrogen load level of the Three Gorges Reservoir.How to effectively control nitrogen in the Jialing River basin for a long time is the key to environmental management of the Three Gorges Reservoir and the middle and lower reaches of the Yangtze River.Therefore,quantifying the temporal and spatial distribution characteristics of nitrogen in the river and analyzing its source are the key to effectively reduce nitrogen,and clarifying the law of nitrogen reduction in the river-reservoir system in the river basin is the premise of river excavation and reservoir construction.Taking Jialing River Basin as the research object,this paper selected 1988-2020 as the research period,collected and used DEM data,land use data,meteorological and hydrological data,statistical yearbooks of relevant counties,hydrological yearbooks and Hydro SHEDS database,etc.Combined with the Global News-2 model and the intra-basin water nitrogen removal model under the Earth system,GIS,ARCSWAT and other tools were used to explore the nitrogen load,spatial and temporal distribution characteristics and sources of nitrogen in the Jialing River Basin during 1988-2020,as well as the rules of nitrogen reduction in the river-reservoir system of the basin.The main conclusions were drawn as follows:(1)During 1988-2020,nitrogen input in the Jialing River Basin increased first and then decreased.Since the 1980 s,in order to meet the needs of people’s production and life,a large amount of fertilizer has been applied to farmland,large-scale livestock breeding has emerged,and nitrogen input in the basin has grown steadily.After the2010 s,under the call of national economic structure transformation,the rise of green environmental protection awareness and the introduction of relevant policies gradually reduced the nitrogen input in the basin.(2)The Global News-2 model revealed the main sources of nitrogen in the Jialing River Basin.From 1988 to 2020,the proportion of nitrogen sources entering the river in Jialing River Basin was: fertilizer > livestock manure > atmospheric nitrogen deposition > domestic sewage > crop nitrogen fixation.The contribution of agriculture and animal husbandry to nitrogen in the Jialing River Basin was more than 70%,indicating that the anthropogenic influence such as fertilizer application and livestock breeding was far greater than that of natural agriculture such as crop nitrogen fixation.Among the four provinces through which the Jialing River Basin flows,the intensity of nitrogen input in Sichuan and Chongqing is higher than that in Shaanxi and Gansu.Sichuan Province was the largest source of nitrogen input in the basin.However,due to the implementation of relevant policies,the amount of fertilizer applied in Sichuan province gradually decreased after the 20110 s,and its contribution to nitrogen in the basin also decreased year by year,from 82.74% to 31.65%.Under the influence of rapid urbanization,the nitrogen contribution of Chongqing increased year by year from 2% to26.82%.Nitrogen contribution was relatively stable in Shaanxi and Gansu provinces.(3)According to the rainfall data of the state control water quality monitoring stations in the basin and the nearby meteorological stations,ARCSWAT hydrological analysis tool and spline function method were used to divide the whole basin into 16 sections according to the river trend and fit the annual rainfall of the Jialing River Basin in relevant years.The results showed that under the influence of rainfall and runoff,the imported nitrogen entered the water body of the basin after being intercepted by soil and vegetation.In this process,rainfall and runoff coefficient play an important role in the nitrogen load into the river basin and the subsequent nitrogen reduction in the river-reservoir system.(4)The effect of nitrogen reduction in the Jialing River Basin was obvious.The nitrogen removal model of water body in the basin under the Earth system showed that the nitrogen interception rate of the river-reservoir system in the basin was about 72.21%to 79.61% during 1988-2020.Due to the lack of some data,we took 2020 as an example to calculate the nitrogen reduction of river network and reservoir respectively.The results showed that the nitrogen interception rate of river network and reservoir was about 53.8% and 37.43% respectively.Reservoir capacity,river network density and rainfall were the main factors affecting nitrogen reduction in river-reservoir system.The effect of non-main stream reservoir group on river nitrogen reduction was no less than that of main stream cascade reservoir group.In the densely populated and agriculturally developed areas such as Qujiang River and Fujiang River basin,rainfall is abundant and river network is dense,and the nitrogen reduction effect of river-reservoir system is good.