Nitrate Nitrogen Isotopes And Nitrogen Cycle Tracing In The Chanhe River Basin,Xi’an

The nitrogen cycle is a biogeochemical process in which nitrogen is recycled into various chemical forms among the atmosphere,land,and marine ecosystems.With the rapid development of industry and agriculture in our country,excessive emissions of nitrogenous chemicals have caused many environmental problems,such as increasing annual nitrogen deposition and eutrophication of rivers and other water bodies.In response to these risks,domestic researchers have attracted particular interests in the natural nitrogen cyclic process.Nitrogen is one of the most important nutrients that affects and limits the growth of plants.At the same time,climate change continues to affect the carbon and nitrogen cycles.Therefore,nitrogen isotopes of plants can also be related to climate information in the environment.Loess Plateau is located in the northwest of China,with scarce vegetation distribution and further resulting in weak water and soil conservation abilities.Soil erosion is frequent in this area,leading to excessive losses of nutrients such as nitrogen and phosphorus.Under this background,investigations on nitrogen cycle in the Loess Plateau region were carried out.Further understanding of this process can provide a theoretical basis for the solution of problems that arise in real life production.In this paper,using the existing methods in Institute of Earth Environment of Chinese Academy of Sciences,we compared the preservation conditions of the collected fresh soil samples;then we analyzed the methods in determining water-soluble nitrogen isotope of the soil during the nitrogen loss;and finally,we investigated the nitrogen isotope composition of plants,soils and rivers in the Xi’an Chanhe River basin（from source to urban area）,which is the first comprehensive study on the isotopes of nitrogen cycles between plants,soils,and river water cycles in the basin.The main conclusions of the paper are as follows:1)We conducted conditional experiments on four preservation methods for soil samples and found that the soil samples should be preserved at low temperatures from-18°C to-20°C when they frozen.Although the nitrate content of soil can be kept constant at-10°C,the soilδ¹⁵N-NO₃^-value changes significantly with time;the nitrate content andδ¹⁵N-NO₃^-values of soil significantly change by cold storage and room temperature preservation.The drying method not only changes the soil nitrate content,but also change theδ¹⁵N-NO₃^-values significantly.2)We investigated the nitrate and nitrogen isotope compositions of the river-soil-plant system in Chanhe River Basin and compared them with previous results of water-nitrogen isotopes in same river.The results show that the content of NO₃^--N in the soil from the upper reaches to the lower reaches in Chanhe River changes as follows:farming area>forest area>urban area;soilδ¹⁵N-NO₃^-values varies as follows:farming area>urban area>forest area.The difference of land-use types may be an important factors in determining the changes of NO₃^--N content and its nitrogen isotope.The contents of NO₃^--N andδ¹⁵N-NO₃^-values in the Chanhe River both show that the downstream has higher values than that of the upstream.Compared with previous results in Chanhe River,we find that the content of NO₃^--N in the upstream and middle reaches of the river increases as a whole,while the content of NO₃^--N in the downstream is lower than that of the previous period.The spatial distribution characteristics of the riverδ¹⁵N-NO₃^-show that the development of the tourism industry in the upper reaches and increasing agriculture in the middle reaches,has aggravated the pollution of water bodies,while the improved treatments of downstream industrial pollution are effective.Our study on the characteristics and changes of nitrogen isotopes compositions of water-soil-plant around the Chanhe River can provide a theoretical basis for the ecological assessment and management for the river basin.