Study On The Composition Specificity And Evolution Of Soluble Organic Matter In Qinghai Lake By Isotope Technology

Qinghai Lake is the largest inland saltwater lake in China.It is located in the arid and semi-arid areas of northern China,and its annual evaporation is much larger than its precipitation.Given the high salt background of Qinghai Lake,salinity must be one of the important factors affecting the existence form of DOM.In recent years,the salinity of Qinghai Lake shows an increasing trend,and the adjacent Gahai,Keluke,Qarhan and Chaka Salt Lakes also have the same characteristics.The increasing salinization of inland lakes will certainly affect the structure and composition of the DOM in them.As far as we know,the influence of salinity on the DOM composition in Qinghai Lake is still lacking at the molecular level.In view of this,this study in qinghai lake water body as the research object,and choose different salinity of surrounding four lakes as the reference object of qinghai lake salinity changes during different periods,the emerging of isotope tracer technique,supplemented by Fourier transform ion cyclotron resonance mass spectrometry（FT-ICR-MS）detailed characterization of qinghai lake from the molecular level composition and properties of DOM,qinghai lake,and to explore the historical evolution process of the organic structure along with the evolution law of salinity change.This study is helpful to clarify the molecular composition characteristics and influencing factors of DOM in Qinghai Lake,and to further understand and predict the changes of water quality characteristics and ecological service functions of inland saline-oriented lakes in China.This paper characterized the DOM of five lake samples,and found that the composition and properties of DOM in the lake samples were affected by salinity and microbial community.DOM properties of the five samples are relatively similar,mainly containing four types of organic matter:CHO,CHON,CHOS and CHONS.The higher the salinity,the higher the average molecular weight of DOM,the higher the degree of oxidation and the lower the aromaticity.In addition,the proportion of DOM that is susceptible to microbial degradation（lipids）,photodegradation（aromatic structure）,or both processes（carbohydrates and unsaturated hydrocarbons）decreases at higher salinity.In contrast,compounds that are not readily biodegradable（such as lignin/CARM-like structures and tannins）or photodegradable compounds（such as aliphatic compounds）accumulate.The range ofδ¹³C values increases with increasing salinity throughout the entire salinity gradient.Compared with stable carbon isotopic composition,δ¹⁵N values do not show a consistent pattern of variation along the salinity gradient of Qinghai Lake.Althoughδ¹³C varies dynamically,theδ¹⁵N values vary in a larger range and are distributed in a more complex pattern.Theδ¹⁵N values first decrease with increasing salinity and then increase with increasing salinity.The distribution pattern ofδ¹⁵N implies that the values ofδ¹⁵N are not simply a mixture of different DOM sources,instead,microbial processes（and possibly physical sediment resuspitation）lead to isotopic fractionation during DOM transport and degradation,suggesting a more active nitrogen cycle.Therefore,the HMW DOM component with lighterδ¹⁵N accumulated in Qinghai Lake with increasing salinity year by year.In addition to organic sources,biogeochemistry and isotope fractionation process are also important factors for theδ¹⁵N value of the high molecular weight DOM in Qinghai Lake.The overall relationship betweenδ¹³C andδ¹⁵N may be due to mixtures of different sources,although the effects of sources inside and outside the closed lake are small,they are affected by the high sensitivity of isotopes.Isotopic tracer means have strict requirements on the accuracy of experimental samples and the interference of the environment.The isotopic fractionation and enrichment processes of carbon and nitrogen are different in the production,transformation and degradation of DOM in the inland closed salt lake.HMW DOM components with heavyδ¹³C and lightδ¹⁵N accumulated in Qinghai Lake with increasing salinity year by year.