Spatio-temporal Characteristics And Ecological Impact Of Amino Acids In Dry Deposition Of Organic Nitrogen In Xichuan Reservoir Area

Dry deposition is an important link in the"biological-earth-chemical"cycle of atmospheric nitrogen,and it is also an important source of nutrients in aquatic ecosystems.In recent years,the amount of atmospheric nitrogen deposition continues to increase,and the negative ecological effects caused by nitrogen deposition gradually become prominent.As the water source of the Middle Route Project of South-to-North Water Transfer,Danjiangkou Reservoir Area has attracted wide attention for its water quality and influencing factors.Therefore,it is the key to control the water pollution and biogeochemical cycle of the reservoir by studying the spatial and temporal distribution characteristics,sources and potential ecological effects of atmospheric nitrogen deposition.In this paper,72 dry deposition samples of atmospheric nitrogen were collected from September 2020 to August 2021 in the Danjiangkou Xichuan reservoir area.Dissolved total nitrogen（DTN）,ammonia nitrogen（NH₄⁺-N）,nitrate nitrogen（NO₃^--N）,dissolved total amino acids（DTAA）,dissolved free amino acids（DFAA）,dissolved combined amino acids（DCAA）and organic chemical fractions were measured in the dry deposition samples and analyzed the spatial and temporal distribution characteristics,sources,and influencing factors of regional organic nitrogen and amino acid deposition,and explored their bioavailability and potential ecological effects.The following main results were obtained from this study:（1）The annual average deposition of dissolved organic nitrogen（DON）in the reservoir area is 9.028 kg·ha^-1·yr^-1.The deposition of DON at each site is higher in January-February when the air quality is poor,and lower in May-August when more rainfall occurs.The high nitrogen deposition area in the reservoir area is mainly concentrated in the southwest part of the reservoir,and the land use type is mainly farmland and forest.Based on PMF source analysis,backward trajectory of air masses and potential source area analysis,it is determined that the source of DON in the reservoir area is related to the land use types around the observation sites,and the main sources include traffic emissions,biomass and coal combustion,agricultural activities and regional secondary contributions（long-range dust air masses from northern China,atmospheric phototransformation）.（2）The average annual dry deposition of DFAA,DCAA and DTAA in the reservoir area was 0.466 kg·ha^-1·yr^-1,1.031 kg·ha^-1·yr^-1and 1.496 kg·ha^-1·yr^-1,respectively,and the dry deposition of amino acids was dominated by DCAA.The monthly variation patterns of DFAA deposition were different among sites,and agricultural activities,secondary transformation of atmospheric photochemistry and polluting air masses were the main factors influencing the variation of DFAA deposition.the monthly variation trends of DCAA deposition and DTAA deposition were the same,and were mainly influenced by agricultural activities,temperature,wind speed and long-distance transported dust air masses.the spatial variation characteristics of DFAA deposition were influenced by land use types around the observation sites The spatial variation of DCAA and DTAA deposition was higher in agricultural areas than in towns.Among the individual amino acids,Glutamic acid（Glu）and Aspartic acid（Asp）contributed more to the amino acid deposition.PMF source analysis and backward trajectory analysis of air masses showed that amino acids mainly originated from the contribution of land and surrounding activities in the reservoir area,and secondary aerosol sources and long-distance input of dust air masses had some influence on amino acids in the reservoir area.Agricultural sources are the main source of DFAA,biological and plant release sources are the main source of DCAA,and agricultural activities also contribute significantly to DCAA.（3）The contribution of DTAA dry deposition to DON in the reservoir area is16.6%,with an annual influx of 81.68 t yr^-1and providing new productivity of 0.38 g C m^-2yr^-1.The contribution of amino acids to organic nitrogen deposition and its impact on algae and water column eutrophication cannot be ignored.The dry deposition of DTN in the reservoir area far exceeds the critical load of nutrient nitrogen deposition in China’s lakes,and the deposition of DON is close to the upper limit of the critical load in agricultural fields,and the impact of DON deposition on water bodies and the surrounding environment cannot be ignored.（4）The average monthly C/N values of dry deposition in the reservoir area ranged from 3.44 to 15,with a mean value of 7.25,and the nitrogen dry deposition had a high phytoplankton bioavailability.The degradation index（DI）indicated a higher bioavailability of organic matter in autumn and a higher degradation of organic matter in winter.Total dissolved amino acids as a percentage of nitrogen molarity of organic carbon（DTAA（%DOC））values ranged from 0.12 to 0.71,with a mean value of 0.36,indicating a higher degree of organic matter degradation and lower bioavailability in dry deposition.Overall,the bioavailability of dry deposition from different sources and at different degradation states in the reservoir area is at a low level,but the impact of dry deposition on water bodies should still be noted in autumn and in the northern part of the reservoir.