The Fractions Of Nitrogen And Dissolved Organic Matter And The Characteristics Of Microbial Communities In The Sediments Of A Nascent River-type Lake

As urban expansion,some small lakes became landscape lakes and natural river channels were transformed into artificial lakes or reservoirs.These changes lead to eutrophication of some newly-formed lakes or pools.With the deepening of water pollution treatment,the problem of endogenous pollution of sediments becomes increasingly prominent.Organic matter,carbon and nitrogen nutrients are closely related to microbial activities,and they participate in various key physical,chemical and biological processes of the sediment-water interface.Coupled relationships also exist among them as their chemical composition and structure change.In order to improve the status of water quality and eutrophication problem in newly-formed lakes and reservoirs,it is necessary to explore the occurrence characteristics of carbon and nitrogen in sediment-water interface and their biogeochemical cycling processes.For newborn channel-type lake?Lake Longjing?,the endogenous nitrogen load and its contribution were unclear,and there was a gap in the relative study of dissolved organic matter?DOM?and the composition of microbial community.According to the sedimentary background,the sediment was divided into original reservoir area,new submerged bottom and slope area and evaluated in different zones.Using conventional water quality monitoring,nitrogen fractionation,parallel factor analysis?PARAFAC?,denaturing gradient gel electrophoresis?DGGE?,and high-throughput sequencing techniques,the pollution load and source/sink characteristics of sedimentary nitrogen in different regions of Lake Longjing were systematically evaluated,the main forms of transferable nitrogen?TF-N?and the fluorescence DOM?FDOM?components and spatial distribution features were analyzed.In addition,the effects of environmental factors on the composition and structure of bacterial and archaea communities in the original reservoir and newly submerged bottom sediments were discussed,and the conceptual models of nitrogen and methane metabolism were constructed.The main research contents and conclusions were as follows:?1?The distribution characteristics of different forms of nitrogen in Lake Longjing were analysed,the source/sink features of inorganic nitrogen in sediment-water interface were determined,and the diffusion fluxes and annual loading contributions of nitrogen were estimated:1)Combining the vertical distribution characteristics of different nitrogen concentrations in overlying and pore water with Fick's First Law,Lake Longjing sediments were characterized by the"source"of NH₄⁺-N,and the diffusion flux ranged from 0.48 to 7.10 mg??m²?d?^-1.The sequence of NH₄⁺-N flux of in different sedimentary background areas was:the original reservoir area>new submerged bottom>new submerged slope.The diffusion flux of NO₃^--N at the sediment-water interface in each zone was small and its expression was complex,flux ranged from-0.28 to 0.31mg??m²?d?^-1.Among the partitions,the original reservoir sediment was characterized by the"source"of NO₃^--N,while the new submerged bottom and slope sediments were acted as"sink".On the whole,the diffusion flux of NH₄⁺-N was higher than that of NO₃^--N,and the sediment in Lake Longjing presented as the"source"of inorganic nitrogen.2)The total nitrogen?TN?content in the surface sediments of Lake Longjing generally presented as original reservoir area?5265 mg/kg?>new submerged bottom?3390 mg/kg?>new submerged slope?1545 mg/kg?.Based on the assessment of TN content in sediments,the sediments of the original reservoir area and the new submerged bottom were at a heavy pollution level with high risk of eutrophication;while the new submerged slope sediment was in a mild pollution state.3)The convertible nitrogen?TF-N?was the main existing form of TN in the surface sediment of Long Jing Lake,and the sequence of nitrogen contents in each formwas residual nitrogen?Res-N,1332 mg/kg?,>strong oxidant leached nitrogen?SOEF-N,984mg/kg?>strong alkali leached nitrogen?SAEF-N,447 mg/kg?>ion exchange nitrogen?IEF-N,240 mg/kg?>weak acid leached nitrogen?WAEF-N,46mg/kg?.The nitrogen morphology of sediments was determined by fractional leaching method,and TF-N was the main form of TN in surface sediments.The sequence of nitrogen contents in the sediments was Residual nitrogen?Res-N,1332 mg/kg?>organic matter-sulfide form nitrogen?SOEF-N,984mg/kg?>iron-manganese oxide form nitrogen?SAEF-N,447 mg/kg?>ion exchange nitrogen?IEF-N,240 mg/kg?>weak acid extractable form nitrogen?WAEF-N,46 mg/kg?.?2?The fluorescence components and spectral characteristics of DOM in the pore water of Lake Longjing sediments of different sedimentary types?the original reservoir area,newly submerged bottom and slope areas?were analyzed by means of UV-Vis and three-dimensional fluorescence spectroscopy:1)The content of organic matter in the surface sediments of Lake Longjing was relatively high,ranging from 1.08 to 8.90%,with an average value of 5.09±2.25%.The sequence of sediment organic matter in each subdivision was original reservoir area>new submerged bottom area>new submerged slope area.The average DOC concentrations in surface water,sediment overlying water and pore water were respectively 22.72±0.51,23.14±1.37,and 53.70±16.61 mg/L,showing a trend of increasing from surface water to bottom sediment.The DOC concentrations of overlying water and surface pore water in each zone were in line with those of sediment organic matter.DOC concentrations of the original reservoir area,the newly submerged bottom and the slope were 65.05,45.76 and 33.93 mg/L,approximately 2.7,1.9 and 1.6times of the corresponding overlying water.It showed that DOC could diffuse from sediment to overlying water.2)The changing feature of spectrum parameters in Lake Longjing showed that the DOM aromaticity and humification degree of pore water were higher than the corresponding overlying water and surface water.Migration of low-molecular weight DOM from sediments to overlying water may occur at the bottom interface.DOM in overlying and pore water was dominated by biological sources,and the original reservoir area has more obvious self-generating characteristics than the new submerged area.With the increase of sediment depth,the proportion of authigenic components decreased gradually.3)Using the parallel factor analysis model,FDOM in surface water,overlying water and pore water was identified to be mainly composed of four fluorescence components:terrestrial humic-like components C1?<225,415?and C3?260/360,445?;protein-like component C2?<225/275,330?and microbially humic-like components C4?255/295,385?.The dominant fluorescence components were various in different areas,the surface water,overlying water from new submerged areas and pore water from new submerged slope was mainly from land source(C1was 37%of F_Tot),while the overlying water and pore water in the original reservoir area and the newly submerged bottom pore water were mainly from the authigenic source(C2was 40%of F_Tot).?3?Based on 16S rRNA DGGE and Illumina MiSeq high-throughput sequencing technology,the community diversity and structural characteristics of bacteria and archaea in the original reservoir area and the newly submerged bottom sediments were analyzed.The distribution of bacterial population was combined with environmental parameters to analyse its function and main influencing factors:1)The results of DGGE showed that the diversity of bacterial community in the original reservoir of Lake Longjingwas higher than that in the new submerged bottom.OR surface sediments had the highest diversity of bacteria,and the Shannon index from the surface to the deep layer decreased first and then increased,which agreed with the changing tendency of abundance and evenness index.High-throughput sequencing results can obtain more sequencing information than DGGE.The species of bacteria belonged to 47 phyla,87 classes,150 orders,232 families and 317 genera.On the whole,the bacteria in the sediments were mainly Proteobacteria,Chlorobacteria,Bacteroidete,Verrucomicrobia and Plancktomycete,accounting for 58.3%⁷1.2%of the total bacterial abundance,the other dominant phyla were Spirochaetae?4.12%⁵.21%?,Cyanobacteria?1.77%⁵.03%?,etc.The archaea were dominated by Euryarchaeota.2)The results of high-throughput sequencing showed that the surface sediments of the original reservoir area were dominated by Proteobacteria,Bacteroidete and Verrucomicrobia,thecorrespondinglymiddlelayerwasproteobacteria,Verrucomicrobia and Chloroflexi,and the deep layer was Chloroflexi,Proteobacteria and Bacteroidete.In contrast,the surface layer of the newly submerged bottom was dominated by Proteobacteria,Bacteroidete and Plancktomycete,and the middle layer was consistent with the corresponding layer in the original reservoir area.In addition,Cyanobacteria,Firmicutes,Parcubacteria and Latescibacteria were mainly distributed in the surface layer of the original reservoir sediments,while Acetothermia and chlorophyta were mainly concentrated in its middle layer.The archaea in the original reservoir area and the new submerged bottom surface sediment were dominated by Methanoregula,and the abundance decreased along the vertical direction.3)Correlation analysis of microbial community structure and environmental variables showed that the bacteria in Lake Longjing sediments were mostly related to the degradation of organic matter,and the bacterial community structure and distribution were affected by total nitrogen,total sulfur content and particle size.The aromatic degree of organic matter in sediments was more related to chlorophenium and acetothermia,that is,the abundance of chlorophenium and acetothermia increased with the increase of depth.This tendencywas consistent with the previous analysis of the community.The positive correlation between Dehalococcoidia and fluorescent component C4 implyed the production of C4 was probably related to the physiological metabolism of Dehalococcoidia.Moreover,by combining high-throughput sequencing technology with KEGG function prediction,a coupling conceptual model of methane and nitrogen metabolism was constructed,in which substances flowed through formates and electrons were transferred through the respiratory process.