Distinguishing The Compositions, Sources And Microbial Degradation Characteristics Of Dissolved Organic Matter In Shizhong River,Jinfo Mountain

In the karst dynamic system,dissolved inorganic carbon?DIC?in water which originated from CO₂ dissolving carbonate rocks is precipitated under certain conditions and releases CO₂ at the same time,therefore,the stability of karst carbon sequestration has been doubted in recent years.However,DIC in karst water may be affected by aquatic photosynthesis that transfers DIC to DOC?dissolved organic carbon?,to be an very important part of DOM?dissolved organic matter?.In recent years,marine carbon sinks have revealed that some photosynthetic bacteria can convert labile dissolved organic matter?LDOM?to recalcitrant dissolved organic matter?RDOM?efficiently,with a circulating scale of more than a thousand years,forming a stable marine carbon stock.In addition,DOM is widely existed in various environmental media around the world,and its composition and structure are very complicated.They are important ligand and adsorption carrier in water environment,exerting significant impact on the migration of heavy metals and organic pollutants.Therefore,studying the composition,origin and stability of DOM in aquatic environment is of great significance for understanding the nutrition cycle of biosphere,the transfer of environmental pollutants,and even the global carbon cycle.In recent years,spectral methods have been widely used because of their low cost,simple processes,complete information and high sensitivity in DOM analysis.We used 3D-fluorencencemeter EEMs combined with UV-visible absorption spectrum in this paper to analyze the composition and origination of DOM in Shizhong River at Jinfo Moutain.9 sampling sites were selected from September in 2016 to August in 2017 to obtain DOM EEMs and UV-visible absorption spectrums.In addition,hydrechemical characters were observed in combination with spectral information to provide scientific evidence for the study of the stability of karst organic carbon sinks.The main conclusions are as follows:?1?The mass fraction of HCO₃^-was>70%,Ca²⁺+Mg²⁺was>80%,as the main anion and cation in Shizhong River,respectively.The area where the upper reaches of the water flows is dominated by shale and limestone,the middle and lower reaches of the area are dominated by limestone and dolomite.The hydrochemical properties of riverwater were mainly controlled by lithology.DOC accounted for about 90%of the TOC in mass concentration,which was the most important part of TOC in Shizhong River.Under the combined influence of environmental factors such as temperature,pH,DIC concentration,aquatic biomass and human activities,the concentration of DOC showed higher values in the downstream than the upstream and higher in the rainy season than the dry season.?2?The DOM of Shizhong River included 3 components,they were fulvic acid,tyrosine-like acid,tryptophan-like acid,accounting for about 38%,26%and 36%respectively;the proportion of endogenous DOM?tyrosine-like acid,tryptophan-like acid?reflects an increasing trend from upstream to downstream,with an average value of approximately 62%in the dry season.In wet season,the fulvic acid was accounted for about 52%,tyrosine-like acid about 29%,and tryptophan-like acid about 19%;the proportion of endogenous DOM was about 48%.?3?Four fluorescence indexes-FI,BIX,HIX and?:?were caculated,in dry season,the mean values were 2.06,0.87,4.35 and 0.66.In wet season,the mean value weres 1.51,0.76,4.50 and 0.66.In general,relatively high FI,BIX,?:?values and low HIX value implied that the DOM was autochthonous and originated mainly from microbes and aquatic plants.Four the absorption of spectral parameters-SUVA254?S_250²90?E₂₅₃/E₂₀₃?M were caculated,in dry season,the mean values were 1.37,0.021,0.015 and 8.01.In wet season,the mean values were 1.72,0.013,0.028 and 8.47,which showed relatively high S_250²90 and M values and low SUVA254,E₂₅₃/E₂₀₃;indicating that the molecular weight of DOM in Shizhong River was smaller,the fulvic acid was higher than humic acid,and the aromatic characters of the DOM were not significant,but likely to be hydrophobic.?4?According to microbial degradation experiments,the two protein-like components C2and C3 disappeared,the humus-like component C1 was preserved,and a new humus-like component C5?fulvic acid-like?was produced.Experimental results showed that endogenous DOM produced by microbial activity and algae decomposition had high activity,which can be used by microorganisms in the short term,but the newly generated DOM in this process was less active,more stable in the water,the cycle decomposition was between 5 to 15months with an average of about 9 months.