The Flooding Behavior And Structure Characteristics Of Soil Dissolved Organic Matter In Water-Level-Fluctuating Zone Of Three Gorges Reservoir Areas

The Three Gorges Reservoir is a remarkable large regulating reservoir,which has continuously attracted worldwide attention and its flooding areas is 632km2.The water-level fluctuation zone is flooding during September to the next April.As the region of connected with terrestrial and aquatic ecosystem,the water-level-fluctuating zone of Three Gorges Reservoir Areas is an active region of energy and material transformation,which has the buffer zone and vegetation protection function make it is sensitive to the change of external.Due to the huge content of soil organic matter,it plays an extremely important role in global carbon cycle.As the most active portion of soil organic matter,dissolved organic matter?DOM?has an important influence on the adsorption,desorption and transport of nutrients,heavy metals and organic pollutants,as well as the global carbon cycle,which has gradually become the focus and hot spot in the research of soil and environmental pollution.For the typical characteristic of “dry-wet alternation”,the soil DOM will be an important source for the overlying water during the flooding process,but the vegetation growth and land utilization will change the structure and properties of soil DOM during dry period,which will make the process more complicated.This exchange behavior will affect the carbon balance and material transformation between terrestrial and aquatic ecosystem,further significant impact on the migration and transformation of various pollutants and biological availability.Therefore,in the Three Gorges Reservoir area,the flooding behavior and structural characteristics of soil DOM will affect the environmental behavior and effects of various types of substances.In this study,four typical site in water-level-fluctuating zone of Three Gorges Reservoir Areas were selected to illustrate the flooding behavior and structural characteristics of soil DOM combined with ultraviolet-visible absorption spectra and three-dimensional fluorescence spectroscopy,in order to contribute to a better understanding the geochemical characteristics and environmental role of soil DOM in Three Gorges Area.The results showed that:?1?For all soils,DOM releases patterns all showed “rapid release,then decrease,and final dynamic equilibrium” trend.Also,through calculations of DOM releases rates and fluxes,DOM releases clearly showed a feature of “source-sink dynamic exchange”.By comparison between two flooding treatments,although DOC and CDOM didn't show any significantly differences,but higher aromaticity?SUVA280?was observed in anoxic experiment.In contrast,spectra slope ratio?SR?was insensitive to redox conditions of different floodings.Meanwhile,correlation between DOC and CDOM in anoxic was also higher than in ambient flooding.Additionally,obvious differences among soils from different sampling sites was observed,Shibao Zhai?SB?of which showed the greatest releases potential.Considering the various soil properties,soil organic matter amounts in soils is one of the key factors to explain the differences among sampling sites,but other geochemical factors should be also considered in the processes of DOM releases.Summarily,only focusing on the changes of DOC concentrations in submerging process is not enough to comprehensively describe the DOM release characteristics.Importantly,combined with DOM and Hg date,DOM showing higher aromaticity and humification at the initial release stage when flooding occurs,suggests greater terrestrial source characteristics,which further is helpful to explain the environmental fates of pollutants in this environmental sensitive areas.Also it would be useful for unveil the role of DOM in environmental system in future.?2?By using fluorescence spectrum technique,the dynamic and geochemical characteristics of DOM releases from flooding soils were investigated.Results showed that the DOM releases trend or model observed by fluorescence spectrum in all soils from four sampling sites,was similar with observation by UV-Vis spectrum.Fluorescence property of soil DOM releases showed an important contribution to DOM fluorescence of overlying waters.The rapid releases at initial stage and removal mechanism for later dynamic equilibrium,was a crucial to explain the dynamic “source-sink” equilibrium in whole flooding period.Effect of inorganic mineral adsorption-desorption on humic-like components?A and C peaks?was significant.Also,impact of microbial utilization on protein-like components?B and T peaks?was confirmative.Additionally,all DOM samples had “autochthonousness plus allochthonousness” property.Except Shibaozhai?SB?site in Zhongxian county,during the whole flooding period,other three sites didn't show any significantly difference between ambient and anoxic conditions.They could be explained by highly heterogeneity of soil property including minerals and organic components.Meanwhile,both of UV-Vis and fluorescence spectra are so complementary for each other,they evidentially showed that DOM releases from submerged soils involving “source-sink”?release and removal?mechanism is the core to decide dynamics of DOM in underlying waters.?3?The soil DOM biodegradation results showed the decreased of doc was 33.51⁴2.91% in anoxic while 19.92³4.70% in ambient during the 28 d incubation.The difference was significant and the Kai Xian?KX?had the lowest biodegradability.The biodegradability of CDOM ranged from 5.10¹6.98% in open air and 19.92³4.70% in anoxic,also the difference was significant and the non-chromophoric component in the degradation process can be preferentially used.A double exponential model was fitted and the portion of labile was lower in the anoxic while the refractory was greater compared to ambient set.The MRT were 1.02³.45 d and 58.82¹11.11 d between two sets and the oxygen content was an important factor affecting the efficiency of microbial degradation.There was significant increase but no significant differences in aromaticity degree between ambient and anoxic after decomposition.The spectra slope?S275-295?decreased in anoxic but no changed in ambient.The correlation analysis between S275-295?CDOM and SUVA254 showed that aromatic structure having greater contribution to DOM molecular weight,which reflecting the differences in microbial degradation of the polymer component.The degradation ratio difference between two types of fluorescence peaks demonstrated that protein-like fluorescence had higher bioavailability,and the increased of BIX and HIX index showed there was some kinds of new DOM created and humification degree increased.Used the initial portion of protein-like fluorescence has a broader applicability while predict the BDOC.?4?The DOC pool?[DOC]?and the soil adsorption constant?Ks?of four site soil DOM showed significant differences,the [DOC] of Tu Jing?TJ?had the highest,was 1.634 mg·g^-1,while Fu Ling?FL?was the lowest 0.612 mg·g^-1.The Shi Bao?SB?had the highest Ks while Kai Xian?KX?was the lowest.The correlation analysis showed that the chromophoric dissolved organic matter?CDOM?were significantly positively correlated with DOC,which means the proportion of CDOM was more stable.Spectral parameters showed that the hydrophobic components were closely related to the aromatic structure,and the soil DOC was significantly affected by the biological sources.?5?The H/C of soil DOM was 2.13³.83 means higher degree of fatty,while fulvic acid and humic acid has higher aromaticity;The FTIR spectral characteristics showed that they were similar to a certain degree,DOM contained more carbohydrates,organic acids,sugars and other substances,while aromatic structure were rich in fulvic acid and humic acid;Also,the UV and fluorescence spectrum demonstrated that fulvic acid and humic acid had high aromaticity and humification degree.