Effects Of Soil Organic Carbon Source And Molecular Structure Characteristics On Mercury Methylation In Liaohe Estuary Wetland

Among the many factors that affect mercury methylation,DOM（Dissolved Organic Matter,DOM）is considered to play a vital role in the fixation,migration and transformation of mercury（Hg）.It has more active sites and can be used as a carrier for heavy metals.It directly combines with Hg²⁺in the soil and water to produce a series of reactions,so that more mercury is stored in the overlying water or adsorbed by the surface soil.However,the influence mechanism of DOM source,composition and molecular structure characteristics on mercury methylation is still unclear.This paper systematically studied the distribution of Hg and MeHg content in the soil of different depths,different salinity regions and different vegetation types in the wetland of Liaohe Estuary,and studied the biomass and community structure changes of soil microbial populations by means of phospholipid fatty acid（PLFA）.Ultraviolet-visible spectroscopy combined with three-dimensional fluorescence spectroscopy analysis technology was used to explore the source,composition and structural characteristics of DOM,and the relationship between DOM,environmental factors and mercury methylation in soil was revealed through multivariate statistical analysis methods.The main conclusions are as follows:（1）Salinity and vegetation types have a significant impact on the existence of mercury.The content of soil Hg and DOC is highest at a depth of 10-20cm.Vegetation coverage and soil with a depth of 0-10cm are the key to the accumulation of MeHg and soil microorganisms.The high-salt environment is not conducive to the accumulation of MeHg and microorganisms in the soil.The soil PLFAs content is low at the depth of10-20cm,and the soil salinity is too high will significantly reduce the number of species.（2）On the salinity gradient,DOM transforms from humification to non-humification.The degree of molecular condensation tends to decrease,the protein and humic acid components of DOM tend to decrease,while the content of fulvic acid increases.trend.In the vertical distribution,the degree of humification of DOM increased,the degree of molecular condensation decreased,and the difference between components was not obvious.Plant activities will increase the proportion of DOM hydrophobic components in the soil and reduce the degree of DOM molecular condensation.The content of protein-like components in reed soil is significantly higher than that in Guangtan soil.（3）Through fluorescence spectroscopy combined with parallel factor analysis（EEMs-PARAFAC）,the soils of different depths in the Liaohe Estuary wetland were obtained.There are five main types of DOM,including fulvic acid-like,humic acid-like,and protein-like;according to Fmax%,it is found that the DOM is like protein The content is highest,followed by fulvic acid-like,and humic acid-like content is the least.Spectral index analysis shows that DOM is mainly self-generating,with strong microbial availability and weak humification.（4）The increase in salinity facilitates the release of Hg from the soil to the overlying water.In the salinity gradient,the degree of humification and hydrophobic components showed an increasing trend,the protein and humic acid components of DOM had a decreasing trend,and the content of fulvic acid showed an increasing trend.The protein-like components have the highest proportion of fluorescence intensity,and the metabolic activities of microorganisms will produce abundant protein-like substances to make organic matter difficult to be humified.（5）The higher the degree of DOM humification,the more favorable the occurrence of Hg methylation.Inorganic mercury input is an important source of MeHg pollution in the surface soil of the Liaohe Estuary wetland.The occurrence of mercury methylation is greatly affected by the fulvic acid component,and external input from estuaries and coastal areas can inhibit methylation.Different organic matter components have different migration and transformation effects on mercury.Small molecular weight DOM such as protein-like components have more mercury binding sites.Anaerobic bacteria,fungi,and gram-negative bacteria have a strong effect on Hg,and salinity,p H,and organic carbon content have a very important effect on spectral indicators.