Changes Of Spectral Characteristics Of DOM With Different Molecular Weight Levels In Poyang Lake And Their Effects On The Species Of Copper And Lead In The Rhizosphere

Poyang Lake Wetland is the largest freshwater lake wetland in China,which has a very important ecological regulation function.After analyzing and evaluating its heavy metal pollution,it was found that Poyang Lake’s water body,shore soil and sediments were all polluted by heavy metals to varying degrees.Dissolved organic matter(DOM),as a highly mixed organic matter,is widely used to study the interaction with heavy metals.Molecular weight grading is an effective way to divide DOM into different material compositions.Plant-derived DOM has an obvious self-generating source.Therefore,the use of ultrafiltration technology to classify the molecular weight of plant-derived DOM provides a theoretical basis for studying the influence of plant-derived DOM with different molecular weights on heavy metals Cu and Pb.In this study,under the conditions of Cu and Pb pollution,the dominant vegetation Carex cinerascens of Poyang Lake was selected as the raw material for extracting DOM,and the molecular weight of Carex DOM was classified,and the UV,infrared and three-dimensional fluorescence spectrum characteristics of different molecular weight Carex DOM components were compared.Collecting the wetland soil and growing Carex on the shore of Poyang Lake,design pot experiment to add different components of DOM to simulate the results of the release of organic matter into the environment after Carex itself decays.Discuss how different components of DOM affect the absorption and form transformation of Cu and Pb in the plant-soil system,as well as the periodic index changes of the rhizosphere environment(p H,EC,Eh,TOC,A250/A365).The main results are as follows:(1)Using ultrafiltration technology to classify the molecular weight of plant-derived DOM,the p H of different molecular weight DOMs is negatively correlated with the molecular weight,and the F4 component of <1 k Da has the highest p H of 7.11.Total organic carbon(TOC)is positively correlated with molecular weight.The F1 component >10 k Da has the highest TOC concentration of2151.8 mg/L.The results of ultraviolet spectroscopy showed that different molecular weight DOMs may contain aromatic aldehydes and ketones,aliphatic aldehydes and ketones or α,β-unsaturated carbonyl compounds,SUVA254 is positively correlated with molecular weight,the smaller the molecular weight,the lower the degree of humification of DOM.A250/A365 is negatively correlated with molecular weight.The larger the molecular weight,the DOM has higher aromaticity and affinity for organic pollutants.The F0,F1 and F2 components are mainly composed of humin,and the F3 and F4 components are mainly composed of fulvic acid.Infrared spectroscopy results show that there are a large number of C-C unsaturated structures,O-H groups,alkyl compounds,aliphatic hydrocarbons,organic acids,and small sugar molecules in DOMs of different molecular weights that can polymerize with aromatic rings as the core.The F0,F1 and F2 components have more aromatic substances,while the F3 and F4 components contain more proteins,peptides and amino acids.Three-dimensional fluorescence spectroscopy results show that Carex DOM is rich in fulvic acid and protein substances.The biological index BIX and the fluorescence index FI are negatively correlated with the molecular weight of DOM,and the humification index HIX value is positively correlated with the molecular weight of DOM.The F1 and F2 components with larger molecular weight may be input from both internal and external sources,and the F3 and F4 components have obvious characteristics of autogenous origin.Fluorescence area integration results show that in different grades of DOM,the proportions of zone IV microbial metabolites or zone V humic acid substances are the highest,and the proportion of zone I tyrosine substances is the lowest.The F1 component has the highest proportion of aromatic proteins in zone II and fulvic acid in zone III.Microbial metabolites accounted for the highest proportion of F3 and F4 components.(2)The results of the potted rhizosphere bag experiment showed that the absorption of Cu by the lower and above-ground parts of the moss and the enrichment coefficient of roots and leaves increased with the decrease of DOM molecular weight,while the transport coefficient was the opposite.The F1 of the highest molecular weight DOM The treatment group had the highest transport coefficient of 0.479.The uptake of Pb is different from the above ground and underground parts.The above ground part absorption and leaf enrichment coefficient and transport coefficient also increase with the decrease of DOM molecular weight.The F4 treatment group with the smallest molecular weight has the highest transport coefficient 0.571.The absorption and root enrichment coefficient of the underground part decrease with the decrease of DOM molecular weight.The addition of DOM with different molecular weights significantly reduced the enrichment coefficient of Cu and Pb in the roots of Carex.(3)The content and form of Cu and Pb in rhizosphere soil were analyzed.Under different molecular weight DOM treatments,the content of heavy metals in rhizosphere soil decreased significantly(P<0.05).In the F0～F4 treatment group,The most significant effect of Cu absorption in rhizosphere soil was in the F4 treatment group(P<0.05),which decreased by 23.11 mg/kg compared with the 0d treatment group.The most significant effect of Pb absorption was in the F0 treatment group(P<0.05),which was reduced by 37.11 mg/kg compared with the 0d treatment group.With the decrease of the molecular weight of exogenous DOM,the effective state(acid-soluble state)of Cu accounts for 30%～33%,and the stable state(organic combined state and residue state)accounts for 41%～28%,showing a good effect on Cu.activation.The effective state(acid-soluble state)of Pb accounts for 9% to 7%,and the stable state(organic combined state and residue state)accounts for 45% to53%,showing the passivation of Pb.(4)Observe the changes in the physical and chemical indexes(p H,EC,Eh)and characteristic indexes(TOC and A250/A365)of the rhizosphere soil during the culture cycle.The p H range is 4.96～5.56,the overall acidity is relatively acidic,and there is no significant difference between the groups.The conductivity EC first rises,then falls,and finally rises again.The redox potential Eh of the F0,F1,and F2 treatment groups decreased between 7 and 14 days,and then increased after 14 days and 21 days.The F3 and F4 treatment groups were the opposite.Compared with the control group,the F0～F4 treatment group can increase the TOC concentration in the first 7 days,and the increase of F3 and F4 is higher than that of F0～F2.F0 and F1 can improve the humic degree of rhizosphere soil.The F2～F4 components will reduce the aroma and humification degree of the rhizosphere soil at the initial stage.The comparison results of infrared spectra of the rhizosphere soil DOM showed that all the DOM except the F2 component can change the material composition and structure of the DOM in the rhizosphere soil of Carex to some extent.