The Composition And Structure Characteristics Of Soil Dissolved Organic Matter And Its Effect On Cadmium Phytotoxicity

Soil dissolved organic matter(DOM)is the most active portion of soil organic matter,which plays the important roles in the adsorption and complexion of heavy metals in soil.The heterogeneity and variability of the compostion and chemical constitution of DOM may cause the variation of binding capacity of metal and DOM and thus influence the speciation,transportation and bioavailability of soil metals.In this study,13 representative woodland soils in the country were selected,and the structural characteristics of soil DOM were analyzed by infrared,ultraviolet and three-dimensional fluorescence spectroscopy,and the complexation of DOM and its components with heavy metal Cd was analyzed by fluorescence quenching titration technique and nonlinear regression.The effects of DOM active components(fulvic acid and humic acid)on plant toxicity of Cd were evaluated by hydroponics toxicity test.The main conclusions are as follows:(1)Spectral characterization experiments showed that the DOM of woodland soil contained many functional groups such as carboxylic acids,alcohols,alkanes,phenols and polysaccharides.SUVA254 was significantly correlated with SUVA260(r= 0.992,P<0.01),indicating that hydrophobicity was closely related to aromatic structure,and soil p H and Mg2+ were significantly correlated with SUVA254 and SUVA260.Three dimensional fluorescence coupled parallel factor analysis(ems-parafac)model was used to analyze four spectral components,including fulvic acid,two humic acids and tyrosin-like substances.According to the Fluorescence index(FI)and Biological index(BIX),the source of forest soil DOM has two characteristics of endogenous and exogenous.(2)Fluorescence quenching titration experiments showed that the binding ability of fulvic acid and protein-like proteins to heavy metal Cd was stronger than that of humic acid.The complexation reaction of fulvic acid and tyrosine with Cd can be fitted with the modified stern-volmer model.The conditional stability parameters and fluorescence quenching rate indicate that the binding capacity of fulvic acid component to heavy metal Cd is greater than that of tyrosine component,and more fluorescent groups are involved in coordination than that of protein-like component.The complexing ability of DOM and Cd in different regions is affected by the properties of soil / soil solution(p H,conductivity EC and Ca2+).(3)The phytotoxicity experiment showed that Cd had obvious growth inhibition effect on Ligustrum lucidum.The composition of DOM(HA:FA)has a greater influence on the morphology and phytotoxicity of Cd.As the proportion of fulvic acid decreases,the activity of free Cd2+ increases,the phytotoxicity threshold of Cd decreases,and the toxicity increases.