Study On The Mechanism Of Agricultural Dissolved Organic Matter Regulating Mercury Methylation And Bioaccumulation In Paddy Soi

Mercury（Hg）is a toxic heavy metal element with properties such as persistence and global distribution,which can exist in different forms in the natural environment,among which methylmercury,which has strong bioaccumulation and neurotoxicity,can threaten human health through the amplification effect of the food chain.Paddy field is a key site for mercury methylation,and rice exhibits a strong enrichment capacity for methylmercury,so rice consumption may be the main route of methylmercury exposure in populations in contaminated areas.Straw return increased the methylation of soil mercury and the accumulation of methylmercury in rice,and increased the risk of methylmercury exposure produced by edible rice,but previous studies could not directly understand how DOM from agricultural sources such as straw differentially affected the structure and composition characteristics of soil DOM,and then regulated the methylation of soil mercury and the enrichment of methylmercury in rice tissues.Therefore,in this study,the composition characteristics of DOM were characterized from different types of agricultural DOM and the differences between different molecular weight components of DOM from agricultural sources as the research object,and 1)pot experiments were constructed to add different types of agricultural source DOM（rice straw DOM,compost rice straw DOM,cow manure DOM,compost cow manure DOM,maize straw DOM,rape straw DOM）at the rice seedling stage.2)Add rape straw DOM and compost rape straw DOM（F0（ungraded stock solution,< 0.45μm）,F1（30k Da ～ 0.45μm）,F2（3k Da ～ 30 k Da）,F3（<3k Da））potting experiment;3)Adsorption kinetics experiments of different molecular weight components of rape straw DOM and compost rape straw DOM with Hg²⁺,through the quantitative analysis of the characteristics of soil DOM ultraviolet-visible and fluorescence spectra in potting experiments,as well as the total mercury and methylmercury content of the soil-interstitial water-rice system,and the adsorption characteristics of different molecular weights of DOM and Hg²⁺ were analyzed,and obtains the following results:（1）After adding DOM from different agricultural sources to mercury-contaminated soil at the seedling stage,the DOM concentration of soil DOM at the tillering stage was significantly reduced by about 60.16%（P < 0.05）compared with the control,and there was no significant difference in the other treatment groups.After entering the filling stage,the soil DOC concentration in the compost cow manure DOM treatment group was significantly reduced by 40.66% compared with the control group（P < 0.05）.At the maturity stage,the soil DOC concentration in the compost rice straw DOM treatment group was significantly higher than that in the rice straw DOM treatment group by 76.24%（P < 0.05）,while the soil DOC concentration in the compost cow manure DOM treatment group was significantly lower than that in the control group and the cow manure DOM treatment group by 42.40% and 51.21%（P < 0.05）,respectively.All agricultural DOMs increased the aromaticity,and the small molecular weight components of soil DOM（P < 0.05）were significantly increased at the filling and maturation stages.Among them,rape straw DOM also reduced the degree of humification of soil DOM in various rice growth periods.（2）After adding rice straw DOM,compost rice straw DOM,cow manure DOM,compost cow manure DOM,maize straw DOM and rape straw DOM to mercury-contaminated soil at the seedling stage,the concentration of methylmercury in soil increased by 17.93% ～ 227.26%（P < 0.05）in the seedling stage,but only the addition of some agricultural source DOM increased the concentration of total dissolved mercury and dissolved methylmercury in pore water.In all treatments,the rice straw DOM,compost rice straw DOM,and compost cow manure DOM treatment groups significantly increased the total mercury and rice methylmercury contents by33.91% ～ 63.79% and 32.06% ～ 118.02%,respectively.In particular,the bioconcentration coefficients of methylmercury,total mercury and methylmercury in rice grain of the rape straw DOM treatment group were reduced by 59.15%,27.20%,and 24.91%,respectively,compared with the maize straw DOM treatment group,and43.80%,42.03% and 60.32% lower than that of rice straw DOM treatment.（3）Through ultraviolet-fluorescence spectroscopy analysis,among the components of rape and compost rape straw DOM,the CDOM concentration of F1 and F2 components was higher,followed by the F0 component,and the lowest F3 component,the aromaticity showed the law of F1 > F2 > F3,and the E2/E3 values increased with the decrease of molecular weight,following the law of F1 < F2 < F3.The degree of humification of each component of compost rape straw DOM was significantly higher than that of rape straw DOM（P < 0.05）,and the highest degree of humification was the F2 component among the components of rapeseed and compost rape straw DOM.Through parallel factor analysis,the content of protein substances in each component of rape straw DOM was significantly higher than that in compost rape straw DOM（P < 0.05）,among which the proportion of protein substances in the F3 component of rape straw DOM was the highest,reaching 85.32%.（4）In the rape straw DOM and compost rape straw DOM components,DOC concentrations showed F2 > F1 > F0 > F3,and the DOC concentrations of F0,F1,F2 and F3 components of the DOM of composted rape straw were 62.05%,23.80%,18.11%,and 79.50% lower than those of F0,F1,F2 and F3 components of rape straw DOM（P < 0.05）.Through XPS analysis,among the components of rape straw DOM,the S element content of the F3 component accounted for the largest proportion（1.57%）and the S element content of the F0 component accounted for the smallest proportion（0.54%）,but among the components of compost rape straw DOM,the S element content of the F3 component accounted for the smallest proportion（1.31%）,and the S element content of the F0 component accounted for the largest proportion（6.07%）.In the adsorption kinetics experiments of different molecular weight components of DOM and Hg²⁺,the equilibrium adsorption capacity followed the law of F1 > F2 > F3.（5）On the third day after adding different molecular weight components F0,F1,F2 and F3 of rape straw DOM and compost rape straw DOM to mercury-contaminated soil during the filling stage,the soil DOC concentration increased by 6.60% ～ 62.16% compared with the control group.The concentration of SO42-in soil was also significantly reduced,with a reduced rate of 35.41 ～ 89.33%.Compared with the control group,the soil methylmercury content in each treatment group was significantly reduced,and the reduction range was between 42.40% ～62.42%.（6）After adding different molecular weight components F0,F1,F2,and F3 of rape straw DOM and compost rape straw DOM to mercury-contaminated soil during the filling stage,compared with the control group,the enrichment of methylmercury in rice roots（5.58% ～ 91.32%）,and the F2 component of composted rape straw DOM had the greatest promoting effect,promoting 91.32%.However,compared with the control group,the coefficient of methylmercury transport to rice in each treatment group was reduced,with a reduced rate of 7.83% ～ 45.87%,and the inhibitory effect on methylmercury transport increased with the decrease of the molecular weight of rape straw DOM,while the opposite was true for compost rape straw.Among them,the F1 component of DOM of compost rape straw had the strongest inhibitory effect on methylmercury transport compared with the control group,and the inhibitory efficiency was 45.87%.