Methylation And Demethylation Of Mercury In Rhizosphere Soil By Double Isotope Trace Method

Mercury?Hg?is a global pollutant with environmental persistence,high toxicity and bioaccumulation.In the natural environment,inorganic mercury?IHg?can be converted into more toxic methylmercury?MeHg?under the action of biotic and abiotic,and MeHg can be further enriched and enlarged along the food chain,posing a threat to biological and human health.Therefore,the generation and degradation of methylmercury has received increasing attention.Existing studies have shown that diversity,temperature,p H,and organic matter of microbial communities are the key factors affecting Hg methylation in the soil environment.The rhizosphere is a micro-domain soil area in the soil that is affected by the root system and plant growth activities.Its physical and chemical properties,root exudates,and rhizosphere microorganisms are significantly different from the original soil.The special physicochemical properties of the rhizosphere environment may significantly affect the methylation and demethylation of mercury?Hg?.In order to confirm this hypothesis,we investigated the concentration of total mercury?THg?,methyl mercury?MeHg?,and the critical physical and chemical properties in the rhizosphere soil of Cynodon dactylon and bulk soil in the water-fluctuating zone of the Three Gorges Reservoir Region?TGRR?,and further a double-isotope tracer method(¹⁹⁹Hg²⁺and Me²⁰¹Hg)was used to conduct indoor simulated culture experiments to explore and compare the methylation rate of IHg and demethylation rate of Me Hg in rhizosphere soil under different culture conditions?bacteria,sterility,hypoxia,normoxia?,exploring the methylation and demethylation of mercury in rhizosphere soil base effect.At the same time,in order to further explore the influencing factors of Hg methylation and demethylation in rhizosphere soil,this study also used isotope tracer methods(¹⁹⁹Hg²⁺and Me²⁰¹Hg)to conduct culture experiments on the rhizosphere soil bacteria suspension,respectively Exploring the rate of mercury methylation and demethylation in rhizosphere soil culture medium after adding different types of low molecular weight organic acids?citric acid,oxalic acid,tartaric acid?and sulfate ions under non-sterile and sterilized culture conditions Influencing factors of mercury methylation and demethylation in soil.The findings are as follows:?1?The concentrations of MeHg in rhizosphere soil is significantly higher than that in bulk soil?p<0.01?.The rhizosphere microdomain is an active area for IHg methylation.?2?Fe²⁺,Mn²⁺,organic matter concentrations,and catalase activity in rhizosphere soil were significantly higher than those in bulk soil?p<0.05?,and the number of bacteria and fungi was extremely significantly greater than that in bulk soil?p<0.01?;There was a significant positive correlation between the concentrations of MeHg in the rhizosphere soil and the concentrations of Mn²⁺,SO₄^2-,and organic matter?p<0.05?,and there was an extremely significant positive correlation with the activity of catalase,the number of bacteria and fungi?p<0.01?;Hg methylation,demethylation rate,and net methylation potential were significantly greater in rhizosphere soil than in bulk soil?p<0.05?.Rhizosphere effect has an important effect on the methylation process of Hg.Rhizosphere soil can promote the methylation of Hg to a certain extent.?3?The rate of methylation and demethylation of mycorrhizosphere soil was significantly higher than that of aseptic rhizosphere soil?p<0.05?;the rate of methylation of hypoxia cultured rhizosphere soil was significantly higher than that of normoxia cultured rhizosphere soil?p<0.05?,and the demethylation rate of rhizosphere in normal oxygen culture was higher than that in rhizosphere of low oxygen culture.The special oxygen environment in the rhizosphere also affects the methylation rate of soil Hg.Both the methylation of IHg and the demethylation of MeHg in rhizosphere soil are affected by microorganisms and non-microorganisms,among which microorganisms show a major role.?4?Low molecular weight organic acids and sulfates have certain effects on the methylation of IHg and the demethylation of MeHg in rhizosphere soil.Among them,organic acids can promote methylation of rhizosphere soil.The methylation-promoting effect on IHg is shown as citric acid>tartaric acid>oxalic acid;the demethylation effect on MeHg is as follows:citric acid inhibits demethylation of MeHg,and tartaric acid and oxalic acid weakly promote demethylation.Sulfate showed a methylation-promoting effect on IHg,while inhibiting the demethylation effect of MeHg.The combined effect of citric acid and sulfate has a significant enhancement effect on the methylation rate of IHg,and also enhances the demethylation inhibition effect on MeHg.