Transfer And Transformation Mechanism Of Exogenous Sulfur On Methylmercury In Soil-rice System

Mercury（Hg）is a toxic,non-essential element and one of the world’s harmful heavy metals.It can be transported over long distance in the atmosphere and has the characteristics of accumulation and persistence.Due to long-term Hg mining,Hg pollution in the surrounding farmland is becoming more and more serious.Since researchers has realized that rice has the capability to bio-accumulate methylmercury（Me Hg）,and then the consumption of rice in the Hg mining area was considered as the main route of Me Hg exposure to residents in Hg mining areas.Sulfur is an essential nutrient element for plants.Increasing attention has been paid to understand the effects of agricultural application of sulfur fertilizer on heavy metals transportation in soil-crop systems.Recently,some studies have reported that sulfur input can inhibit the accumulation of Me Hg in rice,while others reported that sulfur input can promote the accumulation of Me Hg in rice.Therefore,the specific influencing mechanism of the effect of sulfur input on the uptake of Me Hg by rice remains unclear.The security of paddy ecosystem plays a crucial role in food safety.Considering the increasing food demand worldwide,decreasing arable land area and increasing pollution of water,soil and air,it is of great importance to understand production of Me Hg in soil rice ecosystem and its migration and transformation mechanism in soil-rice system to ensure the health and rice quality safety.In this study,two kinds of soil,GX soil（dominated by new deposited Hg）and WK soil（mainly affected by the slag Hg）,were collected in Wanshan Hg mining areas.Pot experiments were performed in Gouxi（high atmospheric Hg deposition flux）and Shenchongkou（middle atmospheric Hg deposition flux）in Wanshan Hg mining areas,Tongren,and Institute of Geochemistry Chinese Academy of sciences（low atmospheric Hg deposition flux）in Guiyang,Guizhou Province.The concentration and distribution characteristics of total mercury（THg）and Me Hg in roots,stems,leaves,and grains of rice after different sulfur（sodium sulfate）treated were revealed;the abundances of C,N,P,and S functional genes were also determined to reveal the impacts of sulfur on microbiome;stable Hg isotope techniques were employed to investigate mechanism that how does the sulfur addition influence the Hg fractionation during THg and Me Hg transportation.Generally,in this study,the effects of exogenous sulfur on the transfer and transformation of Me Hg in paddy field-soil system and its influencing mechanism were systematically studied.The following conclusions are drawn:（1）Soil pot experiment showed that THg,Me Hg,and the ratio of Me Hg in rice grains could be significantly reduced in GX soil and WK soil after adding exogenous sulfur at different gradients（0,50mg/kg,100mg/kg,300mg/kg,500mg/kg,and 1000mg/kg）.Experimental GX soil showed that the THg content of rice in Gouxi,Shenchongkou and Guiyang pot treatment groups decreased by 25.4%～46.7%,52.2%～72.7%and24.8%～56%,respectively,compared with the control group.Compared with the control group,Me Hg in rice were decreased by 65.9%～74.5%,29%～70.7%and 13.9%～73.1%,respectively.When sulfur input was 50mg/kg,the maximum reduction of THg concentration was 72.7%,and the corresponding THg of rice was 0.024 mg/kg.When sulfur input at 100mg/kg,the maximum reduction of Me Hg in rice was 74.5%,and the corresponding Me Hg content of rice was 17±2.55μg/kg.Experimental WK soil showed that the THg content of rice in Gouxi,Shenchongkou,and Guiyang treatment groups decreased by 7.1%～45.3%,1%～77.6%and10.3%～55.8%,respectively,compared with the control group.The reduction ranges of Me Hg contents in rice compared with the control group were as follows:31.4%～71.7%,15.4%～73.7%,and 22.8%～65.8%;when sulfur input at 1000 mg/kg,the maximum decrease of THg and Me Hg in rice were observed.The THg decreased 77.6%,corresponding to 0.018mg/kg of rice THg,Me Hg decreased 73.7%,corresponding to 13±1.26μg/kg of rice Me Hg.The addition of exogenous sulfur can reduce the accumulation of Me Hg in rice stem and leaf,but increase the accumulation of Me Hg in soil and root.The maximum reduction of Me Hg in stem and leaf was 91.2%and 50.8%,respectively,and the corresponding Me Hg concentrations in stem and leaf were 1.13±0.04μg/kg and 5.53±1.10μg/kg.The experimental results showed that exogenous sulfur input could effectively reduce Me Hg and THg in rice grains,and with the decrease of atmospheric Hg concentration,Me Hg in grains of experimental GX soil group decreased more significantly than that of WK soil group,suggesting that atmospheric deposited Hg was highly active,acting as an important source of Hg methylation in soil microorganisms.After exogenous sulfur input,Me Hg increased in soil and roots,and it was speculated that the reason for the decrease of Me Hg in stems,leaves and grains might be related to the fact that sulfur input promoted the formation of iron plaque on root,and the iron plaque can block the migration of Me Hg to the aboveground part.（2）Non-conventional mass fractionation of Hg isotope(MDF,δ²⁰²Hg)and non-mass fractionation(MIF,δ¹⁹⁹Hg,δ²⁰²Hg)showed that the MDF and MIF of Hg in rice grains were significantly affected by the addition of exogenous sulfur.The values of Me Hgδ²⁰²Hg were more negative than those of inorganic Hgδ²⁰²Hg.Theδ²⁰²Hg values of grains showed a significant decreasing trend.Non-mass fractionation of“root-leaf”Hg isotope was significantly different in paddy soils with different types of Hg contamination and in the same atmospheric Hg condition,suggesting the dominant role of soil.（3）Field diurnal temperature difference analysis showed that with the decrease of diurnal temperature difference（6.11℃～3℃）,the concentrations of THg and Me Hg in rice in the control groups showed an increasing trend.The THg content in rice grains in the control groups ranged from 0.8±0.25 mg/kg to 1.2±0.16 mg/kg.The concentrations of THg in rice grains in control groups are as follows:0.40±0.013 mg/kg～0.57±0.051 mg/kg,the content of Me Hg in rice grains of sulfur-treated groups is 218.51±8.28μg/kg～480±143.08μg/kg,and the Me Hg in rice grains of sulfur-treated groups are within the range of 20.33±1.43μg/kg～37.70±10.30μg/kg.With the decrease of diurnal temperature difference,the rising trend of THg and Me Hg in rice was offset by sulfur addition,indicating that the sulfur addition played a more important role than temperature on THg and Me Hg in grains in this study,that is to say,temperature did not substantially influence the effect of sulfur addition.The THg and Me Hg in the sulfur-treated groups were significantly lower than those in the control groups,indicating that the addition of exogenous sulfur could reduce the accumulation of THg and Me Hg in rice grains under the gradually decreasing diurnal temperature difference.With the decrease of diurnal temperature difference,the Me Hg content in rice grains,stems,leaves decreased significantly after sulfur addition compared with that without sulfur addition.When the diurnal temperature difference was 5.08℃,the Me Hg content in rice grains decreased by 93.8%,and when the diurnal temperature difference was3.9℃,the Me Hg content in rice stems was decreased by 97.2%.When the diurnal temperature difference was 3.9℃,the Me Hg content in rice leaves decreased by 90.5%,and when the diurnal temperature difference was5.66℃,the Me Hg content in rice rhizosphere soil was decreased by 58.3%.The results showed that adding sulfur could reduce the accumulation of Me Hg in rice grains,stems and leaves.（4）Experiment of soil microbes,71 C,N,P,and S key functional genes analysis showed that WK soil had a rich initial microbial functional gene abundance than a GX soil,the 57 C,N,P and S related functional genes were detected.Of them,29 were carbon cycle genes,19 nitrogen cycle related genes,6 phosphorus cycle related genes,and 5 sulfur cycle related genes.With the addition of exogenous sulfur,the abundance of microbial C,N,P and S functional genes did not change significantly,suggesting that the total abundance of soil functional genes was not affected by exogenous sulfur.Moreover,the abundance of sulfur circulating genes did notchange with sulfur input,and there was no correlation between exogenous sulfur,functional genes and Me Hg,suggesting sulfur addition affect the Me Hg by means other than sulfur cycle related microorganisms.