| Paddy soil is a critical mercury（Hg）methylation hotspot and a vital methylmercury（Me Hg）"source"for terrestrial ecosystems.Rice is more likely to accumulate Me Hg than other crops,leading to a significant pathway for human Me Hg exposure by consuming rice in Hg mining areas.Many researchers have been concerned about the influence mechanism of mercury migration and transformation in rice fields to reduce the mercury exposure risk by rice consumption.Due to the extremely high affinity of organic matter for Hg,organic matter in the soil becomes an important influencing factor for mercury’s migration and transformation behavior in paddy fields.However,due to the complexity of organic matter structure and composition,organic matter’s effects on Hg in rice paddy soi are heterogeneous.Therefore,it is necessary to explore the role of organic matter in mercury migration and transformation in rice field soil from the perspective of different organic matter components and structures.To explore these scientific problems,the following experiments were conducted:（1）Different fractions of organic matter,including fulvic acid（FA）,humic acid（HA）,huminin（HM）,and purified huminin（PHM）,were extracted from composted straw（FJ）,composted cow manure（FN）,lignite（HE）,and peat soil（NT）.Their molecular structures such as functional groups and elemental compositions,were characterized to reveal their different physicochemical properties.（2）Artificial soil was prepared by controlling the inorganic components,including iron-manganese oxide,quartz,kaolinite,and calcium carbonate,as well as organic components,including peat soil FA,peat soil HA,peat soil HM,and humic acid to preliminary explore their coupling effects on the bio-availability of Hg in soil.（3）Pot experiments were carried out by adding different organic matter fractions in the mercury-contaminated soil.Mercury and methylmercury concentrations in the water,soil,and plant in different incubation periods were detected.Combined with the variations of the DOM composition and structure in pore water,organic acids,and mercury methylating gene abundance in soil,the effects and mechanisms of different organic matter fractions on mercury methylation,mercury migration,and bioavailability in the soil-water-plant system of rice fields were understood.（4）Through microcosmic experiments,the mechanisms of huminin in soil mercury methylation were further studied from the perspectives of soil microorganisms,soil structure,water conditions,and the rhizosphere excitation effect.This paper systematically explores the effects of different components of organic matter on mercury migration and transformation in rice fields,provides a scientific theoretical reference for the practical application of organic amendments in soil,and is of great significance for the restoration of mercury in rice fields and the safety of rice products.The research results mainly include:（1）Fulvic acid,humic acid,and humin from different sources have similar elemental composition and functional group composition.However,the percentage of elemental composition,functional groups,and physical structure differ in different organic matter fractions from various sources.Aromatic,N,C,and S were ranked by content size:humic acid>humin>fulvic acid.The surface of most humic substances is dominated by C-C/C-H（aromatic carbon）.All fulvic acid has a high percentage of carboxyl/carbonyl C and alkyl C and has the highest hydrophilic oxi-disability.The percentages of alkyl C in all humic acids were similar to that in humins,but the alkoxy C in the humins was the highest.Among the different sources of organic matter,the organic fraction compost straw had the highest sulfur levels.Fulvic acid and humic acid extracted from lignite had the lowest sulfur content but contained more pyridine,with a relatively weak surface pore structure.Among fulvic acid and humic acid fractions,the surface of fulvic acid and humic acid extracted from peat soil contained the highest amount of C-C/C-H（aromatic carbon）.Among each fulvic acid and humic acid,the surface of fulvic acid and humic acid extracted from peat soil contained the highest amount of C-C/C-H（aromatic carbon）.However,the surface of all humins extracted from peat soil had a higher percentage of C-O while a lower content of C-C/C-H（aromatic carbon）.（2）Pot experiments showed that inorganic fractions mainly had mercury adsorption capacity.The lack of kaolinite or ferromanganese oxides and the addition of iron sulfide reduced the bioavailability of mercury.The coupling of organic and inorganic components had different effects on mercury’s effectiveness,transport,and transformation.Replacing peat soil with fulvic acid and humic acid in artificial soil significantly reduced the total mercury and methylmercury concentrations in water spinach by 57-93%and 57-93%,respectively,compared with the control.Replacing peat soil with humic acid and humic acid also significantly reduced the concentration of total mercury in rice(44%and 43%,respectively,compared with the control.While replacing peat soil in artificial soil with humin significantly increased total mercury and methylmercury concentrations in rice and water spinach,ranging from 29%to 137%.Adding fulvic acid and humin to the artificial soil significantly increased the overlying water’s total mercury concentration and soil methylmercury concentration by 198%,163%,and 198%,163%,respectively.（3）Different organic matter fractions from various sources increased the concentration of DOC,cysteine,and the abundance of gene hgc A in the soil to varying degrees during the rice filling period.This result led to the elevated soil mercury methylation by fulvic acid,humic acid,and humin from each source.The effects of the same humic fraction from different sources on the transport and transformation of Hg in soil were essentially the same.Still,the degree of impact varied significantly for various sources of the same fraction.All fulvic acids from different sources promoted the release of Hg from the soil at the tillering stage.In contrast,humic acids inhibited the release of Hg from the soil at the filling stage,and each humin promoted the release of Hg at the tillering and maturity stages.The functional groups and elemental composition from different organic matter sources and fractions were different,which led to their different effects on the transport and transformation of Hg.Correlation analysis showed that the sulfur content of organic matter fractions was significantly negatively correlated with the total mercury concentration in pore water at the rice tillering stage（r=-0.701,p<0.05,n=12）.The alkoxy C content was positively correlated with the total mercury concentration of pore water at the rice maturity stage（r=0.661,p<0.05,n=12）.While the surface C-O functional group was significantly correlated with the methylmercury concentration in pore water at the rice tillering stage（r=0.681,p<0.05,n=12）,and total mercury concentration in pore water at rice maturity were significantly positively correlated（r=-0.589,p<0.05,n=12）.The input of different organic fractions also caused significant changes in the composition and structure of pore water DOM,resulting in different Hg transport transformation characteristics.The E2/E3values（r=0.416,p<0.05,n=38）,SUVA254values（r=0.56,p<0.01,n=38）,and a（355）values（r=0.67,p<0.01,n=37）of DOM had a significantly positive correlation with total mercury concentration in pore water throughout the rice cropping cycle.This result indicated that DOM,with many small molecules,high aromaticity,and high photoactivity in pore water,was more favorable for soil mercury release.（4）The effects of different organic fractions and from different sources on mercury bioaccumulation in the plant were different.Fulvic acids from composted cow manure and peat soil promoted the transport of methylmercury by 33%and 41%,respectively,leading to higher concentrations of total and methylmercury in rice.While fulvic acids from lignite and composted straw sources significantly inhibited the transport of inorganic mercury（53%and 66%,respectively）and methylmercury（47%and 36%,respectively）from the roots to the rice grains,leading to a decrease in total and methylmercury concentrations in rice.Humins extracted from composted cow manure and peat soil significantly increased the uptake and translocation of soil Hg by rice,ultimately increasing total rice Hg and Me Hg concentrations by 53%to 147%and 66%to 250%,respectively,compared to the control.The C-O content on the surface of each organic matter fraction had significantly positive correlations with total Hg concentration（r=0.756,p<0.01,n=12）and methylmercury concentration（r=0.634,p<0.05,n=12）in rice.This result indicated that organic fractions’elemental composition and structural characteristics significantly impacted Hg’s transport and transformation in the paddy system.（5）The soil culture experiment results showed that adding humin did not cause significant changes in methylmercury concentrations in soil under dryland conditions.However,adding humin significantly increased methylmercury concentration in the pore water in the flooded soils by 141-198%compared to the control.While co-addition of humin and microorganisms（fungi or/and sulfate-reducing bacteria）to the soil matrix further increased the methylmercury concentration in pore water by 359-815%compared to the control.Rice cultivation under this condition resulted in the highest methylmercury concentration in pore water（1061%）compared to other treatments.It was much higher than that in the treatments treated with only microorganisms and rice cultivation.In the solution incubation experiments,we found that,in the presence of microorganisms,the methylmercury concentration in the treatments with adding humin was significantly higher than that without humin（p<0.05）.And the mercury methylation was enhanced mainly by fungal-pretreated humin,and the methylmercury concentration in this treatment increased by 67%compared with that without fungal pretreatment.These results suggested that humin can provide a carbon source to mercury methylating sulfate-reducing bacteria.It can also be decomposed by the fungal and used then more quickly by mercury-methylating bacteria,thus promoting mercury methylation.This process is more likely to occur under flooded conditions and is not influenced by soil structure.Also,the coupling effect of plant growth and humin may further promote mercury methylation in soil. |
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