| Mercury（Hg）has been considered as a global pollutant for a long time due to its volatile property.Hence,Hg pollution has always been one of the crucial environmental issues for researchers and residents.Hg is a unique volatile heavy metal in the natural environment;hence,it can widely propagate in the atmosphere with the atmospheric turbulences.Inorganic Hg can be transformed into methylmercury（MeHg）by Hg methylating microorganisms,such as Sulfate reducting bacteria（SRB）,Iron-reducing bacteria（SRB）and Methanogens.MeHg is a strong neurotoxic pollutant,and it readily binds to some proteins and amino acids,which include the sulfydryl（-SH）.The strong binding between MeHg and proteins/amino acids results in MeHg transferring and accumulating into the food web easily,as a result,higher MeHg concentration occurs in the higher trophic level organisms.While the doses of MeHg in animals and humans reach to a high level,irreparable toxicity for animals and humans will occur,even cause death.Therefore,MeHg pollution is considered as the key of Hg environmental pollution.To date,it has been demonstrated that most of Hg methylating microorganisms are anaerobic bacteria.Hence,the anaerobic regions are usually important places for Hg methylation,such as the sediment of rivers and lakes,wetland ecosystems.Generally,MeHg is produced in sediment and can then diffuse to porewater and overlying water.Aquatic organisms in rivers and lakes accumulate the MeHg in water from sediment,leading to MeHg accumulating in aquatic products,such as fish.Therefore,consuming aquatic products has been considered as the main pathway of human exposure to MeHg.However,in recent decades,researchers found that rice grain can strongly accumulate MeHg from paddy soil.MeHg in rice grain is higher than that in other crops which grow in the same THg level soil.Hence,MeHg strongly accumulated in rice grain has gradually attracted the attention of numerous researchers and people.Traditionally,the paddy soil is flooding during the rice-growing.At the fallow period of rice,the water in paddy fields will discharge.This alternant flooding for paddy soil,poses a special redox environment for the paddy soil.A high organic matter concentration is always found in paddy soil,which enhances the oxidation environment and promotes the microbial Hg methylation.China is a big country around the world for its huge rice production and consumption.In China,65%of the population live on rice,hence,the issue of MeHg pollution in rice cannot be ignored.The MeHg in rice is mainly from paddy soil,and there are many factors affecting the Hg methylation in paddy soil.The rotation systems and fertilizer application can affect the physical and chemical properties of paddy soil,and then Hg methylation in paddy soil.The microorganism activity in paddy soil controls the accumulation of MeHg in rice grains.Chongqing is one of the important industrial cities in southwest China.Its warm and humid climate is suitable for the growth of rice,so rice is one of the main crops in this region.Due to the complex geographical environment in Chongqing,a variety of rice planting patterns have been formed in Chongqing.Based on the investigation of Hg pollution in paddy fields in Chongqing,this paper analyzed the influencing factors of Hg transportation and transformation in paddy fields,and combined a pot experiment to explore the effects of different fertilizer application on Hg methylation in paddy soil and the effect of MeHg accumulation in rice grain.We assessed the potential risk of MeHg exposure to human through rice consumption with different rotation systems,and recommend the optimal rotation system to reduce the risk of human exposure to MeHg through rice consumption.Finally,to reduce the Hg methylation that is caused by straw turnover,straw of different sizes was used as the organic fertilizer.We hope found the optimal straw treatment to reduce the Hg methylation and MeHg accumulation in rice grain on account of straw turnover.The main results and conclusions of this paper are as follows:1)The atmospheric Hg concentration in paddy fields in the research area ranged from 3 ng·m-3 to 121 ng·m-3,with a geomean of 11 ng·m-3.The atmospheric Hg concentration in research area is higher than the global atmospheric Hg background value(0.7-1.1 ng?m-3).The highest atmospheric Hg concentration was observed in the southeast of Chongqing due to the presence of Hg mine.The atmospheric Hg concentration gradually decreased from southeastern to west.Atmospheric mercury concentrations are elevated in urban areas and Hg mine areas and tend to spread around.The paddy water is acidic,and the average pH of the overlying water is 6.4,and the porewater is 5.2.Both MeHg concentration and MeHg/THg in porewater were higher than those in overlying water.THg concentration in overlying water controlled by soil and atmospheric Hg.MeHg/THg in overlying water has a negative correlation with dissolved oxygen（DO）and a positive correlation with dissolved organic carbon（DOC）;MeHg/THg in porewater had a significant negative correlation（p<0.01）with Eh and pH,while a significant positive correlation with DOC concentration（p<0.01）.THg concentration in paddy soil widely ranged from 24.02 ng?g-1 to 12308.75 ng?g-1,with a geomean of 96.75 ng?g-1.The highest THg concentration of paddy soil was observed in Xiushan,where located in southeastern Chongqing.Several Hg mines distribute in Xiushan county.THg concentration in paddy soil in Xiushan county was higher than the standard Hg value of agricultural safe soil(500 ng·g-1).In this investigation,91.36%of the paddy soil is safe as the standard Hg value of agricultural safe soil in China.Except several sampling paddy soils in the Hg mine region,the paddy soils in Chongqing did not occur significant Hg pollution.MeHg/THg in paddy soil varied from 0.12%to 3.67%,which higher than other agricultural soil.The flooding led to an anaerobic acidic environment in paddy soil,which is beneficial to Hg methylation.Strongly bound Hg（Hg-e）and mercuric sulfide Hg（Hg-s）dominated in paddy soil,and their ratios to THg were 24.67%-69.99%and 21.51%-55.13%,respectively.The ratio of organically bound Hg（Hg-o）to THg ranged from 11.20%to 38.96%,and bio-available Hg（water-soluble Hg,Hg-w and gastric acid-bound Hg,Hg-h）is relatively low in paddy soils,especially in paddy soils in Hg mine areas.The Hg methylation（MeHg/THg）in paddy fields was significantly positively correlated with the proportion of soil organic matter and bioavailable Hg components（p<0.01）,while the soil pH and inert Hg components.Both THg and MeHg concentrations in paddy soil in the Hg mine areas is very high,but MeHg/THg is very low,indicating that the Hg mine has a high methylation rate of Hg due to the high proportion of Hg-s/THg.2)MeHg concentration gradually increased from root,stem,leaf and rice grain.Generally,the MeHg concentration in rice grain was the highest value,while the THg concentration was different from MeHg.THg concentration in root of rice plants was also the highest.However,THg concentrations of stems and leaves are different from the change of MeHg concentrations.In the area with high THg concentration in paddy soil,the THg concentrations in stems were higher than that in leaves.On the contrary,THg concentrations in leaves were higher than that in stems,indicating that Hg in rice leaves affected by the atmospheric Hg and soil Hg concentrations.The roots of rice have a repulsive effect on the absorption of THg in paddy soil.THg concentrations in roots of the areas not contaminated with Hg were higher than that in the paddy soil,and then gradually decreased upwards.Strong paddy soil fertility can cause bio-dilution of THg and MeHg concentrations in rice plants,but it also can enhance the MeHg accumulation in rice grain,especially available nitrogen and available potassium in paddy soil.After a long-time consumption of rice with high Hg concentration,the THg concentrations in hair enhanced significantly and further MeHg accumulation in hair.The rice consumption caused MeHg accumulation in hair,but it is weaker in contrast to the meat consumption.3)Different rotation systems have impacts on the physical and chemical properties of paddy soil,which in turn lead to differences in Hg methylation in paddy soils.The long-time flooding for winter paddy field made it form a strict anaerobic reduction environment,which was beneficial to the activity of Hg methylating microorganisms and the production of MeHg.The long-time decomposition of straw in winter paddy soil resulted in the enhancement of humus-like organic matter in paddy soil,and then the MeHg accumulation in paddy soil is elevated.The paddy-upland rotation（rice-rape rotation and rice-wheat rotation）temporarily changed the redox environment of soil due to the planting of other dryland crops after drainage in the rice fallow period,resulting in a decrease of Hg methylation in soil.But the growth of crop promoted more root exudates release to soil,the protein-like organic matter in the root exudates easily stimulated the activity of microorganisms in the soil,as a result,the MeHg in paddy soil increased.The pattern of sunning soil in winter led to the MeHg demethylation in the paddy soil.The paddy-upland mainly enhances the Hg methylation in paddy soil through the enhancement of microorganism activation from root exudates.However,the winter paddy field was a long-time Hg methylation with the straw decomposing.The protein-like organic matter in root exudates can promote Hg methylation in a short time,while the humus-like organic matter for a long time.The strong binging between MeHg and humus-like organic matter leads to high accumulation of MeHg.4)We classified these sample paddies into five rotation systems.We found the difference of rotation systems led to the change of organic matter in paddy soils,which further led to the difference in Hg methylation in paddy soils.The winter paddy field presented a strong Hg methylation in paddy soils and MeHg accumulation in rice plants.The paddy-upland rotation（rice-rape rotation and rice-wheat rotation）alleviated the high Hg methylation caused by flooding in paddy soil,and the pattern of sunning paddy soil in winter effectively reduced the Hg methylation in paddy soil.In terms of reducing the Hg methylation in paddy soil,the pattern of sunning paddy soil in winter may be the optimal rotation system for rice farmers.However,considering the economic value of paddy fields,the paddy-upland may be the optimal choice for rice farmers to reduce Hg methylation in paddy soil.The effects of chemical fertilizers and organic fertilizers on soil Hg methylation have significant differences.The application of chemical fertilizers increased the organic matter from root exudates because chemical fertilizers stimulated rice growth.The root exudates stimulated the microorganism activity resulting in more MeHg production.The application of nitrogen fertilizer shown the strongest effect on the Hg methylation in paddy soil,followed by phosphate fertilizer and potassium fertilizer.The chemical fertilizers also increased the protein concentration of rice grain,which in turn led to enhanced MeHg accumulation in rice grain.Organic fertilizers caused higher Hg methylation in paddy soil than chemical fertilizers.Due to the high organic matter concentration of organic fertilizers,the addition of organic fertilizers could rapidly increase the concentration of soil organic matter and stimulate the rapid growth of microorganisms in soil,resulting in a rapid increase in MeHg concentration in soil.The significant differences among different organic fertilizers controlled the efficiency of enhanced Hg methylation for diverse organic fertilizers.Both protein and lipid in straw occupy small proportions,and they are easily decomposed by microorganisms and rapidly released into soil.While,lignin and cellulose dominate in straw,and are difficultly decomposed by microorganisms.The straw could sustain in soil for a long time,and stimulate Hg methylation slowly.This led to high MeHg concentration in soil at the grain-filling stage,as a result,high MeHg accumulation in rice grain.The enhanced effect on Hg methylation of pig manure is weaker than that of rice straw,because pig manure decomposed rapidly in soil.Less MeHg accumulated in rice grain in response to the decrease of MeHg concentration in soil at the grain-filling stage.5)The size controlled the time of decomposition of straw in soil.The smaller straw degraded in soil with a shorter time and discharged organic matter similarly.Synchronously,the activity of Hg methylating microorganisms was accompanied with the organic matter discharged from straw.The rapidly decomposing of straw led to low MeHg concentration in soil when the rice grain filling,hence low MeHg concentration in rice grain.On the contrary,the big-size straw in soil immobilized Hg and reduced the loss of Hg.MeHg concentration in soil still sustained high level at harvest time,namely,more MeHg accumulated in rice grain in the filling stage.According to our results,we concluded that the crushed straw could reduce the enhancement of Hg methylation in paddy soil and MeHg accumulation in rice grain that caused by straw turnover.That is,crushed straw could reduce the potential risk of MeHg exposure to human through rice consummation. |
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