Soil MeHg Research Articles

Effects of Microbial Activities on Mercury Methylation in Farmland near Mercury Mining Area

In order to study the main effect of microbial activities on mercury(Hg) methylation in farmland, mercury contaminated upland soils and paddy soils near Hg mining area were sampled as experimental soils. Four treatments were designed including only sterilization as the control, accelerating the activities of sulfate reducing bacteria(SRB), inhibiting the SRB's activities, and accelerating the activities of iron-reducing bacteria(FeRB), to know the effects of microbial and non-microbial factors on mercury methylation in soils. The results were as follows:the highest concentration of methylmercury(MeHg) was observed in soils with SRB accelerated treatment, and the increments of MeHg concentrations in upland soils and paddy soils ranged from 0.15 μg·kg-1 to 0.38 μg·kg-1 and 1 μg·kg-1 to 2 μg·kg-1, respectively. Comparatively, little increments of MeHg concentration were seen in soils with SRB inhibited treatment and FeRB accelerated treatment, which were lower than 0.025 μg·kg-1. Compared with upland soils, more MeHg was formed in Paddy soils and the concentrations of MeHg in paddy soils were 4-9 times of that in upland soils. Variation in the number of SRB in soils was similar to that in the concentration of MeHg in soils, and the number of SRB was positively correlated with the concentration of MeHg concentrations in soils(R2=0.57,P<0.01). The above results indicated that activities of reducing bacteria, especially SRB, played key role in the methylation in soils. In addition, more attention should be paid to paddy soils due to the high potential of methylation when conducting any assessment and taking any measure to manage the health risk caused by the exposure to mercury.

Advances in understanding Hg dynamics in mercury contaminated paddy soils under straw amendment

It is well-known that interactions between Hg and organic matter could greatly affect biogeochemistry of Hg in soils, and its subsequent uptake by plants. Crop straw is mainly composed of organic matter, and its incorporation into soils may have impacts on Hg dynamics in soil-plant systems. Straw return is being widely encouraged, in order to mitigate straw-burning induced air pollution, as well as to promote plant growth by increasing soil fertility and improving the soil. Meanwhile, potential impacts of straw return on the ecological risk of metals in agricultural soils warrant investigation. While much progresses have been made in understanding the reduced metal bioavailability (e.g., for Cu, Cd and Zn) under straw amendment, little is known about the potential effects of straw return on Hg dynamics in soils and the associated ecological risks. Here, we summarize recent advances in understanding Hg dynamics in soil-rice systems under straw amendment. Recent studies revealed evidently enhanced ecological risk of Hg in paddy soils under straw amendment. Especially, straw return was found to have positive effects on methylmercury (MeHg) concentrations in soils, as well as those in crops. The underlying mechanisms may include: (1) Straw amendment has evident effects on the geochemical fractionation of Hg in soils, by facilitating transformation of Hg from the refractory fractions (e.g., residual fractions, including HgS) to more mobile fractions (e.g., organic-complexed fraction), which may increase availability of inorganic Hg to microbial methylators. (2) Transformation of inorganic Hg to MeHg was facilitated under straw amendment, which could be mainly attributed to the increased availability of inorganic Hg as well as enhanced activities of microbial methylators (e.g., sulfate reducing bacteria). (3) Straw return was found to have positive effects on Hg accumulation in grains of crops. The observed effects could be explained by the increased soil MeHg levels, enhanced mobility of MeHg in the presence of dissolved straw-derived organic matter, as well as changes in plant physiology, e.g., increased uptake of MeHg by root and MeHg transformation from other tissues to grains. Information summarized in this manuscript would improve mechanistic understanding about dynamics and risk of Hg in soil-plant systems and ensure appropriate straw management in Hg-contaminated areas. Particularly, more attentions should be paid to the interactions between ‘fresh’ organic matter (e.g., those from crop straw), which may have great impacts on Hg speciation, methylation, mobility, and bioavailability in soils. Furthermore, results gained in those pioneering studies highlight the importance to re-consider the policy of straw return in farming soils, especially in Hg-contaminated areas.

Mercury flow through an Asian rice-based food web

Mercury (Hg) is a globally-distributed pollutant, toxic to humans and animals. Emissions are particularly high in Asia, and the source of exposure for humans there may also be different from other regions, including rice as well as fish consumption, particularly in contaminated areas. Yet the threats Asian wildlife face in rice-based ecosystems are as yet unclear. We sought to understand how Hg flows through rice-based food webs in historic mining and non-mining regions of Guizhou, China. We measured total Hg (THg) and methylmercury (MeHg) in soil, rice, 38 animal species (27 for MeHg) spanning multiple trophic levels, and examined the relationship between stable isotopes and Hg concentrations. Our results confirm biomagnification of THg/MeHg, with a high trophic magnification slope. Invertivorous songbirds had concentrations of THg in their feathers that were 15x and 3x the concentration reported to significantly impair reproduction, at mining and non-mining sites, respectively. High concentrations in specialist rice consumers and in granivorous birds, the later as high as in piscivorous birds, suggest rice is a primary source of exposure. Spiders had the highest THg concentrations among invertebrates and may represent a vector through which Hg is passed to vertebrates, especially songbirds. Our findings suggest there could be significant population level health effects and consequent biodiversity loss in sensitive ecosystems, like agricultural wetlands, across Asia, and invertivorous songbirds would be good subjects for further studies investigating this possibility.

Mercury in soil, vegetable and human hair in a typical mining area in China: Implication for human exposure

Concentrations of total mercury (T-Hg) and methylmercury (MeHg) in soil, vegetables, and human hair were measured in a mercury mining area in central China. T-Hg and MeHg concentrations in soil ranged from 1.53 to 1054.97mg/kg and 0.88 to 46.52μg/kg, respectively. T-Hg concentrations was correlated with total organic carbon (TOC) content (R2=0.50, p<0.01) and pH values (R2=0.21, p<0.05). A significant linear relationship was observed between MeHg concentrations and the abundance of sulfate-reducing bacteria (SRB) (R2=0.39, p<0.05) in soil. Soil incubation experiments amended with specific microbial stimulants and inhibitors showed that Hg methylation was derived from SRB activity. T-Hg and MeHg concentrations in vegetables were 24.79–781.02μg/kg and 0.01–0.18μg/kg, respectively; levels in the edible parts were significantly higher than in the roots (T-Hg: p<0.05; MeHg: p<0.01). Hg species concentrations in rhizosphere soil were positively correlated to those in vegetables (p<0.01), indicating that soil was an important source of Hg in vegetables. Risk assessment indicated that the consumption of vegetables could result in higher probable daily intake (PDI) of T-Hg than the provisional tolerable daily intake (PTDI) for both adults and children. In contrast, the PDI of MeHg was lower than the reference dose. T-Hg and MeHg concentrations in hair samples ranged from 1.57 to 12.61mg/kg and 0.04 to 0.94mg/kg, respectively, and MeHg concentration in hair positively related to PDI of MeHg via vegetable consumption (R2=0.39, p<0.05), suggesting that vegetable may pose health risk to local residents.

The impact of an abandoned mercury mine on the environment in the Xiushan region, Chongqing, southwestern China

Mercury contamination is a serious problem in the Hg mining area of Xiushan County, Chongqing, southwestern China. The concentrations of total mercury (THg) and methylmercury (MeHg) in paddy soil, surface water, and rice (grain) samples were determined to investigate the regional distribution of Hg contamination. Simultaneously, gaseous elemental Hg (GEM) in ambient air near the sampling sites was measured. The total Hg concentrations in surface water were highly elevated, ranging from 13 to 2390 ng/L, and the total MeHg concentrations varied between 0.17 and 1.1 ng/L. The dissolved Hg and MeHg concentrations in surface water ranged from 4.7 to 470 ng/L and 0.14–0.35 ng/L, respectively. High THg and MeHg concentrations were also obtained in paddy soils from mining areas, ranging from 0.45 to 68 μg/g and 0.13–4.8 ng/g, respectively. Similar to the high concentrations in water and soil, the THg concentration in rice (grain) ranged from 4.7 to 550 ng/g and MeHg from 2.9 to 26 ng/g. Elevated Hg concentrations in rice, as a staple food of local residents, confirmed that rice consumption could be a vital pathway for MeHg exposure to native people. Humic acid and fulvic acid had significant correlations with soil MeHg, implying that they have important roles that influence MeHg production in soil. The spatial distribution characteristics of Hg and MeHg pollution in the local environment indicated their origins from historic Hg mining sites in the Xiushan area.

The local impact of a coal-fired power plant on inorganic mercury and methyl-mercury distribution in rice (Oryza sativa L.)

Emission from coal-fired power plants is one of the major anthropogenic sources of mercury (Hg) in the environment, because emitted Hg can be quickly deposited nearby the source, attention is paid to the effects of coal-burning facilities on levels of toxic methyl-mercury (MeHg) in biota near such sources. Since rice is an agricultural crop that can bio-accumulate MeHg, the potential effects of a large Hg-emitting coal-fired power plant in Hunan Province, China on both inorganic Hg (Hg(II)) and MeHg distributions in rice was investigated. Relatively high MeHg (up to 3.8 μg kg−1) and Hg(II) (up to 22 μg kg−1) concentrations were observed in rice samples collected adjacent to the plant, suggesting a potential impact of Hg emission from the coal fired power plant on the accumulation of Hg in rice in the area. Concentrations of MeHg in rice were positively correlated with soil MeHg, soil S, and gaseous elemental Hg (GEM) in ambient air. Soil MeHg was the most important factor controlling MeHg concentrations in rice. The methylation of Hg in soils may be controlled by factors such as the chemical speciation of inorganic Hg, soil S, and ambient GEM.

The influence of sample drying and storage conditions on methylmercury determination in soils and sediments

The separate influences of drying and storage conditions on methylmercury (MeHg) concentrations in soil and sediment samples were investigated. Concentrations of MeHg and total Hg were determined in various soil and sediment samples that had been stored or dried under differing conditions. The influence of drying conditions (oven-drying (40 °C) versus freeze-drying) on MeHg concentrations in marine sediments, river sediments, soils, and paddy field soils was investigated (n = 43). The ratio of the MeHg concentration in oven-dried sub-samples divided by the concentration in freeze-dried sub-samples ranged from 0 to 336%. In order to confirm the production of MeHg during storage in some samples, Hg2+ was added at 15 mg kg−1 to a paddy soil, and the sample was then stored at 30 °C. The concentrations of MeHg at 1-h, 1-day, 4-days and 7-days after Hg2+ spiking were 2.0 ± 0.1, 13.8 ± 1.0, 36.0 ± 5.0, and 24.9 ± 1.6 μg kg−1 (n = 3), respectively. The concentration of MeHg at 4-days after Hg spiking and sterilizing (121 °C, 30 min) was 1.8 μg kg−1, similar to the original value. These results indicate that bacterial Hg methylation and MeHg demethylation occurred within days in the soil. In addition, tests of the stability of MeHg in wet and dry samples during storage were also performed. Overall, our results indicate that the best way to preserve MeHg in soil and sediment samples is to freeze the samples immediately after collection, followed subsequently by freeze-drying, grinding, homogenization, and storage of the dry material in cool, dark conditions until analysis.

Distribution of total mercury and methylmercury around the small-scale gold mining area along the Cikaniki River, Bogor, Indonesia.

This study investigates the distribution of total mercury (T-Hg) and methylmercury (MeHg) in the soil and water around the artisanal and small-scale gold mining (ASGM) area along the Cikaniki River, West Java, Indonesia. The concentration of T-Hg and MeHg in the forest soil ranged from 0.07 to 16.7mgkg-1 and from <0.07 to 2.0μgkg-1, respectively, whereas it ranged from 0.40 to 24.9mgkg-1 and from <0.07 to 56.3μgkg-1, respectively, in the paddy field soil. In the vertical variation of the T-Hg of forest soil, the highest values were observed at the soil surface, and these values were found to decrease with increasing depth. A similar variation was observed for MeHg and total organic carbon content (TOC), and a linear relationship was observed between them. Mercury deposited on the soil surface can be trapped and retained by organic matter and subjected to methylation. The slope of the line obtained for the T-Hg vs. TOC plot became larger near the ASGM villages, implying a higher rate of mercury deposition in these areas. In contrast, the plots of MeHg vs. TOC fell along the same trend line regardless of the distance from the ASGM village. Organic carbon content may be a predominant factor in controlling MeHg formation in forest soils. The T-Hg concentration in the river water ranged from 0.40 to 9.6μgL-1. River water used for irrigation can prove to be a source of mercury for the paddy fields. The concentrations of Hg0 and Hg2+ in river water showed similar variations as that observed for the T-Hg concentration. The highest Hg0 concentration of 3.2μgL-1 can be attributed to the waste inflow from work sites. The presence of Hg0 in river water can become a source of mercury present in the atmosphere along the river. MeHg concentration in the river water was found to be 0.004-0.14% of T-Hg concentration, which was considerably lower than the concentrations of other Hg species. However, MeHg comprised approximately 0.2% of the T-Hg in paddy field soil. Mercury deposited from the atmosphere and the river water can be subjected to methylation. Paddy fields are very important ecosystems; therefore, the effect of MeHg on these ecosystems and human beings should be further investigated.

Longitudinal occurrence of methylmercury in terrestrial ecosystems of the Tibetan Plateau

Methylmercury (MeHg), a neurotoxin, is a global concern because of its potential risk to human and ecological health. Elevated mercury (Hg) concentrations were recently reported in the Tibetan Plateau (TP) due to increasing Hg input from distant regions, yet little is known about MeHg production and distribution in the terrestrial ecosystems of the TP. Here, we report longitudinal occurrence of MeHg and the factors regulating net MeHg production in 23 grassland sites from eastern to western TP. The soil MeHg content varied from 0.002 to 0.058 ng g−1, with different distribution patterns between the eastern and western TP. There was a positive correlation between the MeHg concentration and the longitude after 90 °E, which is similar to the distribution patterns of the total mercury (THg), water and organic carbon in this region. Average MeHg concentration in topsoil is generally higher than that in subsoil. Our results show that MeHg concentration in soils of the TP is directly affected by soil water, potential microbial methylators and THg, while indirectly regulated by soil organic carbon through the microbial community and the longitude-dependent precipitation through soil water. Our study suggests that soil water is the most important driver regulating net MeHg production in the grasslands of the TP. These findings have important implications for unraveling the mechanism of net production of MeHg in high-altitude environments.

Accumulation and translocation of methylmercury and inorganic mercury in Oryza sativa: An enriched isotope tracer study

Methylmercury (MeHg) accumulation in rice is an emerging human health issue, but uptake pathways and translocation into the grain remain poorly understood. We grew Oryza sativa plants in pots of wetland soil amended with an enriched mercury isotope (94.3% 200Hg) tracer, alongside unvegetated control pots, and assessed both ambient and tracer MeHg and inorganic Hg (IHg) concentrations in soil and plant tissues at three growth stages. Based on similar ratios of ambient:tracer MeHg concentrations in soil and plant tissues, we provide the first direct evidence that MeHg is first synthesized in saturated soil and subsequently translocated to rice grains. There is no evidence of in planta methylation of IHg, but significant losses of MeHg from plant tissues between flowering and maturity indicates likely in planta demethylation. In this greenhouse experiment, lower percent of tracer MeHg in vegetated soils at late growth stages suggests that rice plants reduce the net MeHg accumulation capacity of soils, although the mechanism remains unclear. For IHg, roots accumulated Hg from the soil, straw from the soil and the atmosphere, and grain almost entirely from the atmosphere. Management strategies that aim to reduce MeHg accumulation in rice should focus on mercury methylation in paddy soils, but IHg reductions will depend on regional controls of atmospheric Hg.

Methyl Mercury Formation in Hillslope Soils of Boreal Forests: The Role of Forest Harvest and Anaerobic Microbes.

Final harvest (clear-cutting) of coniferous boreal forests has been shown to increase streamwater concentrations and export of the neurotoxin methyl mercury (MeHg) to freshwater ecosystems. Here, the spatial distribution of inorganic Hg and MeHg in soil as a consequence of clear-cutting is reported. A comparison of soils at similar positions along hillslopes in four 80 years old Norway spruce (Picea abies) stands (REFs) with those in four similar stands subjected to clear-cutting (CCs) revealed significantly (p < 0.05) enhanced MeHg concentrations (ng g(-1)), MeHg areal masses (g ha(-1)), and percent MeHg of HgTOT in O horizons of CCs located between 1 and 41 m from streams. Inorganic Hg measures did not differ between REFs and CCs at any position. The O horizon thickness did not differ between CCs and REFs, but the groundwater table and soil water content were significantly higher at CCs than at REFs. The largest difference in percent MeHg of HgTOT (12 times higher at CCs compared to REFs, p = 0.003) was observed in concert with a significant enhancement in soil water content (p = 0.0003) at intermediate hillslope positions (20-38 m from stream), outside the stream riparian zone. Incubation experiments demonstrated that soils having significantly enhanced soil pools of MeHg after clear-cutting also showed significantly enhanced methylation potential as compared with similarly positioned soils in mature reference stands. The addition of inhibitors demonstrated that sulfate-reducing bacteria (SRB) and methanogens were key methylators. Rates of demethylation did not differ between CCs and REFs. Our results suggest that enhanced water saturation of organic soils providing readily available electron donors stimulate Hg-methylating microbes to net formation and buildup of MeHg in O horizons after forest harvest.

Fractionation of Mercury Stable Isotopes during Microbial Methylmercury Production by Iron- and Sulfate-Reducing Bacteria.

The biological production of monomethylmercury (MeHg) in soils and sediments is an important factor controlling mercury (Hg) accumulation in aquatic and terrestrial food webs. In this study we examined the fractionation of Hg stable isotopes during Hg methylation in nongrowing cultures of the anaerobic bacteria Geobacter sulfurreducens PCA and Desulfovibrio desulfuricans ND132. Both organisms showed mass-dependent, but no mass-independent fractionation of Hg stable isotopes during Hg methylation. Despite differences in methylation rates, the two bacteria had similar Hg fractionation factors (αr/p = 1.0009 and 1.0011, respectively). Unexpectedly, δ(202)Hg values of MeHg for both organisms were 0.4‰ higher than the value of initial inorganic Hg after about 35% of inorganic Hg had been methylated. These results indicate that a (202)Hg-enriched pool of inorganic Hg was preferentially utilized as a substrate for methylation by these organisms, but that multiple intra- and/or extracellular pools supplied inorganic Hg for biological methylation. Understanding the controls of the Hg stable isotopic composition of microbially produced MeHg is important to identifying bioavailable Hg in natural systems and the interpretation of Hg stable isotopes in aquatic food webs.

Mercury in western North America: A synthesis of environmental contamination, fluxes, bioaccumulation, and risk to fish and wildlife

Western North America is a region defined by extreme gradients in geomorphology and climate, which support a diverse array of ecological communities and natural resources. The region also has extreme gradients in mercury (Hg) contamination due to a broad distribution of inorganic Hg sources. These diverse Hg sources and a varied landscape create a unique and complex mosaic of ecological risk from Hg impairment associated with differential methylmercury (MeHg) production and bioaccumulation. Understanding the landscape-scale variation in the magnitude and relative importance of processes associated with Hg transport, methylation, and MeHg bioaccumulation requires a multidisciplinary synthesis that transcends small-scale variability. The Western North America Mercury Synthesis compiled, analyzed, and interpreted spatial and temporal patterns and drivers of Hg and MeHg in air, soil, vegetation, sediments, fish, and wildlife across western North America. This collaboration evaluated the potential risk from Hg to fish, and wildlife health, human exposure, and examined resource management activities that influenced the risk of Hg contamination. This paper integrates the key information presented across the individual papers that comprise the synthesis. The compiled information indicates that Hg contamination is widespread, but heterogeneous, across western North America. The storage and transport of inorganic Hg across landscape gradients are largely regulated by climate and land-cover factors such as plant productivity and precipitation. Importantly, there was a striking lack of concordance between pools and sources of inorganic Hg, and MeHg in aquatic food webs. Additionally, water management had a widespread influence on MeHg bioaccumulation in aquatic ecosystems, whereas mining impacts where relatively localized. These results highlight the decoupling of inorganic Hg sources with MeHg production and bioaccumulation. Together the findings indicate that developing efforts to control MeHg production in the West may be particularly beneficial for reducing food web exposure instead of efforts to simply control inorganic Hg sources.

Mercury methylation in paddy soil: source and distribution of mercury species at a Hg mining area, Guizhou Province, China

Abstract. Rice paddy plantation is the dominant agricultural land use throughout Asia. Rice paddy fields have been identified as important sites for methylmercury (MeHg) production in the terrestrial ecosystem and a primary pathway of MeHg exposure to humans in mercury (Hg) mining areas. We compared the source and distribution of Hg species in different compartments of the rice paddy during a complete rice-growing season at two different typical Hg-contaminated mining sites in Guizhou province, China: an abandoned site with a high Hg concentration in soil but a low concentration in the atmosphere and a current-day artisanal site with a low concentration in soil but a high concentration in the atmosphere. Our results showed that the flux of new Hg to the ecosystem from irrigation and atmospheric deposition was insignificant relative to the pool of old Hg in soil; the dominant source of MeHg to paddy soil is in situ methylation of inorganic Hg (IHg). Elevated MeHg concentrations and the high proportion of Hg as MeHg in paddy water and the surface soil layer at the artisanal site demonstrated active Hg methylation at this site only. We propose that the in situ production of MeHg in paddy water and surface soil is dependent on elevated Hg in the atmosphere and the consequential deposition of new Hg into a low-pH anoxic geochemical system. The absence of depth-dependent variability in the MeHg concentration in soil cores collected from the abandoned Hg mining site, consistent with the low concentration of Hg in the atmosphere and high pH of the paddy water and irrigation water, suggested that net production of MeHg at this site was limited. We propose that the concentration of Hg in ambient air is an indicator for the risk of MeHg accumulation in paddy rice.

Investigation of biogeochemical controls on the formation, uptake and accumulation of methylmercury in rice paddies in the vicinity of a coal-fired power plant and a municipal solid waste incinerator in Taiwan

Recent studies have shown that rice consumption is another critical route of human exposure to methylmercury (MeHg), the most toxic and accumulative form of mercury (Hg) in the food web. Yet, the mechanisms that underlie the production and accumulation of MeHg in the paddy ecosystem are still poorly understood. In 2013 and 2014, we conducted field campaigns and laboratory experiments over a rice growing season to examine Hg and MeHg cycling, as well as associated biogeochemistry in a suite of paddies close to a municipal solid waste incinerator and a coal-fired power plant station in Taiwan. Concentrations of total Hg and MeHg in paddy soil and rice grain at both sites were low and found not to exceed the control standards for farmland soil and edible rice in Taiwan. However, seasonal variations of MeHg concentrations observed in pore water samples indicate that the in situ bioavailability of inorganic Hg and activity of Hg-methylating microbes in the rhizosphere increased from the early-season and peaked at the mid-season, presumably due to the anoxia created under flooded conditions and root exudation of organic compounds. The presence of Hg-methylators was also confirmed by the hgcA gene detected in all root soil samples. Subsequent methylation tests performed by incubating the root soil with inorganic Hg and an inhibitor or stimulant specific for certain microbes further revealed that sulfate-reducers might have been the principal Hg-methylting guild at the study sites. Interestingly, results of hydroponic experiments conducted by cultivating rice in a defined nutrient solution amended with fixed MeHg and varying levels of MeHg-binding ligands suggested that chemical speciation in soil pore water may play a key role in controlling MeHg accumulation in rice, and both passive and active transport pathways seem to take place in the uptake of MeHg in rice roots.

Biochar amendment reduced methylmercury accumulation in rice plants

There is growing concern about methylmercury (MeHg) accumulation in rice grains and thus enhanced dietary exposure to MeHg in Asian countries. Here, we explored the possibility of reducing grain MeHg levels by biochar amendment, and the underlying mechanisms. Pot (i.e., rice cultivation in biochar amended soils) and batch experiments (i.e., incubation of amended soils under laboratory conditions) were carried out, to investigate MeHg dynamics (i.e., MeHg production, partitioning and phytoavailability in paddy soils, and MeHg uptake by rice) under biochar amendment (1–4% of soil mass). We demonstrate for the first time that biochar amendment could evidently reduce grain MeHg levels (49–92%). The declines could be attributed to the combined effects of: (1) increased soil MeHg concentrations, probably explained by the release of sulfate from biochar and thus enhanced microbial production of MeHg (e.g., by sulfate-reducing bacteria), (2) MeHg immobilization in soils, facilitated by the large surface areas and high organosulfur content of biochar, and (3) biodilution of MeHg in rice grains, due to the increased grain biomass under biochar amendment (35–79%). These observations together with mechanistic explanations improve understanding of MeHg dynamics in soil-rice systems, and support the possibility of reducing MeHg phytoaccumulation under biochar amendment.

Selenium inhibits sulfate-mediated methylmercury production in rice paddy soil

There is increasing interest in understanding factors controlling methylmercury (MeHg) production in mercury-contaminated rice paddy soil. Sulfate has been reported to affect MeHg biogeochemistry under anoxic conditions, and recent studies revealed that selenium (Se) could evidently reduce MeHg production in paddy soil. However, the controls of sulfate and Se on net MeHg production in paddy soil under fluctuating redox conditions remain largely unknown. Microcosm experiments were conducted to explore the effects of sulfate and Se on net MeHg production in rice paddy soil. Soil was added with 0–960 mg/kg sulfate, in the presence or absence of 3.0 mg/kg selenium (selenite or selenate), and incubated under anoxic (40 days) or suboxic conditions (5 days), simulating fluctuating redox conditions in rice paddy field. Sulfate addition moderately affected soil MeHg concentrations under anoxic conditions, while reoxidation resulted in evidently higher (18–40%) MeHg levels in sulfate amended soils than the control. The observed changes in net MeHg production were related to dynamics of sulfate and iron. However, Se could inhibit sulfate-mediated MeHg production in the soils: Se addition largely reduced net MeHg production in the soils (23–86%, compared to the control), despite of sulfate addition. Similarly, results of the pot experiments (i.e., rice cultivation in amended soils) indicated that soil MeHg levels were rather comparable in Se-amended soils during rice growth period, irrespective of added sulfate doses. The more important role of Se than sulfate in controlling MeHg production was explained by the formation of HgSe nanoparticles irrespective of the presence of sulfate, confirmed by TEM-EDX and XANES analysis. Our findings regarding the effects of sulfate and Se on net MeHg production in rice paddy soil together with the mechanistic explanation of the processes advance our understanding of MeHg dynamics and risk in soil-rice systems.

Mechanistic understanding of MeHg-Se antagonism in soil-rice systems: the key role of antagonism in soil.

Methylmercury (MeHg) accumulation in rice has great implications for human health. Here, effects of selenium (Se) on MeHg availability to rice are explored by growing rice under soil or foliar fertilization with Se. Results indicate that soil amendment with Se could reduce MeHg levels in soil and grain (maximally 73%). In contrast, foliar fertilization with Se enhanced plant Se levels (3–12 folds) without affecting grain MeHg concentrations. This evidence, along with the distinct distribution of MeHg and Se within the plant, demonstrate for the first time that Se-induced reduction in soil MeHg levels (i.e., MeHg-Se antagonism in soil) rather than MeHg-Se interactions within the plant might be the key process triggering the decreased grain MeHg levels under Se amendment. The reduction in soil MeHg concentrations could be mainly attributed to the formation of Hg-Se complexes (detected by TEM-EDX and XANES) and thus reduced microbial MeHg production. Moreover, selenite and selenate were equally effective in reducing soil MeHg concentrations, possibly because of rapid changes in Se speciation. The dominant role of Se-induced reduction in soil MeHg levels, which has been largely underestimated previously, together with the possible mechanisms advance our mechanistic understanding about MeHg dynamics in soil-rice systems.

Production and retention of methylmercury in inundated boreal forest soils.

The Flooded Uplands Dynamics Experiment (FLUDEX) was an ecosystem-scale study examining the production of methylmercury (MeHg) and greenhouse gases from reservoirs constructed on an upland boreal forest landscape in order to quantify their dependence upon carbon stores. We detail the within-reservoir production and storage of MeHg before, during, and nine years after the experiment. The reservoirs were net MeHg producers during the first two years of flooding, and net demethylating systems afterward. During years 1-3, a rapid pulse of MeHg and total Hg was observed in floodwater, followed by substantial increases in MeHg in seston and sediment. Resampling of the dry reservoirs nine years after the experiment ended indicated that organic soil MeHg was still 8 to 52-fold higher than preflood conditions, and averaged 86% of the levels recorded at the end of the third flooding year. Both total Hg and MeHg retention in soil were a strong function of organic carbon content. The time scale of soil MeHg retention may help explain the decadal time lag frequently observed for the decrease of piscivorous fish Hg concentrations in new reservoirs. Predicted extreme precipitation events associated with climate change may serve to make landscapes more susceptible to this process.

Influence of soil mercury concentration and fraction on bioaccumulation process of inorganic mercury and methylmercury in rice (Oryza sativa L.).

Recent studies showed that rice is the major pathway for methylmercury (MeHg) exposure to inhabitants in mercury (Hg) mining areas in China. There is, therefore, a concern regarding accumulation of Hg in rice grown in soils with high Hg concentrations. A soil pot experimental study was conducted to investigate the effects of Hg-contaminated soil on the growth of rice and uptake and speciation of Hg in the rice. Our results imply that the growth of rice promotes residual fraction of Hg transforming to organic-bound fraction in soil and increased the potential risks of MeHg production. Bioaccumulation factors deceased for IHg but relatively stabilized for MeHg with soil total mercury (THg) increasing. IHg in soil was the major source of Hg in the root and stalk, but leaf was contributed by Hg from both atmosphere and soil. Soluble and exchangeable Hg fraction can predict the bioavailability of IHg and MeHg in soils, and that can provide quantitative description of the rate of uptake of the bioavailable Hg. Soluble and exchangeable Hg fraction in paddy soil exceeding 0.0087 mg kg(-1) may cause THg concentration in rice grain above the permissible limit standard, and MeHg concentration in paddy soil more than 0.0091 mg kg(-1) may have the health risks to humans.