Soil MeHg Research Articles

Key active mercury methylating microorganisms and their synergistic effects on methylmercury production in paddy soils

Rice contamination with neurotoxic methylmercury (MeHg) from paddy soils is an escalating global concern. Identifying the microorganisms responsible for mercury (Hg) methylation in these soils is essential for controlling Hg contamination in the food chain and mitigating health impacts. Current research often focuses on total Hg-methylating microorganisms, overlooking the contributions of active ones, which can lead to either overestimating or neglecting the specific roles of microorganisms in Hg methylation within paddy soils. In this study, active Hg-methylating microorganisms in paddy soils were identified using a combination of DNA-SIP, Hg isotope labelling, and Hg methylation gene sequencing techniques. Our findings revealed that Geobacter and Anaerolinea are pivotal active Hg-methylating microorganisms across a contamination gradient in paddy soils. Transcriptomic analysis of soils from major rice-producing provinces in China confirmed the widespread and synergistic involvement of these microorganisms. Microbial incubation further validated their interaction significantly enhances Hg methylation, with Me198Hg concentrations increasing 2.8-fold compared to Geobacter alone and 5.2-fold compared to Anaerolinea alone. These findings enhance our understanding of microbial Hg methylation in paddy soils, providing critical insights for accurately predicting soil MeHg load, rice grain MeHg contamination, and human MeHg exposure risks.

Application of biochar and selenium together at low dose efficiently reduces mercury and methylmercury accumulation in rice grains

Irrespective of cost and ecological risk, literatures have reported that both biochar and selenium (Se) alone at high application rate exhibited positive effects on decreasing rice mercury (Hg) uptake in high Hg contaminated paddy soil. In this study, we investigated whether biochar and Se together at low dose could efficiently reduce the rice grain Hg and MeHg accumulation in the slight Hg-contaminated soil. Compared with control (CK), the Hg concentration of grains in the BC3, Se0.5, and BC3 + Se0.5 treatments decreased by 5.4 %, 38.3 %, and 48.5 %, respectively. Co-application of biochar and Se also decreased the methylmercury (MeHg) concentration in rice grains by 29.1–91.6 %. The decrease of Hg and MeHg level in rice grains for biochar and Se treatments could be attributed to the following mechanisms: (1) high Hg (primarily inorganic Hg) adsorption on biochar through its high hydroxyl groups and large specific surface area; (2) Increased dissolved organic carbon and cysteine contents in pore water after biochar application, which reduced the availability of soil Hg through complexation; (3) Decreased bioavailability of Hg in soil due to the formation of HgSe precipitation which inhibited Hg uptake and translation by rice plant; (4) Both biochar and Se facilitated the reduction of MeHg in soil. Our results indicate that co-application of biochar and Se at low dose is a promising method to effectively mitigate Hg accumulation in rice grains from the slight Hg-contaminated soil.

Precipitation patterns strongly affect vertical migration and methylation of mercury in legacy contaminated sites

Legacy-contaminated sites act as significant sources of mercury (Hg) to their surrounding surface and underground environments. Intensified extreme precipitation is posing great threats to the environment and human health by changing the fate of pollutants, yet little is known about its effect on the vertical migration and methylation of Hg in contaminated sites. Here, we applied a range of simulated extreme precipitation patterns (frequency and intensity) to column leaching assays with soils collected near a contaminated site. We observed that precipitation with high frequency but low intensity resulted in more vertical migration of Hg through the soil profile than that with low frequency but high intensity. The majority (> 90%) of leached Hg was prone to migrate vertically within the top 10 cm of the soil profile. Furthermore, rainfall stimulated microbial Hg methylation, as demonstrated by enhanced production of methylmercury (MeHg) in both simulated and field-contaminated soils. We identified specific microbial taxa including Geobacteraceae, Desulfuromonadaceae, Syntrophaceae, Oscillospiraceae, and Methanomicrobiaceae as key predictors of MeHg production, which differed from those typically observed in overlying water of croplands. Particularly, the relative abundance of these dominant Hg methylators significantly increased during rainfall-induced leaching compared to that of the control, suggesting the crucial yet previously overlooked impacts of increased precipitation events on the process of microbial Hg methylation in industry-contaminated sites. Given the rising incidences of extreme precipitation events worldwide due to climate change, this study highlights the significance of assessing Hg mobility and microbial transformation in legacy contaminated sites.

Predictive modeling of methylmercury in rice (Oryza sativa L.) and species-sensitivity-distribution-based derivation of the threshold of soil mercury in karst mountain areas.

The bioavailable mercury (Hg) in the soil is highly active and can affect the formulation of methyl-Hg (MeHg) in soil and its accumulation in rice. Herein, we predicted the concentration of MeHg in rice using bioavailable Hg extracted from soils; additionally, we determined the threshold value of soil Hg in karst mountain areas based on species sensitivity distribution. The bioavailable Hg was extracted using calcium chloride, hydrochloric acid (HCl), diethylenetriaminepentaacetic acid mixture, ammonium acetate, and thioglycolic acid. Results showed that HCl is the best extractant, and the prediction model demonstrated good predictability of the MeHg concentration in rice based on the HCl-extractable Hg, pH, and soil organic matter (SOM) data. Compared with the actual MeHg concentration in rice, approximately 99% of the predicted values (n = 103) were within the 95% prediction range, indicating the good performance of the rice MeHg prediction model based on soil pH, SOM, and bioavailable Hg in karst mountain areas. Based on this MeHg prediction model, the safety threshold of soil Hg was calculated to be 0.0936mg/kg, which is much lower than the soil pollution risk screening value of agricultural land (0.5mg/kg), suggesting that a stricter standard should be applied regarding soil Hg in karst mountain areas. This study presents the threshold of soil Hg pollution for rice safety in karst mountain areas, and future studies should target this threshold range.

Influence of irrigation water and soil on annual mercury dynamics in Sacramento Valley rice fields.

Methylmercury (MeHg) is a human and environmental toxin produced in flooded soils. Little is known about MeHg in rice (Oryza Sativa L.) fields in Sacramento Valley, California. The objectives of this study were to quantify mercury fractions in irrigation water and within rice fields and to determine their mercury pools in surface water, soil, and grain. Soil, grain, and surface water (dissolved and particulate) MeHg and total mercury (THg) were monitored in six commercial rice fields throughout a winter fallow season and subsequent growing season. Both dissolved and particulate mercury fractions were higher in fallow season rice field water. Total suspended solids and particulate mercury concentrations were positively correlated (r=0.99 and 0.98 for THg and MeHg, respectively), suggesting that soil MeHg was suspended in the water column and potentially exported. Dissolved THg and MeHg concentrations were positively correlated with absorbance at 254nm (r=0.47 and 0.58, respectively) in fallow season field water. In the growing season, fields with higher irrigation water MeHg concentrations (due to recycled water use) had elevated field-water MeHg (r=0.86) and grain MeHg concentrations (r=0.96). Based on a mass balance analysis, soil mercury pools were orders of magnitude larger than surface water or grain mercury pools; however, fallow season drainage and grain harvest were the primary pathways for MeHg export. Based on these findings, reducing (1) discharge when water is turbid, (2) straw inputs, and (3) use of recycled irrigation water could help reduce mercury exports in rice field drainage water.

Effects of elevated CO2 on MeHg and IHg in rice

Methylmercury (MeHg) and, to a lesser extent, inorganic mercury (IHg) contamination of rice is a global public health concern, but little is known about how soil and grain Hg concentrations respond to elevated CO2 (ECO2), or how ECO2 alters movement of Hg through the soil-plant-grain system. To advance knowledge of how Hg contamination of rice will change in the future, this study explored the effect of elevated CO2 (ECO2, c. 800 ppm) on soil, iron plaque, root, stem/leaf, and grain concentrations of MeHg and IHg. We observed evidence that ECO2 increased accumulation of MeHg, but not IHg, in rice grain. For IHg, ECO2 did not alter its uptake from the soil, translocation through the plant, or concentration in rice grain. However, ECO2 did reduce uptake of IHg from the air into leaf tissues, likely as a result of the reduced stomatal conductivity and thus more limited direct uptake from the air.Methylmercury concentrations in the grain of plants grown at ECO2 were significantly higher than those of plants grown at ambient CO2. Moreover, MeHg concentrations were also elevated in stem/leaf (82 %) and root tissue (37 %) for ECO2 plants, although the root-tissue results were not statistically significant. In contrast, soil MeHg concentrations were virtually indistinguishable between treatments, indicating that higher rice grain MeHg concentrations were not likely due to higher microbial IHg methylation in soil. Plant uptake of MeHg into stem/leaves and grain from the soil was significantly greater in the ECO2 treatment; however, translocation patterns of MeHg within the plant itself did not differ between treatments. Notably, these patterns existed despite consistently lower transpiration in the ECO2 treatment, and thus less mass flow of solute towards and through the plant. Our results indicate that as CO2 concentrations rise, the human health risks related to MeHg in grain will likely increase.

Suppression of mercury methylation in soil and methylmercury accumulation in rice by dissolved organic matter derived from sulfur-rich rape straw

Straw amendment significantly enhances mercury (Hg) methylation and subsequent methylmercury (MeHg) bioaccumulation in Hg-contaminated paddy fields by releasing dissolved organic matter (DOM). This study comprehensively investigates the regulatory mechanisms of DOM and its different molecular weights derived from sulfur-rich rape straw (RaDOM) and composted rape straw (CRaDOM) applied in the rice-filling stage on soil MeHg production and subsequent bioaccumulation in rice grains. The results indicated that the amendment of RaDOM and CRaDOM significantly reduced soil MeHg content by 42.40–62.42%. This reduction can be attributed to several factors, including the suppression of Hg-methylating bacteria in soil, the supply of sulfate from RaDOM and CRaDOM, and the increase in the humification, molecular weight, and humic-like fractions of soil DOM. Additionally, adding RaDOM increased the MeHg bioaccumulation factor in roots by 27.55% while inhibiting MeHg transportation by 12.24% and ultimately reducing MeHg content in grains by 21.24% compared to the control group. Similarly, CRaDOM enhanced MeHg accumulation by 25.19%, suppressed MeHg transportation by 39.65%, and reduced MeHg levels in the grains by 27.94%. The assimilation of sulfate derived from RaDOM and CRaDOM into glutathione may be responsible for the increased retention of MeHg in the roots. Over the three days, there was a significant decrease in soil MeHg content as the molecular weight of RaDOM increased; conversely, altering the molecular weight of CRaDOM demonstrated an inverse trend. However, this pattern was not observed after 12 days. Applying sulfur-rich rape DOM can help mitigate MeHg accumulation in paddy fields by regulating the quality of soil DOM, sulfur cycling, and Hg-methylating bacteria.

Selenium–phosphorus modified biochar reduces mercury methylation and bioavailability in agricultural soil

Biochar is a frequently employed for solidifying and stabilizing mercury (Hg) contamination in soil. However, it often results in an elevated presence of soil methylmercury (MeHg), which introduces new environmental risks. Consequently, there is a necessity for developing a safer modified biochar for use in Hg-contaminated soil. This study employed sodium selenite (at a safe dosage for soil) and hydroxyapatite to modify straw biochar (BC) based on the interaction between selenium (Se) and phosphorus (P). This process led to the formation of Se-modified biochar (Se-BC), P-modified biochar (P-BC), and Se and P co-modified biochar (Se-P-BC). Additionally, solvent adsorption experiments and pot experiments (BC/soil mass ratio: 0.5 %) were conducted to investigate the impacts of these soil amendments on soil Hg methylation and bioavailability. Se and P co-modification substantially increased the surface area, pore volume, and Hg adsorption capacity of BC. BC treatment increased the simulated gastric acid-soluble Hg, organo-chelated Hg, and MeHg in the soil. Conversely, Se-P-BC significantly reduced these forms of Hg in the soil, indicating that Se-P-BC can transform soil Hg into less bioavailable states. Among the different biochar treatments, Se-P-BC exhibited the most pronounced reductions in soil MeHg, total Hg, and MeHg in water spinach, achieving reductions of 63 %, 71 %, and 70 %, respectively. The co-modification of Se and P displayed a synergistic reduction effect in managing soil Hg pollution, which is associated with the increase of available Se in the soil due to phosphorus addition. The significantly reduced dissolved organic carbon and the abnormally high SO42− concentration in the soil of Se-P-BC treatment also inhibited Hg methylation and bioavailability in the soil. In summary, Se-P-BC substantially increased reduction percentage in plant Hg content while mitigating the risk of secondary pollution arising from elevated soil MeHg.

Selenium- and chitosan-modified biochars reduce methylmercury contents in rice seeds with recruiting Bacillus to inhibit methylmercury production

Biochar could reshape microbial communities, thereby altering methylmercury (MeHg) concentrations in rice rhizosphere and seeds. However, it remains unclear whether and how biochar amendment perturbs microbe-mediated MeHg production in mercury (Hg) contaminated paddy soil. Here, we used pinecone-derived biochar and its six modified biochars to reveal the disturbance. Results showed that selenium- and chitosan-modified biochar significantly reduced MeHg concentrations in the rhizosphere by 85.83% and 63.90%, thereby decreasing MeHg contents in seeds by 86.37% and 75.50%. The two modified bicohars increased the abundance of putative Hg-resistant microorganisms Bacillus, the dominant microbe in rhizosphere. These reductions about MeHg could be facilitated by biochar sensitive microbes such as Oxalobacteraceae and Subgroup_7. Pinecone-derived biochar increased MeHg concentration in rhizosphere but unimpacted MeHg content in seeds was observed. This biochar decreased the abundance in Bacillus but enhanced in putative Hg methylator Desulfovibrio. The increasing MeHg concentration in rhizosphere could be improved by biochar sensitive microbes such as Saccharimonadales and Clostridia. Network analysis showed that Saccharimonadales and Clostridia were the most prominent keystone taxa in rhizosphere, and the three biochars manipulated abundances of the microbes related to MeHg production in rhizosphere by those biochar sensitive microbes. Therefore, selenium- and chitosan-modified biochar could reduce soil MeHg production by these microorganisms, and is helpful in controlling MeHg contamination in rice.

The Effects of Different Soil Component Couplings on the Methylation and Bioavailability of Mercury in Soil.

Soil composition can influence the chemical forms and bioavailability of soil mercury (Hg). However, previous studies have predominantly focused on the influence of individual components on the biogeochemical behavior of soil Hg, while the influence of various component interactions among several individual factors remain unclear. In this study, artificial soil was prepared by precisely regulating its components, and a controlled potted experiment was conducted to investigate the influence of various organic and inorganic constituents, as well as different soil textures resulting from their coupling, on soil Hg methylation and its bioavailability. Our findings show that inorganic components in the soils primarily exhibit adsorption and fixation effects on Hg, thereby reducing the accumulation of total mercury (THg) and methylmercury (MeHg) in plants. It is noteworthy that iron sulfide simultaneously resulted in an increase in soil MeHg concentration (277%). Concentrations of THg and MeHg in soil with peat were lower in rice but greater in spinach. A correlation analysis indicated that the size of soil particles was a crucial factor affecting the accumulation of Hg in plants. Consequently, even though fulvic acid activated soil Hg, it significantly increased the proportion of soil particles smaller than 100.8 μm, thus inhibiting the accumulation of Hg in plants, particularly reducing the concentration of THg (93%) and MeHg (85%) in water spinach. These results demonstrate that the interaction of organic and inorganic components can influence the biogeochemical behavior of soil Hg not only through their chemical properties, but also by altering the soil texture.

Insights into the reduction of methylmercury accumulation in rice grains through biochar application: Hg transformation, isotope fractionation, and transcriptomic analysis

Methylmercury (MeHg), a potent neurotoxin, easily moves from the soil into rice plants and subsequently accumulates within the grains. Although biochar can reduce MeHg accumulation in rice grains, the precise mechanism underlying biochar-mediated responses to mercury (Hg) stress, specifically regarding MeHg accumulation in rice, remains poorly understood. In the current study, we employed a 4% biochar amendment to remediate Hg-contaminated paddy soil, elucidate the impacts of biochar on MeHg accumulation through a comprehensive analysis involving Hg isotopic fractionation and transcriptomic analyses. The results demonstrated that biochar effectively lowered the levels of MeHg in paddy soils by decreasing bioavailable Hg and microbial Hg methylation. Furthermore, biochar reduced the uptake and translocation of MeHg in rice plants, ultimately leading to a reduction MeHg accumulation in rice grains. During the process of total mercury (THg) uptake, biochar induced a more pronounced negative isotope fractionation magnitude, whereas the effect was less pronounced during the upward transport of THg. Conversely, biochar caused a more pronounced positive isotope fractionation magnitude during the upward transport of MeHg. Transcriptomics analyses revealed that biochar altered the expression levels of genes associated with the metabolism of cysteine, glutathione, and metallothionein, cell wall biogenesis, and transport, which possibly enhance the sequestration of MeHg in rice roots. These findings provide novel insights into the effects of biochar application on Hg transformation and transport, highlighting its role in mitigating MeHg accumulation in rice.

Relative importance of aceticlastic methanogens and hydrogenotrophic methanogens on mercury methylation and methylmercury demethylation in paddy soils

The accumulation of methylmercury (MeHg) in paddy soil results from a subtle balance between inorganic mercury (e.g., HgII) methylation and MeHg demethylation. Methanogens not only act as Hg methylators but may also facilitate MeHg demethylation. However, the diverse methanogen flora (e.g., aceticlastic and hydrogenotrophic types) that exists under ambient conditions has not previously been considered. Accordingly, the roles of different types of methanogens in HgII methylation and MeHg degradation in paddy soils were studied using the Hg isotope tracing technique combined with the application of methanogen inhibitors/stimulants. It was found that the response of HgII methylation to methanogen inhibitors or stimulants was site-dependent. Specifically, aceticlastic methanogens were suggested as the potential HgII methylators at the low Hg level background site, whereas hydrogenotrophic methanogens were potentially involved in MeHg production as Hg levels increased. In contrast, both aceticlastic and hydrogenotrophic methanogens facilitated MeHg degradation across the sampling sites. Additionally, competition between hydrogenotrophic and aceticlastic methanogens was observed in Hg-polluted paddy soils, implying that net MeHg production could be alleviated by promoting aceticlastic methanogens or inhibiting hydrogenotrophic methanogens. The findings gained from this study improve the understanding of the role of methanogens in net MeHg formation and link carbon turnover to Hg biogeochemistry in rice paddy ecosystems.

Assessing microbial degradation potential of methylmercury in different types of paddy soil through short-term incubation

The neurotoxic methylmercury (MeHg) in paddy soils can accumulate in rice grains. Microbial demethylation is an important pathway of MeHg degradation in soil, but the effect of soil type on microbial degradation of MeHg remains unclear. Therefore, we investigated MeHg degradation in eight typical paddy soils and analyzed the associations between soil physiochemical properties and microbial degradation efficiencies of MeHg. Results showed that MeHg was significantly degraded in unsterilized paddy soils, and the microbial degradation efficiency ranged from 10.8% to 64.6% after a 30-day incubation. The high microbial degradation efficiency of MeHg was observed in the soils with high levels of clay content, whereas relatively low degradation efficiency was found in the red paddy soils. We identified that Paenibacillaceae was the most important microbial predictor of MeHg degradation and was positively correlated with the degradation efficiency in the soils. The abundances of these microbial taxa associated with MeHg degradation were positively correlated with clay content. In addition, Eh, pH, and SOC could influence microbial degradation of MeHg by regulating certain microbial communities. Our results indicate that soil type is crucial in driving MeHg degradation, which has important implications for the mitigation of MeHg pollution in various croplands.

Aging rice straw reduces the bioavailability of mercury and methylmercury in paddy soil

Straw amendment is a prevalent agricultural practice worldwide, which can reduce air pollution and improve soil fertility. However, the impact of aging straw amendment on the bioavailability of mercury (Hg) and methylmercury (MeHg) in paddy soil remains unclear. To investigate this, incubation experiments were conducted using the diffusive gradient in thin-film technique. Results showed that amendments of dry-wet aging (DRS), photochemical aging (LRS), and freeze-thaw aging rice straw (FRS) reduced the bioavailable MeHg in paddy soil by 2.2–27.6%, 13.5–69.8%, and 23.5–86.1%, respectively, compared to fresh rice straw (RS) amendment. This result could be due to changes in soil properties such as soil pH and overlying water Fe and Mn as well as microbial abundance (including Clostridiaceae, Firmicutes, and Actinobacteriota). Simultaneously, The LRS and FRS amendments reduced bioavailable Hg in paddy soil by 20.0–40.8% and 17.1–48.6%, respectively, while DRS increased the bioavailable Hg by 15.8–120.0%. This could be attributed to changes in soil oxidation-reduction potential and overlying water SO42− content. Additionally, the results of sand culture experiments showed that the concentrations of Hg uptake by rice seedlings were 97.1–118.2%, 28.1–35.6%, and 198.0–217.1% higher in dissolved organic matter (DOM) derived from DRS, LRS, and FRS than RS, indicating that aging straw leached DOM may promote the Hg bioavailable when straw amendment. This result could be due to lower molecular weight and higher CO functional group content. These results provide new insight into how aging straw amendment affects the bioavailability of Hg and MeHg in paddy soil under different climates.

Controlling Factors and Predictive Models of Total Mercury and Methylmercury Accumulation in Rice (Oryza sativa L.) from Mercury-Contaminated Paddy Soils.

It is urgent to detect the major controlling factors and establish predictive models of mercury (Hg) accumulation in rice. A pot trial was conducted, exogenous Hg was added to 19 paddy soils at 4 concentration levels in this study. The major controlling factors of total Hg (THg) in brown rice were soil THg, pH and organic matter (OM) content, while those of methylmercury (MeHg) in brown rice were soil MeHg and OM. THg and MeHg in brown rice could be well predicted by soil THg, pH and clay content. The data from previous studies were collected to validate the predictive models of Hg in brown rice. The predicted values of Hg in brown rice were within the twofold prediction intervals of the observations, which demonstrated the predictive models in this study were reliable. The results could provide theoretical foundation for the risk assessment of Hg in paddy soils.

Effects of dissolved organic matter on mercury speciation in rice rhizosphere amended with sulfur-rich biochar

Natural sulfur (S)-rich biochar (NRB) can be employed as an alternative for traditional S-modified biochar. However, the effect of dissolved organic matter (DOM) on mercury (Hg) speciation in rice rhizosphere soils under natural S-rich biochar application remains unclear. We conducted a pot experiment to study the effects of NRB application on the chemical composition and structure of DOM and the related speciation and availability of Hg in rice rhizosphere. Applying NRB significantly increased the concentration of methylmercury (MeHg) in the rhizosphere soils, which was enhanced with application frequency. This observation can be explained by MeHg immobilization in response to increasing S content in rice rhizosphere soils. We also observed increased molecular weight and functional group complexity of DOM, likely contributing to the decrease in MeHg mobility. Furthermore, the increase in pH and humification of DOM caused by S-rich biochar application generally reduced the concentrations of water-soluble and mercuric-sulfide fraction (easily-available Hg species) and organo-chelated fraction (potentially-available Hg species). Our findings highlight that the application of NRB can reduce the availability of MeHg in rice rhizosphere, thus providing a practical basis for reducing the potential risk of MeHg toxicity.

New insights into sulfur input induced methylmercury production and accumulation in paddy soil and rice

Sulfur has a high affinity for mercury (Hg) and can serve as effective treating agent for Hg pollution. However, conflict effects between reducing Hg mobility and promoting Hg methylation by sulfur were found in recent studies, and there is a gap in understanding the potential mechanism of MeHg production under different sulfur-treated species and doses. Here, we investigated and compared the MeHg production in Hg-contaminated paddy soil and its accumulation in rice under elemental sulfur or sulfate treatment at a relatively low (500 mg·kg−1) or high (1000 mg·kg−1) level. The associated potential molecular mechanisms are also discussed with the help of density functional theory (DFT) calculation. Pot experiments demonstrate that both elemental sulfur and sulfate at high exposure levels increased MeHg production in soil (244.63–571.72 %) and its accumulation in raw rice (268.73–443.50 %). Coupling the reduction of sulfate or elemental sulfur and decrease of soil redox potential leads to the detachment of Hg-polysulfide complexes from the surface of HgS which can be explained by DFT calculations. Enhancement of free Hg and Fe release through reducing Fe(III) oxyhydroxides further promotes soil MeHg production. The results provide clues for understanding the mechanism by which exogenous sulfur promotes MeHg production in paddies and paddy-like environments and give new insights for decreasing Hg mobility by regulating soil conditions.

Response of methylmercury in paddy soil and paddy rice to pristine biochar: A meta-analysis and environmental implications

Biochar has received increased research attention due to its effectiveness in mitigating the potential risks of mercury (Hg) in agricultural soils. However, there is a lack of consensus on the effect of pristine biochar on the net production, availability, and accumulation of methylmercury (MeHg) in the paddy rice-soil system. As such, a meta-analysis with 189 observations was performed to quantitatively assess the effects of biochar on Hg methylation, MeHg availability in paddy soil, and the accumulation of MeHg in paddy rice. Results suggested that biochar application could significantly increase the production of MeHg in paddy soil by 19.01%; biochar could also decrease the dissolved and available MeHg in paddy soil by 88.64% and 75.69%, respectively. More importantly, biochar application significantly inhibited the MeHg accumulation in paddy rice by 61.10%. These results highlight that biochar could decrease the availability of MeHg in paddy soil and thus inhibit MeHg accumulation in paddy rice, although it might facilitate the net production of MeHg in paddy soil. Additionally, results also indicated that the biochar feedstock and its elementary composition significantly impacted the net MeHg production in paddy soil. Generally, biochar with a low carbon content, high sulfur content, and low application rate might be beneficial for inhibiting Hg methylation in paddy soil, meaning that Hg methylation depends on biochar feedstock. These findings suggested that biochar has great potential to inhibit MeHg accumulation in paddy rice, and further research should focus on selecting biochar feedstock to control Hg methylation potential and determine its long-term effects.

Effect of peat and thiol-modified peat application on mercury (im)mobilization in mercury-polluted paddy soil

Mercury (Hg) pollution in paddy soil has gained special attention because methylmercury (MeHg) can accumulate in rice grains. Therefore, there is an urgent need to explore the remediation materials of mercury-polluted paddy soil. In this study, herbaceous peat (HP), peat moss (PM), and thiol-modified HP/PM (MHP/MPM) were selected to investigate the effects and probable mechanism of their application on Hg (im)mobilization in mercury-polluted paddy soil through pot experiments. The results showed that HP, PM, MHP and MPM addition increased MeHg concentrations in the soil, indicating that the addition of peat and thiol-modified peat might increase the exposure risk of MeHg in soil. The addition of HP could significantly decrease the total mercury (THg) and MeHg concentrations in rice, with average reduction efficiencies of 27.44% and 45.97%, respectively, while adding PM slightly increased the THg and MeHg concentrations in rice. In addition, the addition of MHP and MPM significantly decreased the bioavailable Hg concentrations in the soil and THg and MeHg concentrations in rice, with reduction efficiencies of rice THg and MeHg of 79.14∼93.14% and 82.72∼93.87%, respectively, indicating that thiol-modified peat had good remediation potential. The possible mechanism is that Hg can bind with thiols in MHP/MPM and form steady compounds in the soil, reducing Hg mobility in the soil and inhibiting its uptake by rice. Our study showed the potential value of HP, MHP and MPM addition for Hg remediation. Additionally, we must weigh the pros and cons when adding organic materials as remediation agents to mercury-polluted paddy soil.

Effects of varying amounts of different biochars on mercury methylation in paddy soils and methylmercury accumulation in rice (Oryza sativa L.)

There is growing evidence for the potential of biochars (BCs) in remediating mercury-contaminated paddy soils, but the high doses commonly used in laboratory studies discourage BC application in practice. To address these difficulties, we compared the effects of varying amounts of BCs from different sources on the formation of methylmercury (MeHg) in soil and its accumulation in rice through microcosm and pot experiments. The addition of a wide range of added doses (0.3, 0.6, 1, 2, 4 and 5 %, w/w) of BCs derived from different biomass feedstocks (i.e., corn stalk, wheat straw, bamboo, oak and poplar) significantly decreased the fraction of ammonium thiosulfate ((NH4)2S2O3)-extractable MeHg in the soil, although the MeHg contents varied with BC types and doses during soil incubation. However, the extractable MeHg in the soil did not continuously decrease with increasing BC doses, especially at doses of >1 %, resulting in limited further reductions. Moreover, a relatively low application rate (0.3–0.6 %, w/w) of BCs (i.e., corn stalk, wheat straw and bamboo-derived BC), especially of bamboo-derived BCs, significantly decreased the MeHg levels (42–76 %) in rice grains (brown rice). Meanwhile, the extractable soil MeHg decreased (57–85 %), although the MeHg in the soil varied under BC amendment during rice cultivation. These results provide further evidence that applying BC produced from different raw carbon materials (e.g., lignocellulosic biomass) could effectively reduce MeHg accumulation in rice grains, possibly due to MeHg bioavailability reduction in the soil. Our results suggest the possibility of mitigating MeHg accumulation in rice with a low dose of BCs, with great potential for use in remediating moderately contaminated paddy soils.