DOM influences Hg methylation in paddy soils across a Hg contamination gradient

Regulation of straw-derived DOM and clay mineral complexation on mercury accumulation in vegetables

Straw return-to-field releases substantial dissolved organic matter (DOM), which can interact with clay minerals and influence mercury (Hg) dynamics in soil-plant systems. However, its detailed mechanisms remain poorly understood. In this study, DOM-montmorillonite (DOM-M) complexes were synthesized using DOM extracted from composted rice straw (DOMrice) and rape straw (DOMrape). The objective of this study was to investigate their impacts on Hg methylation in soil and the accumulation of total Hg (THg) and methylmercury (MeHg) in vegetables. The results demonstrated that straw-derived DOM significantly increased MeHg levels in the soil and water spinach. However, humified straw-derived DOM effectively suppressed this elevation by 29.0-64.5%. Specifically, humified DOMrice resulted in lower MeHg concentrations in the soil and reduced THg and MeHg levels in water spinach compared to humified DOMrape. Natural montmorillonite reduced Hg methylation in the soil but increased the accumulation of THg and MeHg in water spinach. In contrast, the humified DOMrape-M complex significantly mitigated the MeHg accumulation in water spinach that was enhanced by montmorillonite, with a reduction percentage of 25.8-52.0%, while the humified DOMrice-M complex did not demonstrate a similar advantage. This discrepancy could be attributed to certain molecular components in DOMrape, such as higher thiol-rich protein-like fractions and oxidized S species, which could promote Hg retention within mineral layers. The reduced adsorption capacity of humified DOMrice-M for Hg2+ also emphasized the unique role of humified DOMrape-M. Overall, this study highlights the importance of humified straw-derived DOM and its interaction with soil minerals in shaping Hg dynamics within the plant-soil system.

New insights into aqueous Hg(II) photoreduction from paddy field system to natural water: Gear effect of straw returning and soil tillage

Soil dissolved organic matter (SDOM) has a strong complex with divalent mercury (Hg(II)) and can affect the fate of aqueous Hg(II) photoreduction. However, little is known about the influence of straw returning and soil tillage on the composition of SDOM in paddy soil and Hg(II) photoreduction in paddy water. Here, we demonstrate that the combined drivers of long-term straw returning and tillage can result in higher degrees of aromatization, and the enrichment of oxygen-containing functional groups in surface SDOM. Hg(II) photoreduction under low Hg/DOC conditions is mainly constrained by the composition of SDOM, whereas solar radiation emerged as a dominant controlling factor associated with high ratio of Hg/DOC. By increasing the release of SDOM and mobility of Hg(II), reducing the stability of Hg(II)-SDOM complexes, and potentially enhancing generation of reactive intermediates, gear effect of straw returning and soil tillage significantly enhanced Hg(II) photoreduction in the presence of surface SDOM from 0-40 cm (maximum photoreduction percentage can reach 44.76 ± 2.24 %). Previous inventories of Hg(0) emissions from paddy field system may have overlooked or underestimated this critical process. Future modeling work should be carried out to evaluate the role of straw returning and soil tillage on global Hg cycle.

Elevated methylmercury production in seasonally inundated sediments: Insights from DOM molecular composition

Seasonally inundated areas (SIA) within aquatic systems are characterized by elevated methylmercury (MeHg) production. Nevertheless, the response characteristics of dissolved organic matter (DOM) quality in SIA sediments, including its molecular compositions and structure, and their impacts on the MeHg production are not yet fully understood. This research gap has been addressed through field investigations and microcosm experiments conducted in a metal-polluted plateau wetland. The results revealed that DOMSIA had lower levels of chromophoric DOM concentrations, protein-like fractions, molecular complexity, and debris size while exhibiting higher humic-like fractions, molecular weight, COO- groups, and bioavailability than DOM in permanently inundated areas (PIA). Compared with DOMPIA, DOMSIA was more easily biodegraded, and exhibited a higher adsorption capacity while lower binding affinity for Hg(Ⅱ). Moreover, MeHg synthesis by Desulfomicrobium escambiense was 29.6-fold higher in DOMSIA than that in DOMPIA, and DOMSIA amendment also resulted in a higher MeHg production in the sediment. The PLS-PM model demonstrated that DOM compositions positively showed high contributions to MeHg levels in sediment porewater (0.51), while binding affinity had a negative pattern (-0.83), but adsorption capacity had a lower contribution (0.09). These findings provide an updated explanation for the elevated MeHg level in the SIA of aquatic systems, which are closely related to the adaptive response of DOM molecular composition and structure in the sediment.

Anaerobic fermentation of straw with sulfate addition: A suitable approach for straw utilization in mercury-contaminated areas

Returning raw straw to the soil can significantly elevate soil methylmercury (MeHg) and crop mercury (Hg) levels, underscoring the need to investigate safer approaches to straw utilization in mercury-contaminated regions. In this study, rice straw underwent anaerobic fermentation with the addition of sulfate, and the resulting fermentation products were utilized in a pot experiment involving water spinach to assess the impact of anaerobically fermented straw return on soil Hg methylation and its bioaccumulation. Findings revealed that the addition of sulfate during straw fermentation markedly increased the fermentation degree of the products, and sulfate was converted into organic sulfur-containing ligands that can functionalize the fermentation residuals. These changes enhanced adsorption or complexation of the fermentation products with Hg. Consequently, compared with raw straw returning to the soil, adding co-fermentation products of straw and sulfate to the soil can significantly reduce the bioavailable Hg and MeHg in the soil, the total mercury (THg) and MeHg in plants, with the maximum reduction rates being 68%, 92%, 66% and 78%, respectively. Therefore, returning the straw that has been anaerobically co-fermented with sulfate to the soil can effectively mitigate Hg methylation and bioaccumulation, while simultaneously increasing biomass, offering a suitable straw utilization method in Hg-contaminated cultivation areas.

Assessment of mercury methylation and methylmercury demethylation potentials in water and sediments along the Wabigoon River system

Monomethylmercury (MMHg) plays a crucial role in the accumulation of mercury (Hg) within aquatic food chains. Since ambient levels of methylmercury are governed by the balance of simultaneous methylation and demethylation processes, determining in situ methylation and demethylation rates is critically important to understand the dynamics of methylmercury in the environment. This is especially important in the Wabigoon River system in Ontario, Canada, which is severely contaminated with Hg by a chlor-alkali facility operating in the 1960s, and still exhibits some of the highest recorded fish mercury concentrations in Canada. This work used a simultaneous addition of isotope enriched Hg and MMHg tracers to ascertain Hg methylation and MMHg demethylation potentials. At the locations investigated for this study, the most favourable conditions for Hg methylation were found at the Hydroelectric dam, being able to transform 4.2 % and 4.4 % of added Hg in water and sediments per day, respectively, to MMHg. This could correspond to 1.9 ng/L and 29 ng/g of new MMHg being produced from current ambient Hg. Clay Lake, which is considered a sink for mercury and exhibiting a seasonal anoxic environment at its bottom waters, also demonstrated significant MMHg generation, being able to produce 2.7 ng/L and 13 ng/g of MMHg per day, respectively. Demethylation rates in sediments of riverbed and wetland locations showed an average half-life for methylmercury of 2.1 days, indicating a rapid turnover of MMHg in the Wabigoon River. However, significantly lower demethylation rates were also measured near the inflow of Clay Lake, where it took up to 144 days for MMHg to decrease by 50 %. Generally, most of the investigated locations downstream of the pollution source displayed the potential to generate methylmercury, which could be distributed throughout the Wabigoon River system and therefore require attention with respect to future remediation activities.

Using Mercury Stable Isotopes to Quantify Directional Soil-Atmosphere Hg(0) Exchanges in Rice Paddy Ecosystems: Implications for Hg(0) Emissions to the Atmosphere from Land Surfaces

Gaseous elemental mercury [Hg(0)] emissions from soils constitute a large fraction of global total Hg(0) emissions. Existing studies do not distinguish biotic- and abiotic-mediated emissions and focus only on photoreduction mediated emissions, resulting in an underestimation of soil Hg(0) emissions into the atmosphere. In this study, directional mercury (Hg) reduction pathways in paddy soils were identified using Hg isotopes. Results showed significantly different isotopic compositions of Hg(0) between those produced from photoreduction (δ202Hg = -0.80 ± 0.67‰, Δ199Hg = -0.38 ± 0.18‰), microbial reduction (δ202Hg = -2.18 ± 0.25‰, Δ199Hg = 0.29 ± 0.38‰), and abiotic dark reduction (δ202Hg = -2.31 ± 0.25‰, Δ199Hg = 0.50 ± 0.22‰). Hg(0) exchange fluxes between the atmosphere and the paddy soils were dominated by emissions, with the average flux ranging from 2.2 ± 5.7 to 16.8 ± 21.7 ng m-2 h-1 during different sampling periods. Using an isotopic signature-based ternary mixing model, we revealed that photoreduction is the most important contributor to Hg(0) emissions from paddy soils. Albeit lower, microbial and abiotic dark reduction contributed up to 36 ± 22 and 25 ± 15%, respectively, to Hg(0) emissions on the 110th day. These novel findings can help improve future estimation of soil Hg(0) emissions from rice paddy ecosystems, which involve complex biotic-, abiotic-, and photoreduction processes.

Role of low-proportion, hydrophobic dissolved organic matter components in inhibiting methylmercury uptake by phytoplankton

Uptake of methylmercury (MeHg), a potent neurotoxin, by phytoplankton is a major concern due to its role as the primary pathway for MeHg entry into aquatic food webs, thereby posing a significant risk to human health. While it is widely believed that the MeHg uptake by plankton is negatively correlated with the concentrations of dissolved organic matter (DOM) in the water, ongoing debates continue regarding the specific components of DOM that exerts the dominant influence on this process. In this study, we employed a widely-used resin fractionation approach to separate and classify DOM derived from algae (AOM) and natural rivers (NOM) into distinct components: strongly hydrophobic, weakly hydrophobic, and hydrophilic fractions. We conduct a comparative analysis of different DOM components using a combination of spectroscopy and mass spectrometry techniques, aiming to identify their impact on MeHg uptake by Microcystis elabens, a prevalent alga in freshwater environments. We found that the hydrophobic components had exhibited more pronounced spectral characteristics associated with the protein structures while protein-like compounds between hydrophobic and hydrophilic components displayed significant variations in both distributions and the values of m/z (mass-to-charge ratio) of the molecules. Regardless of DOM sources, the low-proportion hydrophobic components usually dominated inhibition of MeHg uptake by Microcystis elabens. Results inferred from the correlation analysis suggest that the uptake of MeHg by the phytoplankton was most strongly and negatively correlated with the presence of protein-like components. Our findings underscore the importance of considering the diverse impacts of different DOM fractions on inhibition of phytoplankton MeHg uptake. This information should be considered in future assessments and modeling endeavors aimed at understanding and predicting risks associated with aquatic Hg contamination.

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.

Composition of DOM along the depth gradients in the paddy field treated with crop straw for 10 years

Crop straw return is a widely used agricultural management practice. The addition of crop straw significantly alters the pool of dissolved organic matter (DOM) in agricultural soils and plays a pivotal role in the global carbon (C) cycle, which is sensitive to climate change. The DOM concentration and composition at different soil depths could regulate the turnover and further storage of organic C in terrestrial systems. However, it is still unclear how crop straw return influences the change in DOM composition in rice paddy soils. Therefore, a field experiment was conducted in which paddy soil was amended with crop straw for 10 years. Two crop straw-addition treatments [NPK with 50% crop straw (NPK+1/2S) and NPK with 100% crop straw (NPK + S)], a conventional mineral fertilization control (NPK) and a non-fertilized control were included. Topsoil (0-20 cm) and subsoil (20-40 cm) samples were collected to investigate the soil DOM concentration and compositional structure of the profile. Soil nutrients, iron (Fe) fraction, microbial biomass carbon (MBC), and concentration and optical properties (UV-Vis and fluorescence spectra) of soil DOM were determined. Here, we found that the DOM in the topsoil was more humified than that in the subsoil. The addition of crop straw further decreased the humidification degree of DOM in the subsoil. In crop straw-amended topsoil, microbial decomposition controlled the composition of DOM and induced the formation of aromatic DOM. In the straw-treated subsoil, selective adsorption by poorly crystalline Fe(oxyhydr)oxides and microbial decomposition controlled the composition of DOM. In particular, the formation of protein-like compounds could have played a significant role in the microbial degradation of DOM in the subsoil. Overall, this work conducted a case study within long-term agricultural management to understand the changes in DOM composition along the soil profile, which would be further helpful for evaluating C cycling in agricultural ecosystems.

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.

The divergent effects of nitrate and ammonium application on mercury methylation, demethylation, and reduction in flooded paddy slurries

The production of methylmercury (MeHg) in flooded paddy fields determines its accumulation in rice grains; this, in turn, results in MeHg exposure risks for not only rice-eating humans but also wildlife. Nitrogen (N) fertilizers have been widely applied in rice cultivation fields to supply essential nutrients. However, the effects of N fertilizer addition on mercury (Hg) transformations are not unclear. This limits our understanding of MeHg formation in rice paddy ecosystems. In this study, we spiked three Hg tracers (200HgII, Me198Hg, and 202Hg0) in paddy slurries fertilized with urea, ammonium, and nitrate. The influences of N fertilization on Hg methylation, demethylation, and reduction and the underlying mechanisms were elucidated. The results revealed that dissimilatory nitrate reduction was the dominant process in the incubated paddy slurries. Nitrate addition inhibited HgII reduction, HgII methylation, and MeHg demethylation. Competition between nitrates and other electron acceptors (e.g., HgII, sulfate, or carbon dioxide) under dark conditions was the mechanism underlying nitrate-regulated Hg transformation. Ammonium and urea additions promoted HgII reduction, and anaerobic ammonium oxidation coupled with HgII reduction (Hgammox) was likely the reason. This work highlighted that nitrate addition not only inhibited HgII methylation but also reduced the demethylation of MeHg and therefore may generate more accumulation of MeHg in the incubated paddy slurries. Findings from this study link the biogeochemical cycling of N and Hg and provide crucial knowledge for assessing Hg risks in intermittently flooded wetland ecosystems.

Sulfate-reduction and methanogenesis are coupled to Hg(II) and MeHg reduction in rice paddies

Methylmercury (MeHg) produced in rice paddies is the main source of MeHg accumulation in rice, resulting in high risk of MeHg exposure to humans and wildlife. Net MeHg production is affected by Hg(II) reduction and MeHg demethylation, but it remains unclear to what extent these processes influence net MeHg production, as well as the role of the microbial guilds involved. We used isotopically labeled Hg species and specific microbial inhibitors in microcosm experiments to simultaneously investigate the rates of Hg(II) and MeHg transformations, as well as the key microbial guilds controlling these processes. Results showed that Hg(II) and MeHg reduction rate constants significantly decreased with addition of molybdate or BES, which inhibit sulfate-reduction and methanogenesis, respectively. This suggests that both sulfate-reduction and methanogenesis are important processes controlling Hg(II) and MeHg reduction in rice paddies. Meanwhile, up to 99% of MeHg demethylation was oxidative demethylation (OD) under the incubation conditions, suggesting that OD was the main MeHg degradative pathway in rice paddies. In addition, [202Hg(0)/Me202Hg] from the added 202Hg(NO3)2 was up to 13.9%, suggesting that Hg(II) reduction may constrain Hg(II) methylation in rice paddies at the abandoned Hg mining site. This study improves our understanding of Hg cycling pathways in rice paddies, and more specifically how reduction processes affect net MeHg production and related microbial metabolisms.