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The water-level fluctuation zone (WLFZ) has been considered as a hotspot for mercury (Hg) methylation. Flooding-tolerant herbs are gradually acclimated to this water-land ecotone, tending to form substantial root systems for improving erosion resistance. Accompanying rhizosphere microzone plays crucial but unclear roles in methylmercury (MeHg) formation in the WLFZ. Thus, we conducted this study in the WLFZ of the Three Gorges Reservoir, to explore effects of the rhizosphere of a dominant flooding-tolerant herb (bermudagrass) on MeHg production. The elevated Hg and MeHg in rhizosphere soils suggest that the rhizosphere environment provides favorable conditions for Hg accumulation and methylation. The increased bioavailable Hg and microbial activity in the rhizosphere probably serve as important factors driving MeHg formation in the presence of bermudagrass. Simultaneously, the rhizosphere environments changed the richness, diversity, and distribution of hgcA-containing microorganisms. Here, a typical iron-reducing bacterium (Geobacteraceae) has been screened, however, the majority of hgcA genes detected in rhizosphere, near-, and non-rhizosphere soils of the WLFZ were unclassified. Collectively, these results provide new insights into the elevated MeHg production as related to microbial processes in the rhizosphere of perennial herbs in the WLFZ, with general implications for Hg cycling in other ecosystems with water-level fluctuations.

Manganese (Mn) ores contain substantial concentrations of mercury (Hg), and mining and smelting of Mn ores can bring Hg into the surrounding aquatic environment through atmospheric deposition, leaching of electrolytic Mn residue and Mn gangue dump. However, limited is known that how these processes influence the environmental behaviors of Hg in waterbody. Therefore, the seasonal distribution and existing form of Hg in water and sediment in one Mn ore area in Xiushan County, Chongqing were investigated. Our results showed that the mean Hg and methylmercury (MeHg) concentrations in water (n=35) were 5.8 ± 4.6 ng/L and 0.22 ± 0.14 ng/L, respectively. The mean Hg concentrations in retained riverbed and fluvial bank sediment (n=35) were 0.74 ± 0.26 mg/kg and 0.63 ± 0.27 mg/kg, respectively (the mean MeHg concentrations were 0.64 ± 0.40 µg/kg and 0.51 ± 0.30 µg/kg, respectively). It indicated that the mining and smelting of Mn ores were the main sources of anthropogenic Hg, and Mn may inhibit Hg methylation in rivers in Mn ore areas. Mercury in the bound to iron/ Mn (Fe/Mn) oxides of low crystallinity fraction (Hg-OX) accounted for 4.01% and 5.25% of the total Hg concentrations in the retained riverbed and fluvial bank sediment, respectively. The amount of Hg bound to Fe/Mn oxides in sediment increased significantly due to the manganese mining activities in the investigated area. Therefore, it could be hypothesized that high Hg concentrations in river sediment in Mn mining areas are closely related to high Mn concentration in sediment.

Both inorganic and organic fertilizers are widely used to increase rice yield. However, these fertilizers are also found to aggravate mercury methylation and methylmercury (MeHg) accumulation in paddy fields. The aim of this study was to reveal the mechanisms of inorganic and organic fertilizers on MeHg accumulation in rice grains, which are not yet well understood. Potting cultures were conducted in which different fertilizers were applied to a paddy soil. The results showed that both inorganic and organic fertilizers increased MeHg concentrations rather than biological accumulation factors (BAFs) of MeHg in mature rice grains. Inorganic fertilizers, especially nitrogen fertilizer, enhanced the bioavailability of mercury and the relative amount Hg-methylating microbes and therefore intensified mercury methylation in paddy soil and MeHg accumulation in rice grains. Unlike inorganic fertilizers, organic matter (OM) in organic fertilizers was the main reason for the increase of MeHg concentrations in rice grains, and it also could immobilize Hg in soil when it was deeply degraded. The enhancement of MeHg concentrations in rice grains induced by inorganic fertilizers (5.18-41.69%) was significantly (p 0.05) lower than that induced by organic fertilizers (80.49-106.86%). Inorganic fertilizers led to a larger increase (50.39-99.28%) in thousand-kernel weight than MeHg concentrations (5.18-41.69%), resulting in a dilution of MeHg concentrations in mature rice grains. Given the improvement of soil properties by organic fertilizer, increasing the proportion of inorganic fertilizer application may be a better option to alleviate MeHg accumulation in rice grains and guarantee the rice yield in the agricultural production.

Previous research has found total mercury (THg) and methylmercury (MeHg) levels increase with litterfall decay, thus suggesting litterfall decomposition plays an essential role in the biogeochemical transformation of mercury (Hg). However, it remains unclear how Hg accumulates in the decaying litter, how bacterial taxa networks vary and what roles various microorganisms play during litterfall decomposition, especially nitrogen (N)-fixing, MeHg-degrading and Hg-methylating microbes. Here, we demonstrated as degradation proceeded, a gradually-complex network evolved for litterfall bacteria for the subtropical mixed broadleaf-conifer (MBC) forest, whereas a relatively static network existed for the evergreen broadleaf (EB) forest. N-fixing and MeHg-degrading bacteria dominated throughout litterfall decomposition process, with relative abundances of N-fixing genera and nifH copies maximum and relative abundances of MeHg-degrading bacteria and merAB copies minimum in summer. Hence, N-fixing bacteria likely mediate THg increase in the decomposing litterfall, while MeHg enhancement may be regulated by aerobic MeHg-degrading microbes which can transform MeHg to inorganic divalent Hg (Hg

Historic coal gangue stacking probably brings heavy metals (HMs) into the surrounding agricultural soil, posing potential harm to human and environmental health. For better controlling and preventing agricultural soil HMs pollution, the screening of priority pollutants and identification of their pollution pathways are urgent in coal gangue stacking areas. Thus, this study selected a coal gangue stacking area in Chongqing, China as the research object and conducted the pollution evaluation, spatial distribution and source apportionment of the HMs (Cd, Cr, Ni, Cu, Zn, As, Pb and Hg) in surrounding agricultural soil. Results showed that the soil was moderately to heavily contaminated by Cd with average concentrations of 1.23 mg/kg, which were 4.1 times higher than the Environmental Quality Standards for Soils of China. Cd was considered as the soil precedent-controlled pollutant in this study area and subsequent soil δ

Methylmercury (MeHg) is mainly produced by anaerobic δ-proteobacteria such as sulfate-reducing bacteria (SRB). However, mercury bio-methylation has also been found to occur in the aerobic soil of the Three Gorges Reservoir (TGR). Using γ-proteobacterial TGR bacteria (TGRB) and δ-proteobacterial Desulfomicrobium escambiense strains, the efficiency of mercury methylation and demethylation was evaluated using an isotope tracer technique. Kinetics simulation showed that the bacterial Hg methylation rate (k

Water and sediment samples were collected from a tributary (Ruxi River) of the Three Gorges Reservoir (TGR) to analyze the concentrations of seven heavy metals (HMs) and their fractions for better understanding the migration, bioavailability and potential environmental risk of HMs. The results indicated that the concentrations of HMs in water were lower than the Environmental Quality Standards for Surface Water Class I standards, except for Ni. Cd in sediment was found to be more sensitive to environmental changes, as the acid-soluble fraction of Cd accounted for about 40% of total Cd, and the sediment–water partition coefficient of Cd was the smallest among all the HMs. Meanwhile, multiple risk assessment methods of HMs indicated that sediment Cd in most sampling sites, significantly influenced by human activities, exhibited heavy pollution, suggesting that the Cd pollution should be attached great importance in the Ruxi River.

To explore the effects of herbaceous plants on mercury (Hg) behaviors after flooding in the water-level fluctuation zone (WLFZ) of Jialing River, three typical local plants (D. pyramidalis, A. philoxeroides and C. dactylon) and their in-situ sediments were collected for flooding experiments to study the Hg dynamics of water and sediment in different treatments. The results showed that flooding increased sediment MeHg concentrations and flooded plants (especially for the D. pyramidalis) promoted this process. Similarly, the highest dissolved MeHg level and proportion of MeHg to total Hg (%MeHg) were observed in plant-water treatments in the presence of D. pyramidalis, indicating the potential for the methylation of Hg in the water body influenced by the decay process of herbaceous plants. These results suggest that the herbaceous plant D. pyramidalis contributes more to Hg methylation and release in the WLFZ of the Jialing River than other plants studied.

Biochar has been used increasingly as a soil additive to control mercury (Hg) pollution in paddy rice fields. As the most active component of soil organic matter, soil dissolved organic matter (DOM) plays a vital role in the environmental fate of contaminants. However, there are very few studies to determine the impact of biochar on the Hg cycle in rice paddies using insights from DOM. This study used original and modified biochar to investigate their effect on DOM dynamics and their potential impact on methylmercury (MeHg) production and bioaccumulation in rice plants. Porewater DOM was collected to analyze the variations in soil-derived DOM in paddy soils. The results showed that the addition of biochar, whether in original or modified form, significantly reduced the bioaccumulation of MeHg in rice plants, especially in hulls and grains (p0.05). However, MeHg production in soils was only inhibited by the modified biochar. Biochar addition induced a significant increase in DOM's aromaticity and molecular weight (p0.05), which decreased Hg bioavailability. Furthermore, enhanced microbial activity was also observed in DOM (p0.05), further increasing MeHg production in the soil. Thus, the effect of biochar on the fate of Hg cycle involves competition between the two different roles of DOM. This study identified a specific mechanism by which biochar affects Hg behavior in rice paddy soil and contributes to understanding the more general influence of biochar in agriculture and contaminant remediation.

Understanding atmospheric mercury (Hg) accumulation in remote montane forests is critical to assess the Hg ecological risk to wildlife and human health. To quantify impacts of vegetation, climatic and topographic factors on Hg accumulation in montane forests, we assessed the Hg distribution and stoichiometric relations among Hg, carbon (C), and nitrogen (N) in four forest types along the elevation of Mt. Gongga. Our results show that Hg concentration in plant tissues follows the descending order of litterleaf, barkrootbranchbole wood, indicating the importance of atmospheric Hg uptake by foliage for Hg accumulation in plants. The foliar Hg/C (from 237.0 ± 171.4 to 56.8 ± 27.7 µg/kg) and Hg/N (from 7.5 ± 3.9 to 2.5 ± 1.2 mg/kg) both decrease along the elevation. These elevation gradients are caused by the heterogeneity of vegetation uptake of atmospheric Hg and the variation of atmospheric Hg° concentrations at different altitudes. Organic soil Hg accumulation is controlled by forest types, topographic and climatic factors, with the highest concentration in the mixed forest (244.9 ± 55.7 µg/kg) and the lowest value in the alpine forest (151.9 ± 44.5 µg/kg). Further analysis suggests that soil Hg is positively correlated to C (r

Dissolved organic matter (DOM) influences the environmental fate and toxic effects of trace metals such as mercury (Hg). However, because of limits in DOM analytical techniques and lack of sample diversity in past studies, it remains unclear whether the binding strength of DOM complexed with Hg(II) is related to the DOM properties. In this study, different DOM isolates (n = 26) from various sources were used to determine the conditional stability constant (logK) of DOM-Hg complexes using the equilibrium dialysis ligand exchange (EDLE) method. UV–Vis and fluorescence spectrometry were used to evaluate the correlation between logK values and DOM properties, such as chromophoric moieties, aromaticity, and molecular weight. Results demonstrated that the DOM from different sources presented an extensive range of binding strengths to Hg(II), because of their heterogeneous properties. Moreover, DOM chromophores, including aromaticity and molecular weight, are critical indicators of the DOM-Hg affinity in ambient-relevant circumstances. Significantly, higher terrestrial DOM led to greater DOM-Hg affinity. Additionally, this study supports that UV–Vis and fluorescence spectroscopy can be used to estimate DOM composition and its binding strength with Hg(II). Furthermore, the observed relationship between logK and DOM properties provided a possible pathway of explanation for the spatial co-variations between Hg(II) concentrations and DOM characters observed in previous field investigations.

Straw sizes were found to affect the methylmercury (MeHg) accumulation in rice grains induced by straw incorporation. The mechanism behind, however, still remains unclear. Here, we incorporated rice straw in different sizes (powder, 2 cm and 5 cm) into a Hg-contaminated paddy soil. Our results showed that straw sizes regulated the release of different fractions of organic matter (OM) in straw residues and further Hg methylation in paddy soil. The easily degradable OM (EDOM) was a key driving factor that facilitated net Hg methylation, though it only occupied a small fraction (1.12–3.12%) of the soil OM. Powdered straw reduced the duration of net Hg methylation by 74.39% compared to 5 cm straw, resulting in a strong and rapid net Hg methylation in paddy soil before the rice flowering. After the release of EDOM, the humified OM dominated in paddy soil and bound to MeHg, leading to less MeHg being transported to rice grains during the grain filling. Powdered straw decreased MeHg accumulation by 25.32% in the mature rice grains compared with 5 cm straw. Our study suggests that straw powdering before incorporation provides a feasible pathway for reducing MeHg accumulation in rice grains induced by straw incorporation.

Agricultural watersheds are a crucial contributor of terrestrial dissolved organic matter (DOM) for the adjacent aquatic environment. Recently, ecological engineering of the buffer zone such as a rice-paddy field was established to reduce the export of nutrients and contaminants from a small agricultural watershed. However, the potential of the rice-paddy field to reduce the terrestrial signature of DOM is unclear. Therefore, two small agricultural sub-catchments (i.e., sub-1 and sub-2) with different land uses and hill slope angles in the Three Gorges Reservoir (TGR) area of China were studied from 2014 to 2015. The results showed that the terrestrial DOM signals are indicated by optical indices (SUVA254, SR, fluorescence index) in the steeper and more forest covered, but rice-paddy field buffered sub-catchment (i.e., sub-2) decreased significantly, as compared to the reference sub-catchment (i.e., sub-1). Regardless of seasonal variations, the rice-paddy field retained a buffering role to reduce the terrestrial property of DOM and the highest capacity was observed during the rice-growth period. However, during storm events, the differences of DOM properties for two sub-catchments were not significant, because the buffer system was weakened. Finally, environmental implications of the role of such a buffer zone in the TGR areas are discussed. These results demonstrate that rice-paddy fields are successful in mitigating the terrestrial property of exported DOM, but the weaker performance during storm events still needs to be considered.

Mercury (Hg) methylation is mainly a microbial process mediated by anaerobes. The continued study of Pseudomonas putida (P. putida) strain TGRB4 genome was inspired by the fact that it can transform Hg into the highly toxic methylmercury (MeHg) under aerobic conditions. P. putida strain TGRB4 is a Gram-negative rod-shaped Gamma-proteobacterium (γ-proteobacterium), isolated from the soil in a typical water level fluctuation zone (WLFZ) of the Three Gorges Reservoir (TGR), which suffered from seasonally water level alternations every year. Draft genome assembly of P. putida strain TGRB4 is presented here, which was sequenced using Illumina Hiseq and PacBio single-molecule real-time (SMRT) platforms. Its genome harbors a total of 5504 genes and a G + C content of 62.6%. We further identified the enzymes related to Hg methylation, and found two well-known methyltransferase, including 5-methyl-tetrahydrofolate (5-methyl-THF) and S-adenosylmethionine (AdoMet), were annotated in the genome of P. putida strain TGRB4. This genome information could be treated as a research material to further study the Hg methylation mechanisms under aerobic environment.

Although mercury (Hg) is toxic at all valence states, organic Hg, especially monomethylmercury (MeHg, CH3Hg+, methylmercury), is more toxic than inorganic Hg (Hg(II), Hg2+). As Hg is predominately ...

As a global toxic pollutant, mercury (Hg) bioaccumulation within food chain could be influenced by human disturbance. Ten typical fish species were collected from Changshou Lake, an artificial lake used to carry out cage fish culture, to investigate the C/N isotopic compositions and Hg bioaccumulation in fish. The results showed that the total Hg (THg) and methylmercury (MeHg) levels in fish muscles ((56.03 ± 43.96) and (32.35 ± 29.57) ng/g, wet weight), comparable with those in most studies in China, were significantly lower than the international marketing limit (0.5 mg/kg). Past human input for cage culture in this lake led to abnormal 15N enrichment in food chain, as the quantitative trophic levels based on δ15N were different with that classified by feeding behaviors. This phenomenon subsequently demonstrated that it should be considered thoughtfully with respect to the application of the traditional method for understanding Hg bioaccumulation power by the slope of log10[Hg] with δ15N regression in specific water body (i.e., Changshou Lake). In addition, no significant linear correlation between Hg and body weight or length of some fish species was observed, suggesting that the fish growth in the eutrophic environment was disproportionate with Hg bioaccumulation, and fish length or weight was not the main factor affecting Hg transfer with food web. The occurrence of human disturbance in aquatic system presents a challenge to a better understanding of the Hg bioaccumulation and biomagnification within the food chain.

Mercury (Hg) is one of the most toxic heavy metal pollutants, which can be easily transmitted and enriched through the food chain, posing severe threat to human beings. Forest ecosystems are one of the most active environments for biogeochemical cycles of Hg. It is essential to research on Hg cycling in the forest ecosystem, which contributes to a comprehensive understanding of global biogeochemical cycle of Hg. However, there is still a lack of consensus on whether the forest ecosystem is a "source" or "sink" of Hg in the global Hg cycle so far. Therefore, it is necessary to elucidate the current state of knowledge on Hg deposition, transformation and fate in the forest ecosystem, especially the existing puzzles or issues encountered by scientists worldwide. This review highlights the complexity and uncertainties of Hg cycling in forest ecosystems. It is proposed that a new perspective is required to further understand the role of forest ecosystems in global Hg cycle based on a sufficient understanding of Hg exchange fluxes at the interface of air-soil and air-plant, Hg deposition flux through litterfall, and accurate construction of Hg mass balance system.

Several strains of archaea have the ability to methylate or resist mercury (Hg), and the paddy field is regarded to be conducive to Hg methylation. However, our knowledge of Hg-methylating or Hg-resistant archaea in paddy soils is very limited so far. Therefore, the distribution of archaea and bacteria in the rhizosphere (RS) and bulk soil (BS) of the rice growing in Xiushan Hg-mining area of southwest China was investigated. Bacterial and archaeal 16S rRNA gene amplicon sequencing of the rice rhizosphere along the Hg gradient was conducted. THg concentrations in RS were significantly higher than that in BS at site S1 and S2, while MeHg concentrations in RS was always higher than that in BS, except S6. Bacterial species richness estimates were much higher than that in archaea. The bacterial α-diversity in high-Hg sites was significant higher than that in low-Hg sites based on ACE and Shannon indices. At the genus level, Thiobacillus, Xanthomonas, Defluviicoccus and Candidatus Nitrosoarchaeum were significantly more abundant in the rhizosphere of high-Hg sites, which meant that strains in these genera might play important roles in response to Hg stress. Hg-methylating archaea in the paddy field could potentially be affiliated to strains in Methanosarcina, but further evidence need to be found. The results provide reference to understand archaeal rhizosphere community along an Hg gradient paddy soils.

Manganese (Mn)-related activities would affect the mercury (Hg) cycling in farmlands, whereas this was not well understood. Here, one of the largest Mn ores in China was selected to study the effects of Mn-related activities on the accumulation and distribution of total Hg (THg) and methylmercury (MeHg) in farmland soils. The soil THg concentrations in the mining area were 0.56 ± 0.45, 0.56 ± 0.45, 0.53 ± 0.44, and 0.50 ± 0.46 mg kg−1 in the 0–10, 10–20, 20–30, and 30–40 cm layers, respectively, while they were increased to 0.75 ± 0.75, 0.72 ± 0.60, 0.62 ± 0.46, and 0.52 ± 0.38 mg kg−1 in the smelting area. Similarly, the soil MeHg concentrations in the smelting area were also elevated by 1.04–1.34 times as compared to those in the mining area. Concentrations of THg (0.59 ± 0.50 mg kg−1) and MeHg (0.64 ± 0.82 μg kg−1) in soils were higher than the regional background value but lower than in vicinal Hg-mining areas, while they were largely elevated at the intersection of two rivers in the smelting area. Significant positive Mn-THg relationship (p < 0.01) and negative Mn-MeHg relationship (p < 0.01) favored the conclusion that soil Mn could promote Hg accumulation while inhibiting MeHg production. Approximately 70% of soil Hg was distributed in the residual phase, and the environmental hazard was not elevated according to a geochemical model. Overall, mining and smelting activities of Mn ores have resulted in obvious and distinct effects on the accumulation and methylation of Hg in farmland soils, but the environmental hazards are currently manageable.

The origin and composition of dissolved organic matter (DOM) in porewater of lake sediments is intricate and decisive for fate of pollutants including mercury (Hg). While there are many reports on the relationship between dissolved organic carbon concentration (DOC) and mercury (Hg) concentrations in aquatic systems, there are few in which DOM compositional properties, that may better explain the fate of Hg, have been the focus. In this study, porewaters from sediments of three lakes, Caihai Lake (CH), Hongfeng Lake (HF) and Wujiangdu Lake (WJD), all located in southwest China, were selected to test the hypothesis that DOM optical properties control the fate of Hg in aquatic ecosystems. Porewater DOM was extracted and characterized by UV-Vis absorption and fluorescence spectroscopy. A two end-member (autochthonous and allochthonous DOM) mixing model was used to unveil the origin of DOM in porewaters of the three lakes. Our results show a higher input of terrestrial DOM in the pristine lake CH, as compared to lakes HF and WJD lakes, which were both influenced by urban environments and enriched in autochthonous DOM. While the relationships between the concentrations of DOC and the different chemical forms of Hg forms were quite inconsistent, we found important links between specific DOM components and the fate of Hg in the three lakes. In particular, our results suggest that allochthonous, terrestrial DOM inhibits Hg(II) availability for Hg methylating micro-organisms. In contrast, autochthonous DOM seems to have been stimulated MeHg formation, likely by enhancing the activity of microbial communities. Indeed, DOM biodegradation experiments revealed that differences in the microbial activity could explain the variation in the concentration of MeHg. While relationships between concentrations of DOC and Hg vary among different sites and provide little information about Hg cycling, we conclude that the transport and transformation of Hg (e.g. the methylation process) are more strongly linked to DOM chemical composition and reactivity.

This study evaluated the effect of growth of different tissue compartments on the bioaccumulation of mercury (Hg) and polychlorinated biphenyls (PCBs) in Silver Carp (Hypophthalmichthys molitrix) and Bighead Carp (Hypophthalmichthys nobilis) from the Three Gorges Reservoir (TGR), China. A non-steady state bioenergetics/toxicokinetic model was developed to simulate PCB and Hg concentrations in these two species and compared with field data. Simulations using constant whole body growth rate and constant tissue to whole body weight ratios were contrasted against simulations adopting age specific whole body and tissue/age specific growth rates for their goodness of fit to field data. The simulations using age/tissue specific growth rates demonstrated better fit to field data for PCBs compared to the constant growth rate models (22% improved R2), while both models explained similar variation in Hg concentration data. Both species demonstrated higher growth rates of lipids (on a daily basis) relative to whole body and protein contributing to higher growth dilution of PCBs compared to Hg. Although stable isotope data indicated some degree of diet and/or habitat shift, simulations assuming a constant diet concentration explained between 36 and 40% of the variation in fish concentrations for both contaminants and fish species. This study demonstrates that differences in the bioaccumulation rate of PCBs and Hg by Asian carp can be partially explained by differences in the growth rates of key tissue storage compartments associated with each contaminant. These differences in chemical-specific growth dilution subsequently contribute to differences in chemical retention and bioaccumulation patterns of Hg and PCBs by fish.

To develop high-performance biochar adsorbents, the adsorption ability and rate of untreated (BC-CK) and six modified biochars with amino (BC-NH), epoxy (BC-C2H2O), ethoxy (BC-C2H5O), hydrosulfuryl (BC-SH), selenium (BC-Se), and chitosan (BC-Chitosan) on Hg(II) and MeHg were investigated by simulated experiment. The results indicated that the some modified biochars (BC-NH, BC-C2H2O, BC-C2H5O and BC-Chitosan) showed lower adsorption capacity than the untreated, possibly due to the decreased specific surface area and pore volume. Whereas, BC-SH and BC-Se was improved immensely by forming stable -SH-Hg and Hg-Se with the adsorption capacity 1.26 and 1.51 times as much as BC-CK, respectively. In spite of that, Hg desorption capacities and rates of all biochars were extremely low, exhibiting great adsorption stability of biochars on Hg in another way. In addition, BC-Chitosan performed the highest adsorption speed. These provided insights on the adsorption effectiveness for Hg in the aqueous solution that was critical for evaluating the application of modified biochars.

The heavy metal accumulation in the Tibet Plateau (TP) poses a serious ecologic risk to the health of human and the other biota. Given the TP far away from the large anthropogenic emission sources, the rapid development of traffic activities during last several decades possibly leads to the elevated heavy metal concentration in the roadside soils. Therefore, we comprehensively assessed the heavy metal distribution in the 0–5 cm and 15–20 cm depth soils located at 5 m, 50 m, and 100 m distance to the edge of two major roads among the different vegetation covers and climatic conditions in the TP to verify this hypothesis. Results show that most of heavy metal concentrations in soils of different distance to the major road display an insignificant difference. The Nemero Synthesis indexes which represent the risk of pollution for these regions almost range 1 to 2 (low pollution risk), except 12.7 (extreme pollution risk) at one site. These indicate the limited impacts from the traffic activities for the whole region, but at some specific sites with the elevated traffic pollution. The forest cover at the altitude of 3700–4100 m has the highest mercury accumulation due to the vegetation and climatic factors induced the higher atmospheric depositions and stronger complexation with the organic matters. The statistical analysis finally suggests the geogenic weathering processes, climate, terrain and vegetation play an important role in shaping heavy metal distribution along the roadside of the TP.

The biodegradation of dissolved organic matter (DOM) in natural environments is determined by its molecular composition and reactivity. Redox oscillations are common in the water-level-fluctuation zone (WLFZ) of the Three Gorges Reservoir (TGR). As a consequence, the soil DOM released is degraded under both anaerobic and aerobic conditions. The DOM compounds available for degradation under contrasting redox conditions and the resulting DOM composition still need to be elucidated. By combining laboratory experiments with an in-depth characterization of DOM optical properties, we show that different pathways controlled the depletion and enrichment of the DOM optical components under different oxygen regimes. In particular, 28-day dark biodegradation assays showed that up to 39.5 ± 4% DOM was degraded under anaerobic conditions, while 55.5 ± 6% DOM was biodegraded under aerobic conditions. Aerobic biodegradation resulted in a higher aromaticity and degree of humification of the DOM compared to anaerobic degradation. The specific UV absorbance at a wavelength of 254 (SUVA254) and biological index (BIX) could be used to track DOM biodegradation under anaerobic conditions. Under aerobic conditions, the SUVA254, BIX and concentration of coloured DOM (CDOM, reflected by a (355)) could track DOM biodegradation, and significant amounts of CDOM could be aerobically biodegraded., This research was financially supported by the National Natural Science Foundation of China (41977275, 41771347 and 41877384) and Internal Funding for Early Careers from Department of Forest Ecology and Management (FEM) of Swedish University of Agricultural Science (SLU). Dr. Tao Jiang personally gives his appreciation to the funding of the Sino-Swedish Mercury Management Research Framework (SMaRef) from the Swedish Research Council (VR) (No. D697801) for support his position in SLU. The authors thank Jinzhu Wang for plotting the map of sampling sites.

Tropical forest contributes to 50% of global litterfall mercury (Hg) inputs and surface soil Hg storage, while with limited understanding of Hg biogeochemical processes. In this study, we displayed the 5-m resolution of Hg spatial distribution in three 1-ha tropical forest plots across the latitudinal gradient in Southwest China, and determined Hg isotopic signatures to understand factors driving Hg spatial distribution and sequestration processes. Our results show that tropical forest at the lowest latitude has the highest litterfall Hg input (74.95 versus 34.14-56.59 μg m

The identification of dominant microbes in anaerobic mercury (Hg) methylation, methylmercury (MeHg) demethylation, and methane oxidation as sulfate-reducing bacteria, methanogens or, probably, anaerobic methanotrophic archaea (ANMEs) is of great interest. To date, however, the interrelationship of bacteria and archaea involved in these processes remains unclear. Here, we demonstrated the dynamics of microorganisms participating in these processes. Anaerobic fixed-bed reactors were operated with swine manure and sludge to produce methane stably, and then, sulfate (reactor C), sulfate and Hg(II) (reactor H), and sulfate and MeHg (reactor M) were added, and the reactors were operated for 120 d, divided equally into four periods, P1–P4. The bacterial compositions changed nonsignificantly, whereas Methanosaeta in reactors H and M decreased significantly, revealing that it was irrelevant for Hg transformation. The abundances of Syntrophomonadaceae, Methanoculleus, Candidatus Methanogranum and Candidatus Methanoplasma increased continuously with time; these species probably functioned in these processes, but further evidence is needed. Desulfocella and Desulfobacterium dominated first but eventually almost vanished, while the dominant archaeal genera Methanogenium, Methanoculleus and Methanocorpusculum were closely related to ANME–1 and ANME–2. PLS-DA results indicated that both bacteria and archaea in different periods in the three reactors were clustered separately, implying that the microbial compositions in the same periods were similar and changed markedly with time.

Biochar, an effective and low-cost amendment for immobilizing heavy metals, has been extensively studied. However, the simultaneous inhibition effects of biochar on the plant uptake for arsenic (As) and cadmium (Cd) in co-contaminated soils are still ambiguous due to their distinct environmental behaviors. A meta-analysis was conducted to quantitatively assess the effects using 1030 individual observations from 52 articles. On average, biochar application significantly decreased the bioavailability of Cd in soils by 50.12%, while slightly increased the bioavailability of As in soils by 2.39%. The more instructive result is that biochar application could also simultaneously reduce the concentration of As and Cd in plants by 25.48% and 38.66%, respectively. The orders of the decreased percentage of As and Cd in various tissues were rootstemleafgrain, and rootleafstemgrain, respectively. According to the analysis of critical factors, manure biochar, low pyrolysis temperature (at400 °C), low application rate (2%), and high SOC (30 g/kg) were more conducive to reduce the bioaccumulation of As and Cd simultaneously in co-contaminated soils. Pristine and modified biochar could inhibit As and Cd accumulation in crops, but their efficiencies need to be further improved to ensure the safety of crop productions. Overall, the meta-analysis suggests that biochar has the potential to remedy the As and Cd co-contaminated soils.

Water and sediment samples were collected from a tributary (Ruxi River) of the Three Gorges Reservoir (TGR) to analyze the concentrations of seven heavy metals (HMs) and their fractions for better understanding the migration, bioavailability and potential environmental risk of HMs. The results indicated that the concentrations of HMs in water were lower than the Environmental Quality Standards for Surface Water Class I standards, except for Ni. Cd in sediment was found to be more sensitive to environmental changes, as the acid-soluble fraction of Cd accounted for about 40% of total Cd, and the sediment-water partition coefficient of Cd was the smallest among all the HMs. Meanwhile, multiple risk assessment methods of HMs indicated that sediment Cd in most sampling sites, significantly influenced by human activities, exhibited heavy pollution, suggesting that the Cd pollution should be attached great importance in the Ruxi River.

Recently, there has been increasing interest in reducing methylmercury (MeHg) phytoavailability using biochar, although the underlying mechanisms are not fully understood. By combining lab-scale batch incubation with pot and field validations, we demonstrate that biochar-impacted sulfur cycling in soils and MeHg-soil binding play key roles in controlling MeHg phytoavailability. (1) Under anoxic conditions, biochar-associated sulfate and biochar-facilitated microbial sulfate reduction enhanced the production of reduced inorganic sulfur species as acid-volatile sulfide (AVS) in soils by 122%, facilitating MeHg binding with soils and thus reducing MeHg phytoavailability. (2) In contrast, under oxic conditions, the reduced inorganic sulfur was oxidized (resulting in a 68-91% decrease in AVS), which released soil-bound MeHg and increased MeHg phytoavailability. The proposed mechanisms could explain the distinct effects of biochar amendment on MeHg bioaccumulation observed under anoxic (10-88% lower in rice grains) and oxic conditions (48-84% higher in wheat grains). Our results dispute the commonly held assumption that reduced MeHg phytoavailability under biochar amendment can be primarily attributed to MeHg-biochar binding. Therefore, the potential increased risk of MeHg in oxic soils following biochar amendment should be evaluated in more detail.

Litterfall mercury (Hg) input has been regarded as the dominant Hg source in montane forest floor. To depict combining effects of vegetation, climate and topography on accumulation of Hg in montane forests, we comprehensively quantified litterfall Hg deposition and decomposition in a serial of subtropical forests along an elevation gradient on both leeward and windward slopes of Mt. Ailao, Southwest China. Results showed that the average litterfall Hg deposition increased from 12.0 ± 4.2 μg m−2 yr−1 in dry-hot valley shrub at 850–1000 m, 14.9 ± 6.8 μg m−2 yr−1 in mixed conifer-broadleaf forest at 1250–2400 m, to 23.1 ± 8.3 μg m−2 yr−1 in evergreen broadleaf forest at 2500–2650 m. Additionally, the windward slope forests had a significantly higher litterfall Hg depositions at the same altitude because the larger precipitation promoted the greater litterfall biomass production. The one-year litter Hg decomposition showed that the Hg mass of litter in dry-hot valley shrub decreased by 29%, while in mixed conifer-broadleaf and evergreen broadleaf forests increased by 22–48%. The dynamics of Hg in decomposing litter was controlled by the temperature mediated litter decomposition rate and the additional adsorption of environmental Hg during decomposition. Overall, our study highlights the litterfall mediated atmospheric mercury inputs and sequestration increase with the montane elevation, thus driving a Hg enhanced accumulation in the high montane forest.

Hydrological management of the Three Gorges Dam has resulted in the interception of heavy metals in the Three Gorges Reservoir (TGR). However, the exposure to heavy metals and health risks among local residents remained poorly understood. Here we collected 208 biomarker samples (hair) and 20 food species from typical regions in the TGR to assess the exposure levels of three toxic metals (Cr, Pb and As) in residents of the TGR, and subsequently investigated their health risk via dietary intake. Results indicated that hair Cr and As levels were below the reference value for normal people and threshold of skin lesions, respectively, whereas about 22% hair Pb exceeded the reference for clinical medicine, indicating a potential Pb exposure of local residents. Smoking habit and fish consumption were found to be predictors for hair Pb. In addition, the concentrations of heavy metals in all investigated food samples were below the limits of contaminants in food in China, except for Pb in the sweet potato and fish. The estimated daily intake of metals (DIMs) revealed that the intakes of Cr and As from studied food were under the recommended thresholds of Cr and As. However, the intake of Pb via diet exceeded the limit of the prevalence of chronic kidney disease and closed to the threshold for cardiovascular, which was probably associated with the high Pb concentrations of fish and sweet potato. Overall, residents around the TGR were at low exposure to Cr and As, but Pb exposure may need more attention.

Soil-derived dissolved organic matter (DOM) has a major influence in biogeochemical processes related to contaminant dynamics and greenhouse gas emissions, due to its reactivity and its bridging role between the soil and aquatic systems. Within the Three Gorges Reservoir (TGR, China) area, an extensive water-fluctuation zone periodically submerges the surrounding soils. Here we report a characterization study of soil-derived DOM across the TGR areas, using elemental and optical analysis, infrared spectroscopy (FTIR), pyrolysis-GC-MS (Py-GC-MS) and thermally assisted hydrolysis and methylation (THM-GC-MS). The results showed that the soil DOM from the TGR area is a mixture of "allochthonous" (i.e., plant-derived/terrigenous) and "autochthonous" (i.e., microbial) origins. The terrigenous DOM is composed primarily of phenolic and aliphatic structures from lignin and aliphatic biopolymers (i.e. cutin, suberin), respectively. Multivariate statistics differentiated between two fractions of the microbial DOM, i.e. chitin-derived, perhaps from fungi and arthropods in soil, and protein-derived, partially sourced from algal or aquatic organisms. Molecular proxies of source and degradation state were in good agreement with optical parameters such as SUVA254, the fluorescence index (FI) and the humification index (HIX). The combined use of elemental analysis, fluorescence spectroscopy, and Py-GC-MS provides rigorous and detailed DOM characterization, whereas THM-GC-MS is useful for more precise but qualitative identification of the different phenolic (cinnamyl, p-hydroxyphenyl, guaiacyl, syringyl and tannin-derived) and aliphatic materials. With the multi-methodological approach used in this study, FTIR was the least informative, in part, because of the interference of inorganic matter in the soil DOM samples. The soil DOM from the TGR's water fluctuation zone exhibited considerable compositional diversity, mainly related to the balance between DOM source (microbial- or plant-derived), local vegetation and anthropogenic activities (e.g., agriculture). Finally, the relationship between DOM composition and its potential reactivity with substances of environmental concerns in the TGR area are also discussed.