Mercury evasion from a boreal peatland shortens the timeline for recovery from legacy pollution

Substantial Mercury Releases and Local Deposition from Permafrost Peatland Wildfires in Southwestern Alaska

Increasing wildfire activity at high northern latitudes has the potential to mobilize large amounts of terrestrial mercury (Hg). However, understanding implications for Hg cycling and ecosystems is hindered by sparse research on peatland wildfire Hg emissions. In this study, we used measurements of soil organic carbon (SOC) and Hg, burn depth, and environmental indices derived from satellite remote sensing to develop machine learning models for predicting Hg emissions from major wildfires in the permafrost peatland of the Yukon-Kuskokwim Delta (YKD) in southwestern Alaska. Wildfire Hg emissions during summer 2015─estimated as the product of Hg:SOC (0.38 ± 0.17 ng Hg g C1-), predicted SOC stores (mean [5th-95th] = 9.1 [5.3-11.2] kg C m-2), and burn depth (11.3 [8.2-13.9] cm)─were 556 [164-1138] kg Hg or approximately 6% of Hg emissions from wildfire activity >60°N. Modeling estimates suggest that wildfire nearly doubled summertime Hg deposition within 10 km, despite advection of more than 75% of total emissions beyond Alaska. YKD areal emissions combined with remote sensing estimates of burned area suggest that wildfire Hg emissions from northern peatlands (25.4 [14.9-33.6] Mg y-1) are an important component of the northern Hg budget. Additional research is needed to refine these estimates and understand the implications for Arctic and global Hg cycling.

Predicting Behavior and Fate of Atmospheric Mercury in Soils: Age-Dating METAALICUS Hg Isotope Spikes with Fallout Radionuclide Chronometry

Soils accumulate anthropogenic mercury (Hg) from atmospheric deposition to terrestrial ecosystems. However, possible reemission of gaseous elemental mercury (GEM) back to the atmosphere as well as downward migration of Hg with soil leachate influence soil sequestration of Hg in ways not sufficiently understood in global biogeochemical models. Here, we apply fallout radionuclide (FRN) chronometry to understand soil Hg dynamics by revisiting the METAALICUS experiments 20 years after enriched isotope tracers (198Hg, 200Hg, 201Hg, and 202Hg) were applied to two boreal watersheds in northwestern Ontario, Canada. Hg spikes formed well-defined peaks in organic horizons of both watersheds at depths of 3-6 cm and were accurately dated to the year of spike application in 6 of 7 cases (error = -0.8 ± 1.2 years). A seventh site was depleted by ca. 90% of both the 200Hg spike and background Hg, and the spike was dated 16 years older than its application. Robust FRN age models and mass balances demonstrate that loss of Hg is attributable to its specific physicochemical behavior at this site, but more work is required to attribute this to reemission or leaching. This study demonstrates the potential of FRN chronometry to provide insights into Hg accumulation, mobilization, and fate in forest soils.

Quantifying soil accumulation of atmospheric mercury using fallout radionuclide chronometry

Soils are a principal global reservoir of mercury (Hg), a neurotoxic pollutant that is accumulating through anthropogenic emissions to the atmosphere and subsequent deposition to terrestrial ecosystems. The fate of Hg in global soils remains uncertain, however, particularly to what degree Hg is re-emitted back to the atmosphere as gaseous elemental mercury (GEM). Here we use fallout radionuclide (FRN) chronometry to directly measure Hg accumulation rates in soils. By comparing these rates with measured atmospheric fluxes in a mass balance approach, we show that representative Arctic, boreal, temperate, and tropical soils are quantitatively efficient at retaining anthropogenic Hg. Potential for significant GEM re-emission appears limited to a minority of coniferous soils, calling into question global models that assume strong re-emission of legacy Hg from soils. FRN chronometry poses a powerful tool to reconstruct terrestrial Hg accumulation across larger spatial scales than previously possible, while offering insights into the susceptibility of Hg mobilization from different soil environments.

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.

Distribution and Homogenization of Multiple Mercury Species Inputs to Freshwater Wetland Mesocosms

Mercury (Hg)-impaired aquatic ecosystems often receive multiple inputs of different Hg species with varying potentials for transformation and bioaccumulation. Over time, these distinct input pools of Hg homogenize in their relative distributions and bioaccumulation potentials as a result of biogeochemical processes and other aging processes within the ecosystem. This study sought to evaluate the relative time scale for homogenization of multiple Hg inputs to wetlands, information that is relevant for ecosystem management strategies that consider Hg source apportionment. We performed experiments in simulated freshwater wetland mesocosms that were dosed with four isotopically labeled mercury forms: two dissolved forms (Hg2+ and Hg-humic acid) and two particulate forms (nano-HgS and Hg adsorbed to FeS). Over the course of one year, we monitored the four Hg isotope endmembers for their relative distribution between surface water, sediment, and fish in the mesocosms, partitioning between soluble and particulate forms, and conversion to methylated mercury (MeHg). We also evaluated the reactivity and mobility of Hg through sequential selective extractions of sediment and the uptake flux of aqueous Hg in a diffusive gradient in thin-film (DGT) passive samplers. We observed that the four isotope spikes were relatively similar in surface water concentration (ca. 3000 ng/L) immediately after spike addition. At 1-3 months after dosing, Hg concentrations were 1-50 ng/L and were greater for the initially dissolved isotope endmembers than the initially particulate endmembers. In contrast, the Hg isotope endmembers in surface sediments were similar in relative concentration within 2 months after spike addition. However, the uptake fluxes of Hg in DGT samplers, deployed in both the water column and surface sediment, were generally greater for initially dissolved Hg endmembers and lower for initially particulate endmembers. At one year postdosing, the DGT-uptake fluxes were converging toward similar values between the Hg isotope endmembers. However, the relative distribution of isotope endmembers was still significantly different in both the water column and sediment (p < 0.01 according to one-way ANOVA analysis). In contrast, selective sequential extractions resulted in a homogeneous distribution, with >90% of each endmember extracted in the KOH fraction, suggesting that Hg species were associated with sediment organic matter. For MeHg concentrations in surface sediment and fish, the relative contributions from each endmember were significantly different at all sampling time points. Altogether, these results provide insights into the time scales of distribution for different Hg species that enter a wetland ecosystem. While these inputs attain homogeneity in concentration in primary storage compartments (i.e., sediments) within weeks after addition, these input pools remain differentiated for more than one year in terms of reactivity for passive samplers, MeHg concentration, and bioaccumulation.

Mercury deposition and redox transformation processes in peatland constrained by mercury stable isotopes

Peatland vegetation takes up mercury (Hg) from the atmosphere, typically contributing to net production and export of neurotoxic methyl-Hg to downstream ecosystems. Chemical reduction processes can slow down methyl-Hg production by releasing Hg from peat back to the atmosphere. The extent of these processes remains, however, unclear. Here we present results from a comprehensive study covering concentrations and isotopic signatures of Hg in an open boreal peatland system to identify post-depositional Hg redox transformation processes. Isotope mass balances suggest photoreduction of HgII is the predominant process by which 30% of annually deposited Hg is emitted back to the atmosphere. Isotopic analyses indicate that above the water table, dark abiotic oxidation decreases peat soil gaseous Hg0 concentrations. Below the water table, supersaturation of gaseous Hg is likely created more by direct photoreduction of rainfall rather than by reduction and release of Hg from the peat soil. Identification and quantification of these light-driven and dark redox processes advance our understanding of the fate of Hg in peatlands, including the potential for mobilization and methylation of HgII.

Comparing ecosystem gaseous elemental mercury fluxes over a deciduous and coniferous forest

Sources of neurotoxic mercury in forests are dominated by atmospheric gaseous elemental mercury (GEM) deposition, but a dearth of direct GEM exchange measurements causes major uncertainties about processes that determine GEM sinks. Here we present three years of forest-level GEM deposition measurements in a coniferous forest and a deciduous forest in northeastern USA, along with flux partitioning into canopy and forest floor contributions. Annual GEM deposition is 13.4 ± 0.80 μg m^-2 (coniferous forest) and 25.1 ± 2.4 μg m^-2 (deciduous forest) dominating mercury inputs (62 and 76% of total deposition). GEM uptake dominates in daytime during active vegetation periods and correlates with CO₂ assimilation, attributable to plant stomatal uptake of mercury. Non-stomatal GEM deposition occurs in the coniferous canopy during nights and to the forest floor in the deciduous forest and accounts for 24 and 39% of GEM deposition, respectively. Our study shows that GEM deposition includes various pathways and is highly ecosystem-specific, which complicates global constraints of terrestrial GEM sinks.

Assessing leached TOC, nutrients and phenols from peatland soils after lab-simulated wildfires: Implications to source water protection

Pollutant leaching from wildfire-impacted peatland soils (peat) is well-known, but often underestimated when considering boreal ecosystem source water protection and when treating source waters to provide clean drinking water. Burning peat impacts its physical properties and chemical composition, yet the consequences of these transformations to source water quality through pollutant leaching has not been studied in detail. We combusted near-surface boreal peat under simulated peat smoldering conditions at two temperatures (250 °C and 300 °C) and quantified the concentrations of the leached carbon, nutrients and phenols from 5 g peat L^-1 reverse osmosis (RO) water suspensions over a 2-day leaching period. For the conditions studied, measured water quality parameters exceeded US surface water guidelines and even exceeded EU and Canadian wastewater/sewer discharge limits including chemical oxygen demand (COD) (125 mg/L), total nitrogen (TN) (15 mg/L), and total phosphorus (TP) (2 mg/L). Phenols were close to or higher than the suggested water supply standard established by US EPA (1 mg/L). Leached carbon, nitrogen and phosphorus mainly came from the organic fraction of peats. Heating peats to 250 °C promoted the leaching of carbon-related pollutants, whereas heating to 300 °C enhanced the leaching of nutrients. Post-heated peats leached higher loads of pollutants in water than pre-heated peats, suggesting that fire-damaged boreal peats may be a critical but underappreciated source of water pollution. A simplified Partial Least Squares (PLS) model based on other easily measured parameters provided a simple method for determining the extent of COD and phenolic pollution in bulk water, relevant for water and wastewater treatment plants. Conclusions from this lab study indicate the need for field measurements of aquatic pollutants downstream of peatland watersheds post-fire as well as increased monitoring and treatment of potable water sources for leachable micropollutants in fire-dominated forested peatlands.

Cellular and genetic mechanism of bacterial mercury resistance and their role in biogeochemistry and bioremediation

Mercury (Hg) is a highly toxic element that occurs at low concentrations in nature. However, various anthropogenic and natural sources contribute around 5000 to 8000 metric tons of Hg per year, rapidly deteriorating the environmental conditions. Mercury-resistant bacteria that possess the mer operon system have the potential for Hg bioremediation through volatilization from the contaminated milieus. Thus, bacterial mer operon plays a crucial role in Hg biogeochemistry and bioremediation by converting both reactive inorganic and organic forms of Hg to relatively inert, volatile, and monoatomic forms. Both the broad-spectrum and narrow-spectrum bacteria harbor many genes of mer operon with their unique definitive functions. The presence of mer genes or proteins can regulate the fate of Hg in the biogeochemical cycle in the environment. The efficiency of Hg transformation depends upon the nature and diversity of mer genes present in mercury-resistant bacteria. Additionally, the bacterial cellular mechanism of Hg resistance involves reduced Hg uptake, extracellular sequestration, and bioaccumulation. The presence of unique physiological properties in a specific group of mercury-resistant bacteria enhances their bioremediation capabilities. Many advanced biotechnological tools also can improve the bioremediation efficiency of mercury-resistant bacteria to achieve Hg bioremediation.

Previously unaccounted atmospheric mercury deposition in a midlatitude deciduous forest

Direct measurements of atmospheric deposition of gaseous elemental mercury (GEM) over a temperate forest showed a pronounced annual deposition of 25.1 µg ⋅ m⁻², which dominated as a source of mercury. GEM deposition was five times greater than wet deposition and three times greater than litterfall deposition, which has been used as a proxy for GEM deposition until now. Measured GEM deposition is driven by combined plant GEM uptake and underlying forest floor GEM uptake. Global forests may be a much larger global GEM sink than currently assumed, which may explain high mercury levels in soils across forests. Forest mercury mobilizes via watershed runoff and bioaccumulates in aquatic biota, ultimately leading to mercury exposures in wildlife and humans.

Mercury is toxic to wildlife and humans, and forests are thought to be a globally important sink for gaseous elemental mercury (GEM) deposition from the atmosphere. Yet there are currently no annual GEM deposition measurements over rural forests. Here we present measurements of ecosystem–atmosphere GEM exchange using tower-based micrometeorological methods in a midlatitude hardwood forest. We measured an annual GEM deposition of 25.1 µg ⋅ m⁻² (95% CI: 23.2 to 26.7 1 µg ⋅ m⁻²), which is five times larger than wet deposition of mercury from the atmosphere. Our observed annual GEM deposition accounts for 76% of total atmospheric mercury deposition and also is three times greater than litterfall mercury deposition, which has previously been used as a proxy measure for GEM deposition in forests. Plant GEM uptake is the dominant driver for ecosystem GEM deposition based on seasonal and diel dynamics that show the forest GEM sink to be largest during active vegetation growing periods and middays, analogous to photosynthetic carbon dioxide assimilation. Soils and litter on the forest floor are additional GEM sinks throughout the year. Our study suggests that mercury loading to this forest was underestimated by a factor of about two and that global forests may constitute a much larger global GEM sink than currently proposed. The larger than anticipated forest GEM sink may explain the high mercury loads observed in soils across rural forests, which impair water quality and aquatic biota via watershed Hg export.

A spatial assessment of mercury content in the European Union topsoil

Mapping of surface soil Hg concentrations, a priority pollutant, at continental scale is important in order to identify hotspots of soil Hg distribution (e.g. mining or industrial pollution) and identify factors that influence soil Hg concentrations (e.g. climate, soil properties, vegetation). Here we present soil Hg concentrations from the LUCAS topsoil (0-20 cm) survey including 21,591 samples from 26 European Union countries (one sample every ~200 km²). Deep Neural Network (DNN) learning models were used to map the European soil Hg distribution. DNN estimated a median Hg concentration of 38.3 μg kg^-1 (2.6 to 84.7 μg kg^-1) excluding contaminated sites. At continental scale, we found that soil Hg concentrations increased with latitude from south to north and with altitude. A GLMM revealed a correlation (R² = 0.35) of soil Hg concentrations with vegetation activity, normalized difference vegetation index (NDVI), and soil organic carbon content. This observation corroborates the importance of atmospheric Hg⁰ uptake by plants and the build-up of the soil Hg pool by litterfall over continental scales. The correlation of Hg concentrations with NDVI was amplified by higher soil organic matter content, known to stabilize Hg in soils through thiol bonds. We find a statistically significant relation between soil Hg levels and coal use in large power plants, proving that emissions from power plants are associated with higher mercury deposition in their proximity. In total 209 hotspots were identified, defined as the top percentile in Hg concentration (>422 μg kg^-1). 87 sites (42% of all hotspots) were associated with known mining areas. The sources of the other hotspots could not be identified and may relate to unmined geogenic Hg or industrial pollution. The mapping effort in the framework of LUCAS can serve as a starting point to guide local and regional authorities in identifying Hg contamination hotspots in soils.

Mercury Reduction by Nanoparticulate Vivianite

Mercury (Hg) is a toxic trace element of global environmental concern which has been increasingly dispersed into the environment since the industrial revolution. In aquatic and terrestrial systems, Hg can be reduced to elemental Hg (Hg⁰) and escape to the atmosphere or converted to methylmercury (MeHg), a potent neurotoxin that accumulates in food webs. Fe^II-bearing minerals such as magnetite, green rusts, siderite, and mackinawite are recognized Hg^II reducers. Another potentially Hg-reducing mineral, which commonly occurs in Fe- and organic/P-rich sediments and soils, is the ferrous iron phosphate mineral vivianite (Fe^II₃(PO₄)₂·8H₂O), but its reaction with Hg^II has not been studied to date. Here, nanoparticulate vivianite (particle size ∼ 50 nm; Fe^II content > 98%) was chemically synthesized and characterized by a combination of chemical, spectroscopic, and microscopic analyses. Its ability to reduce Hg^II was investigated at circumneutral pH under anoxic conditions over a range of Fe^II/Hg^II ratios (0.1-1000). For Fe^II/Hg^II ratios ≥1, which are representative of natural environments, Hg^II was very quickly and efficiently reduced to Hg⁰. The ability of vivianite to reduce Hg^II was found to be similar to those of carbonate green rust and siderite, two of the most effective Hg-reducing minerals. Our results suggest that vivianite may be involved in abiotic Hg^II reduction in Fe and organic/P-rich soils and sediments, potentially contributing to Hg evasion while also limiting MeHg formation in these ecosystems.

Mercury biogeochemical cycling: A synthesis of recent scientific advances

The focus of this paper is to briefly discuss the major advances in scientific thinking regarding: a) processes governing the fate and transport of mercury in the environment; b) advances in measurement methods; and c) how these advances in knowledge fit in within the context of the Minamata Convention on Mercury. Details regarding the information summarized here can be found in the papers associated with this Virtual Special Issue of STOTEN.

Formation and mobilization of methylmercury across natural and experimental sulfur deposition gradients

We investigated the influence of sulfate (SO₄^2-) deposition and concentrations on the net formation and solubility of methylmercury (MeHg) in peat soils. We used data from a natural sulfate deposition gradient running 300 km across southern Sweden to test the hypothesis posed by results from an experimental field study in northern Sweden: that increased loading of SO₄^2- both increases net MeHg formation and redistributes methylmercury (MeHg) from the peat soil to its porewater. Sulfur concentrations in peat soils correlated positively with MeHg concentrations in peat porewater, along the deposition gradient similar to the response to added SO₄^2- in the experimental field study. The combined results from the experimental field study and deposition gradient accentuate the multiple, distinct and interacting roles of SO₄^2- deposition in the formation and redistribution of MeHg in the environment.

Recent advances in understanding and measurement of Hg in the environment: Surface-atmosphere exchange of gaseous elemental mercury (Hg0)

The atmosphere is the major transport pathway for distribution of mercury (Hg) globally. Gaseous elemental mercury (GEM, hereafter Hg⁰) is the predominant form in both anthropogenic and natural emissions. Evaluation of the efficacy of reductions in emissions set by the UN's Minamata Convention (UN-MC) is critically dependent on the knowledge of the dynamics of the global Hg cycle. Of these dynamics including e.g. red-ox reactions, methylation-demethylation and dry-wet deposition, poorly constrained atmosphere-surface Hg⁰ fluxes especially limit predictability of the timescales of its global biogeochemical cycle. This review focuses on Hg⁰ flux field observational studies, namely the theory, applications, strengths, and limitations of the various experimental methodologies applied to gauge the exchange flux and decipher active sub-processes. We present an in-depth review, a comprehensive literature synthesis, and methodological and instrumentation advances for terrestrial and marine Hg⁰ flux studies in recent years. In particular, we outline the theory of a wide range of measurement techniques and detail the operational protocols. Today, the most frequently used measurement techniques to determine the net Hg⁰ flux (>95% of the published flux data) are dynamic flux chambers for small-scale and micrometeorological approaches for large-scale measurements. Furthermore, top-down approaches based on Hg⁰ concentration measurements have been applied as tools to better constrain Hg emissions as an independent way to e.g. challenge emission inventories. This review is an up-dated, thoroughly revised edition of Sommar et al. 2013 (DOI: 10.1080/10643389.2012.671733). To the tabulation of >100 cited flux studies 1988-2009 given in the former publication, we have here listed corresponding studies published during the last decade with a few exceptions (2008-2019). During that decade, Hg stable isotope ratios of samples involved in atmosphere-terrestrial interaction is at hand and provide in combination with concentration and/or flux measurements novel constraints to quantitatively and qualitatively assess the bi-directional Hg⁰ flux. Recent efforts in the development of relaxed eddy accumulation and eddy covariance Hg⁰ flux methods bear the potential to facilitate long-term, ecosystem-scale flux measurements to reduce the prevailing large uncertainties in Hg⁰ flux estimates. Standardization of methods for Hg⁰ flux measurements is crucial to investigate how land-use change and how climate warming impact ecosystem-specific Hg⁰ sink-source characteristics and to validate frequently applied model parameterizations describing the regional and global scale Hg cycle.

Recent advances in understanding and measurement of mercury in the environment: Terrestrial Hg cycling

This review documents recent advances in terrestrial mercury cycling. Terrestrial mercury (Hg) research has matured in some areas, and is developing rapidly in others. We summarize the state of the science circa 2010 as a starting point, and then present the advances during the last decade in three areas: land use, sulfate deposition, and climate change. The advances are presented in the framework of three Hg "gateways" to the terrestrial environment: inputs from the atmosphere, uptake in food, and runoff with surface water. Among the most notable advances: These and other advances reported here are of value in evaluating the effectiveness of the Minamata Convention on reducing environmental Hg exposure to humans and wildlife.

Opposing spatial trends in methylmercury and total mercury along a peatland chronosequence trophic gradient

Peatlands are abundant elements of boreal landscapes where inorganic mercury (IHg) can be transformed into bioaccumulating and highly toxic methylmercury (MeHg). We studied fifteen peatlands divided into three age classes (young, intermediate and old) along a geographically constrained chronosequence to determine the role of biogeochemical factors and nutrient availability in controlling the formation of MeHg. In the 10 cm soil layer just below the average annual growing season water table, concentrations of MeHg and %MeHg (of total Hg) were higher in younger, more mesotrophic peatlands than in older, more oligotrophic peatlands. In contrast, total mercury (THg) concentrations were higher in the older peatlands. Partial least squares (PLS) analysis indicates that the net MeHg production was positively correlated to trophic demands of vegetation and an increased availability of potential electron acceptors and donors for Hg methylating microorganisms. An important question for further studies will be to elucidate why there is less THg in the younger peatlands compared to the older peatlands, even though the age of the superficial peat itself is similar for all sites. We hypothesize that ecosystem features which enhance microbial processes involved in Hg methylation also promote Hg reduction that makes previously deposited Hg more available for evasion back to the atmosphere.

Subtropical Forests Act as Mercury Sinks but as Net Sources of Gaseous Elemental Mercury in South China

Comprehensive mercury (Hg) budgets were constructed in two typical subtropical forests in southern China in 2014 to quantify Hg (gaseous elemental Hg, Hg⁰, and reactive Hg, Hg^II) input and output fluxes and Hg retention in forests, consequently exploring the roles of subtropical forests in the global Hg cycle. At site Qianyanzhou, representing a background region with an enhanced atmospheric Hg⁰ concentration, the total Hg^II deposition (67.7 μg·m^-2·year^-1, 73% as dry Hg^II deposition) was found to be slightly higher than the Hg⁰ emission above the canopy (58.5 μg·m^-2·year^-1), indicating that the forest is a minor Hg sink but a significant net Hg⁰ source on a yearly basis. In contrast, the forest in the moderately polluted region (site Huitong) acted as a significant Hg sink but a minor net Hg⁰ source with a higher Hg^II deposition (73.7 μg·m^-2·year^-1) and relatively negligible Hg⁰ emission (2.65 μg·m^-2·year^-1). The decreasing atmospheric Hg⁰ concentrations declined the total Hg sink based on the Hg budgets synthesized of this and previous studies and may promote forest Hg⁰ emissions. Consequently, it was expected that the re-emission of historically deposited Hg may be enhanced from subtropical forests by recent decreases in atmospheric Hg⁰ concentrations throughout China.

The concentrations and characteristics of dissolved organic matter in high-latitude lakes determine its ambient reducing capacity

The reducing capacity (RC) of natural organic matter plays an important role in the carbon cycle and biogeochemical fates of environmental contaminants in the aquatic system. However, the electron donation potentials of dissolved organic matter (DOM) from high-latitude lakes are still uncertain. In this study, we collected DOM samples from high-latitude lakes across the Arctic and boreal regions in Sweden and Norway to investigate the effects of the DOM concentration and characteristics on its ambient reducing capacity (ARC). Mercury (Hg(II)) abiotic reduction in darkness was used to determine the ARC. The results showed that the DOM in Arctic lakes is less terrestrial-dominant than in reference sites (i.e., forest lakes). Between the two categories of Arctic lakes, tundra lakes are more terrestrial-influenced compared to mountain lakes. Additionally, terrestrial-originated DOM is a main controlling factor for enhancing the ambient reducing capacity, whereas the DOM concentration, i.e., dissolved organic carbon (DOC), resulted in variations in the Hg/DOC ratios that also cause the variations of the observed ARC values. Thus, comparisons of the ARC values can be conducted while oxidant/DOC ratios are kept the same and reported through the method using heavy metals as a chemical probe. After correction for Hg/DOC ratio interference, the ambient reducing capacity of DOM followed the order: boreal forest lakes > Arctic tundra lakes > Arctic mountain lakes. This study highlights that the DOM concentration should also be considered when estimating the ARC as compared to the previous that mainly focusing on the properties of DOM such as its origins. As climate change is projected to be severe in high latitudes, this study demonstrates a significant connection between aquatic DOM geochemical reactivity and terrestrial inputs, which is crucial for a better prediction of the role of DOM in high-latitude lakes in the context of climate change.

Enhanced particulate Hg export at the permafrost boundary, western Siberia

Arctic permafrost soils contain large amounts of organic carbon and the pollutant mercury (Hg). Arctic warming and associated changes in hydrology, biogeochemistry and ecology risk mobilizing soil Hg to rivers and to the Arctic Ocean, yet little is known about the quantity, timing and mechanisms involved. Here we investigate seasonal particulate Hg (PHg) and organic carbon (POC) export in 32 small and medium rivers across a 1700 km latitudinal permafrost transect of the western Siberian Lowland. The PHg concentrations in suspended matter increased with decreasing watershed size. This underlines the significance of POC-rich small streams and wetlands in PHg export from watersheds. Maximum PHg concentrations and export fluxes were located in rivers at the beginning of permafrost zone (sporadic permafrost). We suggest this reflects enhanced Hg mobilization at the permafrost boundary, due to maximal depth of the thawed peat layer. Both the thickness of the active (unfrozen) peat layer and PHg run-off progressively move to the north during the summer and fall seasons, thus leading to maximal PHg export at the sporadic to discontinuous permafrost zone. The discharge-weighed PHg:POC ratio in western Siberian rivers (2.7 ± 0.5 μg Hg: g C) extrapolated to the whole Ob River basin yields a PHg flux of 1.5 ± 0.3 Mg y^-1, consistent with previous estimates. For current climate warming and permafrost thaw scenarios in western Siberia, we predict that a northward shift of permafrost boundaries and increase of active layer depth may enhance the PHg export by small rivers to the Arctic Ocean by a factor of two over the next 10-50 years.

Geochemical Factors Controlling Dissolved Elemental Mercury and Methylmercury Formation in Alaskan Wetlands of Varying Trophic Status

The transformations of aqueous inorganic divalent mercury (Hg(II)_i) to volatile dissolved gaseous mercury (Hg(0)_(aq)) and toxic methylmercury (MeHg) govern mercury bioavailability and fate in northern ecosystems. This study quantified concentrations of aqueous mercury species (Hg(II)_i, Hg(0)_(aq), MeHg) and relevant geochemical constituents in pore waters of eight Alaskan wetlands that differ in trophic status (i.e., bog-to-fen gradient) to gain insight on processes controlling dark Hg(II)_i reduction and Hg(II)_i methylation. Regardless of wetland trophic status, positive correlations were observed between pore water Hg(II)_i and dissolved organic carbon (DOC) concentrations. The concentration ratio of Hg(0)_(aq) to Hg(II)_i exhibited an inverse relationship to Hg(II)_i concentration. A ubiquitous pathway for Hg(0)_(aq) formation was not identified based on geochemical data, but we surmise that dissolved organic matter (DOM) influences mercury retention in wetland pore waters by complexing Hg(II)_i and decreasing the concentration of volatile Hg(0)_(aq) relative to Hg(II)_i. There was no evidence of Hg(0)_(aq) abundance directly limiting mercury methylation. The concentration of MeHg relative to Hg(II)_i was greatest in wetlands of intermediate trophic status, and geochemical data suggest mercury methylation pathways vary between wetlands. Our insights on geochemical factors influencing aqueous mercury speciation should be considered in context of the long-term fate of mercury in northern wetlands.

Sorption kinetics of isotopically labelled divalent mercury (196Hg2+) in soil

Understanding the sorption kinetics of Hg²⁺ is the key to predicting its reactivity in soils which is indispensable for environmental risk assessment. The temporal change in the solubility of ¹⁹⁶Hg²⁺ spikes (6 mg kg^-1) added to a range of soils with different properties was investigated and modelled. The sorption of ¹⁹⁶Hg²⁺ displayed a biphasic pattern with a rapid initial (short-term) phase followed by a slower (time-dependent) one. The overall reaction rate constants ranged from 0.003 to 4.9 h^-1 and were significantly correlated (r = 0.94) to soil organic carbon (SOC). Elovich and Spherical Diffusion expressions compellingly fitted the observed ¹⁹⁶Hg²⁺ sorption kinetics highlighting their flexibility to describe reactions occurring over multiple phases and wide timeframes. A parameterized Elovich model from soil variables indicated that the short-term sorption is solely controlled by SOC while the time-dependent sorption appeared independent of SOC and decreased at higher pH values and Al(OH)₃ and MnO₂ concentrations. This is consistent with a rapid chemical reaction of Hg²⁺ with soil organic matter (SOM) which is followed by a noticeably slower phase likely occurring through physical pathways e.g. pore diffusion of Hg²⁺ into spherical soil aggregates and progressive incorporation of soluble organic-Hg into solid phase. The model lines predicted that in soils with >4% SOC, Hg²⁺ is removed from soil solution over seconds to minutes; however, in soils with <2% SOC and higher pH values, Hg²⁺ may remain soluble for months and beyond with a considerable associated risk of re-emission or migration to the surrounding environments.