Mercury Isotopic Fractionation during Pedogenesis in a Tropical Forest Soil Catena (French Guiana): Deciphering the Impact of Historical Gold Mining

Elevated Mercury Deposition, Accumulation, and Migration in a Karst Forest

The karst forest is one of the extremely sensitive and fragile ecosystems in southwest China, where the biogeochemical cycling of mercury (Hg) is largely unknown. In this study, we investigated the litterfall deposition, accumulation, and soil migration of Hg in an evergreen-deciduous broadleaf karst forest using high-resolution sampling and stable isotope techniques. Results show that elevated litterfall Hg concentrations and fluxes in spring are due to the longer lifespan of evergreen tree foliage exposed to atmospheric Hg0. The hillslope has 1-2 times higher litterfall Hg concentration compared to the low-lying land due to the elevated atmospheric Hg levels induced by topographical and physiological factors. The Hg isotopic model suggests that litterfall Hg depositions account for ∼80% of the Hg source contribution in surface soil. The spatial trend of litterfall Hg deposition cannot solely explain the trend of Hg accumulation in the surface soil. Indeed, soil erosion enhances Hg accumulation in soil of low-lying land, with soil Hg concentration up to 5-times greater than the concentration on the hillslope. The high level of soil Hg migration in the karst forest poses significant ecological risks to groundwater and downstream aquatic ecosystems.

Mercury Accumulation and Sequestration in a Deglaciated Forest Chronosequence: Insights from Particulate and Mineral-Associated Forms of Organic Matter

Understanding mercury (Hg) complexation with soil organic matter is important in assessing atmospheric Hg accumulation and sequestration processes in forest ecosystems. Separating soil organic matter into particulate organic matter (POM) and mineral-associated organic matter (MAOM) can help in the understanding of Hg dynamics and cycling due to their very different chemical constituents and associated formation and functioning mechanisms. The concentration of Hg, carbon, and nitrogen contents and isotopic signatures of POM and MAOM in a deglaciated forest chronosequence were determined to construct the processes of Hg accumulation and sequestration. The results show that Hg in POM and MAOM are mainly derived from atmospheric Hg0 deposition. Hg concentration in MAOM is up to 76% higher than that in POM of broadleaf forests and up to 60% higher than that in POM of coniferous forests. Hg accumulation and sequestration in organic soil vary with the vegetation succession. Variations of δ202Hg and Δ199Hg are controlled by source mixing in the broadleaf forest and by Hg sequestration processes in the coniferous forest. Accumulation of atmospheric Hg and subsequent microbial reduction enrich heavier Hg isotopes in MAOM compared to POM due to the specific chemical constituents and nutritional role of MAOM.

Deposition and Re-Emission of Atmospheric Elemental Mercury over the Tropical Forest Floor

Significant knowledge gaps exist regarding the emission of elemental mercury (Hg0) from the tropical forest floor, which limit our understanding of the Hg mass budget in forest ecosystems. In this study, biogeochemical processes of Hg0 deposition to and evasion from soil in a Chinese tropical rainforest were investigated using Hg stable isotopic techniques. Our results showed a mean air-soil flux as deposition of -4.5 ± 2.1 ng m-2 h-1 in the dry season and as emission of +7.4 ± 1.2 ng m-2 h-1 in the rainy season. Hg re-emission, i.e., soil legacy Hg evasion, induces negative transitions of Δ199Hg and δ202Hg in the evaded Hg0 vapor, while direct atmospheric Hg0 deposition does not exhibit isotopic fractionation. Using an isotopic mass balance model, direct atmospheric Hg0 deposition to soil was estimated to be 48.6 ± 13.0 μg m-2 year-1. Soil Hg0 re-emission was estimated to be 69.5 ± 10.6 μg m-2 year-1, of which 63.0 ± 9.3 μg m-2 year-1 is from surface soil evasion and 6.5 ± 5.0 μg m-2 year-1 from soil pore gas diffusion. Combined with litterfall Hg deposition (∼34 μg m-2 year-1), we estimated a ∼12.6 μg m-2 year-1 net Hg0 sink in the tropical forest. The fast nutrient cycles in the tropical rainforests lead to a strong Hg0 re-emission and therefore a relatively weaker atmospheric Hg0 sink.

Mercury Uptake, Accumulation, and Translocation in Roots of Subtropical Forest: Implications of Global Mercury Budget

Plant roots are responsible for transporting large quantities of nutrients in forest ecosystems and yet are frequently overlooked in global assessments of Hg cycling budgets. In this study, we systematically determined the distribution of total Hg mass and its stable isotopic signatures in a subtropical evergreen forest to elucidate sources of Hg in plant root tissues and the associated translocation mechanisms. Hg stored in roots and its isotopic signatures show significant correlations to those found in surrounding soil at various soil depths. The odd mass-independent fractionation (MIF) of root Hg at a shallow soil depth displays a -0.10‰ to -0.50‰ negative transition compared to the values in aboveground woody biomass. The evidence suggests that root Hg is predominantly derived from surrounding soil, rather than translocation of atmospheric uptake via aboveground tissues. The cortex has a more negative mass-dependent fractionation (MDF) of -0.10‰ to -1.20‰ compared to the soil samples, indicating a preferential uptake of lighter isotopes by roots. The similar MDF and odd-MIF signals found in root components imply limited Hg transport in roots. This work highlights that Hg stored in plant roots is not a significant sink of atmospheric Hg. The heterogeneous distribution of Hg mass in roots of various sizes represents a significant uncertainty of current estimates of Hg pool size in forest ecosystems.

Trophic distribution of mercury from an abandoned cinnabar mine within the Záskalská reservoir ecosystem (Czech Republic)

The distribution of mercury species was studied in all aquatic ecosystem components (i.e., water, sediment, emergent aquatic plants, invertebrates and omnivorous and piscivorous fish) of the Záskalská water reservoir (Central Bohemia, Czech Republic) which is in the vicinity of an abandoned cinnabar mine. The results indicate that the transport of mercury from the cinnabar mine is the major source of mercury in the Záskalská reservoir. The legal maximum limit (0.07 μg/L) for total mercury concentration in water samples was exceeded only during rainy periods. The total mercury concentration in the surface sediments was in the range from 0.22 to 9.19 mg/kg in dry matter (up to 0.2% CH₃Hg⁺) and was sample site-specific. The dominant form of mercury in sediments was mercury sulphide (22.9-79.2%). The emergent macrophytes accumulated mercury primarily by the roots from sediments, and no significant translocation of mercury to leaves was observed. The legal maximum limit for mercury content in fish muscle (0.5 mg/kg in the fresh matter) was exceeded up to 4.48 times for piscivorous fish. Hazard index values indicate a health risk concern for children and for people consuming more than 100 g of fish muscle per day. Our results emphasise the need to implement legal restrictions on the consumption of piscivorous fish caught in ecosystems downstream of abandoned cinnabar mines.

Mercury Isotope Fractionation during the Exchange of Hg(0) between the Atmosphere and Land Surfaces: Implications for Hg(0) Exchange Processes and Controls

Atmosphere-surface exchange of elemental mercury (Hg(0)) is a vital component in global Hg cycling; however, Hg isotope fractionation remains largely unknown. Here, we report Hg isotope fractionation during air-surface exchange from terrestrial surfaces at sites of background (two) and urban (two) character and at five sites contaminated by Hg mining. Atmospheric Hg(0) deposition to soils followed kinetic isotope fractionation with a mass-dependent (MDF) enrichment factor of -4.32‰, and negligible mass-independent fractionation (MIF). Net Hg(0) emission generated average MDF enrichment factors (ε²⁰²Hg) of -0.91, -0.59, 1.64, and -0.42‰ and average MIF enrichment factors (E¹⁹⁹Hg) of 0.07, -0.20, -0.14, and 0.21‰ for urban, background, and Hg mining soils and cinnabar tailing, respectively. Positive correlations between ε²⁰²Hg and ambient Hg(0) concentration indicate that the co-occurring Hg(0) deposition (accounting for 10-39%) in a regime of net soil emission grows with ambient Hg(0). The MIF of Hg(0) emission from soils (E¹⁹⁹Hg range -0.27 to 0.14‰, n = 8) appears to be overall controlled by the photochemical reduction of kinetically constrained Hg(II) bonded to O ligands in background soils, while S ligands may have been more important in Hg mining area soils. In contrast, the small positive MIF of Hg(0) emission from cinnabar ore tailing (mean E¹⁹⁹Hg = 0.21‰) was likely controlled by abiotic nonphotochemical reduction and liquid Hg(0) evaporation. This research provides critical observational constraints on understanding the Hg(0) isotope signatures released from and deposited to terrestrial surfaces and highlight stable Hg isotopes as a powerful tool for resolving atmosphere-surface exchange processes.

Hg concentrations and stable isotope variations in tropical fish species of a gold-mining-impacted watershed in French Guiana

The aim of the study was to determine if gold-mining activities could impact the mercury (Hg) concentrations and isotopic signatures in freshwater fish consumed by riparian people in French Guiana. Total Hg, MeHg concentrations, and Hg stable isotopes ratios were analyzed in fish muscles from different species belonging to three feeding patterns (herbivorous, periphytophagous, and piscivorous). We compared tributaries impacted by gold-mining activities (Camopi, CR) with a pristine area upstream (Trois-Sauts, TS), along the Oyapock River. We measured δ¹⁵N and δ ¹³C to examine whether Hg patterns are due to differences in trophic level. Differences in δ ¹⁵N and δ ¹³C values between both studied sites were only observed for periphytophagous fish, due to difference of CN baselines, with enriched values at TS. Total Hg concentrations and Hg stable isotope signatures showed that Hg accumulated in fish from both areas has undergone different biogeochemical processes. Δ¹⁹⁹Hg variation in fish (-0.5 to 0.2‰) was higher than the ecosystem baseline defined by a Δ¹⁹⁹Hg of -0.66‰ in sediments, and suggested limited aqueous photochemical MeHg degradation. Photochemistry-corrected δ²⁰²Hg in fish was 0.7‰ higher than the baseline, consistent with biophysical and chemical isotope fractionation in the aquatic environment. While THg concentrations in periphytophagous fish were higher in the gold-mining area, disturbed by inputs of suspended particles, than in TS, the ensemble of Hg isotope shifts in fish is affected by the difference of biotic (methylation/demethylation) and abiotic (photochemistry) processes between both areas and did therefore not allow to resolve the contribution of gold-mining-related liquid Hg(0) in fish tissues. Mercury isotopes of MeHg in fish and lower trophic level organisms can be complementary to light stable isotope tracers.

Quantification of Atmospheric Mercury Deposition to and Legacy Re-emission from a Subtropical Forest Floor by Mercury Isotopes

Air-soil exchange of elemental mercury vapor (Hg⁰) is an important component in the budget of the global mercury cycle. However, its mechanistic detail is poorly understood. In this study, stable Hg isotopes in air, soil, and pore gases are characterized in a subtropical evergreen forest to understand the mechanical features of the air-soil Hg⁰ exchange. Strong Hg^II reduction in soil releases Hg⁰ to pore gas during spring-autumn but diminishes in winter, limiting the evasion in cold seasons. Δ¹⁹⁹Hg in air modified by the Hg⁰ efflux during flux chamber measurement exhibit seasonality, from -0.33 ± 0.05‰ in summer to -0.08 ± 0.05‰ in winter. The observed seasonal variation is caused by a strong pore-gas driven soil efflux caused by photoreduction in summer, which weakens significantly in winter. The annual Hg⁰ gross deposition is 42 ± 33 μg m^-2 yr^-1, and the corresponding Hg⁰ evasion from the forest floor is 50 ± 41 μg m^-2 yr^-1. The results of this study, although still with uncertainty, offer new insights into the complexity of the air-surface exchange of Hg⁰ over the forest land for model implementation in future global assessments.

Measurement of the Vertical Distribution of Gaseous Elemental Mercury Concentration in Soil Pore Air of Subtropical and Temperate Forests

Solid-gas-water phase partitioning of mercury (Hg) and the processes governing its diffusivity within soils are poorly studied. In this study, landscape and forest species dependences of gaseous elemental Hg (Hg(0)) in soil profiles (0-50 cm) were investigated over four seasons in eight subtropical (130 days) and temperate (96 days) forest plots. The vertical soil pore Hg(0) concentrations differed between subtropical (Masson pine, broad-leaved forest, and open field) and temperate (Chinese pine, larch, mixed broad-leaf forests, and open field) catchments, with annual averages ranging from 6.73 to 15.8 and 0.95 to 2.08 ng m^-3, respectively. The highest Hg(0) concentrations in soil gas consistently occurred in the upper mineral or organic horizons, indicating immobilization of Hg(0) in mineral soils. A strongly positive relationship between pore Hg(0) concentrations and ratios of Hg to organic matter (SOM) in soils suggests that the vertical distribution of Hg(0) is related to soil Hg(0) formation by Hg(II) reduction and sorption to SOM. Temperature was also an important driver of Hg(0) production in soil pores. Based on measurements of soil-air Hg(0) exchange, diffusion coefficients (D_s) of Hg(0) between soil and atmosphere were calculated for field sites, providing a foundation for future development and validation of terrestrial Hg models.

Stable Mercury Isotope Transition during Postdepositional Decomposition of Biomass in a Forest Ecosystem over Five Centuries

Organic soil is an important transient reservoir of mercury (Hg) in terrestrial ecosystems, but the fate of deposited Hg in organic forest soil is poorly understood. To understand the dynamic changes of deposited Hg on forest floor, the composition of stable Hg and carbon (C) isotopes in decomposing litters and organic soil layer was measured to construct the 500 year history of postdepositional Hg transformation in a subtropical evergreen broad-leaf forest in Southwest China. Using the observational data and a multiprocess isotope model, the contributions of microbial reduction, photoreduction, and dark reduction mediated by organic matter to the isotopic transition were estimated. Microbial reduction and photoreduction play a dominant role in the initial litter decomposition during first 2 years. Dark redox reactions mediated by organic matter become the predominant process in the subsequent 420 years. After that, the values of Hg mass dependent fractionation (MDF), mass independent fractionation (MIF), and Δ¹⁹⁹Hg/Δ²⁰¹Hg ratio do not change significantly, indicating sequestration and immobilization of Hg in soil. The linear correlations between the isotopic signatures of Hg and C suggest that postdepositional transformation of Hg is closely linked to the fate of natural organic matter (NOM). Our findings are consistent with the abiotic dark reduction driven by nuclear volume effect reported in boreal and tropical forests. We recommend that the dark reduction process be incorporated in future model assessment of the global Hg biogeochemical cycle.

Climate and Vegetation As Primary Drivers for Global Mercury Storage in Surface Soil

Soil is the largest Hg reservoir globally. Data of Hg concentration in surface soil are fundamental to understanding environmental Hg cycling. However, present knowledge on the quantity and global distribution of Hg in soil remains deficient. Using stable Hg isotopic analyses and geospatial data, the concentration and global spatial distribution of Hg in surface soil of 0-20 cm depth have been developed. It is estimated that 1088 ± 379 Gg of Hg is stored in surface soil globally. Thirty-two percent of the surface Hg storage resides in tropical/subtropical forest regions, 23% in temperate/boreal forest regions, 28% in grassland and steppe and shrubland, 7% in tundra, and 10% in desert and xeric shrubland. Evidence from Hg isotopic signatures points to atmospheric Hg⁰ dry deposition through vegetation uptake as the primary source of Hg in surface soil. Given the influence of changing climate on vegetative development, global climate change can act as an important forcing factor for shaping spatial distribution of Hg in surface soil. This active forcing cycle significantly dilutes the impacts caused by Hg release from anthropogenic sources, and needs to be considered in assessing the effectiveness of reducing Hg use and emissions as specified in Minamata Convention on Mercury.

Mercury Isotope Fractionation in the Subsurface of a Hg(II) Chloride-Contaminated Industrial Legacy Site

To understand the transformations of mercury (Hg) species in the subsurface of a HgCl₂-contaminated former industrial site in southwest Germany, Hg isotope analysis was combined with an investigation of Hg forms by a four-step sequential extraction protocol (SEP) and pyrolytic thermodesorption. Data from two soil cores revealed that the initial HgCl₂ was partly reduced to metallic Hg(0) and that Hg forms of different mobility and oxidation state coexist in the subsurface. The most contaminated sample (K2-8, 802 mg kg^-1 Hg) had a bulk δ²⁰²Hg value of around -0.43 ± 0.06‰ (2SD), similar to published average values for industrial Hg sources. Other sample signatures varied significantly with depth and between SEP pools. The most Hg-rich samples contained mixtures of Hg(0) and Hg(II) phases, and the water-extractable, mobile Hg pool exhibited heavy δ²⁰²Hg values of up to +0.18‰. Sequential water extracts revealed slow dissolution kinetics of mobile Hg pools, continuously releasing isotopically heavy Hg into solution. This was further corroborated by heavy δ²⁰²Hg values of groundwater samples. Our results demonstrate that the Hg isotope signature of an industrial contamination source can be significantly altered during the transformations of Hg species in the subsurface, which complicates source tracing applications but offers the possibility of using Hg isotopes as process tracers in contaminated subsurface systems.