Stable mercury isotopes stored in Masson Pinus tree rings as atmospheric mercury archives

Riverine songbirds capture high levels of atmospheric mercury pollution from brown food webs in forests by mercury isotopic evidence

Elevated methylmercury (MeHg) exposure poses significant risks to bird health, behavior, and reproduction. Still, the risk of MeHg exposure to forest birds, accounting for over 80 % of the world's bird species, is poorly understood. This study combines Hg isotopes and video analysis, aiming to assess MeHg exposure risks to a forest riverine songbird, the spotted forktail (Enicurus maculatus) from a remote subtropical montane forest. Noticeably, 83 % of feather MeHg concentrations of adult forktails exceeded 5000 ng g-1, a threshold level potentially impacting bird reproduction, and 50 % of feather MeHg concentrations in forktail nestlings exceeded the threshold level of 1000 ng g-1, that potentially impacts the nestling growth. Forktail nestlings ingested ∼ 99 % of their MeHg from prey within brown food webs (i.e., from forest floor, aquatic, and emergent aquatic prey). The Hg isotopes reveal that MeHg along the bird food chain is mostly derived from in situ methylation of litterfall deposited atmospheric Hg0, with limited photo-demethylation (i.e., 4-12 %) in shaded forest environments. The risk of MeHg exposure of forest songbirds correlated positively with the proportion of prey consumed from brown food webs. We recommend incorporating resident riverine songbirds in monitoring programs to better evaluate the effectiveness of the Minamata Convention, especially in remote forest ecosystems where in situ MeHg production may be underestimated.

Leaves and Tree Rings as Biomonitoring Archives of Atmospheric Mercury Deposition: An Ecophysiological Perspective

Trees mediate critical biogeochemical cycles involving nutrients, pollutants, water, and energy at the interface between terrestrial biosphere and atmosphere. Forest ecosystems significantly influence the global cycling of mercury (Hg), serving as important sinks and potential sources of re-emission through various biotic and abiotic processes. Anthropogenic Hg emissions, predominantly from industrial activities, mining, and fossil fuel combustion, have substantially altered the natural Hg cycle, intensifying ecotoxicological concerns and establishing forests as primary routes for atmospheric Hg deposition into terrestrial reservoirs. This perturbation profoundly affects global atmospheric Hg concentrations, residence times, and spatial distribution patterns. While early investigations focused on forest stands near heavily polluted areas, contemporary research has expanded to diverse ecosystems, revealing that trees provide tissues that function as temporal archives for atmospheric-terrestrial Hg exchange. Leaves capture high-resolution records of contemporary Hg dynamics at sub-annual timescales, whereas annual growth rings preserve multi-decadal chronologies of historical atmospheric exposure. Incorporating this dual temporal perspective is crucial for analysing Hg deposition trends and assessing the efficacy of environmental policies designed to control and mitigate Hg pollution. This review critically evaluates recent developments concerning the ecophysiological determinants of Hg accumulation in trees, highlighting how combined foliar and dendrochemical analytical methods strengthen our mechanistic understanding of vegetation-atmosphere Hg exchange. To enhance biomonitoring approaches, we emphasised the need for methodological standardisation, deeper integration of ecophysiological variables, and consideration of climate change implications as priority research areas. Furthermore, integrating Hg measurements with functional markers (δ13C and δ18O) and Hg isotope analyses strengthens the capacity to differentiate between physiological and environmental influences on Hg accumulation, thereby refining the mechanistic framework underlying effective tree-based Hg biomonitoring.

Quantifying Altitudinal Mercury Accumulation in Biomonitors along Himalayan Valleys Using Mercury Isotopes

The Himalayan valleys are important transport channels of atmospheric pollutants from South Asia to the Tibetan Plateau. This study aims to demonstrate the use of biomonitors (i.e., tree foliage, bark, mosses, and lichens) in the Himalayas to understand the sources and accumulation of mercury (Hg), including the transboundary atmospheric Hg transport across the Himalayas. Results showed that the significant variability in the physiological characteristics and nutrient uptake pathways, coupled with rapid changes in topography and climate-forced precipitation, led to significant differences in concentrations and isotopic compositions among biomonitor species. Δ199Hg values (-0.32 to -0.10‰) at the lower altitudes were slightly more positive than values at upper altitudes, likely reflecting signals of transboundary transport of anthropogenic Hg from South Asia. The isotope mixing model determined atmospheric Hg0 as the main source of Hg in most biomonitors (67 ± 13% to 88 ± 13%), except for Usnea longissimas (i.e., a unique type of lichen) with 61 ± 16% contribution of atmospheric Hg2+. Additionally, the morphological structure and epiphytic environment of U. longissimas facilitate aqueous Hg secondary reactions. Our results suggest that the Hg cycling in the Himalayan valleys could mix multiple impacts from montane environments and signals of transboundary transport of anthropogenic Hg from South Asia.

Atmospheric Hg(0) dry deposition over environmental surfaces: Insights from mercury isotope fractionation

Atmospheric Hg(0) dry deposition is a vital process that significantly affects the global distribution and cycling of Hg. However, significant knowledge gaps and challenges remain in understanding atmospheric Hg(0) deposition and its subsequent post-deposition processes. Hg isotope fractionation has emerged as the most powerful tool for evaluating the impact of atmospheric Hg(0) deposition and unraveling key processes associated with it. By focusing on Hg isotope fractionation processes, Hg isotopic compositions, and influencing factors, this review presents current knowledge, recent advances, and new insights into atmospheric Hg(0) deposition and post-deposition processes over vegetation, soil, snow, and water surfaces. This review also points out the knowledge gaps pertaining to atmospheric Hg(0) deposition and highlights the need for further investigation into the associated processes, mechanisms, isotope fractionation, and modeling. Further research into Hg isotope fractionation in atmospheric Hg(0) deposition and post-deposition processes will advance source and process tracing, paleoclimate reconstruction, and the modeling of Hg isotope distribution on regional and global scales.

Important accumulated mercury pool in a remote alpine forest and dynamic accumulation revealed by tree rings in China's Qilian Mountains

Quantification mercury (Hg) pools in forests is crucial for understanding the Hg assimilation, flux and even biogeochemical cycle in forest ecosystems. While several investigations focused on Hg pools among broad-leaved, coniferous and mixed forests, there was still absent information on alpine forest. We sampled soil, moss and various tissues of the dominant Qinghai spruce (Picea crassifolia Kom.) to investigate Hg concentrations and pools, and assess Hg accumulation dynamics in the Qilian Mountains, northwestern China. The mean Hg concentration increased in the following order: trunk wood (1.8 ± 0.7 ng g-1) < branch (4.6 ± 0.8 ng g-1) < root (12.2 ± 2.9 ng g-1) < needle (19.3 ± 5.6 ng g-1) < bark (28.7 ± 9.0 ng g-1) < soil (34.1 ± 7.7 ng g-1) < litterfall (42.9 ± 2.9 ng g-1) < moss (62.5 ± 5.0 ng g-1). The soil contained Hg pools two orders of magnitude higher than vegetation and accounted for 92.2 % of the total Hg pool in the alpine forest ecosystem. Moss, despite representing only 2.7 % of total vegetation biomass, contained a disproportionate 16.7 % of the Hg pool. Although species-specific, aboveground spruce tissues exhibited higher Hg pools in alpine forests compared to other forests in China and America. The dynamic accumulation indicated that increasing atmospheric Hg concentration and enhancing tree productivity contributed to rising Hg assimilation in remote alpine forests, particularly after the 1960s. Our results highlight the relatively high levels of Hg pools in aboveground tree tissues of alpine forest and reveal a significant increase in Hg accumulation. We recommend that when assessing Hg dynamics in forest ecosystems, it is crucial to consider both the variability in atmospheric Hg exposure levels and the forest productivity.

Tree Rings Mercury Controlled by Atmospheric Gaseous Elemental Mercury and Tree Physiology

Tree rings are an emerging atmospheric mercury (Hg) archive. Questions have arisen, though, regarding their mechanistic controls and reliability. Here, we report contrasting tree-ring Hg records in three collocated conifer species: Norway spruce (Picea abies), Scots pine (Pinus sylvestris), and European larch (Larix decidua), which are from a remote boreal forest. Centennial atmospheric Hg trends at the site, derived from varved lake sediments, peats, and atmospheric monitoring, indicated a steady rise from the 1800s, peaking in the 1970s, and then declining. Prior to ca. 2005, larch and spruce tree rings reproduced the peak in the atmospheric Hg trend, while pine tree rings peaked in the 1930s, likely due to the prolonged sapwood period and ambiguity in the heartwood-sapwood boundary of pine. Since ca. 2005, tree rings from all species showed increasing Hg concentrations in the physiologically active outer rings despite declining atmospheric Hg concentrations. The good agreement between Hg and nitrogen concentrations in active tree-ring cells indicates a similar transport mechanism and cautions against their applicability as atmospheric Hg archives. Our results suggest that tree-ring Hg records are controlled by atmospheric Hg and tree physiology. We provide recommendations for using tree-ring Hg archives that take tree physiology into account.

Unveiling the Sources and Transfer of Mercury in Forest Bird Food Chains Using Techniques of Vivo-Nest Video Recording and Stable Isotopes

Knowledge gaps in mercury (Hg) biomagnification in forest birds, especially in the most species-rich tropical and subtropical forests, limit our understanding of the ecological risks of Hg deposition to forest birds. This study aimed to quantify Hg bioaccumulation and transfer in the food chains of forest birds in a subtropical montane forest using a bird diet recorded by video and stable Hg isotope signals of biological and environmental samples. Results show that inorganic mercury (IHg) does not biomagnify along food chains, whereas methylmercury (MeHg) has trophic magnification factors of 7.4-8.1 for the basal resource-invertebrate-bird food chain. The video observations and MeHg mass balance model suggest that Niltava (Niltava sundara) nestlings ingest 78% of their MeHg from forest floor invertebrates, while Flycatcher (Eumyias thalassinus) nestlings ingest 59% from emergent aquatic invertebrates (which fly onto the canopy) and 40% from canopy invertebrates. The diet of Niltava nestlings contains 40% more MeHg than that of Flycatcher nestlings, resulting in a 60% higher MeHg concentration in their feather. Hg isotopic model shows that atmospheric Hg0 is the main Hg source in the forest bird food chains and contributes >68% in most organisms. However, three categories of canopy invertebrates receive ∼50% Hg from atmospheric Hg2+. Overall, we highlight the ecological risk of MeHg exposure for understory insectivorous birds caused by atmospheric Hg0 deposition and methylation on the forest floor.

Fate and Transport of Mercury through Waterflows in a Tropical Rainforest

Knowledge gaps of mercury (Hg) biogeochemical processes in the tropical rainforest limit our understanding of the global Hg mass budget. In this study, we applied Hg stable isotope tracing techniques to quantitatively understand the Hg fate and transport during the waterflows in a tropical rainforest including open-field precipitation, throughfall, and runoff. Hg concentrations in throughfall are 1.5-2 times of the levels in open-field rainfall. However, Hg deposition contributed by throughfall and open-field rainfall is comparable due to the water interception by vegetative biomasses. Runoff from the forest shows nearly one order of magnitude lower Hg concentration than those in throughfall. In contrast to the positive Δ199Hg and Δ200Hg signatures in open-field rainfall, throughfall water exhibits nearly zero signals of Δ199Hg and Δ200Hg, while runoff shows negative Δ199Hg and Δ200Hg signals. Using a binary mixing model, Hg in throughfall and runoff is primarily derived from atmospheric Hg0 inputs, with average contributions of 65 ± 18 and 91 ± 6%, respectively. The combination of flux and isotopic modeling suggests that two-thirds of atmospheric Hg2+ input is intercepted by vegetative biomass, with the remaining atmospheric Hg2+ input captured by the forest floor. Overall, these findings shed light on simulation of Hg cycle in tropical forests.

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.

Tree rings as historical archives of atmospheric mercury: A critical review

Historical concentrations of atmospheric mercury (Hg) are uncertain, as monitoring only began a few decades ago. Tree rings can serve as historical archives of Hg, providing centennial trends. The vast majority of tree-ring Hg studies have been published in the last decade, demonstrating the growing use of tree rings for Hg dendrochemistry. Thus, there is a need for a systematic review on current knowledge of tree rings as archives of atmospheric Hg. In this review, the predominant pathways of Hg uptake to tree rings are discussed, including the initial Hg uptake from the surrounding environment, fixation, and subsequent translocation. Foliar uptake of Hg was found to be the most important uptake route for Hg in tree rings, the root and bark route being negligible. Our summary of the suitability of different tree species indicates that radial translocation is the biggest limiting factor for Hg dendrochemistry, shifting and blurring historical Hg trends. Based on the review findings, Picea (spruce) and Larix (larch) are the most promising genera for Hg dendrochemistry. Additionally, the use of tree-ring Hg archives in combination with other co-located archives, namely lake sediments, peat, and ice, is suggested as it enhances the viability of observed tree-ring historical Hg trends. Finally, we propose future directions and recommendations for research using tree-ring Hg, including sampling protocols, experimental designs, and tree selection.

Typical heavy metals accumulation, transport and allocation in a deglaciated forest chronosequence, Qinghai-Tibet Plateau

Understanding heavy metals (HMs) accumulation and transportation is the foundation to assess the ecological risks caused by the pollution of HMs in terrestrial ecosystems. There are large knowledge gaps regarding impacts of vegetation succession on shaping the HMs accumulation, transportation and allocation in the remote alpine regions. Herein, we comprehensively investigated the distribution and source contribution of mercury (Hg), cadmium (Cd) and chromium (Cr) along with vegetation succession in a deglaciated forest chronosequence of Qinghai-Tibet Plateau. Results showed that Hg and Cd were highly enriched in organic soils, while Cr concentrations and pool sizes decreased significantly with the vegetation succession. Atmospheric Hg deposition contributed to the dominant Hg sources in topsoil (74 - 87%), whereas moraine weathering was the main source of Cr (73 - 76%). Both moraine (18 - 48%) and atmospheric deposition inputs (52 - 82%) affected Cd accumulation in topsoil. Over the last century, the accumulation rate of Hg and Cd showed the distinctly decreasing trends due to the vegetation leading to the elevated atmospheric depositions at the earlier deglacial sites. The negative accumulation rate of Cr along with the vegetation succession reflected the formation of organic soil diluting the geogenic inputs of Cr.

Investigation of the biochemical controls on mercury uptake and mobility in trees

Atmospheric elemental mercury (Hg(0)) enters plant stomata, becomes oxidized, and is then transferred to annual growth rings providing an archive of air Hg(0) concentrations. To better understand the processes of Hg accumulation and translocation, the foliage of quaking aspen and Austrian pine were exposed to Hg(0), and methylmercury (MeHg) or Me¹⁹⁸Hg via roots, in controlled exposures during the summer. Isotopic measurements demonstrated, in a laboratory setting, that the natural mass-dependent fractionation observed was the same as that measured in field studies, with the lighter isotopes being preferentially taken up by the leaves. Hg was measured in plant tissues across seasons. Aspen trees moved Hg into new growth immediately after exposure, resorbed Hg in the fall, and then distributed Hg to new growth tissues in the spring. Austrian pine did not reallocate Hg. Mercury measured in aspen leaf fractions of trees exposed to Hg(0) demonstrated that 85 % of Hg was in the cell wall. It was also found that redox-active molecules, such as H₂O₂, could potentiate the release of cell wall-bound Hg from aspen leaves, providing a potential mechanism for remobilization. Regardless of the mechanism, the ability of aspen to reallocate Hg to new tissues indicates that Hg distribution in tree rings from aspen do not provide a reliable record of yearly changes in atmospheric Hg(0).

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.

A Bibliometric Analysis of the Scientific Research on Artisanal and Small-Scale Mining

Mineral resource exploitation is one of the activities that contribute to economic growth and the development of society. Artisanal and small-scale mining (ASM) is one of these activities. Unfortunately, there is no clear consensus to define ASM. However, its importance is relevant in that it represents, in some cases, the only employment alternative for millions of people, although it also significantly impacts the environment. This work aims to investigate the scientific information related to ASM through a bibliometric analysis and, in addition, to define the new lines that are tending to this field. The study comprises three phases of work: (i) data collection, (ii) data processing and software selection, and (iii) data interpretation. The results reflect that the study on ASM developed intensively from 2010 to the present. In general terms, the research addressed focuses on four interrelated lines: (i) social conditioning factors of ASM, (ii) environmental impacts generated by ASM, (iii) mercury contamination and its implication on health and the environment, and (iv) ASM as a livelihood. The work also defines that geotourism in artisanal mining areas is a significant trend of the last decade, explicitly focusing on the conservation and use of the geological and mining heritage and, in addition, the promotion of sustainable development of ASM.