Latitudinal gradient for mercury accumulation and isotopic evidence for post-depositional processes among three tropical forests in Southwest China

Mercury Transport, Transformation and Accumulation Recorded by Stable Isotopes during Retreated Glacier Chronosequence of 250 Years

Vegetative development in regions where glaciers retreated due to global warming forces the mercury (Hg) cycle in the cryosphere. This study depicts the fate of Hg in a glacier-retreated chronosequence over the last 250 years recorded by signals of stable Hg isotopes. Results show that the Hg storage in surface soil increases by 3.2 times over 250 years after the glacier retreated. 53 ± 11% of Hg in grass shoots is from the uptake of atmospheric Hg0 and 47 ± 11%, from the uptake of soil Hg. Atmospheric Hg2+ is the primary source of surface soil Hg (54 ± 13%), followed by atmospheric Hg0 (40 ± 10%) and geogenic Hg. The Hg accumulation in soils increased by a factor of 5 at an accelerating rate from the 1870s to 2010s. The Hg release flux from melting glaciers is 3.51 ± 0.01 μg m-2 yr-1. The highly positive Δ199Hg (1.03 ± 0.49‰) in precipitation due to photoreduction of Hg2+ in water droplets causes all samples in ecosystems to have positive Δ199Hg values. Isotopic evidence suggests that photolytic and abiotic dark reduction processes have driven Hg0 re-emission from glacier and underlying soil after melting. The accelerated Hg release from melting glaciers and soil Hg accumulation caused by global warming alter Hg cycling in the cryosphere.

Climate and vegetation controlling accumulation and translocation of heavy metals in water tower regions of Qinghai-Tibet Plateau

Understanding how climate and vegetation influencing accumulation and translocation of heavy metals (HMs) in soils and vegetation in the Qinghai-Tibet Plateau (QTP) is critical to assess the ecological risk induced by HMs under the global warming. To accompany this goal, we comprehensively determined the accumulation and translocation of HMs within the interface of soil-vegetation in water tower regions of the QTP. The PMF model results show that 54 %-86 % of cobalt (Co), nickel (Ni), arsenic (As), zinc (Zn) and lead (Pb) in the surface soil are mainly from rock weathering and 54 % of cadmium (Cd) comes from effect of litter return. The increase of vegetation biomass significantly promotes the accumulation of HMs in the surface soil. The increase of root biomass significantly enhances the uptake of Co, Ni, As, Cd and Pb by roots, due to the increasing availability of these HMs in surface soil, but reduces the translocation from roots to shoots. The precipitation and temperature influence HMs translocation by controlling the root biomass. Hence, we speculate that the further global warming in the QTP would enhance HMs accumulation in surface soil, but would not significantly increase HMs accumulation in ground vegetation biomass.

Combined Impacts of Climate and Tree Physiology on Mercury Accumulation in Tropical and Subtropical Foliage and Robust Model Parametrization

Atmospheric elemental mercury (Hg0) assimilation by foliage contributes prevalently to the global atmospheric Hg0 sink in forests. Today, little is known about the mechanisms of foliar Hg accumulation and how climate factors and tree physiology interact to impact it. Here, we examined meteorological factors, foliar physiological traits, and Hg accumulation rates from leaf emergence to senescence in a tropical rainforest, tropical savanna, and subtropical evergreen broadleaf forest. Also, robust models for foliar Hg accumulation were parametrized. Generally, foliar Hg accumulation rate in subtropical evergreen forest was highest (16.4 ± 12.3 ng m-2 day-1), followed by the tropical rainforest (14.2 ± 9.8 ng m-2 day-1), and lowest in the tropical savanna (4.7 ± 4.9 ng m-2 day-1). Atmospheric relative humidity, stomatal conductance, and leaf photosynthesis are key drivers of spatial-temporal variations in foliar Hg accumulation. The canopy-structure-induced specific leaf physiological traits drive temporal variations in foliar Hg accumulation, and climate-controlled leaf physiological traits account for spatial variations among three forests. Finally, our robust models enable precise simulation of foliar Hg accumulation rates at both tree species and ecosystem scales facilitating particularly regional and global Hg transport and chemical models to quantify the vegetation's role as a sink for atmospheric Hg0 uptake.

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.

Photoreduction and origin of dissolved and particulate mercury in cloud water: Insights from stable mercury isotopes

The photoreduction of mercury (Hg) in clouds is crucial for determining global Hg cycling. The recently-developed isotope approach provides new insight into the fate of atmospheric Hg, however, limited data have been reported on the dynamics of Hg isotopes in clouds. This study presented the isotopic compositions of dissolved mercury (DHg) and particulate mercury (PHg) in cloud water collected at Mt. Tai (1545 m a.s.l.) in eastern China during summer 2021. Both DHg and PHg exhibited positive mass-independent fractionation of odd isotopes (odd-MIF, denoted as Δ199Hg), with averaged Δ199Hg values of 0.83 ± 0.34‰ and 0.20 ± 0.11‰, respectively. This high odd-MIF likely resulted from aqueous photoreduction in clouds, with DHg being more susceptible to photolysis than PHg. Our findings indicated that the photoreduction was promoted by sunlight and influenced by the chemical compositions of cloud water that controlled the Hg(II) speciation. The isotope mixing model estimation revealed that particulate-bound Hg and reactive gaseous Hg constituted the principal sources of Hg in cloud water, accounting for 55% to 99% of the total, while gaseous element Hg also made a notable contribution. Additionally, cloud water samples with faster reduction rates of Hg(II) were located outside of the isotope mixing models, which indicated an enhanced photoreduction process in cloud water.

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.

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.

Understanding heavy metal distribution in timberline vegetations: A case from the Gongga Mountain, eastern Tibetan Plateau

To quantify impacts of vegetation and topographic factors on heavy metal accumulation in montane forests, we assessed the spatial distribution and determined the sources of mercury (Hg), cadmium (Cd), lead (Pb), chromium (Cr), copper (Cu) and zinc (Zn) in timberline forests of Gongga Mountain. Our results show that vegetation type has little impact on the soil Hg, Cd and Pb concentrations. The soil concentrations of Cr, Cu and Zn are controlled by litter return, moss and lichen biomass, and canopy interception, with the highest concentrations in shrub forest. In contrast to other forests, the soil Hg pool in coniferous forest is significantly high due to the elevated Hg concentration and greater biomass production in litter. However, the soil pool sizes of Cd, Cr, Cu and Zn show a distinct increase along the elevation, which are attributed to the elevated heavy metal inputs from litter and moss, as well as the greater cloud water-induced atmospheric heavy metal depositions. The highest Hg concentrations of the aboveground parts of plant are in the foliage and bark, while the concentrations of Cd, Pb, Cr, Cu and Zn in the branch and bark are the highest. The decreased biomass density leads to a downward trend in the total vegetation pool sizes of Hg, Cd, Pb, Cr, Cu and Zn by 0.4-4.4 times with increasing elevation. The statistical analysis finally suggests that Hg, Cd and Pb mainly originate from anthropogenic atmospheric deposition, whereas Cr, Cu and Zn are mainly from natural sources. Our results highlight the importance of vegetation types and terrain conditions on distribution patterns of heavy metal in alpine forests.

Quantifying Mercury Distribution and Source Contribution in Surface Soil of Qinghai-Tibetan Plateau Using Mercury Isotopes

Long-range transport and atmospheric deposition of gaseous mercury (Hg⁰) result in significant accumulation of Hg in the Qinghai-Tibetan Plateau (QTP). However, there are significant knowledge gaps in understanding the spatial distribution and source contribution of Hg in the surface soil of the QTP and factors influencing Hg accumulation. In this study, we comprehensively investigated Hg concentrations and isotopic signatures in the QTP to address these knowledge gaps. Results show that the average Hg concentration in the surface soil ranks as follows: forest (53.9 ± 36.9 ng g^-1) > meadow (30.7 ± 14.3 ng g^-1) > steppe (24.5 ± 16.1 ng g^-1) > shrub (21.0 ± 11.6 ng g^-1). Hg isotopic mass mixing and structural equation models demonstrate that vegetation-mediated atmospheric Hg⁰ deposition dominates the Hg source in the surface soil, with an average contribution of 62 ± 12% in forests, followed by 51 ± 10% in shrub, 50 ± 13% in steppe, and 45 ± 11% in meadow. Additionally, geogenic sources contribute 28-37% of surface soil Hg accumulation, and atmospheric Hg²⁺ inputs contribute 10-18% among the four types of biomes. The Hg pool in 0-10 cm surface soil over the QTP is estimated as 8200 ± 3292 Mg. Global warming, permafrost degradation, and anthropogenic influences have likely perturbed Hg accumulation in the soil of QTP.

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.