Mercury in a birch forest in SW Europe: Deposition flux by litterfall and pools in aboveground tree biomass and soils

Elevated mercury wet deposition in the biodiversity hotspot of southwestern China

The West China Rain Zone (WCRZ), one of worldwide biodiversity hotspots, is vulnerable to pollutant exposure. Mercury (Hg), as a global toxic pollutant, could enter into this ecosystem by wet and dry deposition, potentially posing ecological risks. Therefore, it is essential to investigate the patterns, controlling factors, and source contributions of Hg deposition in this region. This study presents a three-year comprehensive analysis of atmospheric Hg wet deposition at Mt. Emei of this biodiversity hotspot region. Results revealed the elevated Hg concentrations with volume-weighted mean concentration (VWMC) of 15.6 ± 9.8 ng L-1 and deposition fluxes of 47.8 ± 4.3 μg m-2 in the WCRZ. The wet deposition flux was twice the magnitude of dry deposition flux in this region. This value exceeded the deposition levels observed in many other regions globally, with WCRZ deposition levels being 1.1-15.6 times that of other biodiversity hotspots worldwide. Source apportionment analysis revealed that Hg in wet deposition predominantly originated from local anthropogenic sources. The transboundary Hg of anthropogenic sources in South Asia contributed to ∼20% of Hg source contribution during winter periods. Finally, we suggest the elevated Hg wet deposition inducing a potentially ecological risk to wildlife in this biodiversity hotspot regions of Southwest China.

Quarter century of mercury litterfall at a coniferous forest responding to climate change, Central Europe

This work evaluated the 25-year-long trends (1994-2018) in mercury (Hg) concentrations and fluxes in spruce litterfall at a forest research plot Načetín (NAC) recovering from acidic deposition in the Ore Mountains, Czech Republic. The mean litterfall Hg deposition averaged 51 ± 18 µg m-2 year-1, which has been the highest litterfall Hg deposition reported up to date on the European continent. In contrast, the wet deposition (2017-2019) was an order of magnitude lower averaging at 2.5 ± 1.5 µg m-2 year-1. All the spruce litterfall components bark, twigs, needles, cones, and a mixture of unidentified fragments had elevated mean Hg concentrations relative to background sites averaging 256 ± 77, 234 ± 62, 119 ± 23, 95 ± 14, and 44 ± 15 µg kg-1, respectively. Elevated litterfall Hg deposition and concentrations were attributed to the nearby local Hg emission source-coal-fired power plants. Temporally, the decrease of Czech Hg emissions since the 1990s was reflected by the decreasing trend of Hg concentrations in litterfall bark, cones, and twigs, while in needles and other material, Hg increased but insignificantly. Total litterfall ratios of Hg/C, Hg/N, and Hg/S were lower than those in soil O horizons averaging at 0.23 ± 0.04, 9.5 ± 2.0, and 170 ± 37 μg g-1, respectively. Since the beginning of monitoring, total litterfall Hg/C exhibited no trend, Hg/N decreased, and Hg/S increased. The litterfall biomass deposition averaging at 469 ± 176 g m-2 year-1 increased through time resulting in an increased Hg litterfall deposition at NAC by 1.1 µg m-2 year-1 despite the decreases in Czech Hg emissions. Peaks of annual litterfall Hg deposition up to 96 µg m-2 year-1 at NAC during the 25 years of monitoring resulted from weather extremes such as rime-snow accumulation, wind gusts, droughts, and insect infestation, which all significantly affected the annual biomass deposition. Based on our observations, further increases in biomass and litterfall Hg deposition rates can be expected due to the onset of bark beetle infestation and the increasing number of droughts caused by climate change.

Multi-layered physical factors govern mercury release from soil: Implications for predicting the environmental fate of mercury

Despite the recognised risks of human exposure to mercury (Hg), the drivers of gaseous elemental mercury (GEM) emissions from the soil remain understudied. In this study, we aimed to identify the environmental parameters that affect the GEM flux from soil and derive the correlations between environmental parameters and GEM flux. Principal component analysis (PCA), factor analysis (FA), and structural equation modelling (SEM) were performed on samples from forest and non-forest sites. The associated results revealed the impact of each environmental parameter on GEM flux, either due to the interaction between the parameters or as a coherent set of parameters. An introductory correlation matrix examining the relationship between two components showed a negative correlation between GEM flux and atmospheric pressure at the two sites, as well as strong correlations between atmospheric pressure and soil temperature. In cases of non-forest open sites with no trees, the PCA and FA results were consistent, indicating that atmospheric pressure, solar irradiance, and soil moisture-defined as primary causality-are largely independent drivers of GEM flux. In contrast, the PCA and FA results for the forest areas with high humidity, tree coverage, and shade were inconsistent, confirming the hypothesis that primary causality affects GEM flux rather than consequent parameters driven by primary causality, such as air and soil temperature and atmospheric humidity. The SEM results provided further evidence for primary and consequent causality as crucial drivers of the GEM flux. This study demonstrates the importance of key primary parameters, such as atmospheric pressure, solar irradiance, and soil moisture content, that can be used to predict mercury release from soils, as well as the importance of consequent parameters, such as air and soil temperature and atmospheric humidity. Monitoring the magnitude of these environmental parameters alone may facilitate the estimation of mercury release from soils and be useful for detailed modelling of soil-air Hg exchange.

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.

Predominant contributions through lichen and fine litter to litterfall mercury deposition in a subalpine forest

Litterfall, typically referring to needles/leaves, may stand for >50% of the total mercury (Hg) deposition in forest ecosystems. By detailed categorisation, we reveal for the first time that the contributions through lichens and fine litter, together 9.98 μg Hg m^-2 yr^-1, could be as high as that in needle litter (9.96 μg m^-2 yr^-1) to the annual total Hg deposition (44.6 μg m^-2 yr^-1) in a subalpine forest in Switzerland. Noticeably, needle litter had the highest contribution (53%) to total Hg in the autumn litterfall but lichens and fine litter together predominated in other seasons (47-59%). Such a seasonal pattern is caused by the high ability of lichens and fine litter to accumulate Hg and the high needle litterfall in autumn, which is related to a good rainfall in summer followed by a dry period in autumn. The constantly higher Hg levels in lichens and fine litter than in needle litter together with similar seasonal patterns of litterfall during 2009-2019 and rainfall during 1980-2019 suggest that our finding can be generally valid. Here, we highlight not only the considerable role of non-needle litterfall in Hg deposition but also the association with weather for seasonal Hg dynamics in different litterfall components.