Search for: All records

Abstract 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. 

Abstract. Estuaries are a conduit of mercury (Hg) from watersheds to the coastal ocean, and salt marshes play an important role in coastal Hg cycling. Hg cycling in upland terrestrial ecosystems has been well studied, but processes in densely vegetated salt marsh ecosystems are poorly characterized. We investigated Hg dynamics in vegetation and soils in the Plum Island Sound estuary in Massachusetts, USA, and specifically assessed the role of marsh vegetation for Hg deposition and turnover. Monthly quantitative harvesting of aboveground biomass showed strong linear seasonal increases in Hg associated with plants, with a 4-fold increase in Hg concentration and an 8-fold increase in standing Hg mass from June (3.9 ± 0.2 µg kg−1 and 0.7 ± 0.4 µg m−2, respectively) to November (16.2 ± 2.0 µg kg−1 and 5.7 ± 2.1 µg m−2, respectively). Hg did not increase further in aboveground biomass after plant senescence, indicating physiological controls of vegetation Hg uptake in salt marsh plants. Hg concentrations in live roots and live rhizomes were 11 and 2 times higher than concentrations in live aboveground biomass, respectively. Furthermore, live belowground biomass Hg pools (Hg in roots and rhizomes, 108.1 ± 83.4 µg m−2) were more than 10 times larger than peak standing aboveground Hg pools (9.0 ± 3.3 µg m−2). A ternary mixing model of measured stable Hg isotopes suggests that Hg sources in marsh aboveground tissues originate from about equal contributions of root uptake (∼ 35 %), precipitation uptake (∼ 33 %), and atmospheric gaseous elemental mercury (GEM) uptake (∼ 32 %). These results suggest a more important role of Hg transport from belowground (i.e., roots) to aboveground tissues in salt marsh vegetation than upland vegetation, where GEM uptake is generally the dominant Hg source. Roots and soils showed similar isotopic signatures, suggesting that belowground tissue Hg mostly derived from soil uptake. Annual root turnover results in large internal Hg recycling between soils and plants, estimated at 58.6 µg m−2 yr−1. An initial mass balance of Hg indicates that the salt marsh presently serves as a small net Hg sink for environmental Hg of 5.2 µg m−2 yr−1. 

Abstract 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 CO₂assimilation, 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. 

Mercury (Hg) is an environmental toxicant dangerous to human health and the environment. Its anthropogenic emissions are regulated by global, regional, and local policies. Here, we investigate Hg sources in the coastal city of Boston, the third largest metropolitan area in the Northeastern United States. With a median of 1.37 ng m −3 , atmospheric Hg concentrations measured from August 2017 to April 2019 were at the low end of the range reported in the Northern Hemisphere and in the range reported at North American rural sites. Despite relatively low ambient Hg concentrations, we estimate anthropogenic emissions to be 3–7 times higher than in current emission inventories using a measurement-model framework, suggesting an underestimation of small point and/or nonpoint emissions. We also test the hypothesis that a legacy Hg source from the ocean contributes to atmospheric Hg concentrations in the study area; legacy emissions (recycling of previously deposited Hg) account for ∼60% of Hg emitted annually worldwide (and much of this recycling takes place through the oceans). We find that elevated concentrations observed during easterly oceanic winds can be fully explained by low wind speeds and recirculating air allowing for accumulation of land-based emissions. This study suggests that the influence of nonpoint land-based emissions may be comparable in size to point sources in some regions and highlights the benefits of further top-down studies in other areas. 

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. 

Understanding the processes that influence and control carbon cycling in Arctic tundra ecosystems is essential for making accurate predictions about what role these ecosystems will play in potential future climate change scenarios. Particularly, air–surface fluxes of methane and carbon dioxide are of interest as recent observations suggest that the vast stores of soil carbon found in the Arctic tundra are becoming more available to release to the atmosphere in the form of these greenhouse gases. Further, harsh wintertime conditions and complex logistics have limited the number of year-round and cold season studies and hence too our understanding of carbon cycle processes during these periods. We present here a two-year micrometeorological data set of methane and carbon dioxide fluxes that provides near-continuous data throughout the active summer and cold winter seasons. Net emission of methane and carbon dioxide in one of the study years totalled 3.7 and 89 g C m−2 a−1 respectively, with cold season methane emission representing 54% of the annual total. In the other year, net emission totals of methane and carbon dioxide were 4.9 and 485 g C m−2 a−1 respectively, with cold season methane emission here representing 82% of the annual total – a larger proportion than has been previously reported in the Arctic tundra. Regression tree analysis suggests that, due to relatively warmer air temperatures and deeper snow depths, deeper soil horizons – where most microbial methanogenic activity takes place – remained warm enough to maintain efficient methane production whilst surface soil temperatures were simultaneously cold enough to limit microbial methanotrophic activity. These results provide valuable insight into how a changing Arctic climate may impact methane emission, and highlight a need to focus on soil temperatures throughout the entire active soil profile, rather than rely on air temperature as a proxy for modelling temperature–methane flux dynamics.