Investigation of Uptake and Retention of Atmospheric Hg(II) by Boreal Forest Plants Using Stable Hg Isotopes (original) (raw)
Related papers
The role of ground vegetation in the uptake of mercury and methylmercury in a forest ecosystem
2003
Litterfall from trees has been identified as an important pathway for deposition of mercury (Hg) and methylmercury (MeHg) in forested catchments, but very little is known about the role of ground vegetation in deposition and cycling of Hg compounds. This study was conducted to identify the origin of Hg compounds in the ground vegetation, and to estimate the role of its litterfall with respect to pools and fluxes of Hg in a coniferous forest in the German Fichtelgebirge mountains. Above and below ground biomass of the dominant ground vegetation (Vaccinium myrtillus, Deschampsia flexuosa and Calamagrostis villosa) were sampled at several plots successively during the growing season. The fluxes to the soil via litterfall of the ground vegetation were calculated using contents of Hg and MeHg in the annual fractions of aboveground biomass. With fluxes of 0.4-7.8 mg Hg total ha −1 a −1 and 0.01-0.04 mg MeHg ha −1 a −1 (depending on the plant species) this pathway contributes only a few percent to the total deposition of both compounds in the catchment. To identify the uptake pathways of Hg compounds, the same plant species were grown in a pot experiment with addition of isotope labelled Hg compounds (202 Hg 2+ , Me 198 Hg) to a clean sand substrate. Only small proportions of 202 Hg and Me 198 Hg in the substrate were taken up by the plants, but in all cases the proportion translocated into aboveground biomass after uptake was greater in case of Me 198 Hg. Thus, internal recycling in the plant-soil system is a source especially for MeHg in the ground vegetation. However, as compared to the input of Hg compounds by tree litterfall and storage in the forest floor, Hg total and MeHg in ground vegetation are of minor importance. High volatilization of added Hg isotopes raises the question of a re-emission of Hg compounds by the transpiration flux of the ground vegetation.
Environmental Science & Technology, 2006
This paper presents the design of a dynamic chamber system that allows full transmission of PAR and UV radiation and permits enclosed intact foliage to maintain normal physiological function while Hg(0) flux rates are quantified in the field. Black spruce and jack pine foliage both emitted and absorbed Hg(0), exhibiting compensation points near atmospheric Hg(0) concentrations of ∼2-3 ng m-3. Using enriched stable Hg isotope spikes, patterns of spike Hg(II) retention on foliage were investigated. Hg(0) evasion rates from foliage were simultaneously measured using the chamber to determine if the decline of foliar spike Hg(II) concentrations over time could be explained by the photoreduction and re-emission of spike Hg to the atmosphere. This mass balance approach suggested that spike Hg(0) fluxes alone could not account for the measured decrease in spike Hg(II) on foliage following application, implying that either the chamber underestimates the true photoreduction of Hg(II) to Hg(0) on foliage, or other mechanisms of Hg(II) loss from foliage, such as cuticle weathering, are in effect. The radiation spectrum responsible for the photoreduction of newly deposited Hg(II) on foliage was also investigated. Our spike experiments suggest that some of the Hg(II) in wet deposition retained by the forest canopy may be rapidly photoreduced to Hg(0) and re-emitted back to the atmosphere, while another portion may be retained by foliage at the end of the growing season, with some being deposited in litterfall. This finding has implications for the estimation of Hg dry deposition based on throughfall and litterfall fluxes.
Atmospheric Environment, 2011
Uptake of gaseous elemental mercury (Hg 0 (g)) by three plant species and two soil types was measured using mercury vapor enriched in the 198 isotope (198 Hg 0 (g)). The plant species and soil types were: White Ash (Fraxinus Americana; WA); White Spruce (Picea Glauca; WS); Kentucky Bluegrass (Poa Partensis; KYBG); Plano Silt Loam (4% organic matter; PSL); and Plainfield Sand/Sparta Loamy Sand (1.25e1.5% organic matter: PS). The plants and soils were exposed to isotopically enriched Hg 0 (g) in a 19 m 3 controlled environment room for 7 days under optimal plant growth conditions (20 C, 140 Wm À2 between 300 nm and 700 nm; 70% RH) and atmospherically relevant Hg 0 (g) concentrations. Mercury was recovered from the samples using acidic digestions and surface leaches, and then analyzed for enrichments in 198 Hg by ICPMS. The method was sensitivity enough that statistically significant enrichments in 198 Hg were measured in the plant foliage at the end of Day 1. Whole leaf digestions and surface-selective leaches revealed that accumulative uptake was predominantly to the interior of the leaf under the conditions studied. Uptake fluxes for WA increased between the first and third days and remained constant thereafter (WA; Day 1 ¼ 7 AE 2 Â 10 À5 ng m À2 s À1 ; Days 3e7 ¼ 1.3 AE 0.1 Â 10 À4 ng m À2 s À1 ; where m 2 refers to one sided leaf area). KYBG demonstrated similar behavior although no Day 3 measurement was available (Day 1 ¼ 7.5 AE 0.5 Â 10 À5 ng m À2 s À1 ; Day 7 ¼ 1.2 AE 0.1 Â 10 À4 ng m À2 s À1). Fluxes to White Spruce were lower, with little difference between Days 1 and 3 followed by a decrease at Day 7 (WS; Days 1e3 ¼ 5 AE 2 Â 10 À5 ng m À2 s À1 ; Day 7 ¼ 2.4 AE 0.2 Â 10 À5 ng m À2 s À1). Uptake of Hg to soils was below the method detection limit for those media (PSL ¼ 3 Â 10 À2 ng m À2 s À1 ; PS ¼ 3 Â 10 À3 ng m À2 s À1) over the 7 day study period. Foliar resistances calculated for each species compared well to previous studies.
Journal of Plant Nutrition and Soil Science, 2012
Vegetation type and ecosystem structure affect major aspects of the mercury (Hg) cycle in terrestrial ecosystems which serve as important storage pools for a long-term legacy of natural and anthropogenic Hg release to the environment. The goal of this study was to evaluate the integrated effects of 80 y of different vegetative type on Hg accumulation and partitioning in terrestrial ecosystems by comparing Hg concentrations and pools of two adjacent forests: a coniferous Douglas fir (Pseudotsuga menziesii) and a deciduous red alder (Alnus rubra) stand. These stands grew for > 80 y in close proximity (200 m) with identical site histories, soil parent materials, and atmospheric exposure. Results showed that the Douglas fir stand was characterized by significantly higher Hg concentrations and Hg : C ratios in aboveground biomass compared to the deciduous red alder forest. For foliage, higher Hg concentrations (plus 43 lg kg ±1) were expected due to foliage age, but Hg concentrations also were higher in woody tissues (by 2 to 18 lg kg ±1) indicating increased uptake of atmospheric Hg by coniferous tissues. These differences were reflectedÐand further increasedÐin litter horizons where Hg-concentration differences increased in highly decomposed litter to > 200 lg kg ±1. In soils, no difference in concentrations of Hg was observed, but Hg : C ratios were consistently higher in the coniferous Douglas fir. Estimation of pool sizes of C and Hg in soils and at the whole ecosystem level showed that considerably smaller C pools in the coniferous stand as a result of faster C turnover and lower productivity did not lead to corresponding declines in Hg-pool sizes. The partitioning of Hg among ecosystem componentsÐincluding distribution between aboveground and belowground components and distribution through the soil profileÐwas largely unaffected by forest type. Methyl-Hg concentrations observed in litter layers were also significantly higher in litter of Douglas fir, along with a higher proportion of methylated Hg of total Hg. In soils, methyl-Hg concentrations were similar in both stands. Comparison of these adjacent forest stands highlights that vegetation type affects concentrations of total Hg in otherwise equivalent sites and that differences also exist in respect to methylated Hg.
Environmental Science & Technology, 2008
The purpose of the METAALICUS experiment was to examine the timing and magnitude of the response of fish Hg concentrations to changes in atmospheric Hg loading. Enriched stable Hg isotopes were applied as HgNO 3 to the lake (202 Hg), wetland (198 Hg) and upland (200 Hg) compartments of the L658 watershed at ~22 ug m-2 yr-1 since 2001 (1, 2). However, the present study focused on deposition of ambient Hg, not the experimentally-applied isotopic Hg. Analytical Methods Precipitation and throughfall samples collected between 1992 and 2000 were analysed for total Hg (THg; all forms of Hg in a sample) using EPA Method 1631 (3) by Flett Research Ltd. (Winnipeg, MB). Briefly, all Hg in samples was oxidized to Hg(II) by the addition of BrCl, reduced to Hg(0) using SnCl 2 , purged onto gold traps, thermally desorbed and analyzed by cold vapour atomic fluorescence spectrometry (CVAFS). Samples collected in the METAALICUS watershed between 2001 and 2006 were analysed using inductively coupled plasma mass spectrometry (ICP-MS, 4, 5). Because ICP-MS quantifies concentrations of individual Hg isotopes, it was possible to distinguish concentrations of ambient Hg from the experimentally loaded enriched Hg isotopes. To calculate concentrations of ambient Hg, an isotope that was not experimentally applied to the watershed was used as an ambient Hg surrogate (199 Hg). Samples collected in 2001-2004 were analyzed at the Trent University Worsfold Water Quality Center (Peterborough, ON). Samples were continuously mixed in-line with SnCl 2 using peristaltic pumps, and the reduced gaseous Hg(0) formed was separated in a custom-made gas/liquid separator and swept into the plasma of a Finnigan Element 2 ICP-MS (6). For samples collected between 2005 and 2006, analysis was completed at the University of Alberta Biogeochemical Laboratory (Edmonton, AB). In this laboratory, reduction of Hg and gas/liquid separation was accomplished using an automated Tekran 2600 total Hg analyzer interfaced with a PerkinElmer Elan DRC-e ICP-MS for detection. Precipitation and throughfall samples collected for methylmercury (MeHg) analyses prior to 2001 were distilled, ethylated by additions of sodium tetra-ethyl-borate (NaBEt 4), and volatile Hg species were purged and trapped using either carbo or tenax traps. Samples were thermally desorbed and separated by gas chromatography before quantification by CVAFS (7, 8). Samples collected in the METAALICUS watershed were analyzed for MeHg as described above, except that
Accumulation of atmospheric mercury in forest foliage
Atmospheric Environment, 2003
We used unique mesocosms to examine the role that plants play in accumulating and transforming atmospheric Hg. Several stands of quaking aspen were grown in large gas-exchange chambers in Hg-enriched soil (12.371.3 mg g À1), and the Hg content in the vegetation was determined over time. Foliar Hg concentrations increased as a function of leaf age and leveled off after 2-3 months in the oldest tissue with a mean tissue concentration of 150 ng g À1. Approximately 80% of the total Hg accumulated in the aboveground biomass was found in the leaves, and roughly 1% of that Hg was methylated. Leaves of additional aspen grown within the mesocosms in containers of low Hg soil (0.0370.01 mg g À1) exhibited foliar Hg concentrations similar to those of trees grown in the Hg-enriched soil. Leaf rinses and surrogate Teflon surfaces were analyzed to characterize surface deposition processes. Small gas-exchange systems were used to measure stomatal uptake of Hg vapor, and the mean Hg flux was À3.3 ng m À2 h À1. These experiments showed that almost all of the Hg in foliar tissue originated from the atmosphere. Thus, in the fall when deciduous trees enter dormancy and leaves senesce, litterfall would represent a new Hg input to terrestrial ecosystems.
Litterfall Hg deposition to an oak forest soil from southwestern Europe
Journal of Environmental Management, 2020
Litterfall constitutes one of the main vectors for mercury (Hg) transfer to forested ecosystems, so we studied the deposition of Hg through senescent vegetation (oak leaves, twigs and miscellaneous) in a deciduous forest plot of Southwest Europe dominated by Quercus robur in 2015 and 2016. Total Hg concentrations increased in the following order: bole wood (1.4 μg kg À 1) < bark (8.3 μg kg À 1) < twigs (12.2 μg kg À 1) < miscellaneous (36.0 μg kg À 1) < oak leaves (39.3 μg kg À 1) < mineral soil (42.4 μg kg À 1) < Oi horizons (48.7 μg kg À 1) < Oe þ Oa horizons (71.6 μg kg À 1). Mercury accumulation rates in oak leaves during the growing season were 0.15-0.18 μg kg À 1 day À 1. Mercury deposition fluxes were 26 and 21 μg m À 2 yr À 1 for 2015 and 2016, respectively, with oak leaves being the fraction that contributed the most. Mercury determination in litterfall sorted biomass fractions lead to a more accurate estimation of the total annual Hg deposition fluxes through litterfall. Higher Hg content was obtained for organic horizons (average of 60.2 μg kg À 1) than for mineral soil (mean of 42.4 μg kg À 1), but the soil Hg pool was higher in the latter. The results confirmed the necessity of taking into account the Hg pool in the deeper mineral soil layers as they accumulate substantial quantities of Hg associated to organic C and Al compounds , preventing its mobilization to other compartments of the terrestrial ecosystems.
Water Resources Research, 2005
1] As part of the Mercury Experiment to Assess Atmospheric Loadings in Canada and the United States (METAALICUS) the fate and transport of contemporary mercury (Hg) deposition in a boreal wetland was investigated using an experimentally applied stable mercury isotope. We applied high purity (99.2% ± 0.1) 202 Hg(II) to a wetland plot to determine if (1) the 202 Hg was detectable above the pool of native Hg, (2) the 202 Hg migrated vertically and/or horizontally in peat and pore waters, and (3) the 202 Hg was converted to methylmercury (MeHg) in situ. The 202 Hg was easily detected by ICP/MS in both solid peat and pore waters. Over 3 months, the 202 Hg migrated vertically downward in excess of 15 cm below the water table and traveled several meters horizontally beyond the experimental plot to the lake margin along the dominant vector of groundwater flow. Importantly, at one location, 6% of aqueous 202 Hg was detected as Me 202 Hg after only 1 day. These results indicate that new inorganic Hg in atmospheric deposition can be readily methylated and transported lakeward by shallow groundwater flow, confirming the important role of wetlands as contributors of Hg to aquatic ecosystems. (2005), Speciation and transport of newly deposited mercury in a boreal forest wetland: A stable mercury isotope approach, Water Resour. Res., 41, W06016,
Organic matter (OM) cycling has a large impact on the cycling of mercury (Hg) in the environment. Hence, it is important to have a thorough understanding on how changes in, e.g., catchment vegetation – through its effect on OM cycling – affect the behavior of Hg. To test whether shifts in vegetation had an effect on Hg-transport to lakes we investigated a sediment record from Herrenwieser See (Southern Germany). This lake has a welldefined Holocene vegetation history: at ~8700 years BP Corylus avellana (hazel) was replaced by Quercus robur (oak), which was replaced by Abies alba (fir) and Fagus sylvatica (beech) ~5700 years BP). We were particularly interested in testing if coniferous vegetation leads to a larger export ofHg to aquatic systems than deciduous vegetation. When hazel was replaced by oak, reduced soil erosion and increased transport of DOM-bound mercury from the catchment resulted in increases in both Hg-concentrations and accumulation rates (61 ng g−1 and 5.5 ng cm−2 yr.−1 to 118 ng g−1 and 8.5 ng cm−2 yr.−1). However, even if Hg-concentrations increased also in association with the introduction of fir and beech (173 ng g−1), as a result of higher Hg:C, there was no increase in Hg-accumulation rates (7.6 ng cm−2 yr.−1), because of a decreased input of OM. At around 2500 years BP Hgaccumulation rates and Hg-concentration indicated an additional input of Hg to the sediment (316 ng g−1 and 10.3 ng cm−2 yr.−1),which might be due to increased human activities in the area, e.g., forest burning or mining. Our results contrast those of several paired-catchment studies that suggest a higher release of Hg fromconiferous