their bromide derivatives by acidic potassium bromide. (KBr) and copper sulfate (CuSO4) before subsequent ex- traction into a small volume of dichloromethane.
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 113, D10307, doi:10.1029/2007JD009510, 2008
Mercury concentrations and foliage/atmosphere fluxes in a maple forest ecosystem in Que´bec, Canada Laurier Poissant,1,2 Martin Pilote,1 Emmanuel Yumvihoze,3 and David Lean3 Received 18 October 2007; revised 19 December 2007; accepted 6 February 2008; published 31 May 2008.
 This paper presents mercury (Hg) concentrations and foliage/atmosphere fluxes in
a maple forest ecosystem in southern Que´bec, Canada. The average total gaseous mercury (TGM) concentration measured at an open field site was significantly (p < 0.001) higher than that measured at an adjacent maple forest site, some 300 m away (1.40 ng m3 versus 1.03 ng m3, respectively). Foliage/atmosphere flux of TGM, measured with a dynamic flux bag device, indicated an average deposition flux of 0.39 ± 0.38 ng m2 h1 in the maple tree foliage. Although, bi-directional Hg fluxes were observed, the compensation point in the maple forest was low (0.6 ng m3) explaining a net Hg deposition process at TGM background levels. Foliage mercury concentrations increased from 8.7 ± 1.5 ng g1 to 30.8 ± 3.0 ng g1 during the leaf-growing season. The average Hg deposition flux was in a similar range to the Hg accumulation rate in the maple tree foliage assuming a foliage productivity rate of 220 g m2 a1 (i.e., 0.39 versus 0.55 ng m2 h1). Although significant Hg accumulated in foliage, its transport to non-organic surface topsoil was not significant. When the uptake of mercury measured in this experiment was extrapolated to include all of the maple forest in North America (12.5 million ha), it amounted potentially to more than 600 kg of mercury per year. This represents approximately 0.5% of mercury emissions in Canada and the United States (approximately 120 metric tons). Citation: Poissant, L., M. Pilote, E. Yumvihoze, and D. Lean (2008), Mercury concentrations and foliage/atmosphere fluxes in a maple forest ecosystem in Que´bec, Canada, J. Geophys. Res., 113, D10307, doi:10.1029/2007JD009510.
1. Introduction  Mercury (Hg) is a bio-accumulative global pollutant [Fitzgerald, 1993] of environmental concern due to its high toxicity. A number of studies have shown that natural surfaces (e.g., soil, water, and vegetation) contribute to a considerable amount of mercury cycling in the global pool [e.g., Poissant and Casimir, 1998; Gustin et al., 2000; Lindberg et al., 2002]. Almost 30% of the Earth’s land is covered by vegetation (4 109 ha) [Lindberg et al., 1998]. Understanding the role of plants in atmospheric mercury cycling is, therefore, important in assessing the mercury fates on regional and global scales.  The fate of mercury in vegetation is widely debated but the general understanding indicates that it is largely dependent of plant species [e.g., Millhollen et al., 2006a, 2006b; Greger et al., 2005], the mercury concentrations in soil and in air [e.g., Fay and Gustin, 2007; Millhollen et al., 2006a; Hanson et al., 1997].  Three potential pathways for mercury movement into plants foliage are suggested [Millhollen et al., 2006b]: 1 Science and Technology Branch, Environment Canada, Montre´al, Que´bec, Canada. 2 Department of Earth Sciences, University of Ottawa, Ottawa, Ontario, Canada. 3 Department of Biology, University of Ottawa, Ottawa, Ontario, Canada.
Published in 2008 by the American Geophysical Union.
(1) Mercury transport from soil to foliage through the xylem-sap [e.g., Bishop et al., 1998]; (2) Mercury assimilation into plant tissues (cuticular) either from aerosol and precipitation deposition [e.g., St. Louis et al., 2001]; and (3) Bi-directional mercury air-foliage gas exchange through stomata [e.g., Fleck et al., 1999; Rea et al., 2001, 2002; Poissant et al., 2003, 2004; Graydon et al., 2006].  Ionic and organic forms of mercury might be transported from soil to foliage through the xylem-sap [e.g., Bishop et al., 1998]. Battke et al.  suggested that once in the foliage, some of the ionic mercury might be emitted to the atmosphere due to a reduction reaction with ascorbate, an antioxidant synthesized by the plants. However, with the exception of vegetation growing on highly contaminated soil, the xylem contribution to mercury content in the green or leafy part of plants should be minor compared to that of cuticular mercury uptake from dry/wet deposition [Bishop et al., 1998] or by direct uptake of gaseous mercury through stomata. In other words, even if plants are able to take up Hg from the soil to the roots, the absorbed mercury is not translocated from roots to leaves in any significant amount relative to the available Hg in the root zone [Greger et al., 2005]. Indeed, it has been suggested that the root zone acts as a barrier against mercury entering the intracellular ways. Mercury is generally bound at the acid sites in organic matter. The most commonly encountered acidic functional groups in dissolved organic matter (DOM) include carboxylic acids, phenols, ammonium ions, alcohols,
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and thiols [Ravichandran, 2004]. The possible formation of ligands between mercury and DOM has been suggested as a potential biochemical barrier to the upward translocation of Hg from the root zone [Siegel et al., 1987; Milla´n et al., 2006]. Further works also suggested that roots can effectively uptake Hg and MeHg by sequestration into two classes of cysteine-rich peptides: the metallothioneins and phytochelatins. The mechanism involves Hg binding with organic sulfur (R-SH) groups on the cysteine residues in these peptides being glutathionated and transported into vacuoles for long-term sequestration [Wang and Greger, 2004; Zenk, 1996].  Recently, Millhollen et al. [2006a, 2006b] and Fay and Gustin  demonstrated under environmentally controlled, closed-system growth chambers that foliar Hg concentrations were found to be influenced primarily by atmospheric Hg concentrations and to a lesser extent by soil Hg exposures. Their results also suggested that deciduous species might play a more active role in ecosystem Hg cycling than evergreen trees.  Accordingly, under background conditions, the atmosphere is almost the exclusive source of Hg in foliage [Zhang et al., 2005; Poissant et al., 2004; Ericksen et al., 2003; Fleck et al., 1999]. It has been suggested that gaseous elemental Hg may also migrate into and out of the plant stomates according to the so-called ‘‘atmospheric compensation point’’ (i.e., the atmospheric TGM concentration at which no net exchange of TGM occurs) [Hanson et al., 1995]. Hanson et al.  reported, from laboratory observations, for four tree species growing on low Hg containing soil (