QUANTIFIYING FOLIAR MERCURY ACCUMULATION AND EXCHANGE AS A FUNCTION OF ATMOSPHERIC AND SOIL CONCENTRATIONS

Plant foliage has been shown to accumulate mercury (Hg) over time and to exhibit bi-directional atmospheric fluxes of Hg. The dominant source of Hg in foliage, soil or air, at ambient exposure concentrations is not known. This paper presents the results of two experiments investigating plant-air-soil exchange using enclosed, environmentally-controlled, multiple-plant exposure chambers. Mercury accumulation in foliage was assessed using two herbaceous species, Rudbeckia hirta (Blackeyed Susan) and Chamaecrista nictitans (Partridge Pea), and two grass species, Sorghastrum nutans (Indian Grass) and Andropogon gerardii (Big Bluestem), and three tree species (listed below). Herbaceous and grass species were grown in soils of two Hg concentrations (<0.01 and 0.2 +/- 0.1 µg/g), with the higher soil concentration created by amending the low Hg soil with HgCl2. Plants were also exposed to two atmospheric Hg concentrations (3.2 +/- 0.4 and 10.7 +/- 1.4 ng/m3) and to two atmospheric CO2 concentrations (380 +/- 30 and 710 +/- 50 µmol/mol). All exhibited an increase in foliar Hg over time, however a lesser increase was observed for those plants grown in air with elevated CO2.

Tree species Robinia pseudoacacia (Black Locust), Juniperus scopulorum (Rocky Mountain Juniper) and Pinus Ponderosa (Ponderosa Pine) were grown in three soil Hg concentrations (<0.01, 0.1 +/- 0.02 and 0.9 +/- 0.3 µg/g) and exposed to three atmospheric Hg concentrations (6 +/- 2, 14 +/- 3 and 30 +/- 4 ng/m3). Foliar Hg flux was measured for Robinia sp. and Juniperus sp. (n=3 for each air/soil exposure) over 24 hours using a portable gas exchange system. Midday (1100-1400 hr) fluxes for Juniper sp. were extremely low, ranging from –1.3 to 0.3 ng/m2/hr. A wider range of midday fluxes was observed for Robinia sp. ranging from –6.2 to 4.3 ng/m2/hr. No statistically significant relationship between soil and air Hg in relation to foliar Hg flux was determined.