Foliar exchange of mercury as a function of soil and air mercury concentrations

Previous research has indicated that foliar mercury (Hg) flux is bi-directional, with influence from both atmospheric and soil Hg. This work investigated the role of soil and air Hg concentrations on foliar Hg exchange using a single-plant gas-exchange system. The exchange of Hg vapor with aspen seedlings grown in soil Hg concentrations of 0.03±0.01, 5.8±0.5, and 12.3±1.3 μg g−1 and exposed to atmospheric Hg concentrations of 2.4±0.5, 11.0±0.9, and 30.4±2.2 ng m−3 was measured. At background atmospheric Hg concentrations of 2.4 ng m−3, foliage released Hg at all three soil Hg concentrations and fluxes ranged from 1.6 to 5.5 ng/m2/h. At higher atmospheric Hg concentrations (>11 ng m−3), net deposition to foliage ranged from −9 to −47 ng/m2/h, exhibiting increase uptake with higher air Hg concentrations. Fluxes associated with aspen showed an immediate response to changes in atmospheric Hg concentrations. Compensation points, the air concentration where no net flux of Hg vapor occurred, were 3–4 ng m−3 in the light and 2–3 ng m−3 in the dark for trees grown in soils of 0.03 and 6 μg g−1 Hg content, and 5–6 ng m−3 in the light and 2.5–3.5 ng m−3 in the dark for trees grown in 12 μg g−1 Hg soils.