Accumulation of atmospheric mercury in forest foliage

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 (12.3±1.3 μg g−1), 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−1. 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 (0.03±0.01 μg g−1) 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 −3.3 ng m−2 h−1. 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.