Whole system elemental mercury (Hg-0) flux was measured for similar to1.5 years using two large gas exchange mesocosms containing similar to100 two-year old aspen trees (Populus tremuloides) planted in soil with elevated mercury concentrations (12.3 mug/g). We hypothesized that during leafout, whole mesocosm Hg-0 flux would increase due to movement of Hg-0 in the transpiration stream from the soil to the air. This hypothesis was not supported; plants were found to assimilate Hg-0 from the contaminated air, and whole system Hg-0 emissions were reduced as plants leafed-out due to shading of the soil. Surface disturbance, watering, and increases in soil moisture, light, and temperature were all found to increase whole system Hg-0 flux, with light being a more significant factor. Although surface soils were maintained at 15-20% moisture, daily watering caused pulses of Hg-0 to be released from the soil throughout the experiment. Data developed in this experiment suggested that those processes acting on the soil surface are the primary influence on Hg emissions and that the presence of vegetation, which shields soil surfaces from incident light, reduces Hg emissions from enriched soils.