Application of controlled mesocosms for understanding mercury air-soil-plant exchange

Whole system elemental mercury (Hg0) flux was measured for approximately 1.5 years using two large gas exchange mesocosms containing approximately 100 two-year old aspen trees (Populus tremuloides) planted in soil with elevated mercury concentrations (12.3 microg/g). We hypothesized that during leafout, whole mesocosm Hg0 flux would increase due to movement of Hg0 in the transpiration stream from the soil to the air. This hypothesis was not supported; plants were found to assimilate Hg0 from the contaminated air, and whole system Hg0 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 Hg0 flux, with light being a more significant factor. Although surface soils were maintained at 15-20% moisture, daily watering caused pulses of Hg0 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.