Mercury and plants in contaminated soils: 2. Environmental and physiological factors governing mercury flux to the atmosphere

The objective of this study was to evaluate the role of physiological and environmental factors in governing the flux of elemental mercury from plants to the atmosphere. Five species (Lepidium latifolium, Artemisia douglasiana, Caulanthus sp., Fragaria vesca, and Eucalyptus globulus) with different ecological and physiological attributes and growing in soils with high levels of mercury contamination (34 to 54 μg/g) were examined. Studies were conducted in a whole‐plant, gas‐exchange chamber providing precise control of environmental conditions, and mercury flux was estimated using the mass balance approach. Mercury flux increased linearly as a function of temperature within the range of 20 to 40°C, and the mean temperature coefficient (Q10) was 2.04. The temperature dependence of mercury flux was attributed to changes in the contaminant's vapor pressure in the leaf interior. Mercury flux from foliage increased linearly as a function of irradiance within the range of 500 to 1,500 μmol m/s, and the light enhancement of mercury flux (flux ratio at 1,500:500 μmol m/s) was within a factor of 2.0 to 2.5 for all species. Even though the leaf‐to‐atmosphere diffusive path for mercury vapor from foliage is similar to that of water vapor, stomatal conductance played a secondary role in governing mercury flux. In a quantitative comparison with other studies in both laboratory and field settings, a strong linear relationship is evident between mercury vapor flux and the natural logarithm of soil mercury concentration, and this relationship may have predictive value in developing regional‐ and continental‐scale mercury budgets. The most critical factors governing mercury flux from plants are mercury concentration in the soil, leaf area index, temperature, and irradiance.