Mercury accumulation in grass and forb species as a function of atmospheric carbon dioxide concentrations and mercury exposures in air and soil

The goal of this study was to investigate the potential for atmospheric Hg° uptake by grassland species as a function of different air and soil Hg exposures, and to specifically test how increasing atmospheric CO2 concentrations may influence foliar Hg concentrations. Four common tallgrass prairie species were germinated and grown for 7 months in environmentally controlled chambers using two different atmospheric elemental mercury (Hg°; 3.7 ± 2.0 and 10.2 ± 3.5 ng m−3), soil Hg (<0.01 and 0.15 ± 0.08 μg g−1), and atmospheric carbon dioxide (CO2) (390 ± 18, 598 ± 22 μmol mol−1) exposures. Species used included two C4 grasses and two C3 forbs. Elevated CO2 concentrations led to lower foliar Hg concentrations in plants exposed to low (i.e., ambient) air Hg° concentrations, but no CO2 effect was apparent at higher air Hg° exposure. The observed CO2 effect suggests that leaf Hg uptake might be controlled by leaf physiological processes such as stomatal conductance which is typically reduced under elevated CO2. Foliar tissue exposed to elevated air Hg° concentrations had higher concentrations than those exposed to low air Hg°, but only when also exposed to elevated CO2. The relationships for foliar Hg concentrations at different atmospheric CO2 and Hg° exposures indicate that these species may have a limited capacity for Hg storage; at ambient CO2 concentrations all Hg absorption sites in leaves may have been saturated while at elevated CO2 when stomatal conductance was reduced saturation may have been reached only at higher concentrations of atmospheric Hg°. Foliar Hg concentrations were not correlated to soil Hg exposures, except for one of the four species (Rudbeckia hirta). Higher soil Hg concentrations resulted in high root Hg concentrations and considerably increased the percentage of total plant Hg allocated to roots. The large shifts in Hg allocation patterns—notably under soil conditions only slightly above natural background levels—indicate a potentially strong role of plants in belowground Hg transformation and cycling processes.