Experimental evidence against diffusion control of Hg evasion from soils.

Elemental Hg (Hg(0)) evolution from soils can be an important process and needs to be measured in more ecosystems. The diffusion model for soil gaseous efflux has been applied to modeling the fluxes of several gases in soils and deserves testing with regard to Hg(0). As an initial test of this model, we examined soil gaseous Hg(0) and CO(2) concentrations at two depths (20 and 40 cm) over the course of a controlled environment study conducted in the EcoCELLs at the Desert Research Institute in Reno, Nevada. We also compared small, spatially distributed gas wells against the more commonly used large gas wells. In this study, two EcoCELLs were first watered (June 2000) and then planted (July 2000) with trembling aspen (Populus tremuloides). Following that, trees were harvested (October 2000) and one EcoCELL (EcoCELL 2) was replanted with aspen (25 April 2001). During most of the experiment, there was a strong vertical gradient of CO(2) (increasing with depth, as is typical of a diffusion-driven process), but no vertical gradient of soil gaseous Hg(0). Strong diel variations in soil gas Hg(0) concentration were noted, whereas diel variations in CO(2) were small and not statistically significant. Initial watering and planting caused increases in both soil gas CO(2) and Hg(0). Replanting in EcoCELL 2 caused a statistically significant increase in soil gas CO(2) but not Hg(0). Calculated Hg(0) effluxes using the diffusion model produced values two orders of magnitude lower than those measured using field chambers placed directly on the soil or whole-cell fluxes. Neither soil gas Hg(0) concentrations nor calculated fluxes were correlated with measured Hg(0) efflux from soil or from whole EcoCELLs. We conclude that (1) soil gas Hg(0) flux is not diffusion-driven and thus soil gas Hg(0) concentrations cannot be used to calculated soil Hg(0) efflux; (2) soil gas Hg(0) concentrations are increased by watering dry soil, probably because of displacement/desorption processes; (3) soil gas Hg(0) concentrations were unaffected by plants, suggesting that roots and rhizosphere processes are unimportant in controlling Hg(0) evasion from the soil surface. We recommend the use of the small wells in all future studies because they are much easier to install and provide more resolution of spatial and temporal patterns in soil gaseous Hg(0).

Investigation of effects of trifluoroacetate on vernal pool ecosystems.

Trifluoroacetate (CH3COO-, TFA) is a breakdown product of hydrochlorofluorocarbons and hydrofluorocarbons and is released by the heating of Teflon products. Because of its chemical properties, concentrations in evaporative wetlands are predicted to increase with time. This study focused on assessing the impact of this haloacetic acid on vernal pool soil microbial communities as well as vernal pool and wetland plant species. Microbial respiration for three vernal pool soils and an agricultural soil was not affected by TFA exposures (0, 10, 100, 1,000, and 10,000 microg/L), and degradation of TFA by microbial communities was not observed in soils incubated for three months. Trifluoroacetate accumulated in foliar tissue of wetland plant species as a function of root exposure concentration (100 and 1,000 microg/L TFA), and accumulation was found to stabilize or decrease after the second or third month of exposure. Seeds accumulated TFA as a function of root exposure concentration; however, germination success was not affected. No adverse physiological responses, including general plant health and photosynthetic and conductance rates, were observed for root exposures at the TFA concentrations used in this study. Based on the soils and plant species used in this study, predicted TFA concentrations will not adversely affect the development of soil microbial communities and vernal pool plant species.