Air-Surface exchange of elemental mercury in uncontaminated grasslands : determination of fluxes and identification of forcing factors with micrometeorological methods and controlled laboratory studies

The burning of fossil fuels, incineration of waste, smelting of metals and other industrial processes and applications have been adding considerable amounts of mercury to the atmosphere. Of the total atmospheric mercury, Hg0 (elemental mercury) represents more than 95%, a species which is highly volatile and dispersed globally. Eventually Hg0 is transformed and deposited to land and sea where various processes may produce organic mercury species that have the power to bioaccumulate to levels that are toxic for humans. In Europe and North America mission controls are in place for more than two decades and have reduced mercury emissions substantially. However, due to large uncertainties in global emission estimates and uncertainties regarding the potential of different ecosystems to act as sources or sinks for atmospheric mercury, it is yet not known if deposited Hg0 is stored permanently in soils and if the atmospheric pool is actually reduced. Attempts to estimate the magnitude of the air-surface mercury exchange have focused on polluted sites, boreal regions and arid zones of North America. In contrast, uncontaminated, continental regions of the temperate climate belt haven’t received much attention and respective studies have been mostly limited to spot measurements with flux chambers. The first objective of our study was to describe and evaluate the influence of microbiological activity on the emission of Hg0 from terrestrial background soils. It has been discussed that apart from physically and chemically mediated Hg0 emission, microbial activity might contribute to the emission flux. The importance of this contribution in uncontaminated terrestrial soils is still unclear. Under controlled laboratory conditions it was tested how stimulation and inhibition of microbial activity would affect Hg0 emissions. This was done by comparing sterilised with intact soil samples in an incubation chamber and investigating the response of Hg0 emissions to environmental variables such as temperature and soil moisture. The results of these experiments showed consistent changes of Hg0 emissions with stimulation and inhibition of microbiological activity. Stimulatory effects were observed after addition of glucose, after inoculation of sterilised soil as well as upon temperature shifts and re-moistening of dried samples. We conclude that Hg0 emissions from uncontaminated, terrestrial soils are partly controlled by microbiological activity. Microorganisms might reduce Hg2+ either directly in order to detoxify their immediate environment, or they might indirectly induce Hg0 evasion by producing reductive soil compounds such as humic and fulvic acids.
To obtain a comprehensive picture of elemental mercury exchange of background
areas we performed measurements on an ecosystem scale at three temperate lowland
and subalpine grassland sites. A subalpine meadow at Fruebuel in central Switzerland
was chosen to record the seasonal cycle of the Hg0 exchange and with two additional
sites in Oensingen/Switzerland and Neustift/Austria the spacial variability
was addressed. By measuring concentration gradients, fluxes of elemental mercury
and CO2 were estimated by application of two micrometeorological methods – the flux
gradient method and the modified Bowen ratio method. Due to the low atmospheric
concentrations (between 1.2 and 1.7 ng m