Evidence of microbial control of Hg 0 emissions from uncontaminated terrestrial soils

It is known that in wetland soils and soils contaminated with mercury (Hg), direct biotic reduction of Hg 2+ to Hg 0 leads to Hg 0 emissions to the atmosphere. In terrestrial soils, numerous factors have been reported that control Hg 0 emissions, but it is still unclear if biotic processes are also important. In this study, microbiological activity of Cambisol monoliths from a subalpine grassland with Hg concentrations of approx. 100 ng g –1 were manipulated in laboratory incubation experiments. Elemental Hg emissions were recorded together with CO 2 emission rates as proxy for microbiological respiration. Emissions of Hg 0 increased from approx. 5 ng m –2 h –1 up to 130 ng m –2 h –1 with stimulated biological activity (glucose addition, increase in temperature) and decreased with inhibited activity (chloroform fumigation, autoclaving, drying). Similar patterns with evasion rates of >90 ng m –2 h –1 were observed after dried soils were remoistened again. Our results indicated that processes leading to Hg 0 emissions from uncontaminated terrestrial soils are at least partly controlled by biotic processes. However, it is still uncertain if Hg 0 emission is caused directly by biotic reduction of Hg 2+ or indirectly by abiotic reduction, induced by products of microbiological degradation, e.g. , humic acids.