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 Hg2+ 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 grass land with Hg concentrations of approx. 100 ng g(-1) were manipulated in laboratory incubation experiments. Elemental Hg emissions were recorded together with CO2 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 Hg2+ or indirectly by abiotic reduction, induced by products of microbiological degradation, e.g., humic acids.