Contamination by mercury affects methane oxidation capacity of aerobic arable soils

Effects of long term Hg contamination and addition of Hg2+ on soil CH4 oxidation were investigated in laboratory incubation experiments. Non contaminated soils had CH4 oxidation rate constants (k) between 5.2 and 24.5 x 10-3 h-1, whereas Hg contaminated soils had rates ranging from 2.4 to 8.0 x 10-3 h-1. Specific oxidation rates (qK=k/Cbio), normalized with respect to soil microbial biomass C (Cbio), allowed complete separation between the two groups and ranged between 32.6 and 56.2 x 10-6 h-1 μg-1 Cbio in non contaminated soils and between 6.5 and 14.3 x 10-6 h-1 μg-1 Cbio in long-term Hg contaminated soils. To test short-term toxicity effects and eventual adaptation of methane oxidizers, Hg2+ was added at two levels, 50 and 100 μg g-1, to two soils, one non contaminated and the other contaminated for centuries by Hg, having about the same Cbio. Methane oxidation capacity and Cbio of the soil exposed for centuries to high levels of Hg, were not affected by Hg2+ additions. In the non contaminated soil Cbio was not affected, but methane oxidation rate decreased from 21.4 to 9.8 x 10-3 h-1. Our results show that Hg contamination, either long-term or short term, may negatively impair the CH4 oxidizing capacity of aerobic arable soils.