Activity of Type I Methanotrophs Dominates under High Methane Concentration: Methanotrophic Activity in Slurry Surface Crusts as Influenced by Methane, Oxygen, and Inorganic Nitrogen

Livestock slurry is a major source of atmospheric methane (CH4), but surface crusts harboring methane-oxidizing bacteria (MOB) could mediate against CH4 emissions. This study examined conditions for CH4 oxidation by in situ measurements of oxygen (O-2) and nitrous oxide (N2O), as a proxy for inorganic N transformations, in intact crusts using microsensors. This was combined with laboratory incubations of crust material to investigate the effects of O-2, CH4, and inorganic N on CH4 oxidation, using (CH4)-C-13 to trace C incorporation into lipids of MOB. Oxygen penetration into the crust was 2 to 14 mm, confining the potential for aerobic CH4 oxidation to a shallow layer. Nitrous oxide accumulated within or below the zone of O-2 depletion. With 10(2) ppmv CH4 there was no O-2 limitation on CH4 oxidation at O-2 concentrations as low as 2%, whereas CH4 oxidation at 10(4) ppmv CH4 was reduced at <= 5% O-2. As hypothesized, CH4 oxidation was in general inhibited by inorganic N, especially NO2-, and there was an interaction between N inhibition and O-2 limitation at 10(2) ppmv CH4, as indicated by consistently stronger inhibition of CH4 oxidation by NH4+ and NO3- at 3% compared with 20% O-2. Recovery of C-13 in phospholipid fatty acids suggested that both Type I and Type II MOB were active, with Type I dominating highconcentration CH4 oxidation. Given the structural heterogeneity of crusts, CH4 oxidation activity likely varies spatially as constrained by the combined effects of CH4, O-2, and inorganic N availability in microsites.