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 (CH 4 ), but surface crusts harboring methane‐oxidizing bacteria (MOB) could mediate against CH 4 emissions. This study examined conditions for CH 4 oxidation by in situ measurements of oxygen (O 2 ) and nitrous oxide (N 2 O), 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 , CH 4 , and inorganic N on CH 4 oxidation, using 13 CH 4 to trace C incorporation into lipids of MOB. Oxygen penetration into the crust was 2 to 14 mm, confining the potential for aerobic CH 4 oxidation to a shallow layer. Nitrous oxide accumulated within or below the zone of O 2 depletion. With 10 2 ppmv CH 4 there was no O 2 limitation on CH 4 oxidation at O 2 concentrations as low as 2%, whereas CH 4 oxidation at 10 4 ppmv CH 4 was reduced at ≤5% O 2 . As hypothesized, CH 4 oxidation was in general inhibited by inorganic N, especially NO 2 – , and there was an interaction between N inhibition and O 2 limitation at 10 2 ppmv CH 4 , as indicated by consistently stronger inhibition of CH 4 oxidation by NH 4 + and NO 3 – at 3% compared with 20% O 2 . Recovery of 13 C in phospholipid fatty acids suggested that both Type I and Type II MOB were active, with Type I dominating high‐concentration CH 4 oxidation. Given the structural heterogeneity of crusts, CH 4 oxidation activity likely varies spatially as constrained by the combined effects of CH 4 , O 2 , and inorganic N availability in microsites. Core Ideas Oxygen penetration into surface crusts is shallow. Nitrous oxide accumulates at oxic–anoxic interfaces in surface crusts. Oxygen availability is important to high‐concentration CH 4 oxidation. Microbial CH 4 oxidation is affected by interactions of inorganic N and O 2 . Activity of Type I methanotrophs dominates under high CH 4 concentration.