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 (O₂) 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 O₂, CH₄, and inorganic N on CH₄ oxidation, using ¹³CH₄ 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 O₂ depletion. With 102 ppmv CH₄ there was no O₂ limitation on CH₄ oxidation at O₂ concentrations as low as 2%, whereas CH₄ oxidation at 104 ppmv CH₄ was reduced at =5% O₂. As hypothesized, CH₄ oxidation was in general inhibited by inorganic N, especially NO₂-, and there was an interaction between N inhibition and O₂ limitation at 102 ppmv CH₄, as indicated by consistently stronger inhibition of CH₄ oxidation by NH₄ ⁺ and NO₃ ^- at 3% compared with 20% O₂. Recovery of ¹³C in phospholipid fatty acids suggested that both Type I and Type II MOB were active, with Type I dominating high-concentration CH₄ oxidation. Given the structural heterogeneity of crusts, CH₄ oxidation activity likely varies spatially as constrained by the combined effects of CH₄, O₂, and inorganic N availability in microsites.