Nitrous Oxide Emission in Response to pH from Degrading Palsa Mire Peat Due to Permafrost Thawing

N₂O, a greenhouse gas, is increasingly emitted from degrading permafrost mounds of palsa mires because of the global warming effects on microbial activity. In the present study, we hypothesized that N₂O emission could be affected by a change in pH conditions because the collapse of acidic palsa mounds (pH 3.4-4.6) may result in contact with minerogenic ground water (pH 4.8-6.3), thereby increasing the pH. We compared the effects of pH change on N₂O emission from cultures inoculated with peat suspensions. Peat samples were collected on a transect from a still intact high part to the collapsing edge of a degrading palsa mound in northwestern Finland, assuming the microbial communities could be different. We adjusted the pH of peat suspensions prepared from a collapsing palsa mound and compared the N₂O emission in a pH gradient from 4.5 to 8.5. The collapsing edge had the highest N₂O emission from the peat suspensions among all points on the transect under natural acidic conditions (pH 4.5). The N₂O emission was reduced with a moderate rise in pH (pH 5.0-6.0) by approximately 85% compared with natural acidic level (pH 4.5). The bacterial communities in acidic cultures differed considerably from those in alkaline cultures. When pH was adjusted to alkaline conditions, N₂O-emitting bacteria different from those present in acidic conditions appeared to emit N₂O. The bacterial communities could be characterized by changing pH conditions after thawing and collapse of permafrost have contrasting impacts on N₂O production that calls for further attention in future studies.