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

NO, 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 NO 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 NO 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 NO emission in a pH gradient from 4.5 to 8.5. The collapsing edge had the highest NO emission from the peat suspensions among all points on the transect under natural acidic conditions (pH 4.5). The NO 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, NO-emitting bacteria different from those present in acidic conditions appeared to emit NO. The bacterial communities could be characterized by changing pH conditions after thawing and collapse of permafrost have contrasting impacts on NO production that calls for further attention in future studies.