Archaeal nitrification is a key driver of high nitrous oxide emissions from arctic peatlands

Bare peat surfaces created by frost action and wind erosion in permafrost peatlands have been shown to emit high amounts of nitrous oxide (N2O). With global warming, emissions of this highly potent greenhouse gas are expected to increase in Arctic permafrost peatlands. In natural unmanaged soils with low nitrogen deposition, such as Arctic soils, nitrification is the main source of nitrite and nitrate, and thus a key driver of N2O emissions. Here, we investigated nitrification, ammonia oxidizer populations and N2O production in vegetated and bare peat soils from four distant Arctic geographic locations. Through a combination of molecular analyses and group-specific inhibitor assays, we show that ammonia oxidation, the first step in nitrification, is mainly performed by ammonia-oxidizing archaea (AOA). All soils from different locations, including bare peat soils with high N2O emissions, harbored only two AOA phylotypes, including an organism closely related to Ca. Nitrosocosmicus spp. This indicates that high N2O emissions from these ecosystems are primarily fueled by nitrification mediated by very few archaeal species. To our knowledge, arctic peat soils in this study are the first natural environments where high N2O emissions have been linked to AOA. Any changes in archaeal nitrification induced by global warming will therefore impact on N2O emissions from the permafrost peatlands.