Isotopic signatures of N2O produced by ammonia-oxidizing archaea from soils

N₂O gas is involved in global warming and ozone depletion. The major sources of N₂O are soil microbial processes. Anthropogenic inputs into the nitrogen cycle have exacerbated these microbial processes, including nitrification. Ammonia-oxidizing archaea (AOA) are major members of the pool of soil ammonia-oxidizing microorganisms. This study investigated the isotopic signatures of N₂O produced by soil AOA and associated N₂O production processes. All five AOA strains (I.1a, I.1a-associated and I.1b clades of Thaumarchaeota) from soil produced N₂O and their yields were comparable to those of ammonia-oxidizing bacteria (AOB). The levels of site preference (SP), δ¹⁵N^bulk and δ¹⁸O -N₂O of soil AOA strains were 13–30%, −13 to −35% and 22–36%, respectively, and strains MY1–3 and other soil AOA strains had distinct isotopic signatures. A ¹⁵N-NH₄⁺-labeling experiment indicated that N₂O originated from two different production pathways (that is, ammonia oxidation and nitrifier denitrification), which suggests that the isotopic signatures of N₂O from AOA may be attributable to the relative contributions of these two processes. The highest N₂O production yield and lowest site preference of acidophilic strain CS may be related to enhanced nitrifier denitrification for detoxifying nitrite. Previously, it was not possible to detect N₂O from soil AOA because of similarities between its isotopic signatures and those from AOB. Given the predominance of AOA over AOB in most soils, a significant proportion of the total N₂O emissions from soil nitrification may be attributable to AOA.