Archaea produce lower yields of N ₂ O than bacteria during aerobic ammonia oxidation in soil

Nitrogen fertilisation of agricultural soil contributes significantly to emissions of the potent greenhouse gas nitrous oxide (N₂ O), which is generated during denitrification and, in oxic soils, mainly by ammonia oxidisers. Although laboratory cultures of ammonia oxidising bacteria (AOB) and archaea (AOA) produce N₂ O, their relative activities in soil are unknown. This work tested the hypothesis that AOB dominate ammonia oxidation and N₂ O production under conditions of high inorganic ammonia (NH₃ ) input, but result mainly from the activity of AOA when NH₃ is derived from mineralisation. 1-octyne, a recently discovered inhibitor of AOB, was used to distinguish N₂ O production resulting from archaeal and bacterial ammonia oxidation in soil microcosms, and specifically inhibited AOB growth, activity and N₂ O production. In unamended soils, ammonia oxidation and N₂ O production were lower and resulted mainly from ammonia oxidation by AOA. The AOA N₂ O yield relative to nitrite produced was half that of AOB, likely due to additional enzymatic mechanisms in the latter, but ammonia oxidation and N₂ O production were directly linked in all treatments. Relative contributions of AOA and AOB to N₂ O production, therefore, reflect their respective contributions to ammonia oxidation. These results suggest potential mitigation strategies for N₂ O emissions from fertilised agricultural soils.