Archaea produce lower yields of N 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 (N 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 N O, their relative activities in soil are unknown. This work tested the hypothesis that AOB dominate ammonia oxidation and N O production under conditions of high inorganic ammonia (NH ) input, but result mainly from the activity of AOA when NH is derived from mineralisation. 1-octyne, a recently discovered inhibitor of AOB, was used to distinguish N O production resulting from archaeal and bacterial ammonia oxidation in soil microcosms, and specifically inhibited AOB growth, activity and N O production. In unamended soils, ammonia oxidation and N O production were lower and resulted mainly from ammonia oxidation by AOA. The AOA N O yield relative to nitrite produced was half that of AOB, likely due to additional enzymatic mechanisms in the latter, but ammonia oxidation and N O production were directly linked in all treatments. Relative contributions of AOA and AOB to N O production, therefore, reflect their respective contributions to ammonia oxidation. These results suggest potential mitigation strategies for N O emissions from fertilised agricultural soils.