Ammonia-oxidizing bacteria are the primary N O producers in an ammonia-oxidizing archaea dominated alkaline agricultural soil.

Most agricultural N O emissions are a consequence of microbial transformations of nitrogen (N) fertilizer, and mitigating increases in N O emission will depend on identifying microbial sources and variables influencing their activities. Here, using controlled microcosm and field studies, we found that synthetic N addition in any tested amount stimulated the production of N O from ammonia-oxidizing bacteria (AOB), but not archaea (AOA), from a bioenergy crop soil. The activities of these two populations were differentiated by N treatments, with abundance and activity of AOB increasing as nitrate and N O production increased. Moreover, as N O production increased, the isotopic composition of N O was consistent with an AOB source. Relative N O contributions by both populations were quantified using selective inhibitors and varying N availability. Complementary field analyses confirmed a positive correlation between N O flux and AOB abundance with N application. Collectively, our data indicate that AOB are the major N O producers, even with low N addition, and that better-metered N application, complemented by selective inhibitors, could reduce projected N O emissions from agricultural soils.