Mechanisms underlying the responses of soil N2O production by ammonia oxidizers to nitrogen addition are mediated by topography in a subtropical forest.

• AOB are primarily responsible for soil N 2 O production. • N addition stimulated AOA–derived N 2 O in the organic horizon of the valley. • N addition suppressed AOA and AOB–derived N 2 O in the organic horizon of the slope. • N addition increased AOB–derived N 2 O in the mineral horizon of the valley. • N addition enhanced AOA and AOB–derived N 2 O in the mineral horizon of the slope. Anthropogenic nitrogen (N) deposition may substantially affect the contributions of ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) to soil nitrous oxide (N 2 O) production. Nevertheless, it is still unclear whether topography modulates the responses of AOA– and AOB–derived N 2 O to elevated N deposition. We conducted an ex-situ experiment with organic and mineral soils collected from the valley and slope, respectively, of a subtropical karst forest with three N addition levels, i.e., 0 (control), 50 and 100 kg N ha−1 year−1 for each topographic position. Soil ecoenzymatic stoichiometry indexes were calculated as indicators of microbial resource limitation. AOA– and AOB–derived N 2 O were distinguished by inhibitors. AOB was primarily responsible for soil N 2 O production under the control regardless of topographic position. For the organic horizon, N addition stimulated AOA–derived N 2 O by 112.5–138.2% in the valley because of increased N mineralization due to alleviated microbial carbon limitation; but suppressed AOA and AOB–derived N 2 O by 40.2–53.5% and 35.6–46.8%, respectively, on the slope because of decreased N mineralization attributed to aggravated microbial phosphorus limitation. For the mineral horizon, N addition enhanced AOB–derived N 2 O by 104.5–143% in the valley because of increased ammonia availability, but stimulated AOA and AOB–derived N 2 O by 149.8–1162.5% and 26–64.5%, respectively, on the slope because of increased N mineralization and ammonia availability owing to aggravated microbial C limitation and alleviated phosphorus limitation. Our results indicate that the mechanisms underlying the impacts of N deposition on soil N 2 O production by ammonia oxidizers are topography–dependent, so that topography-specific niche specialization between AOA and AOB should be integrated into Earth system models in order to better predict soil N 2 O production under elevated atmospheric N deposition. [ABSTRACT FROM AUTHOR]