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

Abstract

Anthropogenic nitrogen (N) deposition may substantially affect the contributions of ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) to soil nitrous oxide (N2O) production. Nevertheless, it is still unclear whether topography modulates the responses of AOA– and AOB–derived N2O 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 N2O were distinguished by inhibitors. AOB was primarily responsible for soil N2O production under the control regardless of topographic position. For the organic horizon, N addition stimulated AOA–derived N2O 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 N2O 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 N2O by 104.5–143% in the valley because of increased ammonia availability, but stimulated AOA and AOB–derived N2O 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 N2O 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 N2O production under elevated atmospheric N deposition.