Ammonia oxidation pathways and nitrifier denitrification are significant sources of N 2 O and NO under low oxygen availability

The continuous increase of nitrous oxide (N 2 O) abundance in the atmosphere is a global concern. Multiple pathways of N 2 O production occur in soil, but their significance and dependence on oxygen (O 2 ) availability and nitrogen (N) fertilizer source are poorly understood. We examined N 2 O and nitric oxide (NO) production under 21%, 3%, 1%, 0.5%, and 0% (vol/vol) O 2 concentrations following urea or ammonium sulfate [(NH 4 ) 2 SO 4 ] additions in loam, clay loam, and sandy loam soils that also contained ample nitrate. The contribution of the ammonia (NH 3 ) oxidation pathways (nitrifier nitrification, nitrifier denitrification, and nitrification-coupled denitrification) and heterotrophic denitrification (HD) to N 2 O production was determined in 36-h incubations in microcosms by 15 N- 18 O isotope and NH 3 oxidation inhibition (by 0.01% acetylene) methods. Nitrous oxide and NO production via NH 3 oxidation pathways increased as O 2 concentrations decreased from 21% to 0.5%. At low (0.5% and 3%) O 2 concentrations, nitrifier denitrification contributed between 34% and 66%, and HD between 34% and 50% of total N 2 O production. Heterotrophic denitrification was responsible for all N 2 O production at 0% O 2 . Nitrifier denitrification was the main source of N 2 O production from ammonical fertilizer under low O 2 concentrations with urea producing more N 2 O than (NH 4 ) 2 SO 4 additions. These findings challenge established thought attributing N 2 O emissions from soils with high water content to HD due to presumably low O 2 availability. Our results imply that management practices that increase soil aeration, e.g., reducing compaction and enhancing soil structure, together with careful selection of fertilizer sources and/or nitrification inhibitors, could decrease N 2 O production in agricultural soils.