Ammonia oxidation pathways and nitrifier denitrification are significant sources of N2O and NO under low oxygen availability

The continuous increase of nitrous oxide (N2O) abundance in the atmosphere is a global concern. Multiple pathways of N2O production occur in soil, but their significance and dependence on oxygen (O-2) availability and nitrogen (N) fertilizer source are poorly understood. We examined N2O and nitric oxide (NO) production under 21%, 3%, 1%, 0.5%, and 0% (vol/vol) O-2 concentrations following urea or ammonium sulfate [(NH4)(2)SO4] additions in loam, clay loam, and sandy loam soils that also contained ample nitrate. The contribution of the ammonia (NH3) oxidation pathways (nitrifier nitrification, nitrifier denitrification, and nitrification-coupled denitrification) and heterotrophic denitrification (HD) to N2O production was determined in 36-h incubations in microcosms by N-15-O-18 isotope and NH3 oxidation inhibition (by 0.01% acetylene) methods. Nitrous oxide and NO production via NH3 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 N2O production. Heterotrophic denitrification was responsible for all N2O production at 0% O-2. Nitrifier denitrification was the main source of N2O production from ammonical fertilizer under low O-2 concentrations with urea producing more N2O than (NH4)(2)SO4 additions. These findings challenge established thought attributing N2O 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 N2O production in agricultural soils.