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The continuous increase of nitrous oxide (N2 O) abundance in the atmosphere is a global concern. Multiple pathways of N2 O production occur in soil, but their significance and dependence on oxygen (O2 ) availability and nitrogen (N) fertilizer source are poorly understood. We examined N2 O and nitric oxide (NO) production under 21%, 3%, 1%, 0.5%, and 0% (vol/vol) O2 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 N2 O production was determined in 36-h incubations in microcosms by15 N-18 O isotope and NH3 oxidation inhibition (by 0.01% acetylene) methods. Nitrous oxide and NO production via NH3 oxidation pathways increased as O2 concentrations decreased from 21% to 0.5%. At low (0.5% and 3%) O2 concentrations, nitrifier denitrification contributed between 34% and 66%, and HD between 34% and 50% of total N2 O production. Heterotrophic denitrification was responsible for all N2 O production at 0% O2 . Nitrifier denitrification was the main source of N2 O production from ammonical fertilizer under low O2 concentrations with urea producing more N2 O than (NH4 )2 SO4 additions. These findings challenge established thought attributing N2 O emissions from soils with high water content to HD due to presumably low O2 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 N2 O production in agricultural soils.

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

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