Uncoupling of ammonia oxidation from nitrite oxidation: Impact upon nitrous oxide production in non-cropped Oregon soils

• Soil AOA- and AOB-driven NO2− accumulation represented 8-100% of total nitrification.
• Both AOA- and AOB-driven N2O production were dependent on NO2− accumulation.
• Enhancing soil NO2- oxidizing capacity prevents NO2− accumulation and N2O production.
• AOA- and AOB-driven N2O yields were influenced by NO2− concentration.
• Kinetic parameters were determined for N2O production by AOA and AOB.

The factors controlling the relative contributions of ammonia- (NH3) oxidizing archaea (AOA) and bacteria (AOB) to nitrification and nitrous oxide (N2O) production in soil remain unclear. A study was conducted to examine the contributions of AOA and AOB to nitrification, nitrite (NO2−) accumulation, and NO2−-affected N2O production in three non-cropped Oregon soils. Nitrification potential rates in the three soils ranged seven-fold from 0.15 to 1.08 μmol N g−1 d−1, with AOA contributing 64–71% of the total activity. AOA- and AOB-driven NO2− accumulation represented 8–100% of total NO2− + NO3− accumulation, persisted over 48 h, and was accompanied by acetylene-sensitive, ammonium- (NH4+) stimulated N2O production. Ammonium- and NO2−-dependent N2O production occurred when both AOA and AOB, or AOA alone were active. By adding the NO2−-oxidizing bacteria, Nitrobacter vulgaris, to soil slurries to increase NO2−-oxidizing capacity, both NO2− accumulation and N2O production were prevented, while the overall rate of nitrification was unaffected. Yields of N2O-N amounted to 0.05 ± 0.01% of total NO2− + NO3−-N accumulation in the presence of supplemental NH4+, and 0.28 ± 0.11% in the presence of both supplemental NH4+ + NO2−. Regression analysis of the N2O production against NO2− accumulation over 24 h revealed a positive, non-linear relationship for N2O production by both AOA plus AOB and by AOA alone. Values of Vmax ranged 12-fold from 0.05 to 0.62 nmol N2O g−1 d−1, and predicted Km values for NO2− ranged 15-fold from 0.02 to 0.30 μmol NO2− g−1 soil. These findings provide new insights into the impact of NO2− accumulation in soils on N2O production by both AOA and AOB, and show that NO2− accumulation primarily drives N2O formation in these soils, and increases N2O yield by both AOA and AOB.