Trace amounts of O2 affect NO and N2O production during denitrifying enzyme activity (DEA) assays

We evaluated the potential of trace amounts of O₂ inadvertently introduced into anaerobic incubations to initiate the C₂H₂-catalyzed NO oxidation reaction and affect NO and N₂O production rates during denitrifying enzyme activity (DEA) assays. We measured the rate of NO production in the presence and absence of 10 kPa C₂H₂ and N₂O production in the presence of 10 kPa C₂H₂ by short-term incubations of slurries of humisol and sandy loam soil. NO production, in the absence of C₂H₂, was similar for both soils (0.73-1.32 ng NO-N g dry soil⁻¹ min⁻¹) and replicate measurements of NO production rates were linear and exhibited small standard deviations. It was not possible to consistently measure NO production in the presence of C₂H₂. Replicate measurements of NO production were always lower and exhibited a wide range of variability when C₂H₂ was present. The rate of NO production in the absence of C₂H₂ was only 2.3-3.0% of the rate of N₂O production in the presence of C₂H₂ in humisol soil but was much larger (31.8-35.0%) in the sandy loam soil. Rates of NO production from sandy loam soil were reduced by 58% when trace amounts (30 μl) of O₂ were added to slurry incubations containing C₂H₂. We conclude that trace amounts of O₂ inadvertently introduced into the slurries during sampling could initiate scavenging of NO by the C₂H₂-catalyzed NO oxidation reaction and cause an underestimate of N₂O production during DEA assays in some soils.