Trace amounts of O 2 affect NO and N 2O production during denitrifying enzyme activity (DEA) assays

Abstract

We evaluated the potential of trace amounts of O 2 inadvertently introduced into anaerobic incubations to initiate the C 2H 2-catalyzed NO oxidation reaction and affect NO and N 2O production rates during denitrifying enzyme activity (DEA) assays. We measured the rate of NO production in the presence and absence of 10 kPa C 2H 2 and N 2O production in the presence of 10 kPa C 2H 2 by short-term incubations of slurries of humisol and sandy loam soil. NO production, in the absence of C 2H 2, 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 C 2H 2. Replicate measurements of NO production were always lower and exhibited a wide range of variability when C 2H 2 was present. The rate of NO production in the absence of C 2H 2 was only 2.3–3.0% of the rate of N 2O production in the presence of C 2H 2 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 O 2 were added to slurry incubations containing C 2H 2. We conclude that trace amounts of O 2 inadvertently introduced into the slurries during sampling could initiate scavenging of NO by the C 2H 2-catalyzed NO oxidation reaction and cause an underestimate of N 2O production during DEA assays in some soils.