Production of NO and N2O in the presence and absence of C2H2 by soil slurries and batch cultures of denitrifying bacteria

We evaluated the potential of the C₂H₂-catalyzed NO oxidation reaction to influence N₂O production during denitrification. We measured the total amount of free NO and N₂O produced by slurries of sandy loam soil and by batch cultures of denitrifying bacteria under both anaerobic and low O₂ conditions. The maximum amount of free NO released by anaerobic Nos⁺ (able to reduce N₂O to N₂) and Nos⁻ (unable to reduce N₂O to N₂) batch cultures of Cytophaga johnsonae strains catalyzing denitrification of nitrate was 17-79 nmol NO per bottle. In all cases the maximum headspace concentrations of NO-N measured in anaerobic cultures in the absence of C₂H₂ was less than 0.21% of those of N₂O-N measured in the presence of 10 kPa C₂H₂. Peak NO production was delayed when between 2.0 and 4.5% O₂ was present. Less NO accumulated in cultures in the presence of both O₂ and C₂H₂, and the maximum amount of NO-N measured in the absence of C₂H₂ was less than 0.13% of the total amount of N₂O-N measured in the presence of C₂H₂. For an agricultural sandy loam soil, the maximum concentrations of free NO released from slurries were 598-897 ng NO-N g⁻¹ of dry soil in the absence of C₂H₂ and 118-260 ng NO-N g⁻¹ of dry soil in the presence of 10 kPa C₂H₂. The maximum concentration of NO-N released in the absence of C₂H₂ was between 0.32 and 8.1% of the maximum concentration of N₂O-N accumulated in the presence of 10 kPa C₂H₂. We conclude that scavenging of NO by the C₂H₂-catalyzed NO oxidation reaction in the presence of trace amounts of O₂ does not cause a serious underestimate of long-term measurements of active denitrification in anaerobic soils containing adequate carbon and nitrate sources.