Production of NO and N2O in the presence and absence0f C2H2 by soil slurries and batch cultures of denitrifying bacteria

We evaluated the potential of the C2H2-catalyzed N2O 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 O-2 conditions. The maximum amount of free NO released by anaerobic Nos(+) (able to reduce N2O to N-2) and Nos(-) (unable to reduce N2O to N-2) 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% O-2 was present. Less NO accumulated in cultures in the presence of both O-2 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 O-2 does not cause a serious underestimate of long-term measurements of active denitrification in anaerobic soils containing adequate carbon and nitrate sources. (C) 2003 Elsevier Science Ltd. All rights reserved.