Effect of increasing oxygen concentration on total denitrification and nitrous oxide release from soil by different bacteria

Abstract

The effect of increasing oxygen concentrations (0, 5, 10 and 20 Vol% O2) on total denitrification and N20 release was studied in model experiments using a neutral pH loamy soil relatively rich in easily decomposable organic matter and supplied with nitrate (300 μg nitrate N/g dry soil). The sterilized soil was inoculated with three different denitrifying bacteria (Bacillus licheniformis,Aeromonas “denitrificans” andAzospirillum lipoferum) and incubated (80% WHC, 30°C). The gas volume was analysed for O2, CO2, N2O, NO and N2 by gas chromatography and the soil investigated for changes in ammonium, nitrite, nitrate, pH, total N and C as well as water-extractable C. WithB. licheniformis andAeromonas “denitrificans” total denitrification increased remarkably with increasing pO2 as the result of intensified mineralization.Azospirillum lipoferum, however, showed the highest activity at 5 vol% O2. WithB. licheniformis N2O was released only in anaerobic conditions and at 5 Vol% O2 (maximum) or 10 Vol% 02, but not at 20 Vol%, whereasAeromonas “denitrificans” produced N2O only in the presence of He gas (maximum) or at 5 Vol% O2. In contrast to these bacteria, N2O production withAzospirillum lipoferum was restricted to 10 Vol% O2 (maximum) and to 20 Vol% 02, with some traces at 5 vol% O2. With a certain set of conditions, total denitrification and N2O formation seem to be governed by the mineralization rate of the organisms in question. The increased demand for electron acceptors by a high turnover rate rather than the presence of anaerobic conditions seems to have determined the rate of denitrification.