Production of NO and N2O by Pure Cultures of Nitrifying and Denitrifying Bacteria during Changes in Aeration

Abstract

Peak emissions of NO and N2O are often observed after wetting of soil. The reactions to sudden changes in the aeration of cultures of nitrifying and denitrifying bacteria with respect to NO and N2O emissions were compared to obtain more information about the microbiological aspects of peak emissions. In continuous culture, the nitrifier Nitrosomonas europaea and the denitrifiers Alcaligenes eutrophus and Pseudomonas stutzeri were cultured at different levels of aeration (80 to 0% air saturation) and subjected to changes in aeration. The relative production of NO and N2O by N. europaea, as a percentage of the ammonium conversion, increased from 0.87 and 0.17%, respectively, at 80% air saturation to 2.32 and 0.78%, respectively, at 1% air saturation. At 0% air saturation, ammonium oxidation and N2O production ceased but NO production was enhanced. Coculturing of N. europaea with the nitrite oxidizer Nitrobacter winogradskyi strongly reduced the relative levels of NO and N2O production, probably as an effect of the lowered nitrite concentration. After lowering the aeration, N. europaea produced large short-lasting peaks of NO and N2O emissions in the presence but not in the absence of nitrite. A. eutrophus and P. stutzeri began to denitrify below 1% air saturation, with the former accumulating nitrite and N2O and the latter reducing nitrate almost completely to N2. Transition of A. eutrophus and P. stutzeri from 80 to 0% air saturation resulted in transient maxima of denitrification intermediates. Such transient maxima were not observed after transition from 1 to 0%. Reduction of nitrate by A. eutrophus continued 48 h after the onset of the aeration, whereas N2O emission by P. stutzeri increased for only a short period. It was concluded that only in the presence of nitrite are nitrifiers able to dominate the NO and N2O emissions of soils shortly after a rainfall event.