A comparison of NO and N2O production by the autotrophic nitrifier Nitrosomonas europaea and the heterotrophic nitrifier Alcaligenes faecalis

Soil microorganisms are important sources of the nitrogen trace gases NO and N2O for the atmosphere. Present evidence suggests that autotrophic nitrifiers such as Nitrosomonas europaea are the primary producers of NO and N2O in aerobic soils, whereas denitrifiers such as Pseudomonas spp. or Alcaligenes spp. are responsible for most of the NO and N2O emissions from anaerobic soils. It has been shown that Alcaligenes faecalis, a bacterium common in both soil and water, is capable of concomitant heterotrophic nitrification and denitrification. This study was undertaken to determine whether heterotrophic nitrification might be as important a source of NO and N2O as autotrophic nitrification. We compared the responses of N. europaea and A. faecalis to changes in partial O2 pressure (pO2) and to the presence of typical nitrification inhibitors. Maximal production of NO and N2O occurred at low pO2 values in cultures of both N. europaea (pO2, 0.3 kPa) and A. faecalis (pO2, 2 to 4 kPa). With N. europaea most of the NH4(+) oxidized was converted to NO2(-), with NO and N2O accounting for 2.6 and 1% of the end product, respectively. With A. faecalis maximal production of NO occurred at a PO2 of 2 kPa, and maximal production of N2O occurred at a PO2 of 4 kPa. At these low pO2 values there was net nitrite consumption. Aerobically, A. faecalis produced approximately the same amount of NO but 10-fold more N2O per cell than N. europaea did. Typical nitrification inhibitors were far less effective for reducing emissions of NO and N2O by A. faecalis than for reducing emissions of NO and N2O by N. europaea. A. faecalis produced much less NO and N2O under denitrifying conditions than under nitrifying conditions, and the NO produced appeared to result primarily from chemical interactions involving NO2 at pH 6.95. Once much of the nitrite was consumed, the NO and N2O produced were further reduced to N2. Given the rates of NO and N2O production reported here, our results suggest that heterotrophic nitrification may be a significant source of N2O in aerobic to near-anaerobic soils and water.