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 autotrop hic nitrifiers such as Nitrosomonas europaea are the primary producers of NO and N2O in aerobic soils, whereas denitrifiers such as Pseudomo nas spp. or Alcaligenes spp. are responsible for most of the NO and N2 O emissions from anaerobic soils. It has been shown that Alcaligenes f aecalis, a bacterium common in both soil and water, is capable of conc omitant heterotrophic nitrification and denitrification. This study wa s 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 pa rtial O2 pressure (pO2) and to the presence of typical nitrification i nhibitors. 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 kP a. At these low pO2 values there was net nitrite consumption. Aerobica lly, A. faecalis produced approximately the same amount of NO but 10-f old more N2O per cell than N. europaea did. Typical nitrification inhi bitors 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 conditio ns than under nitrifying conditions, and the NO produced appeared to r esult primarily from chemical interactions involving NO2- at pH 6.95. Once much of the nitrite was consumed, the NO and N2O produced were fu rther reduced to N2. Given the rates of NO and N2O production reported here, our results suggest that heterotrophic nitrification may be a s ignificant source of N2O in aerobic to near-anaerobic soils and water.