Abstract
The N2O production during nitrification was studied in Lake Kizaki by vertical observations and two incubation experiments. The apparent N2O production (ΔN2O), the difference between observed and atmospheric-equilibrium concentration, was highly correlated with apparent oxygen consumption (-ΔO2), in situ NO3- concentration and the amount of NH4+ decrease with time, indicating that N2O was produced by the nitrification process. In a long-term in situ incubation, simultaneous accumulations of NO3- and N2O were observed, the amounts of which were comparable to the in situ changes, suggesting their in situ productions. Both NO3- and N2O productions were greatly enhanced by an NH4+ enrichment after the emergence of nitrification activity.
From both vertical observations and long-term in situ incubations, the yield of N2O relative to NO3- production was estimated to be about 0.1% during the active nitrification period. But thereafter, the obtained yields were appreciably higher. Another incubation experiment also showed that N2O production was delayed several days compared to NO3- production, indicating that N2O production was accelerated in the late phase of nitrification. The physiological state of nitrifiers was suggested to be of some importance for such phenomena. Nitrapyrin, a specific inhibitor of nitrification, was found not to be very effective for N2O formations in contrast with NO3- productions completely inhibited, suggesting a possibility of some N2O sources other than autotrophic nitrification.
