Nitrous oxide in brackish Lake Nakaumi, Japan II: the role of nitrification and denitrification in N2O accumulation

In order to understand the role of nitrification and denitrification in the accumulation of nitrous oxide (N2O) in the hypolimnetic water of brackish Lake Nakaumi, the effects of dissolved oxygen (DO) concentration on these activities were investigated by incubation experiments. N2O was produced during the oxidation of NH4+ to NO2− in nitrification and during the reduction of NO3− to N2 in denitrification. N2O-producing activity by nitrification (N2ON) increased markedly with decreasing concentrations of DO. Low DO (10%–30% saturation) induced high N2ON. In contrast to nitrification, N2O-producing activity by denitrification (N2OD) decreased with decreasing concentrations of DO. Little N2O was accumulated during denitrification under low-level conditions of DO (10%–30%), because of further reduction of N2O to N2. It can therefore be assumed that N2O produced as the by-product of nitrification is concurrently reduced to N2 by denitrification under low-DO conditions. This would result in no substantial accumulation of N2O during active nitrification in the hypolimnetic water of Lake Nakaumi.