N2O accumulation in estuarine and coastal sediments: The influence of H2S on dissimilatory nitrate reduction

The effects of H2S on the production and accumulation of N2O in surface sediments of the coastal lagoons of Lakes Shinji and Nakaumi were studied using sediment suspensions and isolated strains of halophilic N2O producers. Denitrification and N2O accumulation were determined by anaerobic incubations with and without C2H2, respectively. Denitrifying activities in the sediment suspensions of both lakes decreased markedly at an H2S concentration of 3 mg S l 1, whereas N2O accumulations in the sediment suspensions of Lakes Shinji and Nakaumi were highest at 75e100 mg S l 1 and 15e50 mg S l 1 H2S, respectively. In addition, H2S had marked inhibiting (about 30 h) and retarding effects on N2O reduction in the suspensions of both lakes. Two strains of halophilic N2O producers were isolated from the sediment of Lake Shinji (Aeromonas sp. and Vibrio sp.). N2O accumulation rates by Aeromonas sp. and Vibrio sp. were accelerated at 1e5 mgS l 1 and 1e10 mg S l 1 H2S, respectively. Patterns of inorganic nitrogen compounds after the incubations revealed the accumulation of NH4 þ and NO2 , with concomitant N2O, were accelerated by H2S. These results indicate that H2S plays a key role in regulation of N2O accumulation in the eutrophic estuarine or coastal sediments. Furthermore, the accumulation of N2O in the sediments of Lakes Shinji and Nakaumi is predicted to derive through dissimilatory nitrate reduction to ammonium (DNRA) not only denitrification in the presence of H2S.