Effects of methane metabolism on nitrification and nitrous oxide production in polluted freshwater sediment.

We report the effect of CH(4) and of CH(4) oxidation on nitrification in freshwater sediment from Hamilton Harbour, Ontario, Canada, a highly polluted ecosystem. Aerobic slurry experiments showed a high potential for aerobic N(2)O production in some sites. It was suppressed by C(2)H(2), correlated to NO(3) production, and stimulated by NH(4) concentration, supporting the hypothesis of a nitrification-dependent source for this N(2)O production. Diluted sediment slurries supplemented with CH(4) (1 to 24 muM) showed earlier and enhanced nitrification and N(2)O production compared with unsupplemented slurries (</=1 muM CH(4)). This suggests that nitrification by methanotrophs may be significant in freshwater sediment under certain conditions. Suppression of nitrification was observed at CH(4) concentrations of 84 muM and greater, possibly through competition for O(2) between methanotrophs and NH(4) -oxidizing bacteria and/or competition for mineral N between these two groups of organisms. In Hamilton Harbour sediment, the very high CH(4) concentrations (1.02 to 6.83 mM) which exist would probably suppress nitrification and favor NH(4) accumulation in the pore water. Indeed, NH(4) concentrations in Hamilton Harbour sediment are higher than those found in other lakes. We conclude that the impact of CH(4) metabolism on N cycling processes in freshwater ecosystems should be given more attention.