Effects of Methane Metabolism on Nitrification and Nitrous Oxide Production in Polluted Freshwater Sediment

Abstract

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 μM) showed earlier and enhanced nitrification and N 2 O production compared with unsupplemented slurries (≤1 μM 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 μM 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.