Contribution of Methanotrophic and Nitrifying Bacteria to CH(4) and NH(4)(+) Oxidation in the Rhizosphere of Rice Plants as Determined by New Methods of Discrimination

Methanotrophic and nitrifying bacteria are both able to oxidize CH(4) as well as NH(4)(+). To date it is not possible to estimate the relative contribution of methanotrophs to nitrification and that of nitrifiers to CH(4) oxidation and thus to assess their roles in N and C cycling in soils and sediments. This study presents new options for discrimination between the activities of methanotrophs and nitrifiers, based on the competitive inhibitor CH(3)F and on recovery after inhibition with C(2)H(2). By using rice plant soil as a model system, it was possible to selectively inactivate methanotrophs in soil slurries at a CH(4)/CH(3)F/NH(4)(+) molar ratio of 0.1:1:18. This ratio of CH(3)F to NH(4)(+) did not affect ammonia oxidation, but methane oxidation was inhibited completely. By using the same model system, it could be shown that after 24 h of exposure to C(2)H(2) (1,000 parts per million volume), methanotrophs recovered within 24 h while nitrifiers stayed inactive for at least 3 days. This gave an “assay window” of 48 h when only methanotrophs were active. Applying both assays to model microcosms planted with rice plants demonstrated a major contribution of methanotrophs to nitrification in the rhizosphere, while the contribution of nitrifiers to CH(4) oxidation was insignificant.