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

Abstract

ABSTRACT 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.