The coexistence of chemoautotrophic nitrifiers and methanotrophs in a cultivated humisol was investigated. Under laboratory conditions which supported the growth and activity of methanotrophs, the nitrifiers were partially or completely inhibited. The inhibition was related to a competition for available oxygen and a high assimilatory requirement for inorganic nitrogen by the Methanotrophs. Dissolved methane concentrations as high as 250 μMhad no direct effect on the oxidation of ammonium. Simultaneous nitrification and methane oxidation was observed only if relatively high levels of ammonium and oxygen were maintained. Coupled nitrification-assimilatory/dissimilatory nitrate reduction resulted from the high oxygen demand of the actively growing methanotrophs. This study suggests that the potential competitive effects of methanotrophs may influence nitrification by chemoautotrophic nitrifiers in certain environmental systems
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