Landfills are large sources of CH4, but a considerable amount of CH4 can be removed in situ by methanotrophs if their activity can be stimulated through the addition of nitrogen. Nitrogen can, however, lead to increased N2O production. To examine the effects of nitrogen and a selective inhibitor on CH4 oxidation and N2O production in situ, 0.5 M of NH4Cl and 0.25 M of KNO3, with and without 0.01% (w/v) phenylacetylene, were applied to test plots at a landfill in Kalamazoo, MI from 2007 November to 2009 July. Nitrogen amendments stimulated N2O production but had no effect on CH4 oxidation. The addition of phenylacetylene stimulated CH4 oxidation while reducing N2O production. Methanotrophs possessing particulate methane monooxygenase and archaeal ammonia-oxidizers (AOAs) were abundant. The addition of nitrogen reduced methanotrophic diversity, particularly for type I methanotrophs. The simultaneous addition of phenylacetylene increased methanotrophic diversity and the presence of type I methanotrophs. Clone libraries of the archaeal amoA gene showed that the addition of nitrogen increased AOAs affiliated with Crenarchaeal group 1.1b, while they decreased with the simultaneous addition of phenylacetylene. These results suggest that the addition of phenylacetylene with nitrogen reduces N2O production by selectively inhibiting AOAs and/or type II methanotrophs.