The literature is conflicted on the influence of ammonium on the kinetics and microbial ecology of methanotrophy. In this study, methanotrophic cultures were enriched, under ammonium concentrations ranging from 0 to 200 mM, from an inoculum comprising leachate and top-cover soil from a landfill. Specific CH4 biodegradation rates were highest (7.8 x 10(-4) +/- 6.0 x 10(-5)gCH(4)g(x)(-1)h(-1)) in cultures enriched at 4 mM NH4+, which were mainly dominated by type II methanotrophs belonging to Methylocystis spp. Lower specific CH4 oxidation rates (average values of 1.8-3.6 x 10(-4)gCH(4)g(x)(-1)h(-1)) were achieved by cultures enriched at higher NH4+ concentrations (20 and 80 mM), and had higher affinity for CH4 compared to 4 mM enrichments. These lower affinities were attributed to lower diversity dominated by type I methanotrophs, of the Methylosarcina, Methylobacter and Methylomicrobium genera, encountered with increasing concentrations of NH4+. The study indicates that CH4 oxidation biotechnologies applied at low NH4+ concentrations can support efficient abatement of CH4 and high diversity of methanotrophic consortia, whilst enriching type II methanotrophs. (C) 2019 Elsevier Ltd. All rights reserved.