The influence of NH 4 + on microbial CH 4 oxidation is still poorly understood. Therefore, the influence of NH 4 Cl and (NH 4 ) 2 SO 4 on CH 4 oxidation was studied in soils at the different stages of the induction of enhanced methanotrophic activity. After a brief peak in the methanotrophic activity, a steady state was observed in which NH 4 + inhibited CH 4 oxidation at low CH 4 concentrations, and stimulated CH 4 oxidation at high concentrations. Chloride did not strongly inhibit CH 4 oxidation during this phase. During a second phase methanotrophic activity increased again. Ammonium no longer stimulated CH 4 oxidation, and Cl - became an important source of uncompetitive inhibition. It was hypothesized that type I methanotrophs dominated during the first, soil-N-dependent phase while N 2 -fixing type II methanotrophs dominated the second, soil-N-independent phase.