Here we report on a controlled environment experiment in which we applied 13 C-and 15 N-enrichment approaches to quantify methane oxidation rates and source partition N 2 O production in a silt loam soil following application of NH 4 NO 3 , enabling us to look for potential interactions between methane oxidation and nitrifier-N 2 O production. 15 N-N 2 O, 14?15 N-N 2 O and CO 2 fluxes and mineral N concentrations were measured over a 23-day period after application of NH 4 NO 3 (5 at.% excess 15 N) at rates of 0, 5, 10, 20, 30 and 40 g N m -2 to a silt loam soil. Change in 12/13 C-CH 4 concentrations (as indicative of 13 C-CH 4 oxidation rates) and production of 13 C-CO 2 were monitored over the first 72 h after addition of 1.7 ll 13 C-CH 4 l -1 (10 at.% excess 13 C) to these N treatments. Oxidation of applied 13 C-CH 4 was slower in the 5, 10, 20 and 30 g N m -2 (5 at.% excess 15 N) treatments (0.24-0.32 lg 13 C-CH 4 l -1 day -1 ) than in the control (0.40 lg 13 C-CH 4 l -1 day -1 ), suggesting that these N loadings inhibited oxidation. N 2 O production was raised after N addition, and in the 10, 20 and 30 g N m -2 treatments nitrification was the predominant source of N 2 O accounting for 61, 83 and 57% of the total 15 N-N 2 O produced, respectively. Our results point towards the possibility of methylotrophs switching function to oxidise ammonia in the presence of N, which may result in greater atmospheric loading of both CH 4 and N 2 O.