Interactions between N application rate, CH4 oxidation and N2O production in soil

Here we report on a controlled environment experiment in which we applied C-13- and N-15-enrichment approaches to quantify methane oxidation rates and source partition N2O production in a silt loam soil following application of NH4NO3, enabling us to look for potential interactions between methane oxidation and nitrifier-N2O production. N-15-N2O, N14+15-N2O and CO2 fluxes and mineral N concentrations were measured over a 23-day period after application of NH4NO3 (5 at.% excess N-15) at rates of 0, 5, 10, 20, 30 and 40 g N m(-2) to a silt loam soil. Change in C-12/13-CH4 concentrations (as indicative of C-13-CH4 oxidation rates) and production of C-13-CO2 were monitored over the first 72 h after addition of 1.7 mu l C-13-CH4 l(-1) (10 at.% excess C-13) to these N treatments. Oxidation of applied C-13-CH4 was slower in the 5, 10, 20 and 30 g N m(-2) (5 at.% excess N-15) treatments (0.24-0.32 mu g C-13-CH4 l(-1) day(-1)) than in the control (0.40 mu g C-13-CH4 l(-1) day(-1)), suggesting that these N loadings inhibited oxidation. N2O production was raised after N addition, and in the 10, 20 and 30 g N m(-2) treatments nitrification was the predominant source of N2O accounting for 61, 83 and 57% of the total N-15-N2O 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 CH4 and N2O.