Nitrous oxide fluxes from a fertilised maize crop using micrometeorological and chamber methods

Emissions of N2O from maize fields irrigated and fertilised with 200 kg N ha(-1) were measured using several chamber and micrometeorological techniques at a single site in the Landes de Gascogne (France). The methods were compared over similar periods, but the sampling areas covered range from 0.18 m(2) for the chamber methods to 10(4)-10(5) m(2) for the micrometeorological methods. Nitrous oxide concentration was measured using electron capture gas chromatography (GC) for the chamber technique. Micrometeorological fluxes were determined by eddy covariance and gradient methods with N2O mixing ratios measured using fast-response tunable diode laser absorption spectroscopy. The spatial heterogeneity of N2O emissions was studied with a set of 30 closed chambers. This study showed an anisotropic distribution of N2O emissions which was attributed to the pattern of fertiliser application. A sensitivity analysis of the micrometeorological techniques indicated that uncertainties on Aux assessment are mainly due to spatial heterogeneity of N2O emissions. Fluxes measured using chamber and micrometeorological techniques were compared over a 2 h period. Depending on the precision of the techniques, good agreement between the magnitude of the fluxes measured by chamber and micrometeorological methods was observed. For micrometeorological methods, the Aux ranged from 20 to 400 ng N-N2O m(-2) s(-1) and from 25 to 275 ng N-N2O m(-2) s(-1) for chamber method. Average relative variations of the Aux were similar in both methods, and were mainly dependent on the soil water content. Using micrometeorological measurements, large variations of the flux were observed from 1 h to the next related to variations in the wind direction combined with spatial heterogeneity. (C) 1999 Elsevier Science B.V. All rights reserved.