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 h a(-1) were measured using several chamber and micrometeorological technique s at a single site in the Landes de Gascogne (France). The methods were com pared 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 micrometeorologica l methods. Nitrous oxide concentration was measured using electron capture gas chromatography (GC) for the chamber technique. Micrometeorological flux es were determined by eddy covariance and gradient methods with N2O mixing ratios measured using fast-response tunable diode laser absorption spectros copy. The spatial heterogeneity of N2O emissions was studied with a set of 30 closed chambers. This study showed an anisotropic distribution of N2O em issions which was attributed to the pattern of fertiliser application. A se nsitivity analysis of the micrometeorological techniques indicated that unc ertainties on Aux assessment are mainly due to spatial heterogeneity of N2O emissions. Fluxes measured using chamber and micrometeorological technique s were compared over a 2 h period. Depending on the precision of the techni ques, good agreement between the magnitude of the fluxes measured by chambe r and micrometeorological methods was observed. For micrometeorological met hods, 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 th e Aux were similar in both methods, and were mainly dependent on the soil w ater content. Using micrometeorological measurements, large variations of t he flux were observed from 1 h to the next related to variations in the win d direction combined with spatial heterogeneity. (C) 1999 Elsevier Science B.V. All rights reserved.