Micrometeorological measurements of nitrous oxide exchange above a cropland

N2O fluxes in a wheat/maize rotation system were measured using flux gradient methods combined with gas chromatograph (GC) technique. The mean precision of two repeated GC analyses for N2O concentration achieved to 0.27-0.46 ppbv, which could resolve N2O concentration differences in a low range of 0.39-0.65 ppbv. To maximize measurable N2O concentration differences, gradient measurements were conducted only after fertilization or under low wind conditions. During observation period, N2O flux ranged from -4.41 to 4.84 mgN(2)O m(-2) h(-1) for maize field, and from -2.82 to 3.59 mgN(2)O m(-2) h(-1) for wheat field. When gradient observation changed from two layers to four layers, the temporal variation of N2O flux reduced but the mean value changed less. Many negative N2O fluxes were found in maize and wheat fields even after fertilization. Nearly all of them were caused by negative N2O concentration differences. During four days' observation in maize field, a mean N2O flux of -0.75 mgN(2)O m(-2) h(-1) was found in the daytime and could not be simply attributed to the temporal variation of N2O flux. N2O flux determined by the aerodynamic method (F-a) and the Bowen ratio/energy balance method (F-b) were in a good agreement and statistically significant. The ratio of F-a to F-b increased linearly with energy balance ratio (EBR) obtained by the aerodynamic method in the daytime when EBR is larger than 0.3. it is the first time to give a quantitative description for the impact of energy closure on N2O flux, and show a possible way to improve the data quality under the condition of poor energy balance. (C) 2008 Elsevier Ltd. All rights reserved.