NITROUS-OXIDE EMISSION FROM AN AGRICULTURAL FIELD - COMPARISON BETWEEN MEASUREMENTS BY FLUX CHAMBER AND MICROMETEROLOGICAL TECHNIQUES

The soil in a drained fjord area, reclaimed for arable farming, produc ed N2O mainly at 75-105 cm depth, just above the ground water level. S urface emissions of N2O were measured from discrete small areas by clo sed and open-flow chamber methods, using gas chromatographic analysis and over larger areas by integrative methods: flux gradient (analysis by FTIR), conditional sampling (analysis by TDLAS), and eddy covarianc e (analysis by TDLAS). The mean emission of N2O as determined by chamb er procedures during a 9-day campaign was 162-202 mu g N2O-N m(-2) h(- 1) from a wheat stubble and 328-467 mu g N2O-N m(-2) h(-1) from a carr ot field. The integrative approaches gave N2O emissions of 149-495 mu g N2O-N m(-2) h(-1), i.e. a range similar to those determined with the chamber methods. Wind direction affected the comparison of chamber an d integrative methods because of patchiness of the N2O emission over t he area. When a uniform area with a single type of vegetation had a do minant effect on the N2O gradient at the sampling mast, the temporal v ariation in N2O emission determined by the flux gradient/FTIR method a nd chamber methods was very similar, with differences of only 18% or l ess in mean N2O emission, well below the variation encountered with th e chamber methods themselves. A detailed comparison of FTIR gradient a nd chamber data taking into account the precise emission footprint sho wed good agreement. It is concluded that there was no bias between the different approaches used to measure the N2O emission and that the pr ecision of the measurements was determined by the spatial variability of the N2O emission at the site and the variability inherent in the in dividual techniques. These results confirm that measurements of N2O em issions from different ecosystems obtained by the different methods ca n be meaningfully compared. Copyright (C) 1996 Elsevier Science Ltd