Nitrous oxide emissions from managed grassland: a comparison of eddy covariance and static chamber measurements

Managed grasslands are known to be an important source of N₂O with estimated global losses of 2.5 Tg N₂O-N yr⁻¹. Chambers are the most widely used method to measure N₂O fluxes, but also micrometeorological methods have successfully been applied. In this paper we present a comparison of N₂O fluxes measured by non-steady state chambers and eddy covariance (EC) (using an ultra-sonic anemometer coupled with a tunable diode laser) methods from an intensively grazed and fertilised grassland site in South East Scotland. The measurements were taken after fertilisation events in 2003, 2007 and 2008. In four out of six comparison periods a short-lived increase of N₂O emissions after mineral N application was observed, returning to background level within 2–6 days. Highest fluxes were measured by both methods in July 2007 with maximum values of 1300 ng N₂O-N m⁻² s⁻¹ (EC) and 651 ng N₂O-N m⁻² s⁻¹ (chamber method). Frequently, negative fluxes above the detection limit were observed in all comparison periods by EC, while with chambers negative fluxes were always below detection limit. Despite observed negative fluxes, median and average fluxes over each period were always positive. Over all 6 comparison periods 69% of N₂O fluxes measured by EC at the time of chamber closure were within the range of the chamber measurements. Differences between N₂O fluxes calculated from chamber measurements and EC over the same measurement period were never significant. Overall, N₂O fluxes measured by EC during the time of chamber closure were smaller compared to those measured by chambers, however this was the case in only 3 out of 6 comparison periods. This inconsistency observed on the same experimental field at different times can partly be explained by the fact that the different techniques integrate fluxes over different spatial scales. Large fluxes measured by chambers may have represented local hotspots, which made a small contribution to the flux derived by the EC method which integrates fluxes over a larger area. The spatial variability from chamber measurements was high as shown by a coefficient of variation of up to 139%. No diurnal pattern of N₂O fluxes was observed, possibly due to the small diurnal variations of soil temperature. Calculations of cumulative fluxes showed that different integration methods can introduce a large bias in the estimation of cumulative fluxes and therefore emission factors.