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

Abstract. 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 to date the most widely used method to measure N₂O fluxes, but also micrometeorological methods are successfully 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) 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 was observed after mineral N application, returning to background level within 2–6 days. Highest fluxes were measured by both methods in July 2007 with maximum values of 1438 ng N₂O-N m⁻² s⁻¹ (EC) and 651 ng N₂O-N m⁻² s⁻¹ (chamber method). Negative fluxes above the detection limit were observed in all comparison periods by EC, while with chambers, the recorded negative fluxes were always below detection limit. 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. N₂O fluxes measured by EC during the time of chamber closure were not consistently smaller, neither larger, compared to those measured by chambers: this reflects the fact that the different techniques integrate fluxes over different spatial and temporal scales. Large fluxes measured by chambers may be representing local hotspots providing a small contribution to the flux measured by the EC method which integrates 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. The calculation of cumulative fluxes using different integration methods showed EC data provide generally lower estimates of N₂O emissions than chambers.