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

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Abstract

Managed grasslands are known to be an important source of N 2O with estimated global losses of 2.5 Tg N 2O-N yr -1. Chambers are to date the most widely used method to measure N 2O fluxes, but also micrometeorological methods are successfully applied. In this paper we present a comparison of N 2O 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 N 2O 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 N 2O-N m 2 s -1 (EC) and 651 ng N 2O-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 N 2O fluxes measured by EC at the time of chamber closure were within the range of the chamber measurements. N 2O 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 N 2O 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 N 2O emissions than chambers. © 2011 Author(s).

Figures

  • Fig. 1. Site diagram of the study field Easter Bush, showing locations of static chambers in the South and North field in 2003 and 2007/2008, micrometeorological mast (eddy covariance inlet and sonic position) and cabin (containing TDL) on the boundary of the two fields and prevailing wind directions.
  • Table 1. Overview of comparison periods of eddy covariance and chamber N2O flux measurements, fertiliser application dates, amount of N applied, average air and soil temperature (Tair, Tsoil), average soil water content (SWC), average water filled pore space (WFPS) and total rainfall.
  • Fig. 2. N2O fluxes obtained with eddy covariance and static chambers for 6 comparison periods; eddy covariance data are 30 min values (grey line) or values averaged over the one hour period when chambers were closed (between 10:00–12:00, squares). Chamber measurement points represent the average of 14 (2003) or 4 (2007/2008) chambers measured over 1 h (white circles), with error bars representing the range of chamber measurements. Fertiliser applications are indicated with an arrow.
  • Fig. 3. Comparison of N2O fluxes measured at the same times (between 10:00 and 12:00) with eddy covariance (EC) and static chambers for each of the 6 comparison periods and data from all comparison periods. Chamber values represent an average of 14 (2003) or 4 (2007/2008) chambers. Circles represent all comparison points, while crosses represent the retained dataset where outliers were removed. Trend lines represent orthogonal regression (continuous line for all comparison points with non italic model equation, dashed line where outliers were removed with italic model equation). Different shades of circles indicate the contribution of the area in which the chambers were situated to the footprint of the EC measurement (open circles = 0–25 %, medium grey = 26–50 %, dark grey = 51–75 %).
  • Table 2. Statistics of N2O fluxes from chambers and eddy covariance (EC) measurements for all 6 comparison periods. Numbers in brackets represent the number of chambers included in the comparison. EC points are eddy covariance hourly averages during chamber sampling (between 10:00 and 12:00); EC all are half hourly eddy covariance fluxes during the comparison period. The Coefficient of Variation is averaged over all measurements (30 min or daily values).
  • Table 3. Cumulative N2O fluxes from chambers and eddy covariance for all 6 periods. Fluxes were calculated using both non gap-filled and gap-filled data. Values in brackets represent standard deviations amongst 14 (2003) or 4 (2007/2008) chambers.
  • Fig. 4. Comparison of cumulative fluxes from gap-filled data from static chambers and EC, using either comparison points (ECa), daily averages (ECb) or 30 min values (ECc).

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CITATION STYLE

APA

Jones, S. K., Famulari, D., Di Marco, C. F., Nemitz, E., Skiba, U. M., Rees, R. M., & Sutton, M. A. (2011). Nitrous oxide emissions from managed grassland: A comparison of eddy covariance and static chamber measurements. Atmospheric Measurement Techniques, 4(10), 2179–2194. https://doi.org/10.5194/amt-4-2179-2011

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