Eddy Covariance Measurement of Gas Emissions in a Beef Cattle Feedlot: Performance of Gas Analyzers and Spatial Variability of Source Strengths.

Methane (CH₄) emissions from ruminants contribute to one third of the global agricultural greenhouse gas emissions.  Accurate emission measurements from confined animal feeding operations (CAFO) are required to reduce uncertainty in the CH₄ budget and to evaluate mitigation strategies. Micrometeorological methods can be an alternative to measure gas emissions from CAFO’s.   

The eddy covariance (EC) technique is a micrometeorological method that allows precise, direct, nonintrusive and near-continuous flux measurements over large areas. However, the application of the EC technique over feedlots is challenging due to the spatial and temporal variability of gas source strength, induced by changes in wind direction and animal position. More data are required to investigate the use of the EC approach in CAFO’s. The purpose of this study was (i) to assess the performance of a closed-path analyzer to measure CH₄, CO₂, latent and sensible heat fluxes in a beef cattle feedlot; (ii) to investigate the spatial variability of eddy covariance fluxes measured above the surface of a beef cattle feedlot using an analytical flux footprint analysis.

The study was carried out during eight months in a beef cattle feedlot in Western Kansas. Two gas analyzers, a closed-path CH₄/CO₂/H₂O analyzer (G2311-f, Picarro Inc., USA) and an open-path CO₂/H₂O analyzer (LI-7500A, LI-COR Biosciences) were used to measure gas concentrations at 10 Hz.  The raw data was processed using the software Eddy Pro (LI-COR, Biosciences) and fluxes were calculated for 30 min intervals.

Measured CH₄ and CO₂ flux during the study period from the feedlot averaged 0.14 mg m^-2 s^-1 and 4.5 mg m^-2 s^-1 respectively. These values are in agreement with previous studies that used different micrometeorological methods to measure gas emissions from CAFO’s. Co-spectra of CH₄ and CO₂, measured by open and closed path systems, agreed well with theoretical slopes. Furthermore, a very high correlation and agreement (R² = 0.99, slope = 0.98) between CO₂ flux measured using closed and open path analyzer was observed, confirming that the close path analyzer provided reliable concentration measurements. The analytical footprint analysis indicated that the flux magnitudes varied as a result of atmospheric stability, wind speed and direction. Larger flux densities were observed under unstable and near neutral conditions, and smaller flux densities were found during stable condition when the flux footprint extended over a large area in the feedlot.