Eddy covariance measurements of methane emissions from a beef cattle feedlot in Kansas

Methane is an important greenhouse gas (GHG) with a global warming potential 21 times greater than CO₂ over an 100-year period (IPCC, 2014). Methane originated from microbial fermentation in the digestive system of ruminant animals, referred as enteric fermentation, and manure management account for approximately 34% of the total methane anthropogenic emissions in the United States (USEPA, 2015). Micrometeorological methods have been used to measure gas exchange from animal production systems. The eddy covariance (EC) technique is considered the most direct micrometeorological method to measure gas exchange between the land and the atmosphere. Recent advances in optical sensors have allowed the development of fast response sensors capable of measuring other trace gases, such as CH₄, N₂O and ammonia, at a rate suitable for EC measurements. This method may also be an alternative to estimate methane emissions from confined animal feeding operations. However, the eddy covariance technique relies on the assumption that the source strength of the underlying surface is homogeneous, which may not hold true in a feedlot where the surface can have different source strengths depending on animal position and wind direction. The objective of this study was to evaluate the suitability of the eddy covariance technique to measure gas emissions from a commercial beef cattle feedlot. The flux and micrometeorological instrumentation was set up at the north edge of the feedlot, and measurements were carried out for 8 months. The wind velocity components were measured using a 3D sonic anemometer (CSAT3, Campbell Sci.) at 10 Hz. Measurements of H₂O, CO₂ and CH₄ mixing rations were performed using a closed-path gas analyzer (G2311f) at the same frequency. To evaluate the performance of the close-path analyzer, a well-established open-path analyzer (Li7500, Li-cor, Lincoln, NE) was used as reference to provide measurements of CO₂ and H₂O at the same frequency. An automated panoramic camera was set up at the tower to monitor the movement of the cattle in the pans close to the tower. The high frequency data were analyzed using the software Eddy Pro (Li-cor, Lincoln, NE). Our results indicate very good agreement between open-path and closed path CO₂ measurements. The R² and the slope for open-path and closed-path CO₂ fluxes were 0.98 and 0.05, respectively. The flux values obtained are consistent with values found in the literature. The fluxes values were quite variable and dependent on wind direction and animal position in the feedlot. Fluxes of methane showed a very large temporal variation, but the use of a footprint model helped to interpret the flux temporal dynamics.