Methane emissions from sheep pasture, measured with an open-path eddy covariance system

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Methane (CH4) is an important greenhouse gas, contributing 0.4–0.5 W m−2 to global warming. Methane emissions originate from several sources, including wetlands, rice paddies, termites and ruminating animals. Previous measurements of methane flux from farm animals have been carried out on animals in unnatural conditions, in laboratory chambers or fitted with cumbersome masks. This study introduces eddy covariance measurements of CH4, using the newly developed LI-COR LI-7700 open-path methane analyser, to measure field-scale fluxes from sheep grazing freely on pasture. Under summer conditions, fluxes of methane in the morning averaged 30 nmol m−2 s−1, whereas those in the afternoon were above 100 nmol m−2 s−1, and were roughly two orders of magnitude larger than the small methane emissions from the soil. Methane emissions showed no clear relationship with air temperature or photosynthetically active radiation, but some diurnal pattern was apparent, probably linked to sheep grazing behaviour and metabolism. Over the measurement period (days 60–277, year 2010), cumulative methane fluxes were 0.34 mol CH4 m−2, equating to 134.3 g CO2 equivalents m−2. By comparison, a carbon dioxide (CO2) sink of 819 g CO2 equivalents m−2 was measured over the same period, but it is likely that much of this would be released back to the atmosphere during the winter or as off-site losses (through microbial and animal respiration). By dividing methane fluxes by the number of sheep in the field each day, we calculated CH4 emissions per head of livestock as 7.4 kg CH4 sheep−1 yr−1, close to the published IPCC emission factor of 8 kg CH4 sheep−1 yr−1.

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