Eddy covariance methane flux measurements over a grazed pasture: effect of cows as moving point sources

[摘要] Methane (CH₄) from ruminants contributes one-third of globalagricultural greenhouse gas emissions. Eddy covariance (EC) technique hasbeen extensively used at various flux sites to investigate carbon dioxideexchange of ecosystems. Since the development of fast CH₄ analyzers, theinstrumentation at many flux sites has been amended for these gases. However,the application of EC over pastures is challenging due to the spatially andtemporally uneven distribution of CH₄ point sources induced by the grazinganimals. We applied EC measurements during one grazing season over a pasturewith 20 dairy cows (mean milk yield: 22.7 kg d⁻¹) managed in arotational grazing system. Individual cow positions were recorded by GPStrackers to attribute fluxes to animal emissions using a footprint model.Methane fluxes with cows in the footprint were up to 2 orders of magnitudehigher than ecosystem fluxes without cows. Mean cow emissions of423 ± 24 g CH₄ head⁻¹ d⁻¹ (best estimate from thisstudy) correspond well to animal respiration chamber measurements reported inthe literature. However, a systematic effect of the distance between sourceand EC tower on cow emissions was found, which is attributed to the analyticalfootprint model used. We show that the EC method allows one to determine CH₄emissions of cows on a pasture if the data evaluation is adjusted for thispurpose and if some cow distribution information is available.

[学科分类] 地球化学与岩石