Grassland soils are generally considered a small methane (CH4) sink, based mostly on the findings from chamber experiments. However, micrometeorological studies using the eddy covariance (EC) and flux-gradient (FG) methods have often reported upward CH4 fluxes over pasture, in the absence of grazing animals. Here, we collate available EC and FG data from New Zealand’s pasture systems finding a predominance of upwards fluxes, typically ≈ 5–10 mg m−2 d−1. We then investigate whether these fluxes constitute real pasture emissions and conclude that they do not. Rather, these small upward fluxes are likely the result of strong CH4 emissions from upwind sources, at distances on the order of 1 km or more, outside the area usually considered the flux footprint. In other words, horizontal advection is often a non-negligible term in the CH4 mass budget and the homogeneity assumption for EC and FG is violated. This conclusion is based on the following points of evidence: 1) Exploratory surveys with chamber measurements, on three farms where EC and FG measurements operated, found small CH4 uptake rates. 2) Simulations with a dispersion model of cow emissions 800–900 m upwind of a measurement point 2 m above ground showed systematically upward EC fluxes of 1–25 mg m−2 d−1, and upward FG fluxes of similar magnitude for sufficiently stable or unstable stratification, demonstrating that fluxes of the observed magnitude can be entirely caused by advection. 3) Cospectra of CH4 and vertical wind were compared with their carbon dioxide (CO2) counterparts, because CO2 is known to be taken up or emitted locally and spatially homogeneously, and the CH4 cospectra were substantially different, being dominated at small wavenumbers indicating transport by large, organised structures. Based on our analyses, we urge caution when interpreting micrometeorological CH4 flux data in landscapes where large, transient emissions sources are present.
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