Methane and nitrous oxide exchange over a managed hay meadow

Abstract. The methane (CH₄) and nitrous oxide (N₂O) exchange of a temperate mountain grassland near Neustift, Austria, was measured during 2010–2012 over a time period of 22 months using the eddy covariance method. Exchange rates of both compounds at the site were low, with 97% of all half-hourly CH₄ and N₂O fluxes ranging between ±200 and ±50 ng m⁻² s⁻¹, respectively. The meadow acted as a sink for both compounds during certain time periods, but was a clear source of CH₄ and N₂O on an annual timescale. Therefore, both gases contributed to an increase of the global warming potential (GWP), effectively reducing the sink strength in terms of CO₂ equivalents of the investigated grassland site. In 2011, our best guess estimate showed a net greenhouse gas (GHG) sink of −32 g CO₂ equ. m⁻² yr⁻¹ for the meadow, whereby 55% of the CO₂ sink strength of −71 g CO₂ m⁻² yr⁻¹ was offset by CH₄ (N₂O) emissions of 7 (32) g CO₂ equ. m⁻² yr⁻¹. When all data were pooled, the ancillary parameters explained 27 (42)% of observed CH₄ (N₂O) flux variability, and up to 62 (76)% on shorter timescales in-between management dates. In the case of N₂O fluxes, we found the highest emissions at intermediate soil water contents and at soil temperatures close to 0 or above 14 °C. In comparison to CO₂, H₂O and energy fluxes, the interpretation of CH₄ and N₂O exchange was challenging due to footprint heterogeneity regarding their sources and sinks, uncertainties regarding post-processing and quality control. Our results emphasize that CH₄ and N₂O fluxes over supposedly well-aerated and moderately fertilized soils cannot be neglected when evaluating the GHG impact of temperate managed grasslands.