Methane and nitrous oxide exchange over a managed hay meadow

The methane (CH(4)) and nitrous oxide (N(2)O) 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 CH(4) and N(2)O 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 CH(4) and N(2)O 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 CO(2) equivalents of the investigated grassland site. In 2011, our best guess estimate showed a net greenhouse gas (GHG) sink of −32 g CO(2) equ. m(−2) yr(−1) for the meadow, whereby 55% of the CO(2) sink strength of −71 g CO(2)m(−2) yr(−1) was offset by CH(4) (N(2)O) emissions of 7 (32) g CO(2) equ. m(−2) yr(−1). When all data were pooled, the ancillary parameters explained 27 (42)% of observed CH(4) (N(2)O) flux variability, and up to 62 (76)% on shorter timescales in-between management dates. In the case of N(2)O 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 CO(2), H(2)O and energy fluxes, the interpretation of CH(4) and N(2)O exchange was challenging due to footprint heterogeneity regarding their sources and sinks, uncertainties regarding post-processing and quality control. Our results emphasize that CH(4) and N(2)O fluxes over supposedly well-aerated and moderately fertilized soils cannot be neglected when evaluating the GHG impact of temperate managed grasslands.