Methane and nitrous oxide exchange over a managed hay meadow

Abstract

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 more than 95% of the half-hourly fluxes of CH4 and N2O ranging between ±10 and ±1 nmol 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 time scale. 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 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 26/38% of observed CH4/N2O flux variability, and up to 62/75% on shorter time scales in-between management dates. In case of N2O fluxes, we found highest emissions at intermediate soil water contents and at soil temperatures close to zero 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.