Land-use and abandonment alters methane and nitrous oxide fluxes in mountain grasslands

Grasslands cover more than one fifth of total land area in Europe and contribute significantly to the total greenhouse gas budget. The impact of management and land use on the carbon cycle and carbon sequestration in grasslands has been well-studied, however effects on emissions of N₂O and CH₄ remain uncertain. Additionally, the majority of studies have focussed on management differences between intensively managed grasslands, with few results available for lightly managed grasslands and in particular grassland abandonment. We present N₂O and CH₄ flux measurements for an abandonment trajectory at low land-use intensity, comparing meadow (fertilized and cut), pasture (grazed) and abandoned (unmanaged since 1983) grassland sites located in the Austrian Alps. Mean growing season N₂O fluxes were 0.07, 0.07 and - 0.13 nmol m^-2 s^-1 and CH₄ fluxes were - 1.0, - 0.5 and - 1.6 nmol m^-2 s^-1 for the meadow, pasture and abandoned sites respectively. Variability for both gases at the abandoned site was dominated by 'hot moments', while 'hot spots' dominated at the managed meadow and pasture sites. Consideration of the diurnal cycle observed at the abandoned site, linear correlations within all data sets, and principal components analyses of the full data set revealed increased consumption of both N₂O and CH₄ with increasing temperature, but hardly any relationship between fluxes and soil moisture. Upscaled over a year, the observed fluxes correspond to enhanced non-CO₂ greenhouse gas uptake of 172 g CO₂-equiv. m^-2 y^-1 following abandonment. These results show that non-CO₂ greenhouse gases form an important part of the total climate impact of land use change and grassland abandonment, such that abandoned grassland is a net sink for both CH₄ and N₂O.