Temporal and spatial variations of CO2, CH4 and N2O fluxes at three differently managed grasslands.

A profound understanding of temporal and spatial variabilities of CO₂, CH₄ and N₂O fluxes between terrestrial ecosystems and the atmosphere is needed to reliably quantify these fluxes and to develop future mitigation strategies. For managed grassland ecosystems, temporal and spatial variabilities of these three greenhouse gas (GHG) fluxes are due to environmental drivers as well as to fertilizer applications, grazing and cutting events. To assess how these affect GHG fluxes at Swiss grassland sites, we studied three sites along an altitudinal gradient that corresponds to a management gradient: from 400ma.s.l. (intensively managed) to 1000ma.s.l. (moderately intensive managed) to 2000 ma.s.l. (extensively managed). Temporal and spatial variabilities of GHG fluxes were quantified along small-scale transects of 16 static soil chambers at each site. We then established functional relationships between drivers and the observed fluxes on diel and annual time scales. Furthermore, spatial variabilities and their effect on representative site-specific mean chamber GHG fluxes were assessed using geostatistical semivariogram approaches. All three grasslands were N₂O sources, with mean annual fluxes ranging from 0.15 to 1.28 nmol m^-2 s^-1. Contrastingly, all sites were net CH₄ sinks, with uptake rates ranging from -0.56 to -0.15 nmol m^-2 s^-1. Mean annual respiration losses of CO₂, as measured with opaque chambers, ranged from 5.2 to 6.5 µmol m^-2 s^-1. While the environmental drivers and their respective explanatory power for N O emissions differed considerably among the three grasslands (adjusted r² ranging from 0.19 to 0.42), CH₄ and CO₂ fluxes were much better constrained (adjusted r² ranging from 0.41 to 0.83), in particular by soil water content and air temperature, respectively. Throughout the year, spatial heterogeneity was particularly high for N₂O and CH₄ fluxes. We found permanent hot spots for N₂O emissions and CH₄ up- take at the extensively managed site. Including these hot spots in calculating the mean chamber flux was essential to obtain a representative mean flux for this ecosystem. At the intensively managed grassland, management effects clearly dominated over effects of environmental drivers on N₂O fluxes. For CO₂ and CH₄, the importance of management effects did depend on the status of the vegetation. [ABSTRACT FROM AUTHOR]