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

Abstract. A profound understanding of temporal and spatial variabilities of soil carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (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 soil greenhouse gas (GHG) fluxes occur due to changes in environmental drivers as well as fertilizer applications, harvests and grazing. To assess how such changes affect soil GHG fluxes at Swiss grassland sites, we studied three sites along an altitudinal gradient that corresponds to a management gradient: from 400 m a.s.l. (intensively managed) to 1000 m a.s.l. (moderately intensive managed) to 2000 m a.s.l. (extensively managed). The alpine grassland was included to study both effects of extensive management on CH₄ and N₂O fluxes and the different climate regime occurring at this altitude. Temporal and spatial variabilities of soil GHG fluxes and environmental drivers on various timescales were determined along transects of 16 static soil chambers at each site. All three grasslands were N₂O sources, with mean annual soil fluxes ranging from 0.15 to 1.28 nmol m⁻² s⁻¹. Contrastingly, all sites were weak CH₄ sinks, with soil uptake rates ranging from −0.56 to −0.15 nmol m⁻² s⁻¹. Mean annual soil and plant respiration losses of CO₂, 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 soil N₂O emissions differed considerably among the three grasslands (adjusted r² ranging from 0.19 to 0.42), CH₄ and CO₂ soil fluxes were much better constrained (adjusted r² ranging from 0.46 to 0.80) by soil water content and air temperature, respectively. Throughout the year, spatial heterogeneity was particularly high for soil N₂O and CH₄ fluxes. We found permanent hot spots for soil N₂O emissions as well as locations of permanently lower soil CH₄ uptake rates at the extensively managed alpine site. Including hot spots was essential to obtain a representative mean soil flux for the respective ecosystem. At the intensively managed grassland, management effects clearly dominated over effects of environmental drivers on soil N₂O fluxes. For CO₂ and CH₄, the importance of management effects did depend on the status of the vegetation (LAI).