New paper finds grasslands are a net source of CO2 to the atmosphere

A paper published today in Biogeosciences unexpectedly finds farmed grasslands are a net annual source of CO2 to the atmosphere, and also a weak methane sink to the atmosphere. The authors studied 3 Swiss grasslands managed at different levels of intensity, one without fertilizer applied, one with moderate use, and one extensively managed with heavy use of fertilizer, but finds all three acted as net sources of CO2 to the atmosphere, finding "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."

CO2 fluxes at 3 differently managed grasslands shown in bottom row of graphs. Positive values indicate a net source of CO2 to the atmosphere. Top graphs show grasslands are also a net source of the "greenhouse" gas N20, and middle graphs show grasslands are a weak sink for methane [CH4].

Biogeosciences, 10, 5931-5945, 2013 www.biogeosciences.net/10/5931/2013/ doi:10.5194/bg-10-5931-2013 Temporal and spatial variations of soil CO2, CH4 and N2O fluxes at three differently managed grasslands

D. Imer, L. Merbold, W. Eugster, and N. Buchmann
Grassland Sciences Group, Institute of Agricultural Sciences, ETH Zurich, Zurich, Switzerland
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. [positive numbers indicate a net source] 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).



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