Climate and site management as driving factors for the atmospheric greenhouse gas exchange of a restored wetland

Abstract

The atmospheric greenhouse gas (GHG) budget of a restored wetland in western Denmark was established for the years 2009–2011 from eddy covariance measurements of carbon dioxide (CO₂) and methane (CH₄) fluxes. The water table in the wetland, which was restored in 2002, was unregulated, and the vegetation height was limited through occasional grazing by cattle and grass cutting. The annual net CO₂ uptake varied between 195 and 983 g m⁻² and the annual net CH₄ release varied between 11 and 17 g m⁻². In all three years the wetland was a carbon sink and removed between 42 and 259 g C m⁻² from the atmosphere. However, in terms of the full annual GHG budget (assuming that 1 g CH₄ is equivalent to 25 g CO₂ with respect to the greenhouse effect over a time horizon of 100 years) the wetland was a sink in 2009, a source in 2010 and neutral in 2011. Complementary observations of meteorological factors and management activities were used to explain the large inter-annual variations in the full atmospheric GHG budget of the wetland. The largest impact on the annual GHG fluxes, eventually defining their sign, came from site management through changes in grazing duration and animal stocking density. These changes accounted for half of the observed variability in the CO₂ fluxes and about two thirds of the variability in CH₄ fluxes. An unusually long period of snow cover in 2010 had the second largest effect on the annual CO₂ flux, whose interannual variability was larger than that of the CH₄ flux. Since integrated CO₂ and CH₄ flux data from restored wetlands are still very rare, it is concluded that more long-term flux measurements are needed to quantify the effects of ecosystem disturbance, in terms of management activities and exceptional weather patterns, on the atmospheric GHG budget more accurately.