Catchment-Wide Atmospheric Greenhouse Gas Exchange as Influenced by Land Use Diversity

The turbulent fluxes of carbon dioxide between the land surface and the atmosphere were measured with the eddy covariance technique above three contrasting land use types in the Skjern River catchment in western Denmark, namely an agricultural area, a forest plantation, and a wet grassland. The measurements also included the turbulent fluxes of methane above the wet grassland and of nitrous oxide above the agricultural area and ran continuously throughout the year 2009. The highest CO2 uptake rates (around 30 mu mol m(-2) s(-1)) were observed at the agricultural site; however, the site was a CO2 sink only from April to June and a CO2 source during the rest of the year. Over the whole year the forest plantation fixed about 1850 g CO2 m(-2) compared to only 870 g m(-2) at the agricultural site, and it remained a CO2 sink throughout all seasons. The wet grassland site was a CO2 sink from March to October, and its annual CO2 fixation was only marginally higher than that of the agricultural site. The emission of CH4 from the wet grassland showed large seasonal variations. Its annual total corresponded to 276 g CO2 equivalents m(-2) (based on a 100-yr time horizon) and reduced the greenhouse gas sink strength of the site by one-third. At the agricultural site this sink strength was reduced by 9% through the N2O emissions. Scaled up to the catchment, the observed net uptake of CO2 by the land surface was reduced by roughly one-tenth, in terms of CO2 equivalents, due to the emission of CH4 and N2O.