Carbon budget of grassland under two management regimes

Results of the first two years of continuous eddy correlation (EC) measurements of the CO2 exchange over intensively- and extensively-managed grassland fields in Switzerland are presented. Special emphasis is placed on the quality problems of the specific EC measurements like accurate quantification of high frequency loss and appropriate identification of non-turbulent conditions. For both problems, operational spectral methods were deployed. A considerable part of the EC measurements had to be rejected due to a frequent breakdown of turbulence at night and the limited fetch at this site. Thus empirical temperature- and light-response curves were fitted to available EC data in order to fill the gaps and provide continuous time series of the CO2 exchange flux. The total carbon budget of the grassland ecosystems was computed as the sum of the CO2 flux and the non-gaseous carbon exchange, i.e. the C-export by harvest (hay or silage) and the C-import by slurry application to the field. Management depending differences and seasonal variations will be discussed. The resulting annual carbon budget of both fields was found to be closed within the level of experimental uncertainties.