Over a 3-years period, the CO2, N2O and CH4 fluxes exchanged with the atmosphere were studied in an upland semi-natural grassland site which was divided at the start of the experiment in two large paddocks continuously grazed by cattle. The soil at the site is an Andosol with high organic matter content. The intensively managed paddock was supplied with mineral N fertilizer and was grazed at a target sward height of 6 cm. The extensively managed paddock received no N fertilization and was stocked at half the stocking density of the intensive paddock. The net ecosystem exchange of CO2 was continuously measured in each paddock using the eddy covariance technique. Nitrous oxide emissions were measured fortnightly in both paddocks using static chambers. Methane emissions by the grazing cattle were measured four times per year in each paddock using the SF6 tracer method. Averaged across the 3 years, the two paddocks were net sinks of atmospheric CO2 (97 and 75 g CO2-C m-2 year-1 in the intensive and extensive treatments, respectively). Taking into account the LW gain of the cattle and the C loss through methane emissions, the net C storage was estimated at 87 and 69 g C m-2 year-1 in the intensive and extensive treatments, respectively. Emissions of nitrous oxide and methane reduced by 89 and 55% the atmospheric sink activity of the intensive and extensive treatments, respectively. The average greenhouse gas (GHG) balance across the 3 years was -10 and -31 g CO2-C equivalents in the intensive and extensive treatments, respectively. However, the net biome productivity (NBP) and GHG sink activities increased over time in the intensive grazing treatment, whereas they declined after 1 year in the extensive treatment, possibly as a result of a reduced nitrogen status of the vegetation. It is concluded that the suppression of fertilizer N supply combined with a strong reduction in grazing pressure may not be able to increase in the short term the GHG sink per unit land area of managed grasslands. © 2006 Elsevier B.V. All rights reserved.