We describe a unique, one year investigation of CO2 and N2O fluxes over a fertilized grassland in Ireland using two eddy covariance systems. As the global warming potential (GWP) of N2O is 296 ( 100 year time horizon), relatively small N2O emissions have a potentially large impact on overall radiative forcing. Therefore nitrogen fertilizer application practices may possibly turn a site with a net CO2 uptake into a net radiative forcing source. We observed a net annual uptake of 9.45 T CO2 ha(-1). N2O emissions equivalent to 5.42 T ha(-1) CO2 GWP counteracted 57% of the effect of the CO2 uptake. Estimated methane emissions from ruminants (3.74 T ha(-1) CO2 GWP) further counteract the CO2 uptake, making the overall GWP nearly neutral. This delicate balance of the greenhouse gas fluxes underscores the significance of fertilizer application strategies in determining whether a managed grassland is a net GWP source or sink.