We apply a simulation model in order to quantify the patterns of carbon and nitrogen cycling within European grasslands. We map the fluxes of CO$_2$, N$_2$O and CH$_4$ exchanged with the atmosphere as controlled by climate and management conditions. We distinguish between grazing and cutting practice. Because geo-referenced management information for grasslands does not exist at the scale of Europe, we develop a new and robust set of rules defining some management variables. We then perform realistic simulations in term of N fertilization using a national level data set. The model results at European scale are compared with agricultural statistics (yield, animal stocks), which shows that our very simple management calculation is reasonably realistic. We also compare the simulated seasonal cycle of grassland phenology as calculated by PASIM with remote sensing observations from the EOS-TERRA-MODIS satellite, which shows a good general agreement. Emission factors for soil N$_2$O and grazing animals CH$_4$ emissions are diagnosed from the model runs and shown to be comparable to those of previous experimental surveys. We investigate impact of N fertilization on NPP and C storage potential, N$_2$O emissions by soils and CH$_4$ emissions by ruminants. We conclude that, for different time horizon, CH$_4$ and N$_2$O sources are lower than the potential sink of CO$_2$, on a carbon equivalent basis. This result is independent of fertilization intensity but assumes that the current soil C stocks are below the long-term equilibrium values.