Managed grasslands: A greenhouse gas sink or source?

We describe a unique, one year investigation of CO 2 and N 2 O fluxes over a fertilized grassland in Ireland using two eddy covariance systems. As the global warming potential (GWP) of N 2 O is 296 (100 year time horizon), relatively small N 2 O emissions have a potentially large impact on overall radiative forcing. Therefore nitrogen fertilizer application practices may possibly turn a site with a net CO 2 uptake into a net radiative forcing source. We observed a net annual uptake of 9.45 T CO 2 ha −1 . N 2 O emissions equivalent to 5.42 T ha −1 CO 2 GWP counteracted 57% of the effect of the CO 2 uptake. Estimated methane emissions from ruminants (3.74 T ha −1 CO 2 GWP) further counteract the CO 2 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.