N2O Emissions From a Fertilized Grassland: Modeling and Measurements

N2O emissions from a fertilized grassland near Cork, Ireland were continuously measured during 2003 using the eddy covariance method. For most of the year N2O emissions were close to zero and the bulk of the emission occurred in only eight major events, each event lasting from two to twenty days. This data set was used to test the N2O emissions predicted by the DNDC (DeNitrification-DeComposition) model. Good agreement between measured and modeled N2O emissions was found, thus providing a basis for using the model to simulate (a) background N2O emissions and (b) the effect of future climate perturbations on N2O emissions. Our future climate projections are based on the standard Hadley Center model output of the IS92a scenario for Ireland in which the average temperature increases for the whole year and precipitation increases in winter but decreases in summer. The measured annual N2O emission was 11.6 kg N/ha. A total of 335 kg/ha of chemical and organic N was applied, giving an emission factor of 3.4%, uncorrected for background emission. Based on the DNDC model prediction with zero N applications, the background emission accounts for 15% of the total annual observed N2O emission. The DNDC model predicts that the future climate shifts based on the IS92a scenario will increase total annual N2O emission by 45% (7.0 kg N2O-N/ha). By comparison, a decrease of 10% in total N applications (in order to comply with EU water quality legislation) leads to a predicted reduction in current N2O emissions of only 7% (0.9 kg N2O-N/ha). Thus the projected increase in N2O emission caused by climate perturbations is far larger than the decrease expected from reduced fertilization to comply with water quality legislation.