Nitrous oxide emissions from a fertilized and grazed grassland in the South East of Ireland

Agriculture currently accounts for 28% of national greenhouse gas emissions in Ireland. Nitrous oxide (N₂O) emissions from agricultural soils account for 38% of this total. A 2-year study was conducted, using the chamber technique on a fertilized and grazed grassland to quantify the effect of fertilizer application rate, soil and meteorological variables on N₂O emissions. Three N fertilizer regimes (0, 225 & 390 kg N ha^-1) were imposed with three replicates of each treatment. Nitrogen fertilizer was applied as urea (46% N) in spring with calcium ammonium nitrate (CAN-26% N) applied in the summer (June-September). Rotational grazing was practiced using steers. Nitrous oxide emissions arising from the unfertilized plots (0 N) were consistently low. Emissions from the N-fertilized plots (225 & 390 kg N ha^-1) were concentrated in relatively short periods (1-2 weeks) following fertilizer applications and grazing, with marked differences between treatments, relative patterns and magnitudes of emissions at different times of the year and between years. Variation in N₂O emissions throughout both years was pronounced with mean coefficients of variation of 116% in year 1 and 101% in year 2. The study encompassed two climatologically contrasting years. As a result the N₂O-N loss, as a percent of the N applied in the cooler and wetter 2002 (0.2-2.0%), were similar to those used for N-fertilized grasslands under the Intergovernmental Panel on Climate Change (IPCC) N₂O emission inventory calculation methodology (1.25% ± 1). In contrast, the percentage losses in the warmer and drier 2003 (3.5-7.2%) were substantially higher.