Nitrous Oxide Emission from Grazed Grassland Under Different Management Systems

Nitrous oxide (N₂O) emissions from grazed grasslands are estimated to be approximately 28% of global anthropogenic N₂O emissions. Estimating the N₂O flux from grassland soils is difficult because of its episodic nature. This study aimed to quantify the N₂O emissions, the annual N₂O flux and the emission factor (EF), and also to investigate the influence of environmental and soil variables controlling N₂O emissions from grazed grassland. Nitrous oxide emissions were measured using static chambers at eight different grasslands in the South of Ireland from September 2007 to August 2009. The instantaneous N₂O flux values ranged from -186 to 885. 6 μg N₂O-N m^-2 h^-1 and the annual sum ranged from 2 ± 3. 51 to 12. 55 ± 2. 83 kg N₂O-N ha^-1 y^-1 for managed sites. The emission factor ranged from 1. 3 to 3. 4%. The overall EF of 1. 81% is about 69% higher than the Intergovernmental Panel on Climate Change (IPCC) default EF value of 1. 25% which is currently used by the Irish Environmental Protection Agency (EPA) to estimate N₂O emission in Ireland. At an N applied of approximately 300 kg ha^-1 y^-1, the N₂O emissions are approximately 5. 0 kg N₂O-N ha^-1 y^-1, whereas the N₂O emissions double to approximately 10 kg N ha^-1 for an N applied of 400 kg N ha^-1 y^-1. The sites with higher fluxes were associated with intensive N-input and frequent cattle grazing. The N₂O flux at 17°C was five times greater than that at 5°C. Similarly, the N₂O emissions increased with increasing water filled pore space (WFPS) with maximum N₂O emissions occurring at 60-80% WFPS. We conclude that N application below 300 kg ha^-1 y^-1 and restricted grazing on seasonally wet soils will reduce N₂O emissions.