Fertiliser Nitrogen Formulation Affects Nitrous Oxide Emissions from Grassland.

In 2012 Agriculture was responsible for 31% of Ireland’s Agricultural Greenhouse Gas (GHG) emissions, with 39% of these emissions arising from chemical/organic nitrogen fertilizers in the form of nitrous oxide (N₂O). Switching from calcium ammonium nitrate (CAN), the predominant straight source of nitrogen used by Irish farmers, to a urea based fertiliser, with or without a nitrification inhibitor, limits the soil residence period of nitrate, the major substrate for denitrification loss in the N₂O form. However, urea is susceptible to ammonia (NH₃) volatilisation but this risk can be managed using urease inhibitors. The aim of this study was to evaluate the effect of switching from CAN to urea, urea with the urease inhibitor N- (n-butyl) thiophosphoric triamide (n-BTPT) (trade name Agrotain®) and/or the nitrification inhibitor dicyandiamide (DCD) on N₂O emissions in temperate grassland.

The study took place over two years at six permanent pasture sites located at: Johnstown Castle, Co. Wexford, Ireland; Moorepark, Co. Cork, Ireland and Hillsborough, Co. Down, Northern Ireland; covering a range of soil textures and drainage characteristics. The experiment simulated a grazing environment; the annual fertiliser N was applied at 200 kg N ha-1 in five equal splits. The N₂O emissions were measured frequently throughout the year using the static chamber method.

Results from this study, based on two years of data, show that switching from CAN to urea dramatically reduced N₂O emissions.  The N₂O emissions generally following the trend drainage impeded > moderately drained > well drained and fertiliser N formulation direct N₂O fluxes generally followed the trend CAN > urea with n-BTPT > urea > urea with DCD and urea with n-BTPT and DCD. Results indicate that the use of urea based N fertilisers in place of CAN show promise as a strategy to reduce N₂O emissions from intensive grassland, particularly when used in combination with DCD.