Effect of Nitrogen Source on Nitrous Oxide Emissions in Spring Barley.

Agriculture accounts for approximately one third of Ireland’s greenhouse gas (GHG) emissions. The addition of nitrogen (N) fertilizers to soils is one of the main contributors to emissions of N₂O, a GHG approximately 300 times more potent than carbon dioxide. In Ireland calcium ammonium nitrate (CAN) is the dominant N fertilizer source used by farmers. Half of the N in CAN is in the nitrate form and this is immediately available for denitrification loss potentially as N₂O. The use of urea has shown potential to reduce N₂O losses but could result in an increase in N loss through ammonia volatilization. The addition of inhibitors to N fertilizers show promise for mitigating both denitrification and volatilization losses from agricultural systems. The addition of a urease inhibitor can reduce the rate of urea hydrolysis which reduces the loss of N through ammonia volatilization. The addition of a nitrification inhibitor slows down the conversion of ammonium to nitrate thereby reducing the nitrate pool available for denitrification.

The objectives of this study were to assess the effect of nitrogen source (CAN vs. urea) on N₂O emissions and also to assess the effectiveness of urea with inhibitors for reducing N₂O emissions in spring barley production. This study was conducted on two contrasting sites (a long-term arable free-draining loam site and a short-term arable moderately draining sandy loam site) in the south east of Ireland. Fertilizer N was applied at 150 kg N ha^-1 and N₂O emissions were measured using the static chamber technique. Emissions were measured for 365 days and trapezoidal integration was used for calculation of cumulative emissions. Emission factors were generated each year and compared to the IPCC default of 1%.

Results show that emission factors at the long-term arable site were much lower than the IPCC default value of 1%. Using the IPCC default value would overestimate emission factors and there is a need for more accurate measurements for Ireland’s national inventory of N₂O emissions. Emission Factors for the short-term arable site were higher than the IPCC default for CAN, Urea and Urea with the urease inhibitor but lower for both nitrification inhibitor treatments Emission factors for the nitrification inhibitor gave consistently lower emission factors than the other treatments at both sites. The nitrification inhibitor has the potential to further reduce emission factors for spring barley in Ireland.