Use of the 15 N gas flux method to measure the sourceand level of N2O and N2 emissions from grazed grassland

Understanding the contribution of nitrification and denitrification to production of nitrousoxide (N2O), a potent greenhouse gas, is important in devising effective mitigation strategies to reduce emissions. In this study the 15N gas flux method was used to investigate N2O and N2 emissions following an application of 15N labelled ammonium nitrate (0.71 mol N m-2) to intensive grassland swards (grazed at 2.74 or 2.05 livestock units ha-1 year-1) at a site in Southern Ireland. The 15N labelled fertiliser (NO3 moiety 15N labelled at 60 at. %) was applied to designated soil areas in the field, enclosed by staticchambers, in June 2009, September 2009 and March 2010. Fluxes of N2O and N2 were determined over 12 days on each occasion. N2O and N2 emissions were significantly (P\0.001) lower in March 2010 than in June or September 2009. There was little difference between the two swards grazed at different stocking rates on N2O or N2 emissions. Mean cumulative N2O emissions over 103 h were 212.9, 279.5 and 62.06 mg m-2 for June 2009, September 2009 and March 2010, respectively. Mean cumulative N2 emissions for the three time periods were 818.8, 893.8 and 87 mg m-2, respectively. The N2O mole fractionaveraged 0.21 and 0.23 in June 2009 and September 2009, respectively, but increased to 0.41 in March 2010 which may have been due to changes in denitrifier community composition or due to N2O reductase being sensitive to low soil temperatures. Theresults point to denitrification of nitrate as the major source of N2O at this site which may have implications for choice of fertiliser in moist temperate climates.