Differing effects of increasing calcium ammonium nitrate, urea and urea + NBPT fertiliser rates on nitrous oxide emission factors at six temperate grassland sites in Ireland

The present study evaluated the impact of three nitrogen (N) fertiliser formulations, applied at five N rates, on nitrous oxide (N₂O) fluxes and annual direct N₂O-N emission factors (EF) in temperate grassland. Closed static chambers were used to measure direct N₂O fluxes at three geographically dispersed locations in Ireland over a two-year period, generating a total of 90 EFs across the six site-years and treatments. The three fertiliser formulations tested were calcium ammonium nitrate (CAN), urea, and urea amended with the urease inhibitor N-(n-butyl) thiophosphoric triamide (NBPT) at 100, 200, 300, 400 and 500 kg N ha⁻¹ yr⁻¹. All treatments were applied in five equal split applications ranging from 20 to 100 kg N ha⁻¹ split^-1 over the growing season. The N₂O-N EFs for CAN ranged from 0.39 − 4.68 with a mean of 1.62 (cv. 81 %), for urea from 0.04 – 1.7 with a mean of 0.46 (cv. 77 %) and for urea + NBPT from 0.18 – 1.7 with a mean of 0.60 (cv. 59 %). A significant positive relationship was found between the N rate and the annual N₂O-N EFs in three (CAN), five (urea) and two (urea + NBPT) of six the site-years. For the remainder of the site-years EF was unaffected by N rate. These results indicate that fertiliser N choice and rate can be management factors that enable farmers to alter N₂O losses in temperate grassland. Notably, the response of EF to increasing N rate was not consistent across the fertilisers, with the EF from urea being the most sensitive to the increasing N rate, urea + NBPT the least sensitive and CAN being intermediate. The accuracy of national greenhouse gas accounting could be improved by including N fertiliser formulation and its rate of application. Further research is also needed to understand the inconsistency in EF response to N rate across sites.