An evaluation of four years of nitrous oxide fluxes after application of ammonium nitrate and urea fertilisers measured using the eddy covariance method

In this study, we present the first long-term N2O eddy covariance dataset measured from a working farm. The eddy covariance method was used over a four year period to measure fluxes of the greenhouse gas nitrous oxide (N2O) from an intensively managed grazed grassland, to which regular applications of ammonium nitrate or urea fertilisers were spread, for two years each at the field site. The mean emission factors (EFs) reported for ammonium nitrate and urea fertiliser applications in this study over a period of 30 days after fertilisation, were 0.90 and 1.73% of the nitrogen applied, respectively, with EFs of individual events ranging between 0.13 and 5.71%. Our study accurately quantifies emission factors for multiple events and showing unambiguously that large-scale variability is real. EFs do indeed vary from one fertiliser event to another, even at the same site with the same fertiliser type under similar environmental conditions. This makes distinguishing EFs between different fertiliser types for the purposes of developing emission mitigation policy very difficult.