Line-averaged concentrations of N2O and NH3 in the feedlots were measured continuously in campaigns of approximately 10 days in summer and winter at two feedlots in each of 2007 and 2008, using openpath laser systems and an open-path Fourier transform infrared spectrometer. In 2008, the summer campaign at one feedlot was extended for a further 6 months through autumn and winter by using a closed path chemiluminescence analyser to measure concentrations of NH3 and NOx. Emissions of the various gases were inferred through a software package, WindTrax (Thunder Beach Scientific), which simulates trajectories of particles backwards from the sensor using a backward Lagrangian stochastic dispersion model. Fluxes are calculated from the numbers of particle touchdowns inside and outside the source area.
The overall mean of the N2O emissions and its standard deviation were 1.30 ± 1.65 kg N2O-N/ha.d. The corresponding figures for NH3 were 95 ± 36 kg NH3-N/ha.d, and for NOx, 1.20 ± 0.58 kg NOx-N/ha.d. Mosier et al. (1998) suggest that about 1 % of the NH3 and NOx released into the atmosphere is eventually converted to N2O after deposition. Using that figure and the above emission data, we estimate a net contribution of N2O to the atmosphere from Australian cattle feedlots through emissions of NH3, NOx and N2O itself of 0.42 Mt CO2-e, 75 % of which comes from the indirect greenhouse gases NH3 and NOx. These direct and indirect N2O emissions are substantial, more than 60% in terms of CO2-e of the emissions of CH4 from feedlots. Further whole-year studies are needed. As well, the ecological impact of the 99% of the deposited N remaining after emission as NH3 and NOx requires investigation.