Landscape fluxes and dry deposition velocity of ammonia near a cattle feedlot using flux gradient approach

Concentrated animal feeding operations (CAFOs) are emission hotspots of ammonia (NH₃). The NH₃ emitted from these hotspots can be locally recaptured by the surrounding vegetation, mainly due to dry deposition. This deposition can either have beneficial fertilizing effects for N-limited ecosystems or pose adverse impacts on sensitive ecosystems. However, there is a lack of direct measurements of NH₃ deposition near hotspots. We conducted two field campaigns to investigate the landscape NH₃ fluxes over the barley (winter), lentil (winter), and fallow (summer) fields adjacent to an intensive beef cattle feedlot in southeast Australia. The flux measurements were segregated into periods when the measurement location was upwind of the feedlot or downwind. Upwind of the feedlot, we observed upward fluxes (surface emissions) over the fallow and barley sites with daily means (± standard error) of 0.16 ± 0.02 and 0.007 ± 0.012 μg NH₃ m^-2 s^-1, and downward fluxes (deposition) over the lentil site with a daily mean of -0.022 ± 0.007 μg NH₃ m^-2 s^-1. These measurements indicated the NH₃ compensation point for barley was approximately 6.2 μg m^-3 (equivalent to the background atmospheric NH₃ concentration), and the NH₃ compensation point for lentils was lower than 3.4 μg m^-3. Downwind of the feedlot, we observed downward fluxes at all sites with daily means of -0.57 ± 0.09 μg NH₃ m^-2 s^-1 for the barley site, -1.26 ± 0.17 μg NH₃ m^-2 s^-1 for the lentil site, and -0.58 ± 0.12 μg NH₃ m^-2 s^-1 for the fallow site; the mean deposition velocities over the barley, lentil, and fallow sites were 0.74, 0.82 and 0.78 cm s^-1. Based on the frequency of upwind and downwind periods, we estimate that the accumulated N inputs to the barley, lentil and fallow fields during each campaign were 4.5, 14.8 and 4.3 kg N ha^-1, indicating that the deposition of NH₃ emitted from the feedlot serves as a significant source of N input to its adjacent fields. Our study can provide valuable information on NH₃ exchange between vegetation and atmosphere, and extend our understanding of the fate of NH₃ emitted from hotspots.