The relationship between NH₃ emissions from a poultry farm and soil NO and N₂O fluxes from a downwind forest

Abstract. Intensive livestock farms emit large concentrations of NH₃, most of which is deposited very close to the source. The presence of trees enhances the deposition. Rates to downwind forests can exceed 40 kg N ha⁻¹ y⁻¹. The steep gradient in large NH₃ concentrations of 34.3±20.4, 47.6±24.9, 21.7±16.8 µg NH₃ m³ at the edge of a forest 15, 30 and 45 m downwind of the farm to near background concentrations within 270 m downwind (1.15±0.7 µg NH₃ m³) provides an ideal site to study the effect of different rates of atmospheric NH₃ concentrations and inferred deposition on biological and chemical processes under similar environmental conditions. We have investigated the effect of different NH₃ concentrations and implied deposition rates on the flux of NO and N₂O from soil in a mixed woodland downwind of a large poultry farm (160 000 birds) in Scotland, which has been operating for about 40 years. Measurements were carried out for a 6 month period, with hourly NO flux measurements, daily N₂O fluxes close to the farm and monthly at all sites, and monthly cumulative wet and dry N deposition. The increased NH₃ and NH₄⁺ deposition to the woodland increased emissions of NO and N₂O and soil available NH₄⁺ and NO₃⁻ concentrations. Average NO and N₂O fluxes measured 15, 25 and 45 m downwind of the farm were 111.2±41.1, 123.3±40.7, 38.3±28.8 µg NO-N m⁻² h⁻¹ and 9.9±7.5, 34.3±33.3 and 21.2±6.1 µg N₂O-N m⁻² h⁻¹, respectively. At the background site 270 m downwind the N₂O flux was reduced to 1.75±2.1 µg N₂O-N m⁻² h⁻¹. NO emissions were significantly influenced by seasonal and daily changes in soil temperature and followed a diurnal pattern with maximum emissions approximately 3 h after noon. For N₂O no consistent diurnal pattern was observed. Changes in soil moisture content had a less clear effect on the NO and N₂O flux. In spite of the large NO and N₂O emissions accounting for >3% of the N deposited to the woodland downwind of the farm, extrapolation to the entire British poultry flock suggests that these NH₃ emissions contribute to less than 0.5% and 0.02%, respectively of the total annual UK NO_x and N₂O emissions.