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The relationship between ammonia emissions from a poultry farm and soil NO and N 2 O fluxes from a downwind source

Intensive livestock farms emit large concentrations of NH 3 , most of which is deposited very close to the source. The presence of trees enhances the deposition. Rates to down wind forests can exceed 40 kg N ha -1 . The steep gradient in large NH 3 concentration and deposition at the edge of a downwind forest to background concentrations within a few hundred meters provides an ideal site to study the effect of different rates of N deposition on biological and chemical processes under similar environmental conditions. We have investigated the effect of different rates of NH 3 deposition (62, 45, 24 and 5 kg NH 3 -N ha -1 y -1 ) on the flux of NO and N 2 O from soil in a mixed woodland downwind of a large poultry farm (160000 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 N 2 O fluxes close to the farm and monthly at all sites and monthly cumulative wet and dry N deposition. The increased NH 3 and NH 4 + deposition to the woodland increased emissions of NO and N 2 O and soil available NH 4 + and NO 3 - concentrations. Average NO and N 2 O 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 N O -N m -2 h -1 , respectively. At the background site 270 m downwind the N 2 O flux was reduced to 1.75±2.1 µg N 2 O-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 3h after noon. For N 2 O no consistent diurnal pattern was observed. Changes in soil moisture content had a less clear effect on the NO and N 2 O flux. On average the NO emissions expressed as a fraction of the elevated N deposited were 7.1% (at 15 m), 6% (at 25 m) and 2.3% (at 45 m) downwind of the farm, whereas for N 2 O the emissions were only 2.8% (at 15 m), 3% (at 25 m) and 3% (at 45 m) downwind. These emission fractions exceed the emission factor of 1% advised by the IPCC for N 2 O emissions resulting from atmospheric N deposition.