Ammonia (NH3) emissions to the atmosphere increased significantly during the 20th century, largely due to the intensification of agricultural production. Ammonia is a soluble and reactive gas that is emitted by volatilization from various agricultural nitrogen forms including urea, uric acid and mineral fertilizers. Emissions are dependent on various meteorological inputs like temperature and wind speed, and are higher in warmer drying conditions, with smaller emissions occurring under cooler wetter conditions. Impacts of excess nitrogen can include eutrophication and acidification effects on semi-natural ecosystems that can lead to species composition changes. Agroforestry Ammonia Abatement (AAA) is a practical concept which uses both the dispersive effect of a barrier and the uptake of NH3 into the tree canopy to mitigate NH3 emissions. This work built upon the research carried out in Defra project AC0201, bringing together measurements, modelling and agroeconomic analyses to build an assessment of the potential benefits and drawbacks of applying AAA strategies both on a local and national scale. The project objectives were to assess the efficacy of farm woodland features for the recapture of agricultural NH3 emissions. The potential of farm woodlands for NH3 mitigation at a local and the UK scale were assessed. The combined modelling and measurement results from this project show that AAA carefully planned and implemented can lead to a significant decrease in NH3 concentrations downwind from sources and a moderate, up to 20% net decrease in emissions to the atmosphere. AAA systems could be used as a protective measure of downwind sensitive ecosystems where local concentration reductions can be higher. Use of existing woodland plantations and panting new forestry can both be used to mitigate emissions, though scrubbing of NH3 at source and reuse would also be a solution. UK scale modelling shows that targeted application of tree planting around agricultural installations would have a modest effect by modifying ‘on-farm’ emission factors, however when the approach is targeted in regions hot-spot emissions, significant effects on NH3 and N-deposition can be achieved. In many agricultural businesses there are no current economic advantages for converting valuable arable land to woodland without specific opportunity benefits (e.g. woodland egg price margins due to animal welfare considerations, carbon or nitrogen credits). However as the woodland egg example shows, when other considerations become relevant, AAA can be a useful approach. It is noted that mitigating ammonia with trees only addresses one nitrogen flow in the farming systems and the net effect on both the reactive and GHG N budgets over the landscape scale should be considered.
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