Ammonia emission, deposition and impact assessment at the field scale: a case study of sub-grid spatial variability.

A local ammonia (NH3) inventory for a 5x5 km area in central England was developed, to investigate the variability of emissions, deposition and impacts of NH3 at a field scale, as well as to assess the validity of the UK 5-km grid inventory. Input data were available for the study area for 1993 and 1996 on a field by field basis, allowing NH3 emissions to be calculated for each individual field, separately for livestock grazing, livestock housing and manure storage, landspreading of manures and fertiliser N application to crops and grassland. An existing atmospheric transport model was modified and applied to model air concentrations and deposition of NH3 at a fine spatial resolution (50 m grid). From the mapped deposition estimates and land cover information, critical loads and exceedances were derived. to study the implications of local variability for regional NH3 impacts assessments. The results show that the most extreme local variability in NH3 emissions, deposition and impacts is linked to housing and storage losses. However, landspreading of manures and intensive cattle grazing are other important area sources, which vary substantially in the landscape. Overall, the range of predicted emissions from agricultural land within the study area is 0-2000 kg N ha(-1) year(-1) in 1993 and 0-8000 kg N ha(-1) year(-1) in 1996, respectively, with the peak at a poultry farm located in the study area. On average, the estimated field level NH3 emissions over the study area closely match the emission for the equivalent 5-km grid square in the national inventory for 1996. Deposition and expected impacts are highly spatially variable, with the edges of woodland and small "islands" of semi-natural vegetation in intensive agricultural areas being most at risk from enhanced deposition. Conversely the centres of larger nature reserves receive less deposition than average. As a consequence of this local variability it is concluded that national assessments at the 5 km grid level underestimate the occurrence of critical loads exceedances due to NH3 in agricultural landscapes.

Validity and uncertainty in the calculation of an emission inventory for ammonia arising from agriculture in Great Britain.

Emissions of ammonia have received increasing attention recently, following concern about the environmental consequences, especially in The Netherlands where levels are high due to intensive livestock farming. Direct local effects and more widespread consequences for a range of ecosystems have been attributed to ammonia emissions. As the most prevalent alkaline gas in the atmosphere, ammonia interacts with acidic species, changing their characteristics, chemical and physical behaviour, and enhancing their potential for acidification of soils. Ammonia also forms an important component of the nitrogen cycle and of nitrogen deposition. In the UK, as in many other European countries, there has been a considerable increase in the emission of ammonia within the last 30 years, estimated at about 50%. This results mainly from increases in agricultural production based on the steadily rising number of livestock and increasing fertilizer consumption. This paper discusses the various sources to emissions of ammonia from agricultural sources in the United Kingdom, and some of the uncertainties involved in constructing a national emissions inventory.