Assessing atmospheric nitrogen deposition to natural and semi-natural ecosystems: Experience from Danish studies using the DAMOS

Local agricultural emissions contribute significantly to the atmospheric reactive nitrogen loads of Danish terrestrial ecosystems. In the vicinity of the sources this may be up to 6-8 kg N ha(-1) yr(-1) depending on location and ecosystem type. This contribution arises from dry deposition of gas phase ammonia derived from local livestock production. Long-range transport, however, often constitutes the largest contribution to the overall atmospheric terrestrial reactive nitrogen loadings in Denmark. This is often in the range 10-15 kg N ha(-1) yr(-1) and consists mainly of aerosol phase nitrate and ammonium (reaction products of nitrogen oxides and ammonia), but also dry deposition of other reactive nitrogen compounds (mainly nitrogen oxides in the form of gas phase nitric acid and nitrogen dioxide). In Denmark's environmental management of the sensitive terrestrial ecosystems modelling tools are required that account for both the local and the long-range transported contributions. This motivated development of the Danish Ammonia MOdelling System (DAMOS) that has been successfully applied to the assessment of atmospheric nitrogen loadings to sensitive Danish ecosystems. We present here three different examples of such assessments. Our results show that ecosystems located in Western Denmark (Case 1) receive the highest loads of atmospheric nitrogen depositions which generally exceed the critical load. This part of the country has the highest livestock density. In the Eastern part of the country, the atmospheric loadings are often below or close to the lower end of the interval for critical load values. These lower loads in Eastern Denmark (Case 2) are due to lower density of agricultural activities, as well as, lower precipitation rates, which leads to less wet deposition of reactive nitrogen. In general there is a gradient in atmospheric deposition over the country, with the highest depositions in the South-Western part of Denmark (Case 3) due to long-range transport contributions from North-Western Europe, but also due to local ammonia deposition associated with the high local emission from the high density livestock farming in this area.