Regional mass budgets of oxidized and reduced nitrogen and their relative contribution to the nitrogen inputs of sensitive ecosystems

Wet deposition of nitrogen is reasonably well monitored throughout Europe, whereas the dry deposition inputs are provided largely by models. Recent long-term measurements of NO2 and NH4 fluxes to semi-natural vegetation have shown that rates of NH3 deposition exceed those of NO2, typically by an order of magnitude. Incorporating the results of these dry deposition measurements in regional deposition budgets shows that the inputs of reduced nitrogen contribute the dominant fraction of the total nitrogen inputs in most regions of the UK. The results are illustrated by comparing the atmospheric mass-budget for oxidized nitrogen over the UK. Of the annual UK emissions of NOx, amounting to 780 kt N (Salway et al., 1997), only 5% is dry deposited to terrestrial surfaces within the country while 15% is wet deposited, whereas for the reduced nitrogen, 42% of emissions (of the 260 kt N, Salway et al., 1997) are dry deposited and 46% are wet deposited. Even more striking are the relative contributions of oxidized and reduced nitrogen to semi-natural vegetation, which is a particularly efficient sink for NH3. The species composition of semi-natural vegetation is also regarded as very sensitive to nitrogen inputs. The distribution of nitrogen deposition among different land uses shows that the average input to forest in the UK is 33 kg N ha(-1) annually of which 78% is reduced nitrogen. The other land uses receive about 15 kg N ha(-1) of nitrogen of which between 55% and 65% is NHx. Critical loads for nutrient nitrogen are exceeded primarily in forested and moorland areas as a consequence of NH3 dry deposition and wet NH4+ deposition. For forests the area in exceedance of 20 kg N ha(-1) year(-1) critical load represents 70% of the forest area (1.4 x 10(6) ha(-1)) while for moorland the area in exceedance is 13% of the moorland area and occupies 1.04 x 10(6) ha(-1)).