Nitrogen deposition and effects on European forests

Hypotheses about the impacts of elevated atmospheric deposition of nitrogen on the forest ecosystem include an increased sensitivity to natural stress, impacts on roots, reduced species diversity of the ground vegetation, reduced growth, and unbalanced nutritional status due to eutrophication and acidification. The impact of N deposition has gained in ecological importance during recent decades, in part due to the steady decline in S emissions. Results of throughfall and deposition measurements at 163 plots in Europe show that total deposition of S and N compounds ranged from 100 to 3000 mol ha/yr in approximately 90% of the plots, but values up to 4000-8000 mol ha/yr were also observed. Approximately 50% of the plots received N inputs, dominated byNH4, above 1000 mol ha/yr which is a deposition level at which species diversity of the ground vegetation may be at risk. Results of input-output budgets for plots concentrated in Northern and Western Europe indicate that nitrate leaching starts to occurat throughfall inputs above 10 kg ha-1 yr-1, specifically in soils with C/N ratios in the humus layer below 25. Examples are given of field evidence for impacts of elevated N deposition, including elevated N contents in foliage and soil, Al release in soil response to increased nitrate concentrations, reduced shoot/root ratios, and a reduction in species diversity. Although knowledge about the response of forest ecosystems to N inputs has increased over the last decade, there is still a lack of information on the dynamics of N accumulation and related critical N loads in a range of environmental conditions. Furthermore, a European-wide perspective of N saturation in forest ecosystems is still lacking