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 ha1 yr1 in approximately 90% of the plots, but values up to 40008000 mol ha1 yr1 were also observed. Approximately 50% of the plots received N inputs, dominated by NH4, above 1000 mol ha1 yr1, which is a deposition level at which species diversity of the ground vegetation may be at risk. Results of inputoutput budgets for plots concentrated in Northern and Western Europe indicate that nitrate leaching starts to occur at throughfall inputs above 10 kg ha1 yr1, 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.Key words: nitrogen, deposition, inputoutput budgets, nitrogen status, forests, effects.
Microbial Mechanisms Mediating Increased Soil C Storage under Elevated Atmospheric N Deposition