N Deposition Effects on Vegetation and Soils in Alpine Ecosystems

William D. Bowman
Mountain Research Station
Institute of Arctic and Alpine Research
University of Colorado, Boulder, CO 80309-0450

Alpine ecosystems provide important ecosystem and social services, including clean water and recreation. Alpine regions are subjected to elevated rates of N deposition relative to lowlands due to orographic enhancement of precipitation. Thin soils and strong seasonality and low rates of biological activity render alpine ecosystems very sensitive to ecological effects of deposition. Critical loads for compositional changes in vegetation, soils, and surface water determined empirically and by modeling range between 4 and 6 Kg N/ha/year in the southern Rocky Mountains, similar to mountains with granitic parent material in Europe. There is a strong potential for decreases in soil pH and extractable base cation concentrations, and increases in extractable Al3+ with sustained N deposition in alpine soils of the southern Rockies. Increases in plant production are small and transient with increasing N availability. Collectively these results indicate near simultaneous eutrophication and acidification in these alpine sites, with marginal biological buffering. In more heavily impacted alpine sites in the Western Tatra Mountains, Slovakia, increases in N deposition decreases plant growth, and results in decreases in soil extractable Al3+, but gains in extractable Fe2+. The Western Tatras appear to be in a transition stage from Al to Fe buffering of soils, and extreme N saturation causing inhibition of plant growth. Although significant reductions in acid precipitation have been achieved through control of S emissions, greater attention should be given to the strong potential for environmental impacts from N deposition, particularly in sensitive ecosystems such as the alpine.