Exposure to progressive inputs of anthropogenic N deposition elicits a series of changes in terrestrial ecosystems associated with both enhancement and inhibition of biogeochemical processes. Initially, growth of plants is enhanced as the constraint of N supply on NPP is relaxed, sometimes associated with altered dominance of species and gains or losses in diversity. Higher rates of nitrification and leaching of base cations eventually lead to acidification of soils, increases in soluble aluminum, and potentially to decreases in rates of NPP. Experiments in alpine ecosystems of several mountain ranges, characterized by low rates of N cycling, a flora adapted to low N supply, and acidic granitic parent material, indicate that enhancement of NPP is modest, and the potential for soil acidification is high. At the Niwot Ridge LTER site (ambient deposition 8 kg N/ha/yr) and Rocky Mountain National Park (ambient deposition 4 kg N/ha/yr), increases in NPP are modest (< 10 % increase, lower than interannual variability) with N additions. Soils at Niwot Ridge lose base cations, have increases in extractable aluminum, and significant decreases in pH after 8 years of elevated N inputs (> 20 kg N/ha/year). At an ILTER site in the Western Tatra Mountains of Slovakia, which historically received N deposition > 20 kg N/ha/yr, rates of NPP decreases with increases in N supply, associated with loss of nutrient cations and increases in soluble iron. Taken together, the results from these and other studies suggest minimal biological buffering in alpine ecosystems, and the potential for a rapid transition from minimal eutrophication to acidification.