Response to Increased Nitrogen Deposition in Alpine Soils at Three Pacific Northwest National Parks.

As the Western United States increases in population, urbanization, and industrial capacity, the effects of nitrogen pollution are a key concern in the protection of oligotrophic ecosystems such as the alpine. Critical loads for nitrogen deposition have in some cases already been exceeded in the Pacific Northwest. Our 3-year study used fertilization to mimic increasing levels of nitrogen deposition in three different alpine meadow ecosystems (at Mount Rainier, North Cascades, and Olympic National Parks) at levels of 0, 3, 5, and 10 kg NH₄NO₃-N ha^-1 yr^-1.  We examined soil nitrogen pools and inorganic N fluxes with increasing treatment as part of an effort to define critical N loads for the alpine Pacific Northwest. Nitrate flux and some inorganic nitrogen pools show signs of increase with increasing N fertilization at MORA and NOCA, while OLYM soils show little to no treatment effect and high overall organic N availability. No changes in organic nitrogen pools or microbial biomass were observed in response to treatment. Levels of soil extractable nitrate and microbial nitrogen content in summer and of ammonium content in fall are the most significant factors for site differences. Observable soil N pool and flux response to increased N deposition appear to be most significant in areas with low soil N availability and  high C:N ratios. However, high N, sparsely vegetated, moisture-limited alpine soil communities such as those at OLYM may be at highest risk for fall leaching of N deposition.