ANALYSIS OF NITROGEN SATURATION POTENTIAL IN ROCKY-MOUNTAIN TUNDRA AND FOREST - IMPLICATIONS FOR AQUATIC SYSTEMS

We employed grass and forest versions of the CENTURY model under a ran ge of N deposition values (0.02-1.60 g N m(-2) y(-1)) to explore the p ossibility that high observed lake and stream N was due to terrestrial N saturation of alpine tundra and subalpine forest in Loch Vale Water shed, Rocky Mountain National Park, Colorado. Model results suggest th at N is limiting to subalpine forest productivity, but that excess lea chate from alpine tundra is sufficient to account for the current obse rved stream N. Tundra leachate, combined with N leached from exposed r ock surfaces, produce high N loads in aquatic ecosystems above treelin e in the Colorado Front Range. A combination of terrestrial leaching, large N inputs from snowmelt, high watershed gradients, rapid hydrolog ic flushing and lake turnover times, and possibly other nutrient limit ations of aquatic organisms constrain high elevation lakes and streams from assimilating even small increases in atmospheric N. CENTURY mode l simulations further suggest that, while increased N deposition will worsen the situation, nitrogen saturation is an ongoing phenomenon.