ATMOSPHERIC LOADING OF NITROGEN TO ALPINE TUNDRA IN THE COLORADO FRONT RANGE

Dry deposition of atmospheric nitrogen gas and aerosol species was estimated for the alpine tundra of Niwot Ridge, 3525 m elevation in the Colorado Rockies. Comparisons, for the 4 month long growing season and the remaining 8 months of the year, were made with wet deposition and throughfall incident measurements taken during 1987-1989. Dry deposition of N to the tundra is estimated to be equal to or slightly greater than its wet deposition. During the mid-May to mid-September growing season, atmospheric N deposition is >1.0 mg N m(-2)d(-1) directly from the atmosphere with a similar amount contributed indirectly as NO3-N in snowmelt water as a result of dry and wet deposition to the winter snowpack. The total N deposition to Niwot Ridge tundra during the growing season of about 2 mg N m(-2)d(-1) may be compared to an earlier measurement of dry plus fog deposition (1 - 2 mg N m(-2)d(-1)) to a subalpine coniferous canopy at Niwot Ridge. Nitrate yields from two small drainage basins at Niwot Ridge match these fluxes. Seven years of record from an unvegetated glacial cirque suggest an average yield from the alpine of 0.7 mg N m(-2)d(-1). The equivalent estimate for a basin with 50% tundra vegetation cover is 0.4 mg N m(-2)d(-1). The contrast in these two estimates of daily averaged N yields for the entire year suggests the retention of nearly 1 mg N m(-2)d(-1) during the growing season in the more vegetated basin. The sink for this N could be the tundra soil and vegetation where biological activity is often limited by the availability of N.