INPUTS AND STORAGE OF NITROGEN IN WINTER SNOWPACK IN AN ALPINE ECOSYSTEM

Inorganic nitrogen concentrations in winter snow were measured on Niwot Ridge, Colorado, to estimate the potential inputs of this nutrient into an alpine ecosystem for support of early season growth of plants and soil microorganisms. Concentrations of both NO3--N and NH4+-N increased in snow from February to May, reaching maximum values just prior to the onset of the growing season, which commences in early to mid-June. Vertical profiles of nitrogen concentration indicated downward movement of N in meltwater beginning in April. The highest N concentrations from April to May were found just above ice lenses. Potential N inputs into four plant communities from N released in snowmelt in 1990 and 1991 varied primarily according to differences in snow depth, and ranged from 53 mg m-2 yr-1 in a dry meadow community to 606 mg m-2 yr-1 in a snowbed community. This represents approximately 14 to 101% of the potential total atmospheric inputs of N from estimations of bulk deposition without horizontal redistribution of snow, and is as much as 30% of inputs from net N mineralization of soil organic matter. Thus reservoirs of N in snowpack are a potentially large source of N supporting plant growth, and may explain in part the spatial heterogeneity of primary production in alpine ecosystems.