Controls on nitrogen flux in alpine/subalpine watersheds of Colorado

High-altitude watersheds in the Front Range of Colorado show symptoms of ad vanced stages of nitrogen excess, despite having less nitrogen in atmospher ic deposition than other regions where watersheds retain nitrogen. In two a lpine/subalpine subbasins of the Loch Vale watershed, atmospheric depositio n of NO3- plus NH4+ was 3.2-5.5 kg N ha(-1), and watershed export was 1.8-3 .9 kg N ha(-1) for water years 1992-1997. Annual N export increased in year s with greater input of N, but most of the additional N was retained in the watershed, indicating that parts of the ecosystem are nitrogen-limited. Di ssolved inorganic nitrogen (DIN) concentrations were greatest in subsurface water of talus landscapes, where mineralization and nitrification augment high rates of atmospheric deposition of N. Tundra landscapes had moderately high DIN concentrations, whereas forest and wetland landscapes had low con centrations, indicating little export of nitrogen from these landscapes. Be tween the two subbasins the catchment of Icy Brook had greater retention of nitrogen than that of Andrews Creek because of landscape and hydrologic ch aracteristics that favor greater N assimilation in both the terrestrial and aquatic ecosystems. These results suggest that export of N from alpine/sub alpine watersheds is caused by a combination of direct flushing of N from a tmospheric deposition and release of N from ecosystem biogeochemical proces ses (N cycling). Sensitivity of alpine ecosystems in the western United Sta tes to atmospheric deposition of N is a function of landscape heterogeneity , hydrologic flow paths, and climatic extremes that limit primary productiv ity and microbial activity, which, in turn, control retention and release o f nitrogen. Conceptual and mechanistic models of N excess that have been de veloped for forested ecosystems need to be modified in order to predict the response of alpine ecosystems to future changes in climate and atmospheric deposition of N.