Natural variability in N export from headwater catchments: Snow cover controls on ecosystem N retention

The causes of natural variability in catchment scale N export need to be understood and quantified before the effects of increased N deposition in high elevation catchments can be evaluated. This study evaluates controls on the size of the leachable soil N pool concurrent with the spring hydrologic flush that is primarily responsible for the transport of N to surface water. In high elevation catchments in the western United States, sources of N during this snowmelt flush include both atmospheric N deposition stored in the snowpack until melt and mobile soil N pools, and sinks are dominated by biogeochemical processes that occur in soil under snow cover. Because soil processes may serve either as a source or sink for N, controls on the amount of inorganic N leached from soil during the snowmelt period were evaluated in the major landscape types in four catchments in Colorado. Measurements of leached N were inversely related to measurements of over-winter CO₂ flux at all sites, indicating that N was immobilized in soil heterotrophic biomass. Because over-winter soil heterotrophic activity is controlled primarily by the depth and timing of snow accumulation, the importance of these plot scale measurements to catchment scale N export were evaluated using a long-term record of winter precipitation, N deposition, and N export from Loch Vale in Rocky Mountain National Park. This data set identified a strong, linear relationship (r² = 0·68) between catchment scale N retention and winter snow cover, consistent with subnivean, soil based controls on the mobile N pool identified at the plot scale. These results indicate that the winter snow pack is the major control both on hydrologic N export and on soil source/sink relationships for N concurrent with this transport mechanism. The effect of winter snow cover on the fate of both atmospheric and soil N needs to be considered when evaluating potential the effects of increased N deposition on either terrestrial or aquatic ecosystems in seasonally snow-covered watersheds. In these systeems, changes in surface water chemistry are likely to occur in high deposition, snow-covered sites during low years before terrestrial vegetation is affected.