Snowpack controls on nitrogen cycling and export in seasonally snow-covered catchments

Here we provide an overview of current research activities on nitrogen (N) cycling in high-elevation catchments of the Colorado Front Range. We then use this information to develop a conceptual model of how snow cover controls subnivial (below snowpack) microbial processes and N leachate from the snow-soil interface to surface waters. This model is based on research that identi®es subnivial processes as a major control on the leaching loss of N from soil during snowmelt. These subnivial soil processes are controlled by the development of the seasonal snow pack that insulates soil from cold air temperatures and allows heterotrophic microbial activity in the soil to immobilize N. In this model the duration of snow-cover is divided into four snowpack regimes; zone I is characterized by shallow-short duration snowpacks, zone II is characterized by high interannual variability in snow depth and duration, zone III is characterized by early developing, continuous snow cover, and zone IV is characterized by deep, long-duration snow cover verging on perennial snowpacks. In zone I, soils remain frozen and there is little microbial activity and N leachate is high. In zone II, total microbial activity is highly variable and the amount of N leachate is highly variable. In zone III, total microbial activity is high and there is little N leachate. In zone IV, microbial activity is reduced because of carbon limitation and N leachate is high. This model suggests that a portion of the spatial and temporal variability observed in N export from these seasonally snow-covered systems is due to variability in winter snow cover across landscape types and inter-annually within a landscape type. Copyright # 1999 John Wiley & Sons, Ltd.