Links between N Deposition and Export from a High-Elevation Watershed in the Colorado Front Range

Abstract:

Long-term patterns of stream N export and atmospheric N deposition were evaluated over three decades in Loch Vale, a high-elevation watershed in the Colorado Front Range. Flow-normalized concentrations and fluxes were estimated using a regression model that removed the influence of interannual variability in streamflow to reveal underlying patterns of change. Stream N export increased in the early 1990s, peaked in the mid-2000s, and has since declined by over 30%. Similarities in the timing and magnitude of N deposition suggest the watershed is responding to changes in atmospheric deposition. Other possible explanations, including forest disturbance, snow depth, or permafrost or ice thaw could not explain stream N export although a climate signal is revealed in the analysis. High-elevation watersheds have responded rapidly to reductions in sulfur deposition and here we show the response to N deposition may be similar, at least in areas with high rates of deposition where surface-water nitrate concentrations are elevated. Our results show that stream N export responds rapidly to reductions in N deposition in high elevation watersheds, similar to patterns observed for reductions in sulfur deposition.