Critical Loads for Inorganic Nitrogen Deposition in the Colorado Front Range, Usa

Abstract

We suggest an empirical approach for determining critical loads for inorganic nitrogen (N) deposition in wetfall to the central Rocky Mountains (USA). We define “critical loads” as a deposition amount above which natural resources can be negatively affected. The arithmetic average from 1992 to 1996 of annual inorganic N deposition in wetfall at the eight National Acid‐Deposition Program (NADP) sites located at elevations >2500 m in the central Rocky Mountains ranged from 2.5 to 3.5 kg·ha−1·yr−1. In contrast, inorganic N deposition was <2.5 kg·ha−1·yr−1 at all 23 NADP sites below 2500 m in elevation. At the Niwot Ridge NADP site in the Colorado Front Range, a simple linear regression of inorganic N in wetfall with time shows a significant increase in deposition of inorganic N in wetfall at the rate of 0.32 kg·ha−1·yr−1 (r2 = 0.62; P < 0.001, n = 13). In turn, the increasing amount of inorganic N in wetfall is causing episodic acidification in headwater catchments of the Green Lakes Valley in the Colorado Front Range, with acid‐neutralizing capacity (ANC) values below 0 μmolc/L in surface waters during snowmelt runoff at 9‐ha and 42‐ha sampling sites. At present rates of ANC decrease, we can expect the 9‐ha and 42‐ha sites to become chronically acidified within the next decade and the 220‐ha basin of Green Lake 4 to become episodically acidified. A synoptic survey in 1995 of 91 high‐elevation lakes in the central Rocky Mountains suggests that water quality is being affected by inorganic N in wetfall throughout the region. Federal land managers are required to “err on the side of protection” when assessing the amount of deposition that will alter ecosystem processes. However, given the political and economic ramifications of policy decisions, land managers are aware of the need to provide a scientific basis for these decisions and to balance conflicting needs. To achieve this balance and to allow for natural‐resource protection, we make a conservative recommendation that critical loads of inorganic N in wetfall to Class 1 areas in the central Rocky Mountains be set at 4 kg·ha−1·yr−1. Target loads may be set at lower levels of inorganic N deposition in wetfall to allow a margin of safety to protect extremely sensitive natural resources.

Storage and release of solutes from a subalpine seasonal snowpack: soil and stream water response, Niwot Ridge, Colorado

“…The lack of nitrate in surface waters at this treeline site is in sharp contrast to other highelevation catchments in the Colorado Front Range (Williams et al 1996c;Williams and Tonnessen 2000;Baron 2006) and to mountain catchments in northeastern US (e.g. Rascher et al 1987).…”

“…Edwards et al 2007). In alpine streams of the Colorado Front Range of Colorado, several investigations have shown that nitrate in surface waters is elevated above background concentrations, suggesting that these areas are approaching N saturation (Williams et al 1996b;Williams and Tonnessen 2000). Elevated amounts of ammonium and nitrate in wetfall are considered a major reason for the excess nitrate in surface waters (Burns 2004;Baron 2006).…”

The consequence of species loss on ecosystem nitrogen cycling depends on community compensation

“…N deposition from anthropogenic sources is increasing on Niwot Ridge (currently approximately 6 kg N/ha per year) (Baron et al 2000;Williams et al 1996;Williams and Tonnessen 2000). N fertilization experiments suggest that Deschampsia may increase more than Acomastylis with increased N deposition .…”

White on green: under-snow microbial processes and trace gas fluxes through snow, Niwot Ridge, Colorado Front Range

“…Oxidized nitrogen inputs from precipitation have significantly increased over the past century (Galloway et al 2008), and enhancements of nitrate in atmospheric deposition have been reported in most environmental compartments, including precipitated snow (Williams and Tonnessen 2000). As the CO 2 flux studies have provided convincing evidence for an active, wintertime respiration processes in the soil underneath the snow, it is likely that similar nitrogen cycling occurs.…”