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.

[1] Snowpack, snowmelt, precipitation, surface water, and groundwater samples from the Loch Vale watershed in Colorado were analyzed for d 15 N and d 18O of nitrate to determine the processes controlling the release of atmospherically deposited nitrogen from alpine and subalpine ecosystems. Although overlap was found between the d 15 N (NO3) values for all water types (À4 to +6%), the d 18 O (NO3) values for surface water and groundwater (+10 to +30%) were usually distinct from snowpack, snowmelt, and rainfall values (+40 to +70%). During snowmelt, d18 O (NO3) indicated that about half of the nitrate in stream water was the product of microbial nitrification; at other times that amount was greater than half. Springs emerging from talus deposits had high nitrate concentrations and a seasonal pattern in d18 O (NO3) that was similar to the pattern in the streams, indicating that shallow groundwater in talus deposits is a likely source of stream water nitrate. Only a few samples of surface water and groundwater collected during early snowmelt and large summer rain events had isotopic compositions that indicated most of the nitrate came directly from atmospheric deposition with no biological assimilation and release. This study demonstrates the value of the nitrate double-isotope technique for determining nitrogencycling processes and sources of nitrate in small, undisturbed watersheds that are enriched with inorganic nitrogen.

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The major challenge to stewardship of protected areas is to decide where, when, and how to intervene in physical and biological processes, to conserve what we value in these places. To make such decisions, planners and managers must articulate more clearly the purposes of parks, what is valued, and what needs to be sustained. A key aim for conservation today is the maintenance and restoration of biodiversity, but a broader range of values are also likely to be considered important, including ecological integrity, resilience, historical fidelity (ie the ecosystem appears and functions much as it did in the past), and autonomy of nature. Until recently, the concept of “naturalness” was the guiding principle when making conservation‐related decisions in park and wilderness ecosystems. However, this concept is multifaceted and often means different things to different people, including notions of historical fidelity and autonomy from human influence. Achieving the goal of nature conservation intended for such areas requires a clear articulation of management objectives, which must be geared to the realities of the rapid environmental changes currently underway. We advocate a pluralistic approach that incorporates a suite of guiding principles, including historical fidelity, autonomy of nature, ecological integrity, and resilience, as well as managing with humility. The relative importance of these guiding principles will vary, depending on management goals and ecological conditions.

In an effort to understand sources of nitrate (NO3−) in surface waters of high‐elevation catchments, nitrogen (N) transformations in and under seasonal snow were investigated from 1993 to 1995 on Niwot Ridge, an alpine ecosystem at 3,500 m located in the Colorado Front Range of the Rocky Mountains. Ammonium (NH4+) and NO3− labeled with 15N applied as nonconservative tracers to the snow showed no evidence of nitrification in the snowpack. Furthermore, NH4+ movement through the amended snowpack was highly correlated with a conservative chloride tracer (r2 = 0.99). In an unamended snowpack NH4+ concentrations in meltwater before contact with the ground were highly correlated with NO3− concentrations (r2 = 0.98), which is consistent with no nitrification in the snowpack. The isotopically labeled 15NH+4 applied to the snowpack was found in underlying soils, showing that NH4+ released from snow can be rapidly immobilized. Resin bag (mixed‐bed ion‐exchange resins) measurements (n = 22) showed that 80% of the mobile inorganic N in unamended subnivial soils was NO3−. Measurements of KCl‐extractable inorganic N from surface soils showed that highest values were prior to the initiation of snowmelt and lowest values were during the growing season. The natural δ15N abundance of unamended soils was negative and ranged from −12 to −2, suggesting that atmospheric deposition of δ15N‐depleted N is an important component of N cycling in these alpine soils. These results suggest that soil mineralization under seasonal snow, rather than snowmelt release of NO3−, may control NO3− concentrations in surface waters of high‐elevation catchments.