Critical loads for inorganic nitrogen deposition in the Colorado Front Range, USA

We suggest an empirical approach for determining critical loads for inorgan ic nitrogen (N) deposition in wetfall to the central Rocky Mountains (USA). We define "critical loads" as a deposition amount above which natural reso urces can be negatively affected. The arithmetic average from 1992 to 1996 of annual inorganic N deposition in wetfall at the eight National Acid-Depo sition Program (NADP) sites located at elevations >2500 m in the central Ro cky Mountains ranged from 2.5 to 3.5 kg.ha(-1).yr(-1). In contrast, inorgan ic 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 signif icant increase in deposition of inorganic N in wetfall at the rate of 0.32 kg.ha(-1).yr(-1) (r(2) = 0.62; P < 0.001, n = 13). In turn, the increasing amount of inorganic N in wetfall is causing episodic acidification in headw ater catchments of the Green Lakes Valley in the Colorado Front Range, with acid-neutralizing capacity (ANC) values below 0 mu mol(c)/L in surface wat ers during snowmelt runoff at 9-ha and 42-ha sampling sites. At present rat es of ANC decrease, we can expect the 9-ha and 42-ha sites to become chroni cally 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 M ountains suggests that water quality is being affected by inorganic N in we tfall throughout the region. Federal land managers are required to "err on the side of protection" when assessing the amount of deposition that will a lter ecosystem processes. However, given the political and economic ramific ations 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 wetfal l 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 w etfall to allow a margin of safety to protect extremely sensitive natural r esources.