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Title: Nitrogen saturation in the Rocky Mountains

Author: Williams, Mark W.; Baron, Jill S.; Caine, Nel; Sommerfeld, Richard; Sanford, Robert Jr.;

Date: 1996

Source: Environmental Science and Technology. 30(2): 640-646.

Publication Series: Scientific Journal (JRNL)

Description: Nitrogen saturation is occurring throughout high-elevation catchments of the Colorado Front Range. Annual inorganic N loading in wet deposition to the Front Range of ~4 kg ha-1 yr-1 is about twice that of the Pacific States and similar to many sites in the northeastern United States. In the last ten years at Niwot Ridge/Green Lakes Valley and Glacier Lakes, annual minimum concentrations of NO3- in surface waters during the growing season have increased from below detection limits to ¡­10 ¥ìequiv L-1, indicating that these two catchments are at the threshold of N saturation. The Loch Vale watershed is N saturated, with annual minimum concentrations of NO3- in surface waters generally above 10 ¥ìequiv L-1; annual volume-weighted mean (VWM) concentrations of 16 ¥ìequiv L-1 in surface waters are greater than that of ¡­11 ¥ìequiv L-1 NO3- in wet deposition. At these high-elevation catchments, there has been a shift in ecosystem dynamics from an N-limited system to an N-saturated system as a result of anthropogenically fixed N in wetfall and dryfall. Results from the Western Lakes Survey component of the National Surface Water Survey show that N saturation is a regional problem in the Colorado Front Range, with many lakes having (NO3-) concentrations greater than 10 ¥ìequiv L-1. Foliar N:P ratios in bristlecone pine increase with elevation in the Colorado Front Range, indicating that at higher elevations P is translocated from foliar tissue more efficiently than N and that increasing atmospheric deposition of N with elevation is causing a change from N limitation to P limitation in the highest-elevation bristlecone pines. Current concepts of critical loads need to be reconsidered since only modest atmospheric loadings of N are sufficient to induce N leaching to surface waters in high-elevation catchments of the western United States.

Keywords: nitrogen saturation, high-elevation catchments

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Williams, Mark W.; Baron, Jill S.; Caine, Nel; Sommerfeld, Richard; Sanford, Robert, Jr. 1996. Nitrogen saturation in the Rocky Mountains. Environmental Science and Technology. 30(2): 640-646.

 


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