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Nitrogen Saturation in the Rocky Mountains

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...

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Bibliographic Details
Published in:Environmental science & technology 1996-02, Vol.30 (2), p.640-646
Main Authors: Williams, Mark W, Baron, Jill S, Caine, Nel, Sommerfeld, Richard, Sanford, Robert
Format: Article
Language:English
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Summary: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 μequiv 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 μequiv L-1; annual volume-weighted mean (VWM) concentrations of 16 μequiv L-1 in surface waters are greater than that of ∼11 μequiv 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 μequiv 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.
ISSN:0013-936X
1520-5851
DOI:10.1021/es950383e