Nitrogen Saturation in the Rocky Mountains

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• 作者：Mark W. Williams ; Jill S. Baron ; Nel Caine ; Richard Sommerfeld ; and Robert Sanford ; Jr.
• 刊名：Environmental Science & Technology
• 出版年：1996
• 出版时间：February 1996
• 年：1996
• 卷：30
• 期：2
• 页码：640 - 646
• 全文大小：450K
• 年卷期：v.30,no.2(February 1996)
• ISSN：1520-5851

文摘

Nitrogen saturation is occurring throughout high-elevation catchments of the Colorado Front Range.Annual inorganic N loading in wet deposition to theFront Range of ~4 kg ha^-1 yr^-1 is abouttwice thatof the Pacific States and similar to many sites in thenortheastern United States. In the last ten yearsat Niwot Ridge/Green Lakes Valley and Glacier Lakes,annual minimum concentrations of NO₃^- insurfacewaters during the growing season have increasedfrom below detection limits to ~10 µequiv L^-1,indicatingthat these two catchments are at the threshold ofN saturation. The Loch Vale watershed is N saturated,with annual minimum concentrations of NO₃^-insurface waters generally above 10 µequiv L^-1;annualvolume-weighted mean (VWM) concentrations of16 µequiv L^-1 in surface waters are greater thanthatof ~11 µequiv L^-1 NO₃^- inwet deposition. At thesehigh-elevation catchments, there has been a shift inecosystem dynamics from an N-limited system toan N-saturated system as a result of anthropogenicallyfixed N in wetfall and dryfall. Results from theWestern Lakes Survey component of the NationalSurface Water Survey show that N saturation is aregional problem in the Colorado Front Range, withmany lakes having (NO₃^-) concentrationsgreater than10 µequiv L^-1. Foliar N:P ratios in bristleconepineincrease with elevation in the Colorado Front Range,indicating that at higher elevations P is translocatedfrom foliar tissue more efficiently than N and thatincreasing atmospheric deposition of N with elevationis causing a change from N limitation to P limitationin the highest-elevation bristlecone pines. Currentconcepts of critical loads need to be reconsidered sinceonly modest atmospheric loadings of N are sufficientto induce N leaching to surface waters inhigh-elevation catchments of the western UnitedStates.