Nitrogen saturation is occurring throughout high-elevation catchments
of the Colorado Front Range. Annual inorganic N loading in wet deposit
ion to the Front Range of similar to 4 kg ha(-1) yr(-1) is about twice
that of the Pacific States and similar to many sites in the northeast
ern United States. In the last ten years at Niwot Ridge/Green Lakes Va
lley and Glacier Lakes, annual minimum concentrations of NO3- in surfa
ce waters during the growing season have increased from below detectio
n limits to similar to 10 mu equiv L(-1), indicating that these two ca
tchments 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 mu equiv L(-1); annual volume-weighted mean
(VWM) concentrations of 16 mu equiv L(-1) in surface waters are great
er than that of similar to 11 mu equiv L(-1) NO3- in wet deposition. A
t these high-elevation catchments, there has been a shift in ecosystem
dynamics from an N-iimited system to an N-saturated system as a resul
t of anthropogenically fixed N in wetfall and dryfall. Results from th
e Western Lakes Survey component of the National Surface Water Survey
show that N saturation is a regional problem in the Colorado Front Ran
ge, with many lakes having (NO3-) concentrations greater than 10 mu eq
uiv L(-1). Foliar N:P ratios in bristlecone pine increase with elevati
on in the Colorado Front Range, indicating that at higher elevations P
is translocated from foliar tissue more efficiently than N and that i
ncreasing atmospheric deposition of N with elevation is causing a chan
ge from N limitation to P limitation in the highest-elevation bristlec
one 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 wester
n United States.