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 similar to 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 similar to 10 mu equiv 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 mu equiv L(-1); annual volume-weighted mean (VWM) concentrations of 16 mu equiv L(-1) in surface waters are greater than that of similar to 11 mu equiv L(-1) NO3- in wet deposition. At these high-elevation catchments, there has been a shift in ecosystem dynamics from an N-iimited 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 mu equiv 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.