Nitrogen Saturation in the Rocky Mountains

Abstract

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.

Response of alpine lakes and soils to changes in acid deposition: the MAGIC model applied to the Tatra Mountain region, Slovakia-Poland

“…The catchments of AS lakes have the lowest percentage of meadows (27%) and sparse soils 36±15 kg m -2 , while the catchments of NS and ES have significantly larger amounts of soil (49±13 and 74±24 kg m -2 , respectively; Table 2). The ability of alpine catchments to retain N is positively related to vegetation and amount of soil (e.g., Baron et al 1994;Williams et al 1996;Kopáček et al 2000). The sparse vegetation and low amount of soil thus explain the low terrestrial retention of N in the AS catchments, resulting in the highest NO 3 -(and, consequently, also SSA) concentration in these lakes compared to the other categories (Fig.…”

Ecological issues related to N deposition to natural ecosystems: research needs

“…Tundra ecosystems have received more research attention (Williams et al, 1996;Bowman et al, 1995;Baron et al, 1994), but much of the research on these ecosystems is in its early stages and comes from very few sites. There is a notable paucity of research evaluating N deposition effects on North American wetlands, although much fine research has been done on European wetlands.…”

“…I will discuss what we do Nitrogen-saturated terrestrial ecosystems that have been know about these topics and where I perceive the research identified include a number of forest types (Stein and van gaps to be. Breemen, 1993;Peterjohn et al, 1996;, alpine tundra (Baron et al, 1994;Williams et al, 1996), chaparral (Riggan et al, 1994) and riparian wetlands (Han-2. The nitrogen cycle son et al, 1994).…”

Temporal and spatial variation of nitrogen transformations in nitrogen-saturated soils of a central Appalachian hardwood forest

“…Although much of the N-saturation research in the United States has been in conifer and hardwood forests of the Northeast (Kahl et al 1993;McNulty and Aber 1993;Rustad et al 1993;Norton et al 1994;Currie 1999;Magill et al 2000), several ecosystems across the United States have been found to be N saturated, including Colorado Rocky Mountain tundra (Baron et al 1994;Williams et al 1996), chaparral and high-elevation sites in southern California (Riggan et al 1985;Fenn et al 1996;Fenn and Poth 1999), hardwood forests of the Catskill Mountains (Lovett et al 2000), and spruce-fir stands of the Northeast (McNulty et al 1996) and Great Smoky Mountains (Johnson et al 1991). …”