Evaluating regional patterns in nitrate sources to watersheds in national parks of the rocky mountains using nitrate isotopes

Abstract

In the Rocky Mountains, there is uncertainty about the source areas and emission types that contribute to nitrate (NO3) deposition, which can adversely affect sensitive aquatic habitats of high-elevation watersheds. Regional patterns in NO3 deposition sources were evaluated using NO3 isotopes in five National Parks, including 37 lakes and 7 precipitation sites. Results indicate that lake NO3 ranged from detection limit to 38 μeq/L, δ18O (NO3) ranged from -5.7 to +21.3‰, and δ15N (NO3) ranged from -6.6 to +4.6‰. δ18O (NO3) in precipitation ranged from +71 to +78‰. δ15N (NO 3) in precipitation and lakes overlap; however, δ15N (NO3) in precipitation is more depleted than δ15N (NO3) in lakes, ranging from -5.5 to -2.0‰. δ15N (NO3) values are significantly related (p < 0.05) to wet deposition of inorganic N, sulfate, and acidity, suggesting that spatial variability of δ15N (NO3) over the Rocky Mountains may be related to source areas of these solutes. Regional patterns show that NO3 and δ15N (NO3) are more enriched in lakes and precipitation from the southern Rockies and at higher elevations compared to the northern Rockies. The correspondence of high NO 3 and enriched δ15N (NO3) in precipitation with high NO3 and enriched δ15N (NO3) in lakes, suggests that deposition of inorganic N in wetfall may affect the amount of NO3 in lakes through a combination of direct and indirect processes such as enhanced nitrification. © 2008 American Chemical Society.