Evaluating regional patterns in nitrate sources to watersheds in national parks of the Rocky Mountains using nitrate isotopes

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 mu eq/L, delta O-18 (NO3) ranged from -5.7 to +21.3 parts per thousand, and delta N-15 (NO3) ranged from -6.6 to +4.6 parts per thousand. delta O-18 (NO3) in precipitation ranged from +71 to +78 parts per thousand. delta N-15 (NO3) in precipitation and lakes overlap; however delta N-15 (NO3) in precipitation is more depleted than delta N-15 (NO3) in lakes, ranging from -5.5 to -2.0 parts per thousand. delta N-15 (NO3) values are significantly related (p < 0.05) to wet deposition of inorganic N, sulfate, and acidity, suggesting that spatial variability of delta N-15 (NO3) over the Rocky Mountains may be related to source areas of these solutes. Regional patterns show that NO3 and delta N-15 (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 NO3 and enriched delta N-15 (NO3) in precipitation with high NO3 and enriched delta N-15 (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.