Evaluating the source of streamwater nitrate using δ15N and δ18O in nitrate in two watersheds in New Hampshire, USA

✍ Scribed by Linda H. Pardo; Carol Kendall; Jennifer Pett-Ridge; Cecily C. Y. Chang

• Publisher: John Wiley and Sons
• Year: 2004
• Tongue: English
• Weight: 328 KB
• Volume: 18
• Category: Article
• ISSN: 0885-6087
• DOI: 10.1002/hyp.5576

✦ Synopsis

Abstract

The natural abundance of nitrogen and oxygen isotopes in nitrate can be a powerful tool for identifying the source of nitrate in streamwater in forested watersheds, because the two main sources of nitrate, atmospheric deposition and microbial nitrification, have distinct δ^18^O values. Using a simple mixing model, we estimated the relative fractions in streamwater derived from these sources for two forested watersheds with markedly different streamwater nitrate outputs. In this study, we monitored δ^15^N and δ^18^O of nitrate biweekly in atmospheric deposition and in streamwater for 20 months at the Hubbard Brook Experimental Forest, New Hampshire, USA (moderate nitrogen export), and monthly in streamwater at the Bowl Research Natural Area, New Hampshire, USA (high nitrogen export). For rain, δ^18^O values ranged from +47 to +77‰ (mean: +58‰) and δ^15^N from −5 to +1‰ (mean: −3‰); for snow, δ^18^O values ranged from +52 to +75‰ (mean: +67‰) and δ^15^N from −3 to +2‰ (mean: −1‰). Streamwater nitrate, in contrast to deposition, had δ^18^O values between +12 and +33‰ (mean: +18‰) and δ^15^N between −3 and +6‰ (mean: 0‰). Since nitrate produced by nitrification typically has δ^18^O values ranging from −5 to +15‰, our field data suggest that most of the nitrate lost from the watersheds in streamflow was nitrified within the catchment. Our results confirm the importance of microbial nitrogen transformations in regulating nitrogen losses from forested ecosystems and suggest that hydrologic storage may be a factor in controlling catchment nitrate losses. Copyright © 2004 John Wiley & Sons, Ltd.