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

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 delta(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 delta(15)N and delta(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, delta(18)O values ranged from +47 to +77parts per thousand (mean: +58parts per thousand) and delta(15)N from -5 to +1parts per thousand (mean: -3parts per thousand); for snow, delta(18)O values ranged from +52 to +75parts per thousand (mean: +67parts per thousand) and delta(15)N from -3 to +2parts per thousand (mean: -1parts per thousand). Streamwater nitrate, in contrast to deposition, had delta(18)O values between +12 and +33parts per thousand (mean: +18parts per thousand) and delta(15)N between -3 and +6parts per thousand (mean: 0parts per thousand). Since nitrate produced by nitrification typically has delta(18)O values ranging from -5 to +15parts per thousand, 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 (C) 2004 John Wiley Sons, Ltd.