ORIGINAL PAPER Isotopic signature of nitrate in two contrasting watersheds of Brush Brook, Vermont, USA Heidi C. Hales Æ Donald S. Ross Æ Andrea Lini Received: 26 June 2006 / Accepted: 12 December 2006 / Published online: 9 March 2007 Ó Springer Science+Business Media B.V. 2007 Abstract We used the dual isotope method to study differences in nitrate export in two subwater- sheds in Vermont, USA. Precipitation, soil water and streamwater samples were collected from two watersheds in Camels Hump State Forest, located within the Green Mountains of Vermont. These samples were analyzed for the d 15 N and d 18 O of NO 3 – . The range of d 15 N–NO 3 – values overlapped, with precipitation –4.5& to +2.0& (n = 14), soil solution –10.3& to +6.2& (n = 12) and stream- water +0.3& to +3.1& (n = 69). The d 18 O of precipitation NO 3 – (mean 46.8 ± 11.5&) was sig- nificantly different (P < 0.001) from that of the stream (mean 13.2 ± 4.3&) and soil waters (mean 14.5 ± 4.2&) even during snowmelt periods. Ex- tracted soil solution and streamwater d 18 O of NO 3 – were similar and within the established range of microbially produced NO 3 – , demonstrating that NO 3 – was formed by microbial processes. The d 15 N and d 18 O of NO 3 – suggests that although the two tributaries have different seasonal NO 3 – concentrations, they have a similar NO 3 – source. Keywords d 15 N Á d 18 O Á Nitrate Á Nitrogen isotopes Á Oxygen isotopes Á Vermont Abbreviations ANOVA Analysis of variance C/N Ratio of carbon to nitrogen d Delta DI Deionized distilled DOC Dissolved organic carbon DOM Dissolved organic matter IAEA International Atomic Energy Agency N Nitrogen 15 N Nitrogen-15 NADP National Atmospheric Deposition Program NH 4 + Ammonium ion NO 3 – Nitrate ion 18 O Oxygen-18 18 Ow Oxygen-18 of H 2 O & Per mil spp Species USGS United States Geological Survey V-SMOW Vienna Standard Mean Ocean Water VT Vermont H. C. Hales (&) Department of Environmental Conservation, 103 S. Main St., Waterbury, VT 05671, USA e-mail: heidi.hales@state.vt.us D. S. Ross Department of Plant and Soil Science, University of Vermont, Burlington, VT 05405, USA A. Lini Department of Geology, University of Vermont, Burlington, VT 05405, USA 123 Biogeochemistry (2007) 84:51–66 DOI 10.1007/s10533-007-9074-6