Isotopic signature of nitrate in two contrasting watersheds of Brush Brook, Vermont, USA

We used the dual isotope method to study differences in nitrate export in two subwatersheds 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 δ¹⁵N and δ¹⁸O of NO ₃ - . The range of δ¹⁵N-NO ₃ - values overlapped, with precipitation -4.5[per thousand] to +2.0[per thousand] (n = 14), soil solution -10.3[per thousand] to +6.2[per thousand] (n = 12) and streamwater +0.3[per thousand] to +3.1[per thousand] (n = 69). The δ¹⁸O of precipitation NO ₃ - (mean 46.8 ± 11.5[per thousand]) was significantly different (P < 0.001) from that of the stream (mean 13.2 ± 4.3[per thousand]) and soil waters (mean 14.5 ± 4.2[per thousand]) even during snowmelt periods. Extracted soil solution and streamwater δ¹⁸O 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 δ¹⁵N and δ¹⁸O of NO ₃ - suggests that although the two tributaries have different seasonal NO ₃ - concentrations, they have a similar NO ₃ - source.