ISOTOPIC SIGNATURE OF NITRATE IN TWO CONTRASTING WATERSHEDS OF BRUSH BROOK, VERMONT, USA

Abstract

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‰ to +2.0‰ (n = 14), soil solution −10.3‰ to +6.2‰ (n = 12) and streamwater +0.3‰ to +3.1‰ (n = 69). The δ18O of precipitation NO 3 − (mean 46.8 ± 11.5‰) was significantly 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. 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.