Tracing sources of nitrate in snowmelt runoff using a high‐resolution isotopic technique

Abstract

The denitrifier method to determine the dual isotopic composition (δ 15 N and δ 18 O) of nitrate is well suited for studies of nitrogen contributions to streams during runoff events. This method requires only 70 nmol of NO 3 − and enables high throughput of samples. We studied nitrate sources to a headwater stream during snowmelt by generating a high‐temporal resolution dataset at the Sleepers River Research Watershed in Vermont, USA. In the earliest phase of runoff, stream NO 3 − concentrations were highest and stream discharge, NO 3 − concentrations, and δ 18 O of NO 3 − generally tracked one another during diurnal melting. The isotopic composition of stream NO 3 − varied in‐between atmospheric and groundwater NO 3 − end members indicating a direct contribution of atmospherically‐derived NO 3 − from the snow pack to the stream. During the middle to late phases of snowmelt, the source shifted toward soil NO 3 − entering the stream via shallow subsurface flow paths.