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

The denitrifier method to determine the dual isotopic composition (δ¹⁵N and δ¹⁸O) of nitrate is well suited for studies of nitrogen contributions to streams during runoff events. This method requires only 70 nmol of NO₃⁻ 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 NO₃⁻ concentrations were highest and stream discharge, NO₃⁻ concentrations, and δ¹⁸O of NO₃⁻ generally tracked one another during diurnal melting. The isotopic composition of stream NO₃⁻ varied in-between atmospheric and groundwater NO₃⁻ end members indicating a direct contribution of atmospherically-derived NO₃⁻ from the snow pack to the stream. During the middle to late phases of snowmelt, the source shifted toward soil NO₃⁻ entering the stream via shallow subsurface flow paths.