Tracing sources of nitrate in snowmelt runoff using a high-resolution isotopic technique
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[ 1] The denitrifier method to determine the dual isotopic composition (delta(15)N and delta(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 ($) over bar (3) and enables high throughput of samples. We studied nitrate sources to a headwater stream during snowmelt by generating a high-temporal resolution dataset at the Sleepers River Research Watershed in Vermont, USA. In the earliest phase of runoff, stream NO ($) over bar (3) concentrations were highest and stream discharge, NO ($) over bar (3) concentrations, and delta(18)O of NO ($) over bar (3) generally tracked one another during diurnal melting. The isotopic composition of stream NO ($) over bar (3) varied in-between atmospheric and groundwater NO ($) over bar (3) end members indicating a direct contribution of atmospherically-derived NO ($) over bar (3) from the snow pack to the stream. During the middle to late phases of snowmelt, the source shifted toward soil NO ($) over bar (3) entering the stream via shallow subsurface flow paths.