[1] We explored catchment processes that control stream nutrient concentrations at an upland forest in northeastern Vermont, USA, where inputs of nitrogen via atmospheric deposition are among the highest in the nation and affect ecosystem functioning. We traced sources of water, nitrate, and dissolved organic matter (DOM) using stream water samples collected at high frequency during spring snowmelt. Hydrochemistry, isotopic tracers, and end-member mixing analyses suggested the timing, sources, and source areas from which water and nutrients entered the stream. Although stream-dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) both originated from leaching of soluble organic matter, flushing responses between these two DOM components varied because of dynamic shifts of hydrological flow paths and sources that supply the highest concentrations of DOC and DON. High concentrations of stream water nitrate originated from atmospheric sources as well as nitrified sources from catchment soils. We detected nitrification in surficial soils during late snowmelt which affected the nitrate supply that was available to be transported to streams. However, isotopic tracers showed that the majority of nitrate in upslope surficial soil waters after the onset of snowmelt originated from atmospheric sources. A fraction of the atmospheric nitrogen was directly delivered to the stream, and this finding highlights the importance of quick flow pathways during snowmelt events. These findings indicate that interactions among sources, transformations, and hydrologic transport processes must be deciphered to understand why concentrations vary over time and over space as well as to elucidate the direct effects of human activities on nutrient dynamics in upland forest streams.

Abstract:The stable isotopic composition of dissolved inorganic carbon (υ 13 C-DIC) was investigated as a potential tracer of streamflow generation processes at the Sleepers River Research Watershed, Vermont, USA. Downstream sampling showed υ 13 C-DIC increased between 3-5‰ from the stream source to the outlet weir approximately 0Ð5 km downstream, concomitant with increasing pH and decreasing PCO 2 . An increase in υ 13 C-DIC of 2Ð4 š 0Ð1‰ per log unit decrease of excess PCO 2 (stream PCO 2 normalized to atmospheric PCO 2 ) was observed from downstream transect data collected during snowmelt. Isotopic fractionation of DIC due to CO 2 outgassing rather than exchange with atmospheric CO 2 may be the primary cause of increased υ 13 C-DIC values downstream when PCO 2 of surface freshwater exceeds twice the atmospheric CO 2 concentration. Although CO 2 outgassing caused a general increase in stream υ 13 C-DIC values, points of localized groundwater seepage into the stream were identified by decreases in υ 13 C-DIC and increases in DIC concentration of the stream water superimposed upon the general downstream trend. In addition, comparison between snowmelt, early spring and summer seasons showed that DIC is flushed from shallow groundwater flowpaths during snowmelt and is replaced by a greater proportion of DIC derived from soil CO 2 during the early spring growing season. Thus, in spite of effects from CO 2 outgassing, υ 13 C of DIC can be a useful indicator of groundwater additions to headwater streams and a tracer of carbon dynamics in catchments.

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.

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