Pathways for nitrate release from an alpine watershed: Determination using δ¹⁵N and δ¹⁸O

[1] Snowpack, snowmelt, precipitation, surface water, and groundwater samples from the Loch Vale watershed in Colorado were analyzed for δ¹⁵N and δ¹⁸O of nitrate to determine the processes controlling the release of atmospherically deposited nitrogen from alpine and subalpine ecosystems. Although overlap was found between the δ¹⁵N_(NO3) values for all water types (−4 to +6‰), the δ¹⁸O_(NO3) values for surface water and groundwater (+10 to +30‰) were usually distinct from snowpack, snowmelt, and rainfall values (+40 to +70‰). During snowmelt, δ¹⁸O_(NO3) indicated that about half of the nitrate in stream water was the product of microbial nitrification; at other times that amount was greater than half. Springs emerging from talus deposits had high nitrate concentrations and a seasonal pattern in δ¹⁸O_(NO3) that was similar to the pattern in the streams, indicating that shallow groundwater in talus deposits is a likely source of stream water nitrate. Only a few samples of surface water and groundwater collected during early snowmelt and large summer rain events had isotopic compositions that indicated most of the nitrate came directly from atmospheric deposition with no biological assimilation and release. This study demonstrates the value of the nitrate double-isotope technique for determining nitrogen-cycling processes and sources of nitrate in small, undisturbed watersheds that are enriched with inorganic nitrogen.