Pathways for nitrate release from an alpine watershed:: Determination using δ15N and δ18O -: art. no. 1052

[1] Snowpack, snowmelt, precipitation, surface water, and groundwater samples from the Loch Vale watershed in Colorado were analyzed for delta(15)N and delta(18)O of nitrate to determine the processes controlling the release of atmospherically deposited nitrogen from alpine and subalpine ecosystems. Although overlap was found between the delta(15)N((NO3)) values for all water types (-4 to +6parts per thousand), the delta(18)O((NO3)) values for surface water and groundwater (+ 10 to +30parts per thousand) were usually distinct from snowpack, snowmelt, and rainfall values (+ 40 to +70parts per thousand). During snowmelt, delta(18)O((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 delta(18)O((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.