Nitrate on Steroids: Evaluating the Contribution of Talus Streams, Rock Glaciers, and Other High Alpine Sources to a Headwater Stream in the Colorado Front Range

High elevation ecosystems throughout the Colorado Front Range are undergoing changes in biogeochemical cycling due to an increase in nitrogen deposition in precipitation and a changing climate, resulting in alterations in alpine biota and water quality. The Niwot Ridge LTER has recorded an increase of 0.27 μmol L^-1 y^-1 between 1985 to 2009 at Green Lakes 4 (GL4) at the headwaters of Boulder Creek, yet the question remains whether the primary source of nitrate in Green Lakes Valley (GLV) is atmospheric or terrestrial. Previous research in the GL4 catchment suggests that nitrification in barren soils, talus and rock glaciers, not atmospheric deposition, is the primary source of nitrate in surface waters. We compare the nutrient content and sources in surface waters draining landscapes characteristic of alpine barren soils, including glaciers, rock glaciers, talus and blockslopes, and subsurface sources to the main stream draining the hydrologic axis of the catchment to evaluate the importance of barren soil contributions to stream water quality. Coinciding with climatic changes (drought), the nitrate-N yield of the GL4 catchment increased by 40%, from a mean of 1.7 to 2.3 kg N ha^-1 yr^-1 between the periods 1985-1999 and 2000-2009. During the dry period, 2000-2009, noticeably high nitrate concentrations were recorded from the rock glacier above GL4 of up to 135 μmol L^-1, as well as high concentrations in talus runoff of up to 94 μmol L^-1. A synoptic survey of talus streams from summer 2012 showed similar results, with a maximum nitrate concentration of 84 μmol L^-1 and a mean concentration of 33 μmol L^-1. In conjunction with nutrient analysis, water samples collected from throughout GLV during summer 2012 will be analyzed for triplicate oxygen isotopes of the nitrate molecule, a new method, which quantifies the proportion of atmospheric versus terrestrially produced nitrate. High nitrate concentration values are consistent with carbon limitation in barren soil areas combined with increasing nitrogen deposition, resulting in an increase in net nitrification and nitrogen export from microbially dominated landscape types—talus, rock glaciers, permafrost—to streams draining alpine watersheds.