Of Microbes and Men: Determining Sources of Nitrate in Surface Waters of an Alpine Basin in the Colorado Front Range
Abstract
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 changes in alpine biota and water quality. Between 1985 to 2009, nitrate concentrations in the outflow of Green Lakes 4 (GL4), at the headwaters of Boulder Creek, have increased 0.27 μmol/L per year, yet the question remains whether the primary source of increasing 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. Additionally, increasing nitrate levels could be attributed to melting ice features—permafrost and a rock glacier—in GLV, associated with increasing hydrologic connectivity. 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/yr 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, as well as high concentrations in talus runoff of up to 94 μmol/L. A synoptic survey of talus streams from summer 2012 showed similar results, with a maximum nitrate concentration of 84 μmol/L and a mean concentration of 33 μmol/L. We aim to characterize nitrate pathways in GLV with an analysis of triple oxygen isotopes of the nitrate molecule, a new method that allows us to discriminate whether nitrate in surface waters is from atmospheric deposition or produced by nitrification by microbial communities. Samples were collected weekly throughout summer 2012 from surface streams, talus streams, snowpack, snowmelt, groundwater wells, soil water and precipitation. High nitrate concentration values found in GLV 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.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.B23H0547H
- Keywords:
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- 0469 BIOGEOSCIENCES / Nitrogen cycling;
- 0470 BIOGEOSCIENCES / Nutrients and nutrient cycling;
- 1041 GEOCHEMISTRY / Stable isotope geochemistry;
- 1871 HYDROLOGY / Surface water quality