Thursday, October 17, 2013, 4:30PM - 5:30PM
RL-1 room 269
Determining sources of nitrate in the Green Lakes Valley, Colorado with triple isotope analysis
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, with cascading effects on downstream ecology. Between 1985 to 2009, nitrate concentrations in the outflow of Green Lakes 4 have increased 0.27 μmol L-1 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. 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. Using triple isotope analysis of NO3-, a new method that allows us to discriminate whether nitrate in surface waters is from atmospheric deposition or produced by nitrification by microbial communities, we determined that primary source of nitrate in GLV is microbial. 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.