Climate Change, Permafrost Melt, and Increased Nitrate Export, Green Lakes Valley, Colorado

Alpine ecosystems are particularly susceptible to disturbance due to their short growing seasons, sparse vegetation, thin soils, and a harsh climate. Warming temperatures and atmospheric nitrogen (N) deposition, two drivers of global change, are currently affecting the Green Lakes Valley within the Colorado Front Range.  Since 1983 average air temperatures have increased by 1.08 °C decade-1 (Clow 2010, J. of Climate), with even larger changes in summer time minimum temperatures. In addition, annual precipitation has generally decreased over the last decade. Total flux of DIN via atmospheric deposition steadily increased through the 1980s and 1990s, but has decreased over the last decade by 0.56 kg ha-1 yr-1 due to summer drought conditions. 

Despite the decrease in N deposition, the concentration and yield of nitrate at the outlet of Green Lake 4 (GL4) has increased by 0.27 µM yr-1 and 0.24 kg N ha-1 yr-1. This constitutes a 40% increase during the last decade as compared to the period from 1985-1999. At the same time there has been a significant decrease in nitrate concentrations of 0.13 µM yr-1 and no significant trend in nitrate yield at Albion, a site located within the subalpine.

In order to assess how weathering responds to drought conditions, average annual yields were compared between the wet and dry periods. Despite a decrease in precipitation, yields increased in all cases: SO4 2- , Ca2+, and Si yields increased by 120%, 92%, and 52%, respectively at GL4. Yield increases were smaller at ALB: 30%, 24%, and 27% for SO42- , Ca2+, and Si, respectively. The increase in geochemical weathering products & fluxes during the dryer-post 2000 period suggests that GLV is responding to climate change through increased hydraulic connectivity due to melting permafrost. This is consistent with the ideas that weathering rates increase in warmer climates (Rogora et al. 2003, Hydro. & Earth Sys Sci) and that melting permafrost can serve as water source during dry years (Baron et al. 2009, GCB).

This increase in hydrologic connectivity through melting permafrost facilitates the export of N and weathering products even during dry summer months in GL4, in accordance with the mechanism proposed by Baron et al. (2009, GBC). A mass balance model suggests there is a 1:1 relationship between ammonium loss and nitrate gain between the ARK and NAV sites, suggesting that microbial nitrification is the likely source of nitrate export from the alpine. Increased rates of N cycling combined with an increase in barren soil due to glacial retreat and melting permafrost, and an increase in hydraulic connectivity provides a mechanism to explain the increases in nitrate export from the alpine despite concurrent decreases in atmospheric deposition.