Thawing glacial and permafrost features contribute to nitrogen export from Green Lakes Valley, Colorado Front Range, USA

Abstract

Alpine ecosystems are particularly susceptible to disturbance due to their short growing seasons, sparse vegetation and thin soils. Increased nitrogen deposition in wetfall and changes in climate currently affect Green Lakes Valley within the Colorado Front Range. Research conducted within the alpine links chronic nitrogen inputs to a suite of ecological impacts, resulting in increased nitrate export. The atmospheric nitrogen flux decreased by 0.56 kg ha -1 year -1 between 2000 and 2009, due to decreased precipitation; however alpine nitrate yields increased by 40 % relative to the previous decade (1990)(1991)(1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999). Long term trends indicate that weathering products such as sulfate, calcium, and silica have also increased over the same period. The geochemical composition of thawing permafrost, as indicated by rock glacial and blockfield meltwater, suggests it is the source of these weathering products. Furthermore, mass balance models indicate the high ammonium loads within glacial meltwater are rapidly nitrified, contributing *0.5-1.4 kg N ha -1 to the growing season nitrate flux from the alpine watershed. The sustained export of these solutes during dry, summer months is likely facilitated by thawing cryosphere providing hydraulic connectivity late into the growing season. This mechanism is further supported by the lack of upward weathering or nitrogen solute trends in a neighboring catchment which lacks permafrost and glacial features. These findings suggest that reductions of atmospheric nitrogen deposition alone may not improve water quality, as cryospheric thaw exposes soils to biological and geochemical processes that may affect alpine nitrate concentrations as much as atmospheric deposition trends.

Assessing the Chemistry and Bioavailability of Dissolved Organic Matter From Glaciers and Rock Glaciers

“…DOM had C:N ratios around 2 (2.35 ± 0.62 for glaciers, 1.85 ± 0.83 for rock glaciers) for both glacier types, which is consistent with the elevated inorganic nitrogen reported from glaciers in Colorado and elsewhere (Barnes et al, ; Fegel et al, ; Saros et al, ; Slemmons et al, ; Williams et al, ). Prior to incubation neither the concentration, chemical diversity, nor the C:N ratio of the DOM was significantly different between glaciers and rock glaciers (Table ).…”

Climate regulates alpine lake ice cover phenology and aquatic ecosystem structure

“…The same climate factors that influence ice cover phenology also affect rates of glacial and permafrost thawing, which have been increasing in the past three decades in the Green Lakes Valley and surrounding areas [ Hoffman et al ., ; Caine , ]. Meltwater from these sources is a primary vector of nitrogen and rock weathering products (e.g., calcium, magnesium, and sulfate) that are transported into lakes, particularly in autumn months [ Williams et al ., ; Mast et al ., ; Slemmons and Saros , ; Barnes et al ., ]. As a result, years with early ice‐off lead to greater inputs of late season ions into the lake.…”

Soil formation and weathering in a permafrost environment of the Swiss Alps: a multi‐parameter and non‐steady‐state approach

“…There is no doubt that temperature (energy) determines mineral and soil weatheringeven in the long-term over thousands of years, as shown by Williams et al (2010). Nevertheless, there is no unanimous agreement whether weathering in cold regions will really be increased by the melting of permafrost: and if not, which other factors will be more dominant (Rasmussen et al 2011;Pokrovsky et al, 2012;Barnes et al, 2014). As shown by previous investigations in the European Alps (Egli et al, 2008), higher temperatures do not necessarily lead to higher weathering rates in cold Alpine regions.…”