Nitrogen transformations in alpine talus fields, Green Lakes Valley, Front Range, Colorado, USA

This study reports the first direct evidence of nitrogen (N) transformations in alpine talus fields. Mineralization (conversion of organic N to inorganic N) and nitrification (conversion of ammonium [NH4+] to nitrate [NO3-]) rates were measured in pockets of fine material in talus slopes of an alpine catchment of the Colorado Front Range, U.S.A., in 1996. Rates of both processes were higher in patches with vegetation covering >80% of the surface area than in patches with vegetation covering <20% of the surface area. In the mid- to late growing season, net mineralization rates at vegetated sites of 2.49 mg N kg(-1) d(-1) were significantly greater than the 0.39 mg N kg(-1) d(-1) at unvegetated sites (P = 0.03). Net nitrification rates of 1.0 mg N kg(-1) d(-1) at sites with vegetation were significantly greater than the 0.27 mg N kg(-1) d(-1) at unvegetated sites (P = 0.01). These rates of N transformation were comparable to mineralization and nitrification rates reported for well-developed tundra soils nearby on Niwot Ridge. Mineralization and nitrification rates varied inversely with elevation along a transect with a 100 m elevational change (P = 0.005, P = 0.025). Nitrogen mineralization and nitrification in talus may help explain elevated levels of NO3- in alpine streams in mid- to late summer.