NITROGEN TRANSFORMATIONS IN ALPINE TALUS FIELDS, GREEN LAKES VALLEY, FRONT RANGE, COLORADO, USA

This study reports the first direct evidence of nitrogen (N) transform ations in alpine talus fields. Mineralization (conversion of organic N to inorganic N) and nitrification (conversion of ammonium [NH4+] to n itrate [NO3-]) rates were measured in pockets of fine material in talu s slopes of an alpine catchment of the Colorado Front Range, U.S.A., i n 1996. Rates of both processes were higher in patches with vegetation covering >80% of the surface area than in patches with vegetation cov ering <20% of the surface area. In the mid- to late growing season, ne t mineralization rates at vegetated sites of 2.49 mg N kg(-1) d(-1) we re significantly greater than the 0.39 mg N kg(-1) d(-1) at unvegetate d sites (P = 0.03). Net nitrification rates of 1.0 mg N kg(-1) d(-1) a t 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 transfo rmation were comparable to mineralization and nitrification rates repo rted for well-developed tundra soils nearby on Niwot Ridge. Mineraliza tion and nitrification rates varied inversely with elevation along a t ransect with a 100 m elevational change (P = 0.005, P = 0.025). Nitrog en mineralization and nitrification in talus may help explain elevated levels of NO3- in alpine streams in mid- to late summer.