Mineral nitrogen transformations in and under seasonal snow in a high-elevation catchment in the Rocky Mountains, United States

In an effort to understand sources of nitrate (NO₃⁻) in surface waters of high-elevation catchments, nitrogen (N) transformations in and under seasonal snow were investigated from 1993 to 1995 on Niwot Ridge, an alpine ecosystem at 3,500 m located in the Colorado Front Range of the Rocky Mountains. Ammonium (NH₄⁺) and NO₃⁻ labeled with ¹⁵N applied as nonconservative tracers to the snow showed no evidence of nitrification in the snowpack. Furthermore, NH₄⁺ movement through the amended snowpack was highly correlated with a conservative chloride tracer (r² = 0.99). In an unamended snowpack NH₄⁺ concentrations in meltwater before contact with the ground were highly correlated with NO₃⁻ concentrations (r² = 0.98), which is consistent with no nitrification in the snowpack. The isotopically labeled ¹⁵NH⁺₄applied to the snowpack was found in underlying soils, showing that NH₄⁺ released from snow can be rapidly immobilized. Resin bag (mixed-bed ion-exchange resins) measurements (n = 22) showed that 80% of the mobile inorganic N in unamended subnivial soils was NO₃⁻. Measurements of KCl-extractable inorganic N from surface soils showed that highest values were prior to the initiation of snowmelt and lowest values were during the growing season. The natural δ¹⁵N abundance of unamended soils was negative and ranged from −12 to −2, suggesting that atmospheric deposition of δ¹⁵N-depleted N is an important component of N cycling in these alpine soils. These results suggest that soil mineralization under seasonal snow, rather than snowmelt release of NO₃⁻, may control NO₃⁻ concentrations in surface waters of high-elevation catchments.