Non-mycorrhizal uptake of amino acids by roots of the alpine sedge Kobresia myosuroides: Implications for the alpine nitrogen cycle

Non-mycorrhizal plants of the alpine sedge, Kobresia myosuroides, take up the amino acid glycine from nutrient solutions at greater rates than NO₃^- or NH₄⁺. The amino acids glutamate and proline were also taken up at high rates. Total plant biomass was twice as high after 4 months of growth on glycine, compared to NH₄NO₃, with significant increases in both root and leaf biomass. By taking advantage of differences in the δ¹³C signature of air in the growth chamber and the glycine used for growth, a two-member mixing model was used to estimate that a significant amount of the glycine was taken up as intact molecules, enough to contribute 16% of the total carbon assimilation over a 4-month growing period. Glycine uptake was inhibited when roots were exposed to N₂ in place of air, and when the protonophore carbonyl cyanide m-chlorophenylhydrazone (CCCP) was added to the root solution. From these results it is concluded that glycine uptake occurs through active transport. Glycine uptake exhibited a Q₁₀ of 2.0 over the temperature range 5-15° C, with relatively high rates maintained at the lowest temperature measured (5° C). Roots of Kobresia were not capable of taking up NH₄⁺ at measureable rates. To our knowledge, this is the first report of a plant whose non-mycorrhizal roots cannot take up NH₄⁺. Measurements of three N fractions (NO₃^-, NH₄⁺, and total amino acids) in the soil pore water were made over two growing seasons in two Kobresia dry meadows using microlysimeters. At the West Knoll site, which is characterized by soils with average amounts of organic matter, the dominant forms of N in the soil pore water were NO₃^- and NH₄⁺ (0-450 μmol L^-1). Amino acid concentrations were generally less than 20 μmol L^-1 at this site. At the East Knoll site, which is characterized by soils with higher than average amounts of organic matter, amino acids were generally present at higher concentrations (17-100 μmol L^-1), compared to NO₃^- and NH₄⁺. The most abundant amino acids were glycine (10-100 μmol L^-1), glutamate (5-70 μmol L^-1), and late in the season cysteine (5-15 μmol L^-1). The results demonstrate that this sedge, which dominates dry meadow communities in many alpine ecosystems, is capable of taking up intact amino acids as a principal N source and has access to high amino acid concentrations in certain alpine soils. Such uptake of organic N may accommodate plant N demands in the face of slow alpine N mineralization rates due to cold soil temperatures.