Free amino acids in Arctic salt-marsh coastal sites and plant nitrogen acquisition

The importance of free amino acids as a source of plant nitrogen was examined in an Arctic coastal salt-marsh. Concentrations of inorganic nitrogen in salt-marsh soils were low relative to those reported for most temperate soils, whereas soluble organic nitrogen concentrations of salt-marsh soils were relatively high; the median ratio of free amino acid nitrogen as a proportion of ammonium nitrogen was 0.36 and amino acid concentrations exceeded those of ammonium in 24% of samples. Growth of the salt-marsh grass 'Puccinellia phryganodes' on glycine in a continuous flow hydroponic medium was similar to growth on ammonium ions at an equivalent concentration of nitrogen. Furthermore, in short-term excised root uptake experiments, rates of glycine uptake were equal to rates of ammonium and nitrate uptake combined when roots were provided with all three nitrogen substrates at equal concentrations. Amino acid uptake relative to ammonium uptake was favoured at high temperatures, high salinity and low pH. Free amino acids turned over rapidly in the soil, with half-lives in the soil solution ranging from 8-23 h for glycine, compared with ranges of 6-15 h and 6-16 h for ammonium and nitrate ions, respectively. Plant incorporation of 15N tracer injected into soil cores was 56, 83, and 68% of incorporation by soil microorganisms for glycine, ammonium and nitrate ions, respectively. The simultaneous incorporation of 13C and 15N into plant roots following injection of 13C15N-glycine into soil cores indicated that at least a portion of this amino acid was absorbed intact. In a model of the dynamics of nitrogen movement in an Arctic salt-marsh grazed and grubbed by geese, the direct uptake of organic nitrogen by plant roots was required to obtain rates of mineralization consistent with empirical estimates. Overall, these results indicate that free amino acids are likely a substantial contribution to plant nitrogen nutrition in Arctic coastal marshes.