NITROGEN UPTAKE BY ARCTIC SOIL MICROBES AND PLANTS IN RELATION TO SOIL NITROGEN SUPPLY

Abstract

In Alaska, evergreen and deciduous shrubs dominate the vegetation of moist acidic arctic tundra (soil pH < 5.5) while graminoids and forbs are important at the more species-rich moist nonacidic arctic tundra (soil pH > 5.5). In this study we compare soil concentrations and microbial and plant uptake of amino acids, ammonium (NH4+), and nitrate (NO3−) in acidic and nonacidic tundra. The objective was to determine any differences between the tundra sites that may relate to the differences in vegetation. We sampled the water-extractable soil N pool over one growing season and found that it at all times was higher at the nonacidic than at the acidic site, while at both sites it was dominated by NH4+ followed in order by amino acid N and NO3−. In addition, we designed an experiment in which a mixture of aspartic acid, glycine, NH4+, and NO3− were injected into the soil in the middle of the growth period. In the mixture, one N form at a time was labeled with 15N and in the case of amino acids also with 13C. Soil and plant samples were collected 4 h following the injection of labeled N. A large portion of the experimental N was recovered in the soil microbial biomass (on average 49% at the acidic site and 40% at the nonacidic site), while less than 1% was recovered in plants. Soil microbes and plants at both acidic and nonacidic tundra were able to take up all isotopically labeled N forms in the presence of added unlabeled N, demonstrating adequate potential to use any N form available. In addition, gas chromatography–mass spectrometry (GC–MS) analysis of plant roots revealed plant uptake of intact glycine, while isotopically labeled aspartic acid was not recovered inside plants.