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Plant uptake of amino acids in the Arctic tundra of Alaska.
Weintraub, Michael*,1, Schimel, Joshua1, 1 University of California, Santa Barbara, Santa Barbara, CA
ABSTRACT- Amino acid uptake by plants was first reported in the Arctic tundra, and is thought to be important there because both plants and soil microbes are strongly N limited, and estimates of plant nitrogen uptake exceed the traditional measures of nitrogen availability. Because estimates for plant amino acid uptake range widely and have not accounted for microbial competition, it is still hard to draw solid conclusions about the importance of amino acids to tundra plants. To assess the competitive partitioning of amino acids between tundra plants and soil microbes, we added either labeled ammonium or amino acids labeled with both carbon and nitrogen isotopes to intact cores of the tundra sedges Eriophorum vaginatum and Eriophorum angustifolium several times in the summers of 2000 and 2001. Recoveries of the amino acids from the plants were not significantly different when calculated independently using either the carbon or nitrogen isotopes, indicating that amino acids were taken up by the plants as intact molecules. Up to 3% of the added amino acids and 5% of the added ammonium were recovered in Eriophorum vaginatum; and up to 1%, but typically less than 0.5% of the amino acids and up to 1.5% of the ammonium were recovered in Eriophorum angustifolium. We also found that Eriophorum vaginatum took up adenine in similar proportions to amino acids. On all but one occasion both plant species took up more of at least one of the added amino acids than they did ammonium, indicating that these tundra species often compete well for amino acid N relative to ammonium. To determine the role of uptake kinetics in the competitive partitioning of amino acids and ammonium, we separately quantified the uptake kinetics of both Eriophorum vaginatum excised roots and soil at the same times as the intact core experiments in the summer of 2000. We hypothesized that plants would compete best for the compounds that had the lowest uptake rates in soil. Arginine and adenine did fit this pattern, and were the two compounds beside ammonium that were best taken up by Eriophorum vaginatum. Ammonium did not fit this pattern, however, and was the compound that had the highest uptake rates in both plants and soil.
Key words: tundra, eriophorum, amino acid, organic nitrogen