Ectomycorrhizal transfer of amino acid-nitrogen to the alpine sedge Kobresia myosuroides

Previous work in the Colorado alpine ecosystem has shown that amino acids a re a potentially important N source for the sedge, Kobresia myosuroides. Th is plant is the only known sedge to harbour associations with ectomycorrhiz al fungi. The aim of the present work was to test the hypothesis that these ectomycorrhizas transfer N from amino acids in the soil solution to the ho st plant, and thereby have an important role in the N nutrition of this spe cies. We used a two-chamber system (rhizoboxes) in which K. myosuroides pla nts were separated from a soil chamber by nylon mesh that allowed fungal hy phae, but not plant roots, to cross it. Injections of [N-15, 2-C-13]glycine were made into the soil chamber. The hyphal crossings on half of the rhizo boxes were regularly disrupted to control for leakage of label across the b arrier. Plants in the intact rhizoboxes showed significantly higher N-15 en richment than those in controls, and mycorrhizal root tips were significant ly more enriched than bulk roots. The mycorrhizas transferred an average of 1.3% of the added N-15 label to plants, a figure comparable to those obtai ned in previous studies in which plant roots were directly exposed to label . We conclude that fungal associations have an important role in the N nutr ition of K. myosuroides by transferring N from amino acids to their hosts.