We assessed the degree to which ectomycorrhizal fungi exploit organic nitrogen in situ. In an Alaskan boreal forest, we identified pairs of sporocarps from five taxa of ectomycorrhizal fungi. We added C-13-labeled alanine to the soil surrounding one sporocarp within each pair; the second served as an unlabeled control. Peak rates of C-13-respiration from alanine were higher in the labeled sporocarp plots than the controls, indicating that the C-13-alanine was detectably respired from the soil. "Reference" plots adjacent to the sporocarps served as an indication of background C-13-respiration rates released by the soil community as a whole. Ectomycorrhizal sporocarps displayed higher C-13-respiration rates than their reference plots. Thus, the sporocarps and associated mycorrhizal mycelium appeared to contribute significantly to the release of alanine-derived (CO2)-C-13, confirming the hypothesis that ectomycorrhizal fungi may access soil amino acid pools under natural conditions. (C) 2008 Elsevier Ltd. All rights reserved.