Studies on nitrogen fixation by Alnus crispa

Root nodules of Alnus crispa (Ait.) Pursh were shown to possess a symbiotic nitrogenfixing organism. The reduction of acetylene to ethylene, as measured by gas chromatography, was used to determine the presence of the nitrogen-fixing system. Ethylene production was measured at 5.1 μmoles/g excised nodule-hr for both field and greenhouse plants. The nodules were found to consist of short nubs usually clustered in masses up to 4 cm in diam. Microscopic examination of nodules revealed some cortical cells fully packed with spherical endophyte cells. The outer cortex and radiating arms of cells in the inner cortex remained uninfected. Nodules examined during the winter were found to be shrunken, with a random distribution of endophyte cells. Soil nitrogen measurements indicated that nitrogen fixation activity by A. crispa does not lead to an increase in soil nitrogen above levels in adjacent areas.