Nitrogen Fixation by Root Nodules of Alnus incana in a Norwegian Forest Ecosystem

Alnus incana nodules were incubated in situ and assayed for nitrogen-fixing activity by the acetylene reduction method. The effects of air and soil temperature and light intensity (at the tree stem) on the acetylene reduction activity were examined, and diurnal and seasonl fluctuations in acetylene reducing activity were studied. Rates of acetylene reduction obtained varied between 0.4 and 84.8 Rmoles of ethylene per g dry weight of nodule per h with a seasonal average of 8.6 moles g-l h-1. Based on field assays an annual rate of fixation of 43 kg N/ha was estimated for a 30 year old A. incana stand.