Measuring Rates of Symbiotic Nitrogen Fixation by Alnus Tenuifolia

Rates of nitrogen fixation were determined for alder (Abuts tenuifolia Nutt.) with the C 2 H 2 assay and verified by the 15 N method. Rates of C 2 H 2 reduction obtained varied between 0.0342 and 10.9 μmoles per gram of nodule tissue (fresh weight) per hour with a seasonal average of 2.70 μmoles g ‐1 hr ‐1 . Expressed as nitrogen fixation rates these are equivalent to 0.0114, 3.63, and 0.9 μmoles g ‐1 hr ‐1 of elemental nitrogen, respectively. Nitrogen fixation ( 15 N) rates averaged 2.20 μmoles N g ‐1 hr ‐1 with a standard deviation of 1.14. The influence of pC 2 H 2 , pO 2 , temperature during incubation, length of incubation time, presence vs. absence of N 2 in the incubation atmosphere, and amount of nodule biomass per sample were investigated.