N-2 FIXATION IN A YOUNG ALNUS-INCANA STAND, BASED ON SEASONAL AND DIURNAL-VARIATION IN WHOLE PLANT NITROGENASE ACTIVITY

N2 fixation by grey alder, Alnus incana (L.) Moench, was studied in the field during two growing seasons in Sweden. Alders were planted in a nitrogen-poor soil. Each alder had its root system enclosed in an open-ended cylinder that was closed with a gas-tight lid around the stem base to serve as cuvette during nitrogenase activity (acetylene reducing activity) measurements. To follow the seasonal variation, nitrogenase activity was measured at noon on 15 occasions for each alder in 1987 and on 15 occasions in 1988. Diurnal variation in nitrogenase activity was studied at six occasions, but no obvious pattern in the diurnal variation was found. Nitrogenase activity began shortly after leaf emergence at the very end of May, increased in June, stayed high although with some variation through July and August, declined during September, and was zero in early October. Cumulative nitrogenase activity over the season was converted to cumulative N2 fixation after determination of molar ratio nitrogenase activity to N2 fixation. This conversion was facilitated as the Frankia chosen as symbiont was lacking hydrogenase activity. Control experiments showed that the introduced symbiont was the only infective Frankia in the soil. N2 fixation was estimated to be 0.23 and 2.83 g N/(alder - year) in the 1st and 2nd year, respectively. Despite its young age, A. incana was apparently capable of high N2 fixation rates at the high latitude studied.