Factors Affecting Nitrogen Fixation in Azolla caroliniana

The effects of environmental variables on nitrogen fixation and growth of Azolla caroliniana were studied. Nitrogenase activity was highest at ~25˚C. Nitrogenase activity was highest at photosynthetic photon flux densities of 400-1000 µmol·m -2 ·s -1 and decreased sharply at lower levels. Azolla showed great sensitivity to drying in that nitrogenase activity was approximately halved as moisture concentration decreased from 95% to 87% of total fresh plant mass. A strong diurnal pattern of nitrogenase activity was detected; this pattern was not due to an endogenous rhythm. High plant density decreased specific nitrogenase activity per unit plant mass and per unit area; the optimal plant density for nitrogen fixation on an area basis is approximately 50-100 g dry ·m -2 . Two or 10.0 mg NH 4 -N·l -1 decreased nitrogenase activity; 10.0 mg NO 3 -N·l -1 decreased nitrogenase activity while 10.0 mg urea-N·l -1 was associated with an increase in nitrogenase activity. Ten mg NH 4 -N·l -1 caused a decline in Azolla growth; 10.0 mg NO 3 N·l -1 resulted in an increased growth rate.