EFFECT OF MINERAL NUTRIENTS AND COMBINED NITROGEN-SOURCES IN THE MEDIUM ON GROWTH AND NITROGEN-FIXATION OF THE AZOLLA-ANABAENA ASSOCIATION

The effect of the mineral nutrient status and combined nitrogen sources in the medium on the growth and nitrogen fixation of Azolla japonica was investigated. Threshold concentrations of K, Mg, Ca. P, and S in the medium for growth were about 0.3, 0.3, 0.4, 0.08, and 0.08 mM, and those for the nitrogen-fixing capacity were about 0.07, 0.06, 1.0, 0.4, and 0.05 mM, respectively. Ammonium in the medium inhibited strongly both growth and nitrogen fixation. The total nitrogen budget, as a consequence of the balance between nitrogen inputs from the atmosphere and medium, was positive at concentrations of ammonium below 10 mM, whereas above this concentration, it was negative. Nitrate in the medium was less effective than ammonium. The addition of urea to the medium resulted in a substantial increase in the growth rate and nitrogen content although nitrogen fixation was appreciably reduced.