Evaluation of Nitrogen-fixation By Different Strains of the Azolla-anabaena Symbiosis in the Presence of a High-level of Ammonium

The proportion of N derived from N2 fixation for 99 strains ofAzolla spp. (comprising all known species) in the presence of ammonium (40 mg/1) was assessed using a15N-dilution technique. The percentage of N derived from air varied from 29.5% to 79.9%. Although the N concentration ofAzolla spp. was not correlated with fertilizer N, it correlated fairly well with N2 fixation. Regression analysis suggests that the N yield ofAzolla spp. is more dependent on N2 fixation than on ammonium assimilation. The high correlation between N yield and isotopically determined, fixed N2 indicates that the N yield could be used as a parameter in the selection ofAzolla spp. strains that are capable of maintaining high N2 fixation in the presence of a high level of ammonium.