Acid-Base Regulation by Azolla spp. with N2 as Sole N Source and with Supplementation by NH+4 or NO−3

Diazotrophic cultures of three species of Azolla (Az. caroliniana, Az. microphylla, Az. pinnata) symbiotic with Anabaena azollae accumulated some 0.10–0.24 mol organic anion per mol N assimilated (0.010–0.018 mol organic anion per mol C assimilated), with a corresponding efflux of 0.05–0.11 mol H⁺ per mol N assimilated (0.006–0.009 mol H⁺ per mol C assimilated). These values are lower than those found for terrestrial diazotrophic vascular plants; this may be related to the decreased possibility of increasing Fe and P availability by rhizosphere acidification in a free-floating plant. Modification of the organic anion content, and of the quantity (and direction) of H⁺ exchange with the medium, with 5 mol m^?3 NH⁺₄ or NO^?₃ added to diazotrophic cultures, are consistent with substantial N acquisition from combined N as well as N₂ assimilation. This conclusion is consistent with previously published work with ¹⁵N.