Okoronkwo, N., Van Hove, C., & Eskew, DL. (1989). Evaluation of Nitrogen-fixation By Different Strains of the Azolla-anabaena Symbiosis in the Presence of a High-level of Ammonium. Biology and Fertility of Soils, 7(3), 275-278. https://doi.org/10.1007/BF00709661 (Original work published 1989)