Availability to Rice Plants of Nitrogen Fixed by Azolla

The decomposition processes of Azolla in water and soil in relation to the availability of Azolla-nitrogen to rice plants were determined using 15N as a tracer. Among the 3 species of Azolla (8 strains) used in this experiment, Azolla pinnata from Malaysia attained the highest biomass and supplied the largest amount of decaying materials to water. Azolla mexicana showed intermediate characteristics in growth and detachment of old tissues. Azolla filiculoides grew very slowly and developed symptoms of decay at early stages. The amount of ammonium excreted from Azolla into the water was negligible. When Azolla was incorporated into soil, A. pinnata underwent the most active mineralization process, followed by A. filiculoides and A. mexicana. Among the A. pinnata strains, the one from Malaysia released the largest amount of ammonium into soils. Within 6 weeks about two thirds of the nitrogen were lost from Azolla that was placed on the surface of flooded soils. Losses were about 30% from Azolla that was inco...