The production of rice has increased considerably in recent years due to the release of improved varieties and the adoption of better fertilization practices. Nevertheless, the production and use of inorganic N fertilizer involves costly investments in terms of energy and transport, the need for complex manufacturing plants, as well as the potential for environmental pollution. The use of agricultural systems that include dinitrogen fixing organisms appears to be an economically sound cultural practice. In the particular case of rice, biological nitrogen fixation by Azolla, blue-green algae...