Nitrogen is the most limiting element for crop production. Traditionally, expensive commercial fertilizers are used to correct soil nitrogen deficiencies. Indeed, 50% of the increase in rice yields after World War II can be attributed to increased fertilizer nitrogen use. Although an increased rate of fertilizer nitrogen application has been advocated to meet the growing demand for food, it is unrealistic to advise the farmers to apply fertilizers they could hardly afford, and whose prices are likely to escalate in the years ahead. In addition, when they are not applied judiciously there are problems of environmental pollution as plants are capable of taking up only a relatively small portion of the applied nitrogen, a substantial amount being lost through various chemical and biological processes. The exploitation of cheaper alternatives or supplements to fertilizers have therefore gained much interest in recent years. Our increased interest in biological nitrogen fixation as a supplement or alternative to nitrogen fertilizers led to the convening of a consultants' meeting on `The Role of Isotopes in Studies on Nitrogen Fixation and Nitrogen Cycling by Blue-Green Algae and the A*zolla-Anabaena azollae Association', in Vienna from 11--15 October 1982. The consultants' group recommended that the Joint FAI/IAEA Division of Nuclear Techniques in Food and Agriculture embark on a coordinated research programme in this field and that initial emphasis should be placed on Azolla-Anabaena* symbiosis. As a result, such a programme was initiated in 1984, which was concluded in 1989. The results and conclusions reported here are those that were generated during the five years of its operation.

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Preface. 1. Introduction. 2. Assessment of nitrogen fixation in Azolla using the 15N isotope dilution technique. 3. Comparison of the direct and indirect 15N methods for evaluation of N uptake by rice from Azolla. 4. 15N labelled ammonium sulphate vs 15N labelled urea as a source for labelling Azolla with 15N. 5. Effects of applying urea and Azolla on N uptake and yield of rice. 6. Coordinated field evaluation of Azolla N uptake and yield response of rice in Bangladesh, China, Indonesia, Pakistan, the Philippines, Sri Lanka and Thailand. 7. Residual effects of Azolla N to a second crop of rice or wheat. 8. A comparative study of the effects of Azolla species and strain on availability of N to rice. 9. Comparison of the availability of N from fresh vs dried Azolla. 10. Effects of phosphorus fertilization on growth and N accumulation in Azolla. 11. Nitrogen balance studies in Azolla/rice cropping systems. 12. Increasing fertilizer use efficiency of urea using Azolla cover. 13. Summary and conclusions. 14. References. Appendix.