1. Alexander D, Sadanandan N, Karumakaran K (1988) Effect ofAzolla and other fertilizers on rice yields. IRRI Newsl 13:5?29
2. Buresh RJ, Austin ER, Craswell ET (1982) Analytical methods in15N research. Fert Res 3:37?62
3. Dewan GI, Subba Rao NS (1979) Seed inoculation withAzospirillum brasilense andAzotobacter chroococcum and the root biomass of rice (Oryza sativa L.). Plant and Soil 53:295?302
4. Galal YGM (1993) Evaluation of some biofertilizers for increasing nitrogen efficiency under Egyptian conditions. PhD Thesis, Ain Shams University, Cairo, Egypt
5. Kumarasinghe KS, Eskew DL (1991) Application of15N isotope techniques inAzolla-rice studies. In: Proc IAEA, Vienna, pp 169?178