During the past three decades there has been a large amount of research on biological nitrogen fixation, in part stimulated by increasing world prices of nitrogen-containing fertilizers and environmental concerns. In the last several years, research on plant--microbe interactions, and symbiotic and asymbiotic nitrogen fixation has become truly interdisciplinary in nature, stimulated to some degree by the use of modern genetic techniques. These methodologies have allowed us to make detailed analyses of plant and bacterial genes involved in symbiotic processes and to follow the growth and persistence of the root-nodule bacteria and free-living nitrogen-fixing bacteria in soils. Through the efforts of a large number of researchers we now have a better understanding of the ecology of rhizobia, environmental parameters affecting the infection and nodulation process, the nature of specificity, the biochemistry of host plants and microsymbionts, and chemical signalling between symbiotic partners.
This volume gives a summary of current research efforts and knowledge in the field of biological nitrogen fixation. Since the research field is diverse in nature, this book presents a collection of papers in the major research area of physiology and metabolism, genetics, evolution, taxonomy, ecology, and international programs.

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Preface. 1. Evolution and diversity in the legume-rhizobium symbiosis: Chaos theory? J.I. Sprent. 2. Recent developments in Rhizobium taxonomy; E. Martínez-Romero. 3. Host range, RFLP, and antigenic relationships between Rhizobium fredii strains and Rhizobium sp. NGR234; H.B. Krishnan, S.G. Pueppke. 4. Rapid identification of Rhizobium species based on cellular fatty acid analysis; B.D.W. Jarvis, S.W. Tighe. 5. Ammonium sensing in nitrogen fixing bacteria: Functions of the glnB and glnD gene products; C. Kennedy, N. Doetsch, D. Meletzus, E. Patriarca, M. Amar, M. Iaccarino. 6. Regulation of nodulin gene expression; F.J. de Bruijn, R. Chen, S.Y. Fujimoto, A. Pinaev, D. Silver, K. Szczyglowski. 7. Synthesis, release, and transmission of alfalfa signals to rhizobial symbionts; D.A. Phillips. F.D. Dakora, E. Sande, C.M. Joesph, J. Zón. 8. Role of rhizobial lipo-oligosaccharides in root nodule formation on leguminous plants; O. Geiger, T. Ritsema, A.A.N. van Brussel, T. Tak, A.H.M. Wijfjes, G.V. Bloemberg, H.P. Spaink, B.J.J. Lugtenberg. 9. Sucrose transport and hydrolysis in Rhizobium tropici; V.I. Romanov, E. Martínez-Romero. 10. Shoot/root assimilate allocation and nodulation of Vigna unguiculata seedlings as influenced by shoot light environment; M.J. Kasperbauer, P.G. Hunt. 11. Mechanism of osmotically regulated N-acetylglutaminylglutamine amide production in Rhizobium meliloti; L. Tombras Smith, A. Ameer Allath, G.M. Smith. 12. What triggers the regulation of nitrogenase activity in forage legumenodules after defoliation? U.A. Hartwig, J. Nösberger. 13. Nodulation and nitrogen fixation in extreme environments; L.M. Bordeleau, D. Prévost. 14. Analysis and regulation of legume inoculants in Canada: The need for an increase in standards; P.E. Olsen, W.A. Rice, L.M. Bordeleau, V.O. Biederbeck. 15. Recent developments in the actinorhizal symbioses; A.M. Berry. 16. Characterization of Bradyrhizobium japonicum chromosomal genes involved in the regulation of hup gene expression in free-living conditions and in controlling hydrogenase activity; C. van Soom, J. Vanderleyden, A.P. van Gool. 17. Characterization of Rhizobium galegae by REP-PCR, PFGE and 165 rRNA sequencing; I. Huber, S. Selenska-Pobell. 18. A hypothesis for the role of malic enzyme in symbiotic nitrogen fixation in soybean nodules; D.A. Day, R.G. Quinnell, F.J. Bergersen. 19. Eastern Canadian soybean field trials of rhizobial strain NS 1 in two commercial carriers; R.E. Sanders. 20. Nitrogen fixation efficiency of cold-adapted rhizobia on sainfoin (Onobrychis viciifolia): Laboratory and field evaluation; D. Prévost, L.M. Bordeleau, R. Michaud, C. Lafrenière, J. Waddington, V.O. Biederbeck. 21. Survival of Bradyrhizobium japonicum in pig slurry used as carrier for soil inoculation; G. Ciafardini, G.C. Turtura. 22. Plasmid DNA content of several agronomically important Rhizobium species that nodulate alfalfa, berseem clover, or Leucaena; F.M. Hashem, D. Kuykendall. 23. International FAO/IAEA programm