In the 100 years since the legume-Rhizobium symbiotic nitrogen fixation interaction was first described, interest in this field has grown rapidly. The types of studies have been cyclical in nature, involving a cross-section of disciplines. The availability of cheap nitrogenous fertilizers caused much of the biological nitrogen fixation research to become more theoretical in the developed world. The high cost of energy, coupled with environmental concerns and the interest in sustainable agriculture, has stimulated research in symbiotic nitrogen fixation. The development of modern genetic techniques has resulted in interdisciplinary research on plant-microbe interactions controlling nitrogen fixation. This has resulted in a better understanding of environmental factors influencing the nodulation process, chemical signalling between the symbiotic partners and the nature of the specificity between host plant and microsymbiotant. This volume summarizes the diverse research efforts in biological nitrogen fixation by presenting a collection of papers in the areas of physiology and metabolism, taxonomy and evolution, genetics and ecology.

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.

For 31 years, the North American Symbiotic Nitrogen Fixation Conference (for merly Rhizobium Conference) has been a forum for scientists and graduate students to discuss their research advances, extending from basic aspects to agricultural appli cations, and dealing with topics ranging from bacterial genetics and metabolism to plant genetics and physiology. Nitrogen fixation, being a major life-supporting process on this planet, has attracted the interest of researchers for more than one century. Nitrogen fixation is responsible for the conversion of "inert" dinitrogen (N ) gas from the atmos z phere into usable ammonia, replacing the fixed nitrogen constantly being lost to the atmosphere by the denitrification process. Worldwide agricultural productivity is deter mined by the availability of fixed nitrogen in all its forms, which the continually increas ing human population depends upon for survival. An international momentum is developing in the use of biofertilizers to improve legume and non-legume crop yields and in the study of genomics in this area. New studies are being undertaken in several laboratories to study novel nitrogen-fixing systems, including non-legume crops. The North American Conferences on Symbiotic Nitrogen Fixation have periodically chronicled the advances in our knowledge of this area. For the first time, this conference was held in Mexico. Thus, all three North Amer ican countries have been hosts for this event. This year conference brought nearly 200 scientists from 18 different countries presenting lectures and over 80 posters.

Microbiology may be described as one of the younger sciences with its history, as a precise subject, only dating as far back as Pasteur in the mid 1800s and his revelation both of the role of microorganisms in nature and their importance to human welfare. Medical scientists rapidly took up the challenge, with their area of microbiology flourishing and expanding almost in complete isolation from the rest of biology. We now know, of course, that microorganisms have always played an important, if not essential role, in the biosphere with fermented foods and beverages, plant and animal diseases and nutrient cycling foremost in their sphere of activities. Within the last twenty years, microbiology has received two enormous boosts with the developments in microbial genetics and genetic engineering probably being the most influential, and the greater awareness of pollution and environmental sustainability following a close second. In 1990, your editor had the privilege and pleasure of being elected as President of The Association of Applied Biologists in the United King dom and, as the topic for his three-day Presidential Conference, chose 'The exploitation of microorganisms in applied biology'. This meeting stimu lated great interest in a wide range of subject areas, from weed control to nematology, from plant breeding to plant pathology, from mushrooms to mycorrhiza. The proceedings of this meeting were published in Aspects of Applied Biology, No. 24, 1990.

Proceedings of the Second Working Group Meeting of the Frontier Project on Nitrogen Fixation in Rice held in Faisalabad, Pakistan, 13-15 October 1996