The AAAS Selected Symposia Series was begun in 1977 to provide a means of more permanently recording and more widely disseminating some of the valuable material which is discussed at the AAAS Annual National Meetings. The volumes in the Series are based on symposia held at theMeetings which address topics of current and continuing significance, both within and among the sciences, and in the areas in which science and technology have an impact on public policy. The Series format is designed to provide for rapid dissemination of information, so the papers are reproduced directly from camera-ready copy. The papers are organized and edited by the symposium arrangers who then become the editors of the various volumes. Most papers published in the Series are original contributions which have not been previously published, although in some cases additional papers from other sources have been added by an editor to provide a more comprehensive view of a particular topic. Symposia may be reports of new research or reviews of established work, particularly work of an interdisciplinary nature, since the AAAS Annual Meetings typically embrace the full range of the sciences and their societal implications.

Soil organic matter is a reservoir for plant nutrients, provides water-holding capacity, stabilizes soil structure against compaction and erosion, and thus determines soil productivity. All agriculture to some degree depends on soil organic matter. It has long been known that soil organic matter declines when land is taken into cultivation, and that the productivity of new agricultural land is governed by fertility contributions from decomposing natural organic matter. The expansion of agriculture to ever new and more fragile lands, particularly in tropical and developing regions, causes environmental degradation with local effects on soil quality, regional effects on landscape integrity and water quality, and global effects on carbon cycles and the atmosphere. This book summarizes current knowledge of the properties and dynamics of soil organic matter in the tropics, its role in determining soil quality, its stability and turnover, and the options for management in the context of tropical landuse systems, for a readership of resource scientists, economists and advanced students. Maintenance of organic matter is critical for preventing land degradation. Case studies and practical applications are therefore an important part of the book, as are the exploration of future directions in research and management.

Reprinted from Plant and Soil, v.174, nos.1-2 (1995), this volume is devoted to discussions on the role of biological nitrogen fixation (BNF) in agricultural sustainability. Papers presented on BNF in crop forage and tree legumes are augmented with discussion of integrated farming systems involving BNF, soil and N management, and recycling of legume residues. BNF by non-legumes is discussed and attempts to transform cereals into nodulating plants are critically reviewed. Also described are advances in the development of new methodologies to understand symbiotic interactions and to assess N-2 fixation in the field; means of enhancing BNF through plant and soil management; breeding and selection; problems encountered in exploiting BNF under farmers' field conditions; and promising approaches to improve BNF exploitation. Lacks a subject index. Annotation copyright by Book News, Inc., Portland, OR