BIOLOGICAL NITROGEN-FIXATION - AN EFFICIENT SOURCE OF NITROGEN FOR SUSTAINABLE AGRICULTURAL PRODUCTION

A fundamental shift has taken place in agricultural research and world food production. In the past, the principal driving force was to incr ease the yield potential of food crops and to maximize productivity. T oday, the drive for productivity is increasingly combined with a desir e for sustainability. For farming systems to remain productive, and to be sustainable in the long-term, it will be necessary to replenish th e reserves of nutrients which are removed or lost from the soil. In th e case of nitrogen (N), inputs into agricultural systems may be in the form of N-fertilizer, or be derived from atmospheric N-2 via biologic al N-2 fixation (BNF). Although BNF has long been a component of many farming systems throughout the world, its importance as a primary sour ce of N for agriculture has diminished in recent decades as increasing amounts of fertilizer-N are used for the production of food and cash crops. However, international emphasis on environmentally sustainable development with the use of renewable resources is likely to focus att ention on the potential role of BNF in supplying N for agriculture. Th is paper documents inputs of N via symbiotic N-2 fixation measured in experimental plots and in farmers' fields in tropical and temperate re gions. It considers contributions of fixed N from legumes (crop, pastu re, green manures and trees), Casuarina, and Azolla, and compares the relative utilization of N derived from these sources with fertilizer N .