Biological nitrogen fixation:: A key input for integrated soil fertility management in the tropics

This paper describes the importance of biological nitrogen fixation (BNF) by legume-Rhizobium symbiosis to tropical agriculture, the evolution of BNF paradigms, creation of strategic alliances to combat soil fertility degradation, and accomplishments of collaborative BNF-related research at CIAT-TSBFI. It suggests that a holistic-multidisciplinary-systems approach is needed to integrate BNF-efficient and stress adapted legumes into smallholder systems. It proposes a number of research and development priorities for achieving improved BNF contributions through integrated soil fertility management, a holistic approach to soil fertility that includes all driving factors and consequences of soil degradation. Although BNF has not proved a solution for strain selection or breeding of host, modest progress has been registered. The technology is economically viable. The environment is at least as limiting on BNF as is the strain and the host. The benefits of BNF are best expressed in the context of an agronomic management system that addresses other components of the crop, especially P supply, drought stress and frequently, starter N. Selection for BNF capacity under physiological stress has revealed genotypes worth exploiting more fully. Research efforts on BNF in tropical forage legumes indicated that the main constraints to their widespread adoption include a lack of legume persistence, presence of anti-quality factors such as tannins, variable Bradyrhizobium requirements, and a lack of acceptability by farmers. Farmer-participatory selection of legumes for increased acceptability is needed. Substantial progress was made in creating an organic resource database and using it to construct a decision support system for organic matter management. Analysis of organic resource data indicated a set of critical values of nitrogen, lignin and polyphenol content for predicting the "fertilizer equivalence" of organic inputs. This provides farmers with guidelines for appropriate use of organic materials for soil fertility improvement. The paper also suggests key interventions that are needed to achieve greater impact of legume-BNF technologies.