Biological nitrogen fixation for sustainable agriculture: A perspective

The economic and environmental costs of the heavy use of chemical N fertilizers in agriculture are a global concern. Sustainability considerations mandate that alternatives to N fertilizers must be urgently sought. Biological nitrogen fixation (BNF), a microbiological process which converts atmospheric nitrogen into a plant-usable form, offers this alternative. Nitrogen-fixing systems offer an economically attractive and ecologically sound means of reducing external inputs and improving internal resources. Symbiotic systems such as that of legumes and Rhizobium can be a major source of N in most cropping systems and that of Azolla and Anabaena can be of particular value to flooded rice crop. Nitrogen fixation by associative and free-living microorganisms can also be important. However, scientific and socio-cultural constraints limit the utilization of BNF systems in agriculture. While several environmental factors that affect BNF have been studied, uncertainties still remain on how organisms respond to a given situation. In the case of legumes, ecological models that predict the likelihood and the magnitude of response to rhizobial inoculation are now becoming available. Molecular biology has made it possible to introduce choice attributes into nitrogen-fixing organisms but limited knowledge on how they interact with the environment makes it difficult to tailor organisms to order. The difficulty in detecting introduced organisms in the field is still a major obstacle to assessing the success or failure of inoculation. Production-level problems and socio-cultural factors also limit the integration of BNF systems into actual farming situations. Maximum benefit can be realized only through analysis and resolution of major constraints to BNF performance in the field and adoption and use of the technology by farmers.