ABSTRACT
The environmental concerns due to the increasing amount of the reactive forms of nitrogen in atmosphere, originating from the manufacture and use of chemical fertilizers have resulted in a re-focus on the importance of biological nitrogen fixation (BNF). The economic costs of the very high use of chemical nitrogenous fertilizers in agriculture system are also a concern in the modern world for the poor farming community. BNF (symbiotic and non-symbiotic), a microbiological process which converts atmospheric nitrogen into a plant-usable form, offers this alternative. Non-symbiotic N2 fixation (by free-living bacteria in soils or associated with the rhizosphere) has the potential to meet several needs, especially in the lower input cropping systems worldwide. There has been considerable research on nonsymbiotic N2 fixation, but still, there is much argument about the amount of N that can potentially be fixed by this process. Largely due to shortcomings of 174indirect measurements; however, isotope-based direct methods indicate agronomically significant amounts of N2 fixation both in annual crop and perennial grass systems. New molecular technologies offer opportunities to increase our understanding of N2-fixing microbial communities (many of them non-culturable) and the molecular mechanisms of non-symbiotic N2 fixation. This knowledge should assist the development of new plant-diazotrophic combinations for specific environments and more sustainable exploitation of N2-fixing bacteria as inoculants for agriculture. Although the ultimate goal might be to introduce nitrogenase genes into significant non-leguminous crop plants, it may be more realistic in the shorter-term to better synchronize plant-microbe interactions to enhance N2 fixation when the N needs of the plant are greatest. The review explores possibilities to maximize potential N inputs from non-symbiotic N2 fixation through improved management practices, identification of better performing microbial strains and their successful inoculation in the field, and plant-based solutions.
