SYMBIOTIC N2 FIXATION AS AN ALTERNATIVE SOURCE OF NITROGEN - A REVIEW
The role of N2 fixation through symbiotic relationship between legumes and root nodule bacteria or rhizobia could not be over emphasized, as an alternative nitrogen source for legumes and other crops. The N2 fixed by legume-rhizobia symbiosis ranges between 16 -145 kg N ha-1 year-1 in fertile soils and 15 -123 kg N ha-1 year-1 in poorly fertilized soils, while annual chemical fertilizer use has been reported as 89.7 kg per ha year-1. This paper reviews legumes, their interaction with rhizobia in N2 fixation and importance to agricultural systems. More than 732 genera of legumes have been discovered, with more than 19, 321 species. Legumes, through N2 fixation,directly influence their own growth and development, simultaneously benefit companion crops in mixed cropping and subsequent crops in rotation. Likewise, there are diverse nature of rhizobia, and more genera and species are being discovered. This therefore, emphasizes the need to intensify legume production, through identifying and utilizing relevant rhizobia as inoculants for particular legumes to optimize the benefits of N2 fixation. The review addresses legumes and rhizobia in relation to their role in N2 fixation and benefits derived from incorporating legumes in farming systems. It also discusses the current taxonomy of rhizobia, the concepts of specificity and promiscuity among both symbionts. Likewise, it gives highlight on the genera and species of rhizobia currently described and the dynamic nature of their discovery. There now exist more than 15 genera containing over 120 species of rhizobia, with discovery rate of 10 species per annum.