Role of Arbuscular Mycorrhizal Fungi in Nitrogen Fixation in Legumes

Abstract

Most herbaceous legumes of family Papillionaceae are symbiotic with nitrogen-fixing rhizobia and nutrient-absorbing arbuscular mycorrhizal (AM) fungi. Rhizobia and AM-fungi often interact synergistically resulting in better root nodulation, nutrient uptake, and plant yield compared to plants symbiotic with either organism used alone. Increased phosphorus and other nutrients, enhanced photosynthesis, beneficial interaction with rhizospheric microorganisms, and alleviation of environmental stresses due to AM colonization could account for enhanced nitrogen fixation and grain yield of legumes. Few studies have also been carried out to investigate the effect of AM-fungi on free-living nitrogen fixers in the rhizosphere of legumes; however, no definite conclusion could be drawn. The effectiveness of co-inoculation of AM-fungi and rhizobia and AM-fungi with free-living diazotrophs depends upon the compatibility among interacting partners in the rhizosphere that varies greatly with physicochemical characteristics of soil, test microorganisms, plant genotypes, and substances exude from host plant species. In this chapter, role of AM in symbiotic and non-symbiotic nitrogen fixation in legumes and the mechanisms involved are described.