Studies on the occurrence and distribution of azospirillum population in the soil of Assam and their associative influence on rice oryza sativa crop

Abstract

Rediscovery of Spirillum lipoferum by Dobereiner and Day (1975) in the roots of forage grasses and cereals renewed the interest in the bacterium as it could fix nitrogen in the root region of cereal crops. Later Spirillum lipoferum was found to contain two sero-types and therefore, it was assigned to the status of a genus named as Azospirillum with two species, viz. A. lipoferum and A. brasilense. Three more species, viz. A. amazonense , A. halopraferense and A. irakense were also isolated later. Interest in Azospirillum grew further because of its ability to colonise the root zone of all types of cereals and can fix good quantity of nitrogen under microaerophilic condition. Further, the organism has competitive advantage over many others, in that, it is resistant to antibiotics produced by some competitive microorganisms in the soil. All these are favourable factors for considering Azospirillum as the most sought after diazotroph. It has been estimated that 20 to 40 Kg nitrogen fertilizer can be saved per hectare through inoculation of the roots of cereals with Azospirillum. Reports are available that Azospirillum occurs in India. But most works are restricted to inoculation of wheat , maize etc., with suitable species of Azospirillum. Their occurrence and distribution received little attention. In Assam, no Azospirillum has been reported so far from natural habitats. Here also some works have been done in Agricultural University with respect to inoculation only. In India and elsewhere occurrence of Azospirillum in the rice rhizosphere also received little attention. The present investigation is an attempt to isolate Azospirillum from diverse habitats of Assam and from the rhizosphere of varied types of wild grasses, rice, wheat, sugarcane and bamboo. The investigation has also been done to know the potentiality of the isolates to colonize the rhizosphere of rice on inoculation, and their role in nitrogen-fixation. For the purpose of investigation, rhizosphere samples of rice, wheat, sugarcane, bamboo and...