Effect of carrier materials, sterilization method, and storage temperature on survival and biological activities of Azotobacter chroococcum inoculant

Azotobacter chroococcum (A101) was examined for some biological activities such as nitrogenase, phosphatase, potassium solubilization, and production of some plant hormones such as indole acetic acid, gibberellic acid, and cytokinin. Six different formulations were prepared, using different carrier materials namely; peat moss, mixture of peat moss plus vermiculite 1:2 (w/w), wheat bran, rice husk, clay, and sodium alginate. Each carrier material was packed using polyethylene pages, and then divided into three groups. The first group was sterilized by autoclaving at 121 °C for 20 min, and the second one was sterilized by gamma irradiation at a dose rate of 4.0 kGy for 1 h. However, the third group was left without sterilization. Half of the inoculated polyethylene bags, containing the tested formulations either sterilized by autoclaving or gamma irradiation, were incubated at 8 °C and the other bags were incubated at 30 °C for 6 months. The non-sterilized bags were incubated under the same condition but only for 3 months. For testing the survival of Az. chroococcum (A101), the prepared formulation samples were taken every month during the storage period. Nitrogenase activity was evaluated in the prepared formulations which exerted survival cells equal to or more than 108 CFU/ml after 6 months of storage period. Results revealed that non-sterile formulations exerted high numbers of total fungi and bacteria along the storage period; however, Az. chroococcum (A101) numbers were decreased over incubation time. No contaminants have been detected in all sterilized carriers. Az. chroococcum (A101) inoculated on wheat bran exhibited the highest densities among the tested carriers. Encapsulated formulation of alginate exerted the high stability in Az. chroococcum (A101) densities up to the end of the incubation period (6 months) at both 30 °C and 8 °C, being 11.905 log 10 CFU/g.