Longevity of Azotobacter cysts and a model for optimization of cyst density in liquid bioinoculants

Species of Azotobacter are known to form heat and desiccation-resistant cys ts that have a long life span. Recently this property has been used to prep are nitrogen-fixing liquid bioinoculants useful for a variety of crops. We determined the survival of Azotobacter cysts in a liquid medium in order to estimate the shelf-life of a liquid cyst bioinoculant. A negative exponent ial model was fitted to the survivorship curve. The death rate was found to increase with the initial density of cysts and ranged from 0.02 to 0.05 pe r month. The nitrogen-fixing ability, on the other hand, dropped after two years. The shelf-life of cyst bioinoculants therefore should be decided by the nitrogen-fixing ability rather than cyst survival. Based on the derived kinetics a model for optimization of cyst density in Azotobacter cyst-base d liquid bioinoculants is proposed.