Ngumah, C.; Orji, J.; Akubuokwuoma, J.; Anoliefoh, U. Impacts of Different Concentrations of Copper and Zinc on in Vitro Responses of Azotobacter Chroococum in Biomass and Nitrogen Fixing Outputs. Ecotoxicol. Environ. Contam. 2019, 13, 79-83.