Examination of root systems and adhering soil associated with unusually green corn plants from nitrogen-deficient areas in fields has revealed higher acetylene-reducing activities than comparable root systems of chlorotic plants that appeared to be nitrogen deficient. From the root systems with acetylene-reducing activity, N₂-fixing Enterobacter cloacae were isolated. Pure cultures of the organism grown in a medium lacking added fixed nitrogen reduced acetylene to ethylene and used N₂ as a sole source of nitrogen under anaerobic conditions. Acetylene reduction was strikingly inhibited by 0.04 atm or greater partial pressures of O₂; however, the bacteria maintained appreciable acetylene-reducing rates in medium exposed to partial pressures of O₂ ranging from 0.005 to 0.015 atm. Nitrogenase activity in cell-free extracts of the bacterium was dependent upon Na₂S₂O₄ and an ATP-generating system. Some environmental conditions expected to influence N₂ fixation by free-living N₂-fixing bacteria on root surfaces of nonlegumes are discussed.

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