Effect of CO2-free air treatment on nitrogen fixation of soybeans inoculated with Bradyrhizobium japonicum and Rhizobium fredii

Soybean (Glycine max L. Merr.) cultivars Akisirome and Peking were inoculated with Bradyrhizobium japonicum Is-21 and Rhizobium fredii USDA 194, respectively, and were grown in cylindrical pots containing sterilized vermiculite which were aerated with CO2-free air or ambient air to study the effects of CO2 deficiency in the rhizosphere on plant growth, nodulation, and nitrogen fixation. The repressive effects of CO2-free air treatment were more conspicuous in Peking than Akisirome, and nodule number, nodule mass, amount of biologically fixed N and plant growth of Peking were reduced remarkably by the CO2-free air treatment. Acetylene reduction activity (ARA) of Peking inoculated with USDA 194 and Akisirome inoculated Is-21 was assayed in the absence and presence of CO2. ARA of Akisirome was not affected by the absence of CO2, while that of Peking decreased drastically. Based on these results, it was concluded that nitrogen fixation by Akisirome inoculated with B. japonicum Is-21 was not appreciably influenced by CO2 deficiency in the rhizosphere, while that of Peking inoculated with R. fredii USDA 194 was severely repressed, and the decrease was estimated to be due to both the reduction of the nodule mass and specific nitrogen-fixing activity.