Effects of CO2 concentration in rhizosphere on nodulation and N2 fixation of soybean and cowpea

Soybean (Glycine max L. Merr.) cvs. Akisengoku and Peking, and cowpea (Vigna unguiculata Walp.) cv. Kegonnotaki were inoculated with Bradyrhizobium japonicum AlO17, Shinorhizobium fredii USDAI93, and B. sp. Vigna MAFF03-03063, respectively and were cultured hydroponically with supply of CO₂-free air, 3dm³ m^-3 CO₂ air, or 25 dm³ m^-3 CO₂ air to study the effects of the CO₂ concentration in the rhizosphere on plant growth, nodulation, and nitrogen fixation. Increase of the CO₂ concentration in the rhizosphere led to the increase of the plant dry weight in the symbiosis between Peking and USDAI93, and that between Kegonnotaki and MAFF03-03063. On the other hand, dry matter accumulation in the symbiosis between Akisengoku and AI017 decreased under the supply of 25 dm³ m^-3 CO₂ air aimed at increasing the CO₂ concentration in the rhizosphere beyond the optimum CO₂ concentration for growth. Nodule mass and nodule number per plant were highest in Akisengoku, followed by Kegonnotaki and lowest in Peking. Also the increase of the CO₂ concentration in the rhizosphere led to the increase of the nodule mass and number in Kegonnotaki, while no changes were observed in Akisengoku and Peking. Biological nitrogen fixation (BNF) was highest in Akisengoku, followed by Kegonnotaki, and lowest or near zero in Peking. BNF in Akisengoku and Kegonnotaki showed a similar tendency to that of dry matter accumulation. BNF of Peking was especially low under the supply of CO₂-free air, and it increased with the increase of the CO₂ concentration in the rhizosphere. For the symbiosis of Bradyrhizobium strains with soybean and cowpea, the most suitable CO₂ concentration for N₂ fixation and plant growth was estimated to be about 10 dm³ m^-3, while for the symbiosis of S. fredii with soybean, the value was estimated to be above 30 dm³ m^-3.