Effect of inoculation with Klebsiella oxytoca and Enterobacter cloacae on dinitrogen fixation by rice-bacteria associations

The influence of Klebsiella oxytoca and Enterobacter cloacae inoculation on dinitrogen fixation by the rice-bacteria association was examined in pots in a greenhouse. For inoculation, K. oxytoca NG13 isolated from a Japan paddy soil, E. cloacae E26 isolated from a China soil and following three strains were employed. K. oxytoca NG1389 is a mutant from NG13 and has no nitrogenase activity (nif). K. oxytoca NG13/pMC71A and E. cloacae E26/pMC71A were produced by inserting a nif A containing K. pneumoniae plasmid (pMC71A) to NG13 and E26, respectively. These two strains were able to fix dinitrogen fixation in the presence of ammonium, whereas nitrogenase activity of wild strains (NG13 and E26) were repressed under this condition. Inoculation effects were tested on two rice (Oryza sativa L.) varieties, a Indica type C5444 and a Japonica type T65. Rice seedlings were planted to nonsterilized potted soil, and grown under flooded conditions. Upon inoculation with NG13 and E26, growth and N increment of plants particularly in T65 were stimulated above NG1389 inoculated plants. Assay by¹⁵N dilution and acetylene reduction techniques indicated that this N increment may be due to fixed N. Inoculation with NG13/pMC71A and E26/pMC71A resulted in more dry weight and fixed N than those of NG13 and E26 inoculated plants.