Dinitrogen fixation activity of Azorhizobium caulinodans in the rice (Oryza sativa L.) rhizosphere assessed by nitrogen balance and nitrogen-15 dilution methods

 Azorhizobium caulinodans was directly inoculated onto rice plants in three short-term pot trials. Addition of increasing amounts of sucrose (23, 46, 92 kg ha^–1) did not influence the N economy of the A. caulinodans-rice association during the early vegetative growth stage. A. caulinodans inoculation alone and in combination with the highest amount of sucrose had a significantly positive effect on the N balance, with small but significant N gains in the system. Application of 60 kg urea-N ha^–1 had a negative impact on the N economy of the inoculated treatments. N losses increased and the amount of atmospheric N₂ fixed and incorporated decreased significantly as compared to the amounts under the 20 kg urea-N ha^–1 regime. However, N losses were low – a maximum of 8% – at the early vegetative growth stage under the conditions of the experiments. C limitation does not seem to be a limiting factor for the incorporation of fixed N₂ in this bacteria-plant association. Biological N₂ fixation caused by A. caulinodans inoculation was responsible for 14% of the plant N at the vegetative growth stage and under low N conditions. Received: 30 January 2000