Isolation and selection of indigenous Azospirillum spp. from a subtropical island, and effect of inoculation on growth of lowland rice under several levels of N application

Thirty-five Azospirillum strains (13 strains from plant roots and 22 strains from soils) were isolated from Ishigaki island, Japan, which has a subtropical climate. These strains were different from each other according to polymerase-chain-reaction band patterns obtained by using a random primer (OPT-08). Two Azospirillum strains (AZ43 and AZ92-2) were also examined for use in further experiments. Inoculation of lowland rice with these strains enhanced early growth of rice to various degrees. Inoculation of strains VIII.P1-2, AZ92-2, V.S2-2, and V.P5 in sterilized soil yielded higher shoot dry weights than the application of 90 mu g N g(-1) soil without inoculation. Only inoculation with strains AZ92-2 and VIII.P1-2 caused higher N uptake than the application of 90 mu g N g(-1) soil. Three strains were selected for the next experiment based on the results of their effect on the early growth of rice. An investigation was conducted to determine the ability of two indigenous Azospirillum strains (V.S2-2 and VIII.P1-2) and one stock strain (AZ92-2) to promote growth and nutrient-uptake of lowland rice in unsterilized soil under several levels of N application (0, 80, 160, and 240 mg N pot(-1)). Inoculation with these strains without N application increased shoot dry weight by 12-15% compared to the uninoculated treatment. Inoculation with Azospirillum V.S2-2 together with the application of 160 mg N pot(-1) resulted in a shoot dry weight as high as that obtained in the treatment with 240 mg N pot(-1) without inoculation. Thus, in this former case, the amount of N applied could be reduced by 80 mg Pot-l due to the effect of the microbial inoculum without a significant change in the high, targeted, yield.