We investigated the effects of the water status (flooded or non-flooded) and presence of the nosZ gene in bradyrhizobia on the bradyrhizobial community structure in a factorial experiment that examined three temperature levels (20°C, 25°C, and 30°C) and two soil types (andosol and gray lowland soil) using microcosm incubations. All microcosms were inoculated with Bradyrhizobium japonicum USDA6 T , B. japonicum USDA123, and B. elkanii USDA76 T , which do not possess the nosZ gene, and then half received B. diazoefficiens USDA110 T wt (wt for the wild-type) and the other half received B. diazoefficiens USDA110ΔnosZ. USDA110 T wt possesses the nosZ gene, which encodes N 2 O reductase; 110ΔnosZ, a mutant variant, does not. Changes in the community structure after 30-and 60-d incubations were investigated by denaturing-gradient gel electrophoresis and an image analysis. USDA6 T and 76 T strains slightly increased in non-flooded soil regardless of which USDA110 T strain was present. In flooded microcosms with the USDA110 T wt strain, USDA110 T wt became dominant, whereas in microcosms with the USDA110ΔnosZ, a similar change in the community structure occurred to that in non-flooded microcosms. These results suggest that possession of the nosZ gene confers a competitive advantage to B. diazoefficiens USDA110 T in flooded soil. We herein demonstrated that the dominance of B. diazoefficiens USDA110 T wt within the soil bradyrhizobial population may be enhanced by periods of flooding or waterlogging systems such as paddy-soybean rotations because it appears to have the ability to thrive in moderately anaerobic soil.

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One of the strategies that is commonly used in the Philippines to improve the production of soybean is by inoculation. However, this technique often fails mainly due to the lack of information about the indigenous soybean rhizobia in the Philippines soil. In this study, the diversity of indigenous bradyrhizobia collected from the non-flooded and flooded soil conditions at 11 locations in the country was investigated using a local soybean cultivar as the host plant. The genetic variation among the 424 isolates was detected through Polymerase Chain Reaction-Restriction Fragment Length Polymorphism (PCR-RFLP) treatment and sequence analysis for 16S rRNA gene, 16S-23S rRNA internal transcribed spacer (ITS) region and rpoB housekeeping gene. All the isolates were classified under the Bradyrhizobium species namely B. elkanii, B. diazoefficiens, B. japonicum, B. yuanmingense and a considerable proportion of the isolates were clustered under Bradyrhizobium sp. The isolates which were classified under Bradyrhizobium sp. were thought to be endemic to Philippines soil as evidenced by their nucleotide divergence against the known rhizobia and the historical absence of rhizobia inoculation in the collection sites. The major influence on the distribution and diversity of soybean bradyrhizobia is attributed to the difference in the flooding period, followed by soil properties such as pH, soil type, and nutrient content. As determined, it is proposed that the major micro-symbiont of soybean in the Philippines are B. elkanii for non-flooded soils, then B. diazoefficiens and B. japonicum for flooded soils.