Effect of Flooding and the <i>nosZ</i> Gene in Bradyrhizobia on Bradyrhizobial Community Structure in the Soil

Abstract

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^Twt (wt for the wild-type) and the other half received B. diazoefficiens USDA110ΔnosZ. USDA110^Twt possesses the nosZ gene, which encodes N₂O 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^Twt strain, USDA110^Twt 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^Twt 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.