Denitrifying ability of indigenous strains of Bradyrhizobium japonicum isolated from fields under paddy-upland rotation

In Japan some paddy fields are used for upland crops for several years and then returned to paddy fields (paddy-upland rotation). Soybeans (Glycine max L.) are an important summer crop. The ability to denitrify and some characteristics of denitrification by isolated strains of Bradyrhizobium japonicum were investigated to clarify the frequency of denitrifiers in indigenous populations of B. japonicum in fileds under paddyupland rotation. Eight field plots with different cropping systems at two sites were used. The fields consisted of a Gray Lowland Soil, and either soybeans or paddy rice (Oryza sativa L.) was grown as a summer crop, with barley (Hordeum distichum L.) or wheat (Triticum aestivum L.) as a winter crop. All B. japonicum strains present in the plots were able to denitrify. Isolated strains fell into two main groups (groups I and II) according to the rate of denitrification. Strains of group I evolved N2O with C2H2 at a rate comparable to that of Alcaligenes denitrificans IAM 12370, whereas the denitrification activity of group II strains was 100 times lower than that of group I strains. Both group I and II strains occurred in each plot. Amounts of N2O produced by indigenous strains with and without C2H2 suggested that strains of group I and II evolved N2 or N2O, respectively, as the end product of denitrification. One strain (S 107) that was isolated had the highest denitrifying ability with an end product of N2O. These results indicate that indigenous bradyrhizobia may partly contribute to denitrification of field soil under a paddy-upland rotation.