After the accident of Fukushima Daiichi Nuclear Power Plant (FDNPP), we screened Japanese soybean mini-core
collections to select cultivars with low 137Cs accumulation in seeds. Field tests at Fukushima Prefecture resulted in two groups of soybeans: one with low and the other with high Cs accumulation in seeds. We continued investigations to find the reasons. A reconfirmation field test and two pot experiments were conducted to evaluate the 137Cs activity concentrations in various organs of soybean cultivars, focusing on the root nodules affected by single/co-inoculation with Bradyrhizobium and arbuscular mycorrhizal (AM) fungi in two types of soils. Oni hadaka with low, and Kuromame and Kurakake with high 137Cs accumulation properties in seeds were the test cultivars. These cultivars had similar biomass and root surface area. Total 137Cs inventory was greater in cultivars Kuromame and Kurakake. Average 137Cs concentrations were in the order: root nodules, plant root, leaf, stem in Kuroboku soil, and root nodules similar/lower to that in the order of plant root, leaf, stem in brown forest soil. Single/co-inoculation of AM fungi did not change the 137Cs activity concentrations of root nodules and plant leaf/stem significantly; however, it increased the 137Cs concentrations of plant roots than those of without AM fungi inoculation and also root nodules significantly. This shows that the 137Cs retained by AM mycelium in the roots is not available to translocate into other parts in soybeans tested.