Growth, Mineral Nutrition, and Yield Responses of Perilla frutescens to Bacillus- and Aspergillus-Based Amendments in Soils Affected by Radiocesium Contamination in Fukushima

The Fukushima nuclear accident caused widespread radiocesium contamination, and subsequent decontamination reduced soil fertility by removing nutrient-rich topsoil. Although biological amendments have been widely investigated for soil improvement, their potential to restore crop productivity in decontaminated Fukushima soils remains poorly understood. This study evaluated a Bacillus-based biofertilizer (Yume-Bio) and an Aspergillus fermentation product (kouji) as biological amendments for restoring crop productivity in decontaminated soils. Pot and field experiments were conducted to assess their effects on the growth, mineral nutrition, and seed yield of Perilla frutescens grown in decontaminated Fukushima soils. In pot experiments, Yume-Bio showed no significant effects on plant growth, although slight root improvement was observed. In contrast, application of kouji alone or in combination with Yume-Bio significantly enhanced plant growth, increasing leaf number by 112% and improving biomass production. Nutrient accumulation was also promoted, with total N and Fe increasing by 170% and 194%, respectively. In field experiments at two sites in Fukushima, treatment effects were limited and generally non-significant. These results indicate that kouji has potential to enhance plant growth under controlled conditions, while the effectiveness of biological amendments under field conditions remains site-dependent, highlighting the need to optimize application strategies under heterogeneous soil conditions.