Forest type effects on the retention of radiocesium in organic layers of forest ecosystems affected by the Fukushima nuclear accident

The Fukushima Daiichi nuclear power plant disaster caused serious radiocesium (Cs-137) contamination of forest ecosystems over a wide area. Forest-floor organic layers play a key role in controlling the overall bioavailability of Cs-137 in forest ecosystems; however, there is still an insufficient understanding of how forest types influence the retention capability of Cs-137 in organic layers in Japanese forest ecosystems. Here we conducted plot-scale investigations on the retention of Cs-137 in organic layers at two contrasting forest sites in Fukushima. In a deciduous broad-leaved forest, approximately 80% of the deposited Cs-137 migrated to mineral soil located below the organic layers within two years after the accident, with an ecological half-life of approximately one year. Conversely, in an evergreen coniferous forest, more than half of the deposited Cs-137 remained in the organic layers, with an ecological halflife of 2.1 years. The observed retention behavior can be well explained by the tree phenology and accumulation of Cs-137 associated with litter materials with different degrees of degradation in the organic layers. Spatial and temporal patterns of gamma-ray dose rates depended on the retention capability. Our results demonstrate that enhanced radiation risks last longer in evergreen coniferous forests than in deciduous broad-leaved forests.