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

Jun Koarashi,Takeshi Matsunaga,Mariko Atarashi-Andoh,Yukihisa Sanada

doi:10.1038/srep38591

Abstract

The Fukushima Daiichi nuclear power plant disaster caused serious radiocesium (137Cs) contamination of forest ecosystems over a wide area. Forest-floor organic layers play a key role in controlling the overall bioavailability of 137Cs in forest ecosystems; however, there is still an insufficient understanding of how forest types influence the retention capability of 137Cs in organic layers in Japanese forest ecosystems. Here we conducted plot-scale investigations on the retention of 137Cs in organic layers at two contrasting forest sites in Fukushima. In a deciduous broad-leaved forest, approximately 80% of the deposited 137Cs 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 137Cs remained in the organic layers, with an ecological half-life of 2.1 years. The observed retention behavior can be well explained by the tree phenology and accumulation of 137Cs 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.

Highlights

The Fukushima Daiichi nuclear power plant disaster caused serious radiocesium (137Cs) contamination of forest ecosystems over a wide area
The striking contrasts in the mobility and bioavailability of 137Cs between the organic and mineral soil layers suggests that the retention of 137Cs in organic layers is a key factor in evaluating radiation risks delivered from 137Cs contamination of forest ecosystems
The canopy-interception effect was less important in deciduous broad-leaved forests compared to evergreen coniferous forests because the deciduous forests were leafless at the time of the accident; a large proportion of 137Cs delivered by the Fukushima nuclear power plant (NPP) accident was directly deposited on the forest floor[16,17,18]

Summary

Introduction

The Fukushima Daiichi nuclear power plant disaster caused serious radiocesium (137Cs) contamination of forest ecosystems over a wide area. One of the most important lessons learned from observational studies of 137Cs migration in European forests after the Chernobyl NPP accident was that forest-floor organic layers retain the largest portion of the fallout 137Cs for a long time (over a decade) because they are a prolonged source for 137Cs recycling (i.e., the uptake of 137Cs by trees from the organic layers and the subsequent redeposition of 137Cs onto the forest floor via litterfall) in forest ecosystems[3,4,5,6,7] This is primarily because organic layers are deficient in clay minerals that offer specific sites for 137Cs adsorption[8,9], and 137Cs in these layers is not strongly fixed and remains potentially mobile and bioavailable[7,10]. It is assumed that the differences in 137Cs deposition behavior between the forests will result in different capacities of 137Cs retention in their organic layers, at least over the first several years following forest contamination

Methods

Results

Conclusion