Temporal changes in the spatial patterns of air dose rate from 2012 to 2016 at forest floors in Fukushima, Japan.

This study investigates temporal changes in the distribution of air dose rates at forest floors from 2012 to 2016 by measuring air dose rates at a height of 10 cm. The study was conducted at four different topography forest sites in Fukushima Prefecture, Japan. At each forest site, the air dose rate was found to have decreased by 7%-22% over time from 2012 to 2016 owing to the movement of radiocesium from organic layers to mineral soil layers in the forest site. However, the spatial distribution patterns of air dose rates did not change at these forest sites over five years. Besides, high correlations between air dose rates and organic plus surface mineral soil inventories were found at these forest sites during most of study years. Therefore, little changes in the spatial distribution of air dose rates could be caused by radiocesium retention at the same location in these forest sites. No statistical correlation between air dose rates and slope gradients was found in the two hilly forest sites with steep slopes above 35°. Accordingly, this study shows that the distribution patterns of air dose rates in the forest floors remained stable depending on the spatial distribution of radiocesium, which formed in the early phase after the Fukushima Daiichi Nuclear Power Plant accident.

Assessment of vertical radiocesium transfer in soil via roots.

Several years after the Fukushima Daiichi Nuclear Power Plant accident, the surface mineral soil layer is believed to be the main reservoir of radiocesium (137Cs) in forest ecosystems in Japan. Dissolved 137Cs combines with clay minerals in the soil, and hence, it is not expected to easily infiltrate over time. However, previous studies have indicated that 137Cs derived from the older global fallout migrated deeper than that of the Chernobyl accident, and this cannot be explained by only the dissolved 137Cs vertical migration in the soil. Considering the carbon and nutrient dynamics in the forest floor, the 137Cs transfer process in soil via roots may alter its vertical distribution on a decadal scale. Therefore, in this study, we investigated the 137Cs activity concentrations in both roots and soil matrix, by considering four (0-20 cm) or six (0-30 cm) mineral soil layers taken at every 5 cm at seven study sites dominated by one of the six plant species (three coniferous forests, one deciduous forest, two deciduous forests covered by Sasa, and one bamboo forest) in eastern Japan in 2013. Comparing the results of 137Cs activity concentrations between roots and soil matrix taken at the same soil layer, roots at the surface (0-5 cm) layer often showed lower values than the soil matrix. However, roots deeper than 5 cm had higher activity concentrations than the soil matrix, conversely. The 137Cs inventories ratio of roots to soil matrix are about 1% at the 0-5 and 5-10 cm soil layer, and about 2% at the soil layers deeper than 10 cm. These results suggest that decomposition of root litter little affect the short-term vertical migration of 137Cs in the forest soil. However, it indicates that continuous production and mortality of roots with relatively high 137Cs activity concentrations have an important role for changing the vertical distribution of 137Cs on time scale of decades, particularly at deeper soil layers.

Characteristics of initial deposition and behavior of radiocesium in forest ecosystems of different locations and species affected by the Fukushima Daiichi Nuclear Power Plant accident.

After the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident, information about stand-level spatial patterns of radiocesium initially deposited in the surrounding forests was essential for predicting the future dynamics of radiocesium and suggesting a management plan for contaminated forests. In the first summer (approximately 6 months after the accident), we separately estimated the amounts of radiocesium ((134)Cs and (137)Cs; Bq m(-2)) in the major components (trees, organic layers, and soils) in forests of three sites with different contamination levels. For a Japanese cedar (Cryptomeria japonica) forest studied at each of the three sites, the radiocesium concentration greatly differed among the components, with the needle and organic layer having the highest concentrations. For these cedar forests, the proportion of the (137)Cs stock in the aboveground tree biomass varied from 22% to 44% of the total (137)Cs stock; it was 44% in highly contaminated sites (7.0 × 10(5) Bq m(-2)) but reduced to 22% in less contaminated sites (1.1 × 10(4) Bq m(-2)). In the intermediate contaminated site (5.0-5.8 × 10(4) Bq m(-2)), 34% of radiocesium was observed in the aboveground tree biomass of the Japanese cedar stand. However, this proportion was considerably smaller (18-19%) in the nearby mixed forests of the Japanese red pine (Pinus densiflora) and deciduous broad-leaved trees. Non-negligible amounts of (134)Cs and (137)Cs were detected in both the sapwood and heartwood of all the studied tree species. This finding suggested that the uptake or translocation of radiocesium had already started within 6 months after the accident. The belowground compartments were mostly present in the organic layer and the uppermost (0-5 cm deep) mineral soil layer at all the study sites. We discussed the initial transfer process of radiocesium deposited in the forest and inferred that the type of initial deposition (i.e., dry versus wet radiocesium deposition), the amount of rainfall after the accident, and the leaf biomass by the tree species may influence differences in the spatial pattern of radiocesium by study plots. The results of the present study and further studies of the spatial pattern of radiocesium are important for modeling future radiocesium distribution in contaminated forest ecosystems.

Changes in radiocesium concentrations in epigeic earthworms in relation to the organic layer 2.5 years after the 2011 Fukushima Dai-ichi Nuclear Power Plant accident.

We reported previously that radiocesium ((137)Cs) concentrations in earthworms increased with those in litter and/or soil in Fukushima Prefecture forests 0.5 y after the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident. This study provides further results for 1.5 and 2.5 y after the accident and discusses temporal changes in (137)Cs concentrations and transfer factors (TF) from litter to earthworms to better understand the mechanisms by which (137)Cs enters soil food webs. The concentration of (137)Cs in accumulated litter on the forest floor rapidly decreased, and the concentration in soil (0-5-cm depth) increased over time from 0.5 to 1.5 y, but changed only moderately from 1.5 to 2.5 y. The concentration of (137)Cs in earthworms consistently decreased during the study period; values 2.5 y after the accident were 18.8-68.5% of those 0.5 y after the accident. The TFs from accumulated litter to earthworms decreased over time: 0.24 ± 0.08 (mean ± SD) at 0.5 y and 0.16 ± 0.04 at 2.5 y. This decrease may be a result of decreases in the bioavailability of (137)Cs in litter and the surface soil layer. Changes in (137)Cs bioavailability should be continuously tracked to determine any changes in the relationship between radiocesium concentrations in earthworms and that in accumulated litter or soil.

Radiocesium concentrations in epigeic earthworms at various distances from the Fukushima Nuclear Power Plant 6 months after the 2011 accident.

We investigated the concentrations of radiocesium in epigeic earthworms, litter, and soil samples collected from forests in Fukushima Prefecture 6 months after the Fukushima Dai-ichi Nuclear Power Plant accident in 2011. Radiocesium concentrations in litter accumulated on the forest floor were higher than those in the soil (0-5 cm depth). The highest average (134+137)Cs concentrations in earthworms (approximately 19 Bq g(-1) of wet weight with gut contents and 108 Bq g(-1) of dry weight without gut contents) were recorded from a plot that experienced an air dose rate of 3.1 µSv h(-1), and earthworm concentrations were found to increase with litter and/or soil concentrations. Average (134)Cs and (137)Cs concentrations (with or without gut contents) were intermediate between accumulated litter and soil. Different species in the same ecological groups on the same plots had similar concentrations because of their use of the same habitats or their similar physiological characteristics. The contribution of global fallout (137)Cs to earthworms with gut contents was calculated to be very low, and most (137)Cs in earthworms was derived from the Fukushima accident. Transfer factors from accumulated litter to earthworms, based on their dry weights, ranged from 0.21 to 0.35, in agreement with previous field studies.