Contamination processes of tree components in Japanese forest ecosystems affected by the Fukushima Daiichi Nuclear Power Plant accident 137Cs fallout

Radiocesium dynamics from canopy to forest floor by main depositional processes in Fukushima forests at quasi-equilibrium state

Accurate measurement of the amounts of radiocesium (137Cs) that transfer from the canopy to the forest floor at the recent quasi-equilibrium state is crucial for improving the accuracy of long-term predictions of 137Cs distribution in forests. This study is the first to detail the 137Cs flux via throughfall, stemflow, and litterfall processes in two Japanese cedar forests (KU1-S and KU2-S) and one deciduous broad-leaved forest (KU1-Q) in the Fukushima Prefecture during the quasi-equilibrium state. From 2020 to 2023, the annual transfer of 137Cs from the canopy to the forest floor, relative to the initial deposition, varied among the study plots but never exceeded 2% (0.433-0.564% in KU1-S, 1.06-1.62% in KU2-S, and 0.421-0.461% in KU1-Q). Since the transfer of 137Cs from trees to soil is balanced with the transfer from soil to trees at the quasi-equilibrium state, these percentages can be interpreted as the root uptake of 137Cs from the soil, which is difficult to observe directly. Analysis of the contributions of litterfall, throughfall, and stemflow to the total transfer of 137Cs revealed that litterfall accounted for more than 50% in both Japanese cedar and deciduous broad-leaved forests. We also found that the proportion of 137Cs flux via litterfall was higher in Japanese cedar forests compared to the deciduous broad-leaved forest (KU1-S:91-92%; KU2-S:74-77%; KU1-Q:57-60%). These findings regarding the 137Cs fluxes at the quasi-equilibrium state will enhance our understanding of the future dynamics of 137Cs within forest ecosystems and improve the accuracy of long-term predictions of 137Cs activity concentration in forests.

Estimation of spatio-temporal distribution of 137Cs concentrations in litter layer of forest ecosystems in Fukushima using FoRothCs model

The nuclear power plant accident in Fukushima had led to pollution of forest ecosystems with ¹³⁷Cs in 2011. In this study, we simulated the spatiotemporal distribution of ¹³⁷Cs concentrations of litter layer in the contaminated forest ecosystems in two decades from 2011, which is one of the key environmental components of ¹³⁷Cs migration in the environment due to the high bioavailability of ¹³⁷Cs in the litter. Our simulations showed that ¹³⁷Cs deposition is the most important factor in the degree of contamination of the litter layer but vegetation type (evergreen coniferous/deciduous broadleaf) and mean annual temperature are also important for changes over time. Deciduous broadleaf trees had higher initial concentrations in the litter layer due to the direct initial deposition on the forest floor. However, the concentrations remained higher than those in evergreen conifers after 10 years due to redistribution of ¹³⁷Cs by vegetation. Moreover, areas with lower average annual temperatures and lower litter decomposition activity retained higher ¹³⁷Cs concentrations in the litter layer. The results of the spatiotemporal distribution estimation of the radioecological model suggest that, in addition to ¹³⁷Cs deposition, elevation and vegetation distribution should also be considered in the long-term management of contaminated watersheds, which can be informative in identifying hotspots of ¹³⁷Cs contamination on a long-term scale.

Effect of soil organic matter on the fate of 137Cs vertical distribution in forest soils

Understanding the fate of the vertical distribution of radiocesium (¹³⁷Cs) in Japanese forest soils is key to assessing the radioecological consequences of the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident. The ¹³⁷Cs behavior in mineral soil is known to be primarily governed by interaction with clay minerals; however, some observations suggest the role of soil organic matter (SOM) in enhancing the mobility of ¹³⁷Cs. Here we hypothesized that soil organic carbon (SOC) concentration profile determines the ultimate vertical pattern of ¹³⁷Cs distribution in Japanese forest soils. In testing this hypothesis, we obtained soil samples that were collected before the FDNPP accident at four forest sites with varying SOC concentration profiles and quantified the detailed vertical profile of ¹³⁷Cs inventory in the soils roughly half a century after global fallout in the early 1960 s. Results showed that the higher the SOC concentration in the soil profile, the deeper the ¹³⁷Cs downward penetration. On the basis of the data for surface soils (0-10 cm), the ¹³⁷Cs retention ratio for each of the 2-cm thick layers was evaluated as the ratio of ¹³⁷Cs inventory in the target soil layer to the total ¹³⁷Cs inventory in and below the soil layer. A negative correlation was found between the ratio and SOC concentration of the layer across all soils and depths. This indicates that the ultimate fate of ¹³⁷Cs vertical distribution can be predicted as a function of SOC concentration for Japanese forest soils, and provides further evidence for SOM effects on the mobility and bioavailability of ¹³⁷Cs in soils.

Mode of Atmospheric Deposition in Forests Demonstrates Notable Differences in Initial Radiocesium Behavior

The March 2011 Fukushima Dai-ichi Nuclear Power Plant accident in Japan released 520 PBq of radionuclides compared to a total release of 5300 PBq from the Chornobyl Nuclear Power Plant accident. Both nuclear accidents resulted in deposition of radiocesium throughout the northern hemisphere, and a plethora of studies have been performed regarding radiocesium (¹³⁷Cs) behavior. However, few studies have assessed the impact of precipitation on ¹³⁷Cs deposition in forests. Wide-scale environmental measurements from 2011 and 2016 were used to determine the differences in ¹³⁷Cs deposition because of precipitation following the Fukushima accident. In areas where wet deposition processes were dominant, dense forests generally had lower ambient dose rates and levels of contamination on forest floors than other stands with fewer stems per hectare in 2011. Similar tendencies were not observed in areas that were primarily subject to dry deposition nor were any trends observed in 2016. ¹³⁷Cs was retained in dense forest canopies for an extended period regardless of the deposition mode. Additionally, it was found that the initial retention of radionuclides by forest canopies is in general higher for areas with predominantly dry deposition. Incorporation of radiocesium into wood tissues was the same for both wet and dry deposition.