5:15 PM - 6:45 PM
[MAG34-P01] Downward migration of 137Cs as a self-cleansing process in forest ecosystems in Fukushima
Keywords:Forest decontamination, Root uptake, Forest soil, Self-cleaning
Approximately 70% of the area highly 137Cs-contaminated by the Fukushima Daiichi Nuclear Power Plant accident is forested. However, decontamination works have not progressed in most of these forests, and the forestry industry remains stagnant. Although the long-term dynamics of 137Cs in the forest ecosystem will be controlled by the amount of 137Cs uptake via roots in the future, temporal changes in 137Cs in tree roots have rarely been reported. In this study, we investigated the 137Cs concentration and inventory in the soil and very fine (VF) roots (< 0.5 mm) of Japanese cedar from 2011 to 2023. Recently, a functional classification of fine roots smaller than 2 mm into absorptive and transport fine roots has been proposed. Absorptive fine roots possess nutrient and water absorption abilities that are highly sensitive to various soil conditions and are current-year roots that are less affected by translocation and accumulation of 137Cs within a tree. The absorptive fine root of Japanese cedar has been reported to be approximately 0.5 mm or less. Moreover, we discussed the potential self-cleaning effects of forest ecosystems as a sustainable remediation strategy based on natural processes and cycles.
An approximately 3 m x 3 m plot was established in a cedar forest (initial deposition 440 kBq m-2) in the Yamakiya district of Kawamata Town, Fukushima Prefecture. Litter and soil samples were collected twice a year during 2011-2012 and once a year after 2013 using a scraper plate at 0.5 cm intervals for 0-5 cm, 1 cm intervals for 5-10 cm, and 5 cm intervals for 10-20 cm. VF root samples were collected by further separating only the roots with tweezers from soil samples in 2012, 2015, 2017, 2020 and 2023, and washed by ultrasonic homogenizer to remove soil particles on the root surface. The 137Cs concentrations in the samples were determined by measuring gamma rays at 662 keV using germanium semiconductor detector and the 133Cs concentrations in samples were measured by ICP-MS after decomposition using concentrated nitric acid and hydrogen peroxide.
The 137Cs concentration in the litter layer was still decreasing exponentially more than 12 years after the accident, its inventory was about 0.2-0.5% of the deposited amount. The depth distribution of 137Cs concentration in the mineral soil layers was fitted with an exponential equation until 2019, but after 2020, the peak concentration shifted slightly downward and was fitted with a hyperbolic function. The 137Cs inventory in the soil tended to increase over time due to the migration from the forest canopy and litter layers, whereas that in the VF roots decreased in 2020 and 2023. Especially, the 137Cs inventory in the VF roots in the 0–2 cm of soil reached 89% in 2012; however, it decreased with time to approximately 43% in 2020 and 55% in 2023. The positive correlation between the137Cs concentration in the VF roots and litter layers was found only at 0–2 cm, but the 137Cs concentration in the VF roots at other depths were correlated with the 137Cs concentration in soils. Whereas the positive correlation between the133Cs concentration in the VF roots and soil was found at all depths. It was concluded that the decrease in the137Cs concentration in the VF roots at 0–2 cm was caused by the decrease in 137Cs concentration in the litter layers. Although the 137Cs concentration in the VF roots below 2 cm increased with increasing 137Cs concentration in the soil, the downward migration of 137Cs within the soil can reduce the amount of 137Cs absorbed by roots because the VF root biomass decreases exponentially with depth. In other words, 137Cs can be removed from the long-term active cycles of forest ecosystems as they migrate deeper into the soil without physical decontamination. This natural downward migration process can be regarded as a “self-cleaning” of the forest ecosystem, resulting in a decrease in the air dose rate and the amount of 137Cs absorbed by roots.
An approximately 3 m x 3 m plot was established in a cedar forest (initial deposition 440 kBq m-2) in the Yamakiya district of Kawamata Town, Fukushima Prefecture. Litter and soil samples were collected twice a year during 2011-2012 and once a year after 2013 using a scraper plate at 0.5 cm intervals for 0-5 cm, 1 cm intervals for 5-10 cm, and 5 cm intervals for 10-20 cm. VF root samples were collected by further separating only the roots with tweezers from soil samples in 2012, 2015, 2017, 2020 and 2023, and washed by ultrasonic homogenizer to remove soil particles on the root surface. The 137Cs concentrations in the samples were determined by measuring gamma rays at 662 keV using germanium semiconductor detector and the 133Cs concentrations in samples were measured by ICP-MS after decomposition using concentrated nitric acid and hydrogen peroxide.
The 137Cs concentration in the litter layer was still decreasing exponentially more than 12 years after the accident, its inventory was about 0.2-0.5% of the deposited amount. The depth distribution of 137Cs concentration in the mineral soil layers was fitted with an exponential equation until 2019, but after 2020, the peak concentration shifted slightly downward and was fitted with a hyperbolic function. The 137Cs inventory in the soil tended to increase over time due to the migration from the forest canopy and litter layers, whereas that in the VF roots decreased in 2020 and 2023. Especially, the 137Cs inventory in the VF roots in the 0–2 cm of soil reached 89% in 2012; however, it decreased with time to approximately 43% in 2020 and 55% in 2023. The positive correlation between the137Cs concentration in the VF roots and litter layers was found only at 0–2 cm, but the 137Cs concentration in the VF roots at other depths were correlated with the 137Cs concentration in soils. Whereas the positive correlation between the133Cs concentration in the VF roots and soil was found at all depths. It was concluded that the decrease in the137Cs concentration in the VF roots at 0–2 cm was caused by the decrease in 137Cs concentration in the litter layers. Although the 137Cs concentration in the VF roots below 2 cm increased with increasing 137Cs concentration in the soil, the downward migration of 137Cs within the soil can reduce the amount of 137Cs absorbed by roots because the VF root biomass decreases exponentially with depth. In other words, 137Cs can be removed from the long-term active cycles of forest ecosystems as they migrate deeper into the soil without physical decontamination. This natural downward migration process can be regarded as a “self-cleaning” of the forest ecosystem, resulting in a decrease in the air dose rate and the amount of 137Cs absorbed by roots.