Revealing the self-cleansing effect of radiocesium in a forest ecosystem after the Fukushima Daiichi Nuclear Power Plant accident

Tsukuba, Japan—In the Hamadori (Pacific coast) area of Fukushima Prefecture, numerous efforts are underway to recover from the impacts of the Great East Japan Earthquake and Fukushima Daiichi Nuclear Power Plant accident. Although some decontamination works, such as the removal of layers of litter (dead leaves), were conducted in part of the forest area, most of the area remained contaminated by radioactive cesium (¹³⁷Cs). Therefore, there are persistent requests from citizens for forest decontamination. However, using existing decontamination methods, the efficiency of reducing ¹³⁷Cs today, more than 13 years after the accident, remains low, rendering the development of new forest decontamination methods crucial.

In this study, a research group at University of Tsukuba investigated ¹³⁷Cs dynamics in the litter and soil layers and absorptive fine roots (<0.5 mm in diameter) in a cedar forest in the Yamakiya district of Kawamata Town, Fukushima Prefecture, from 2011 to 2023. Results show that the downward migration of ¹³⁷Cs from the litter layer to the mineral soil surface occurred rapidly. Further, the ¹³⁷Cs content of the soil increased over time, whereas that in the fine roots of cedar trees decreased since 2020. Specifically, the ¹³⁷Cs content in fine roots at a soil depth of 0-2 cm, where fine roots are dense, decreased considerably because the downward migration of ¹³⁷Cs in the soil caused a shift in the peak of the depth distribution of ¹³⁷Cs in the soil from the depth distribution of roots.

Thus, the downward migration of ¹³⁷Cs in the soil by even a few centimeters reduces ¹³⁷Cs absorption by trees. This phenomenon can be regarded as the self-cleansing effect of the forest ecosystem. Here, the term self-cleansing refers to a reduction in radionuclide concentrations by natural processes. After the Chernobyl nuclear power plant accident, self-cleansing effects, by which the ¹³⁷Cs content is diluted by rainwater and incoming river water, were reported primarily in aquatic ecosystems such as rivers and oceans. However, such effects in forest ecosystems have received little attention.

Therefore, the evaluation of the self-cleansing effects presented in this study and their use as a new forest decontamination measure is expected to contribute to the resumption of the stagnant forestry industry in Hamadori.

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This study was supported by the Commission Study (FMWSE; Fukushima Radiation Monitoring of Water, Soil and Entrainment) of Ministry of Education, Culture, Sports, Science and Technology (MEXT) FY 2011-2012; Japan Atomic Energy Agency-funded FY2015-2021 (27I002); JSPS KAKENHI Grant-in-Aid for Early-Career Scientists (20K19951); and the Japan Science and Technology Agency (JST) as part of the Belmont Forum (ABRESO; Abandonment and rebound: Societal views on landscape and land-use change and their impacts on water and soils).

_{Original Paper}

Title of original paper:
Downward migration of ¹³⁷Cs promotes self-cleaning of forest ecosystem by reducing root uptake of Japanese cedar in Fukushima

Journal:
Science of the Total Environment

DOI:
10.1016/j.scitotenv.2024.174010

_{Correspondence}

Assistant Professor TAKAHASHI, Junko
Center for Research in Radiation, Isotopes, and Earth System Sciences (CRiES) / Institute of Life and Environmental Sciences, University of Tsukuba

_{Related Link}

Center for Research in Radiation, Isotopes, and Earth System Sciences (CRiES) 
Institute of Life and Environmental Sciences