Six-year trends in exchangeable radiocesium in Fukushima forest soils

We investigated the changes in ¹³⁷Cs exchangeable fraction in the soils of contaminated forest ecosystems until 6 years after the Fukushima radioactive atmospheric deposits. For this investigation, we performed chemical extractions of ¹³⁷Cs from both organic and mineral soil layer sampled from two forest stands, Japanese cedar (Cryptomeria japonica) and broadleaf deciduous, mainly konara oak (Quercus serrata), which are located 26-27 km inland from the Fukushima Dai-ichi Nuclear Power Plant. The exchangeable ¹³⁷Cs as a proportion of the total ¹³⁷Cs inventory was only 10% in the organic layer and 6.1% in the mineral soil layer about 5 months after the accident at the Japanese cedar site. We observed an exponential decrease in the proportion of exchangeable ¹³⁷Cs in both organic and mineral soil layer samples over the 6-years observation period at both sites. The proportion significantly decreased within 2-4 years after the accident, becoming almost constant (2-4%). These results support the interpretation that contaminated forests have entered a steady-state phase of ¹³⁷Cs cycling, although we need more integration for an improved vision of the future of Fukushima forests.