Six-year trends in exchangeable radiocesium in Fukushima forest soils

Highlights: • We examined trends in exchangeable ¹³⁷Cs in Fukushima forest soils for 6 years. • The proportion of ¹³⁷Cs in exchangeable form decreased significantly for 2–4 years. • The proportion of exchangeable ¹³⁷Cs was almost constant in the following years. - Abstract: 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.