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Ingestion of edible wild mushrooms collected in areas contaminated with radiocaesium released from the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident may pose a risk of internal dose of the consumers. Species specific aggregated transfer factor (Tag), which is calculated using radiocaesium concentration in a wild mushroom species (Bq kg-1 wet mass [WM]) divided by the total concentration in the soil surface area (Bq m-2), would be a useful tool to estimate the dose from wild mushrooms by ingestion. In this study, we especially focused on the Tag data collected after 2016 to use these values for a long-term dose assessment. It was implied that concentrations of 137Cs after that year were almost the same, thus soil-mushroom system would be in apparent steady-state condition. Finally, we could obtain Tag values of 137Cs in 44 edible wild mushroom species native to Japan. The geometric mean (GM) values were 1.7×10-3 m2 kg-1 WM for saprobic type species (460 Tag data) and 6.2×10-3 m2 kg-1 WM for mycorrhizal type species (447 Tag data). On average, GM of species specific Tag values were 1.9±0.9 times higher to those reported in a previous study of wild mushrooms observed after the FDNPP, probably due to the different approaches for Tag calculation.

Cesium is an element that belongs to the same group as K, and is known to show similar behaviour to that of K in plants. In this study, we conducted a serial leaching test for 120 h to compare the leaching characteristics of dissolved 137Cs and K in forest litter, obtained from Japanese cedar and deciduous broadleaf forests located 40 km from the site of the 2011 Fukushima Dai-ichi nuclear power plant accident. The litter was collected in 2018 and was divided into three groups according to the decomposition level. The cumulative leachable fraction of 137Cs at 120 h ranged from 0.3% to 3.3%, suggesting that most of the 137Cs in the litter was hardly leachable in water. The leachable fraction of 137Cs generally decreased with the decomposition level of the litter, implying that the easily leachable 137Cs eluted into the water during the first stage of decomposition. Meanwhile, the cumulative leachable fraction of K at 120 h was approximately 10 times greater than that of 137Cs and ranged from 22.7% to 54.8%. The leaching speeds of 137Cs and K decreased suddenly with elapsed time regardless of the tree species, decomposition degree, or element. Our findings contribute to the long-term understanding of the 137Cs cycle in forest ecosystems.