Radiocesium Dynamics in Wild Mushrooms During the First Five Years After the Fukushima Accident

Abstract

Dynamics of radiocesium in wild mushrooms, especially in mycorrhizal fungi, in forest ecosystems were investigated for 5 years after the Fukushima nuclear accident, in relation to substrates such as litter, soil and wood debris. Some mushroom species contained a high level of radiocesium in the first or second year, and then the radiocesium content decreased. Changes in radiocesium activities were ambiguous for many other mushrooms. Radiocesium accumulation with time was not common contrary to expectations. Reduction of radiocesium activities in litter and increase in mushrooms and soils, i.e. transfer of radiocesium from litter to mushrooms and soils, was recognized in the first and second year, but it was not obvious in subsequent years. Radiocesium accumulated in several mushroom species, especially in mycorrhizal fungi, while radiocesium in the other mushrooms did not exceed those in the neighboring forest litter. Similar differences in radiocesium level among mushroom species were observed in relation to 40K levels, though 137Cs/40K ratio in mushrooms was lower than in O horizon, but at the same level of the A horizon in general. These facts suggested differences in the mechanisms of cesium accumulation. Residual 137Cs due to nuclear weapons tests or the Chernobyl accident still remained in mushrooms and soils. From the ratio of the past residual 137Cs, it was suggested that the residual 137Cs was tightly retained in the material cycles of forest mushroom ecosystem, whereas 137Cs emitted from the Fukushima accident was still fluid.