Assessment of vertical radiocesium transfer in soil via roots

摘要

Several years after the Fukushima Daiichi Nuclear Power Plant accident, the surface mineral soil layer is believed to be the main reservoir of radiocesium (Cs-137) in forest ecosystems in Japan. Dissolved Cs-137 combines with clay minerals in the soil, and hence, it is not expected to easily infiltrate over time. However, previous studies have indicated that Cs-137 derived from the older global fallout migrated deeper than that of the Chernobyl accident, and this cannot be explained by only the dissolved Cs-137 vertical migration in the soil. Considering the carbon and nutrient dynamics in the forest floor, the Cs-137 transfer process in soil via roots may alter its vertical distribution on a decadal scale. Therefore, in this study, we investigated the Cs-137 activity concentrations in both roots and soil matrix, by considering four (0-20 cm) or six (0-30 cm) mineral soil layers taken at every 5 cm at seven study sites dominated by one of the six plant species (three coniferous forests, one deciduous forest, two deciduous forests covered by Sasa, and one bamboo forest) in eastern Japan in 2013. Comparing the results of Cs-137 activity concentrations between roots and soil matrix taken at the same soil layer, roots at the surface (0-5 cm) layer often showed lower values than the soil matrix. However, roots deeper than 5 cm had higher activity concentrations than the soil matrix, conversely. The Cs-137 inventories ratio of roots to soil matrix are about 1% at the 0-5 and 5-10 cm soil layer, and about 2% at the soil layers deeper than 10 cm. These results suggest that decomposition of root litter little affect the short-term vertical migration of Cs-137 in the forest soil. However, it indicates that continuous production and mortality of roots with relatively high Cs-137 activity concentrations have an important role for changing the vertical distribution of Cs-137 on time scale of decades, particularly at deeper soil layers.