Six-year monitoring of the vertical distribution of radiocesium in three forest soils after the Fukushima Dai-ichi Nuclear Power Plant accident

After the Fukushima Dai-ichi Nuclear Power Plant accident on March 2011, several studies showed that the downward migration of Cs-137 from litter to mineral soil is more rapid in forests in Fukushima than in forests affected by the Chernobyl accident. Therefore, the downward migration within mineral soil layers is more important for predicting long-term dynamics of Cs-137 in forest ecosystems in Fukushima. In the present study, we monitored the detailed vertical distribution of Cs-137 in litter and soil layers for 6 y (2011-2017) following the previous study (2011-2012), and found that temporal changes in those distributions were different among mixed forest (MF), mature cedar (MC) and young cedar (YC) forests. The Cs-137 concentrations and inventories in the litter layer exponentially decreased with time for all sites, with more than 80-95% of the deposited Cs-137 on the forest floor distributed in mineral soil layers by 2017. The percentage of Cs-137 inventory in the litter layer to the total Cs-137 inventory in litter and mineral soil layers was well fitted by a single exponential equation with decreasing rate of 0.22-0.44 y(-1). The slower migration was observed in the YC site, probably because of higher initial interception of Cs-137 fallout by dense canopy. As the downward migration from litter to mineral soil progressed, the Cs-137 concentration in the first few cm of mineral soil surface gradually increased and became higher than the Cs-137 concentration in the litter within 2-3 y of the accident. The Cs-137 concentration in mineral soil layers exponentially decreased with depth throughout survey period, and an exponential equation fitted well. The relaxation depth of Cs-137 concentration in mineral soil layers estimated by the exponential equation were constantly increasing in the MC and YC sites with 0.08 cm y(-1). In contrast, there was no temporal increase in the relaxation depth in the MF site, indicating little migration to subsurface soil layer from not only litter layer but also surface soil layer. Further studies are necessary to identify the forests prone to the downward migration of Cs-137 and its factors regarding both forest and soil characteristics.