Effect of mass of organic layers on variation in 137Cs distribution in soil in different forest types after the Fukushima nuclear accident

Forests over a wide area were contaminated with radiocesium following the Fukushima nuclear accident in March 2011. Most radiocesium in the forests migrated into the soil a year after the accident and it is therefore necessary to clarify the distribution pattern of radiocesium in soil to evaluate the actual state and possible future changes of the radiocesium cycle in forest ecosystems. We investigated the stock and vertical distribution patterns of Cs-137 in soil under evergreen and deciduous tree species from 2012 to 2014 in a mountainous area in Fukushima Prefecture. The Cs-137 stock in soil was higher in evergreen forests than in deciduous forests. This was likely caused by the difference in tree leaf biomass at the time of the initial deposition, when deciduous trees were defoliated. The organic layer (particularly the humified layer) retained Cs-137 in high concentration and stock, indicating limited migration of Cs-137 into the mineral layer during the study period. This may reflect the incorporation of Cs-137 into organic matter and the retention of fungi in the organic layer. Analyses of these data and those from other studies conducted in regions contaminated by the accident showed that the ratio of radiocesium stock in the organic layer to the total amount in soil was positively correlated with the mass of the organic layer (organic layers with a mass of >2.0kg/m(2) retained more than 50% of total radiocesium). Thus, the status of the organic layer is an important factor regulating the downward migration of radiocesium.