Factors affecting vertical distribution of Fukushima accident-derived radiocesium in soil under different land-use conditions

Abstract

The Fukushima Dai-ichi nuclear power plant accident in Japan, triggered by a big earthquake and the resulting tsunami on 11 March 2011, caused a substantial release of radiocesium (137Cs and 134Cs) and a subsequent contamination of soils in a range of terrestrial ecosystems. Identifying factors and processes affecting radiocesium retention in these soils is essential to predict how the deposited radiocesium will migrate through the soil profile and to other biological components. We investigated vertical distributions of radiocesium and physicochemical properties in soils (to 20cm depth) at 15 locations under different land-use types (croplands, grasslands, and forests) within a 2km×2km mesh area in Fukushima city. The total 137Cs inventory deposited onto and into soil was similar (58.4±9.6kBqm−2) between the three different land-use types. However, aboveground litter layer at the forest sites and herbaceous vegetation at the non-forested sites contributed differently to the total 137Cs inventory. At the forest sites, 50–91% of the total inventory was observed in the litter layer. The aboveground vegetation contribution was in contrast smaller (<35%) at the other sites. Another remarkable difference was found in vertical distribution of 137Cs in mineral soil layers; 137Cs penetrated deeper in the forest soil profiles than in the non-forested soil profiles. We quantified 137Cs retention at surface soil layers, and showed that higher 137Cs retention can be explained in part by larger amounts of silt- and clay-sized particles in the layers. More importantly, the 137Cs retention highly and negatively correlated with soil organic carbon content divided by clay content across all land-use types. The results suggest that organic matter inhibits strong adsorption of 137Cs on clay minerals in surface soil layers, and as a result affects the vertical distribution and thus the mobility of 137Cs in soil, particularly in the forest ecosystems.