A new perspective on the (137)Cs retention mechanism in surface soils during the early stage after the Fukushima nuclear accident

The Fukushima Daiichi nuclear power plant accident caused serious radiocesium ((137)Cs) contamination of the soil in multiple terrestrial ecosystems. Soil is a complex system where minerals, organic matter, and microorganisms interact with each other; therefore, an improved understanding of the interactions of (137)Cs with these soil constituents is key to accurately assessing the environmental consequences of the accident. Soil samples were collected from field, orchard, and forest sites in July 2011, separated into three soil fractions with different mineral–organic interaction characteristics using a density fractionation method, and then analyzed for (137)Cs content, mineral composition, and organic matter content. The results show that 20–71% of the (137)Cs was retained in association with relatively mineral-free, particulate organic matter (POM)-dominant fractions in the orchard and forest surface soil layers. Given the physicochemical and mineralogical properties and the (137)Cs extractability of the soils, (137)Cs incorporation into the complex structure of POM is likely the main mechanism for (137)Cs retention in the surface soil layers. Therefore, our results suggest that a significant fraction of (137)Cs is not immediately immobilized by clay minerals and remains potentially mobile and bioavailable in surface layers of organic-rich soils.