Sorption experiments using stemflow: Reproduction and understanding of radiocesium dynamics on the forest floor during the early stage after the Fukushima nuclear accident

The Fukushima Dai-ichi Nuclear Power Plant accident on March 11, 2011 released significant amounts of radiocesium (¹³⁷Cs), which contaminated large forested areas in Fukushima, with large spatial heterogeneity. To understand the characteristics of ¹³⁷Cs sorption to outer bark and fresh plant residues on the forest floor, mainly in the Oi horizon, we performed sorption experiments in the laboratory. To simulate actual conditions, we used ¹³⁷Cs-free organic matter samples from Kochi, far from the power plant, and ¹³⁷Cs-containing stemflow samples from Fukushima. ¹³⁷Cs sorption was highest in the outer bark of Japanese cedar (Cryptomeria japonica) (on average 76.1%) and in the Oi horizon (72.8%), followed by fallen leaves of Konara oak (Quercus serrata) (45.5%), fallen branches of Japanese cedar (37.6%), and fallen needles of Japanese cedar (27.4%). We discuss various factors controlling ¹³⁷Cs sorption characteristics, such as organic chemical contents and sample size, and the chemical composition of the stemflow sample. Shortly after the accident, when most of the ¹³⁷Cs in the forest ecosystem was trapped in the O horizon and the forest canopy, the sorption characteristics of the materials would have influenced the horizontal heterogeneity of ¹³⁷Cs in the Oi horizon under the actual conditions on the forest floor.