Leaching characteristics of 137Cs for forest floor affected by the Fukushima nuclear accident: A litterbag experiment.

Abstract In forest ecosystems, forest litter is considered an active medium for radiocesium (137Cs). To understand discharge mechanisms of highly bioavailable dissolved 137Cs from forests to river systems, we investigated the characteristics of 137Cs leaching from forest litter as observed from litterbag experiments. Leaching experiments with conifer needle and deciduous broadleaf litters were then conducted. After soaking conifer needles and broadleaf litters for 20 min, 140 min, and 1 day, the mean values of the 137Cs leaching ratios were 0.13–2.0% and 0.81–6.6%, respectively, indicating that 137Cs leaching ratios are different between forest litter types. To elucidate the factors affecting 137Cs leaching from forest litter, a multi-regression analysis of 137Cs leaching ratios was conducted against antecedent mean precipitation and temperature before sampling the litterbag and accumulated temperature during the litterbag experiments. The 137Cs leaching ratios showed a negative correlation to the antecedent mean precipitation for both litters and the accumulated temperature for broadleaf litters, whereas it exhibited a positive correlation with the antecedent mean temperature for both litters and the accumulated temperature for conifer needle litters. It was proposed that the fraction of 137Cs in labile sites in forest litter increased/decreased due to litter decomposition by antecedent/accumulated temperature, and that this fraction can be washed off by the antecedent precipitation. The different effects of accumulated temperature on 137Cs leaching from conifer needles and broadleaf litters could be due to their different decomposition rates. Our results contribute further the understanding of the mechanisms associated with dissolved 137Cs discharge from forested catchments.