Topographic heterogeneity effect on the accumulation of Fukushima-derived radiocesium on forest floor driven by biologically mediated processes

The accident at the Fukushima Daiichi nuclear power plant caused serious radiocesium (Cs-137) contamination of forest ecosystems located in mountainous and hilly regions with steep terrain. To understand topographic effects on the redistribution and accumulation of Cs-137 on forest floor, we investigated the distribution of Fukushima-derived Cs-137 in forest-floor litter layers on a steep hillslope in a Japanese deciduous forest in August 2013 (29 months after the accident). Both leaf-litter materials and litter-associated Cs-137 were accumulated in large amounts at the bottom of the hillslope. At the bottom, a significant fraction (65%) of the Cs-137 inventory was observed to be associated with newly shed and less degraded leaf-litter materials, with estimated mean ages of 0.5-1.5 years, added via litterfall after the accident. Newly emerged leaves were contaminated with Fukushima-derived Cs-137 in May 2011 (two months after the accident) and Cs-137 concentration in them decreased with time. However, the concentrations were still two orders of magnitude higher than the pre-accident level in 2013 and 2014. These observations are the first to show that Cs-137 redistribution on a forested hillslope is strongly controlled by biologically mediated processes and continues to supply Cs-137 to the bottom via litterfall at a reduced rate.