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 ((137)Cs) contamination of forest ecosystems located in mountainous and hilly regions with steep terrain. To understand topographic effects on the redistribution and accumulation of (137)Cs on forest floor, we investigated the distribution of Fukushima-derived (137)Cs 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 (137)Cs were accumulated in large amounts at the bottom of the hillslope. At the bottom, a significant fraction (65%) of the (137)Cs 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 (137)Cs in May 2011 (two months after the accident) and (137)Cs 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 (137)Cs redistribution on a forested hillslope is strongly controlled by biologically mediated processes and continues to supply (137)Cs to the bottom via litterfall at a reduced rate.