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

Abstract

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

Forest type effects on the retention of radiocesium in organic layers of forest ecosystems affected by the Fukushima nuclear accident

“…The carbon-to-nitrogen (C/N) ratio generally decreased with depth, indicating that the litter materials in the F layers were older and had experienced higher degrees of microbial decomposition compared to those in the L layers3132. This was consistent with the visual aspect of the litter materials collected from the F layers as a mixture of finely fragmented plant residues and macroscopically unrecognizable organic materials20.…”

“…Trees at the DBF site had no leaves in March 2011 when the Fukushima NPP accident occurred, and therefore, it is assumed that most of the Fukushima-derived 137 Cs was directly deposited onto forest-floor litter materials1718. Because broadleaf litter is known to be relatively rapidly decomposed via microbial activity in the organic layers (a mean residence time of a few years)192031, it is likely that the litter materials contaminated by the direct deposition of 137 Cs have been degraded since March 2011 and that most of the 137 Cs was leached from the organic layers and then immobilized in the topsoil, with a minor fraction (14% of the total deposition) being still retained in the F layers in December 2012171835.…”

“…Deciduous leaf fall is an annual occurrence and the decomposition of broad leaves in such forests has been shown to be rapid1920, whereas the mean leaf longevity of evergreen coniferous trees is longer than a year21 and the decomposition of needle-like leaves appears to be much slower compared to broad leaves2223. The rate of decomposition of litter materials in organic layers is also influenced by climatological factors such as temperature and precipitation.…”

“…In addition, forests have complex stand structures and microtopography, and therefore, the quantity and quality (degrees of degradation) of litter materials accumulated in organic layers are highly spatially variable, even within a forest ecosystem27. Spatial heterogeneity can also be a complicating factor in quantitatively assessing the retention capability of organic layers for 137 Cs and its dependence on the forest type2028.…”

Predicting and Controlling Nuclear Accident Hazards: Issues and Challenges

“…At present, nuclear accident hazard models are mainly global and mesoscale models, but nuclear accidents may actually harm the zone within the range of a few to hundreds of kilometers, which belongs to multi-scale transport problems, and the spread of radioactive materials is affected by a variety of meteorological elements and topographies (Declan, 2011;Morino et al, 2011;Yasunari et al, 2011;Koarashi et al, 2014). For example, precipitation determines the wet scavenging of airborne particles and soluble gases, and thus their wet deposition onto the surface.…”