Distribution of radiocesium in different density fractions of temperate forest soils in Fukushima

In forested areas of Fukushima, Japan, radiocesium emitted by the Fukushima Daiichi nuclear power plant accident has accumulated on the forest floor and in surface mineral soils. To estimate and model the mobility of radiocesium in forest soils, the concept of physical variations in soil organic matter is useful. We applied density fractionation to soil samples collected in two different forest stands in Fukushima; a deciduous broadleaf forest (DBF) and an evergreen needleleaf forest (ENF). To determine the relative contributions of the two density fractions to retention of radiocesium in surface mineral soils in a temperate forest region we investigated the concentrations and stocks of radiocesium in the low- and high-density fractions (LF and HF, respectively) of the soils, as well as in the litter, and temporal changes therein between 2012 and 2014. The 137Cs stock in the litter decreased from 2012 to 2014, whereas in the soil, as the sum of the LF and HF, it increased. On average, more than 40% of the 137Cs stock in surface mineral soil (depth of 0-5 cm) was retained in LF particles, which were composed mainly of particulate organic matter. Although the LF accounted for only 10.3% of the dry mass of the soil on average, it was also characterized by an average concentration of 137Cs that was 7.8 times higher than that of the HF, which was composed of organo-mineral particles. The mean increase in the 137Cs stock in the LF from 2012 to 2014 was equivalent to 70.1% and 52.5% of those in the soils in the DBF and ENF, respectively. The relationship between loss on ignition and 137Cs content indicated that the spatial heterogeneity of radiocesium in bulk soil was substantially affected by the heterogeneity in particulate organic matter. The coarse-size (>250 μm) of the LF suggested that it was less physically protected against microbial attack. Accordingly, the LF was considered to act as a large, but temporal reservoir of radiocesium that was originally retained in the litter layer. In contrast, the accumulation of 137Cs in the HF was low, especially in 2013-2014. The HF (or organo-mineral particles) might have a more important role than the LF in 137Cs fixation over a long period, and should be continuously studied as part of the current radiocesium monitoring network.