Research and Development Relating to the Accident at the TEPCO’s Fukushima Daiichi NPS

1-8

Retention of Mobile Radiocesium in Forest Surface Soils－ Quantifying Microbial Uptake and Abiotic Adsorption －Litter layer
0-3 cm3-10 cm> 10 cm

Non-fumigatedFumigated

Cs extractability (%)

1580
60
40
200

105137

137Cs inventory (kBq/m2)

1000

Soil 1

Soil 2

Soil 3

Soil 4

Soil 5

Soil 1

Soil 2

Soil 3

Soil 4

Soil 5

Fig.1-16 Vertical distribution of 137Cs in forest soils threemonths after the accident
At five forest sites in Fukushima prefecture, 50% –91% and6%–39% of the total 137Cs deposition were observed in theaboveground litter layer ( ) and the topmost (0–3 cm) soil layer( ), respectively.

Fig.1-17 Extractability of 137Cs in five forest surface (0–3 cm)
soils
Cesium-137 was extracted with K2SO4 from soils before (nonfumigated) and after (fumigated) chloroform fumigation. No
increase in the 137Cs extractability was observed after thedestruction of microbial biomass by chloroform fumigation.

The accident at the TEPCO’s Fukushima Daiichi NPSreleased a huge amount of radiocesium into the environmentand caused serious radioactive contamination of forest
ecosystems. Our investigation, which was conducted in
Fukushima Prefecture three months after the accident,
revealed that >89% of the cesium-137 (137Cs) deposited onto
forest floors remained in the aboveground litter and topmost(0–3 cm) soil layers (Fig.1-16). In forest ecosystems affectedby the accident at the Chernobyl NPS in 1986, 137Cs hasreportedly still been observed in surface soils and has becomea major potential source for soil-to-plant transfer. Therefore,
a better understanding of the mechanisms of retention andmigration of 137Cs in forest surface soils is urgently needed.
We focused on microbial involvement in the retention
process of mobile 137Cs in forest surface soils because some
types of microorganisms in soils are believed to accumulate
137
Cs and thus influence the cycling of 137Cs in forest
ecosystems. Surface (0–3 cm) soil samples were collectedfrom five forest sites in Fukushima Prefecture one year after
the accident, and the following experiments were conducted.
First, soils were extracted with potassium sulfate (K2SO4)

solution, and the extracts were analyzed for 137Cs. The
extraction process liberated 2.1%–12.8% of the total 137Cs
as easily exchangeable ions from the soils. Two soils witha higher content of clay-sized particles and organic carbon
showed higher 137Cs extractability (Fig.1-17, Soils 4 and 5).
Next, the soils were fumigated with chloroform to destroy
microbial biomass and then extracted with K2SO4. Microbial
biomass was extracted from all of the soils; however, noincrease in the 137Cs extractability was observed (Fig.1-17).
The results indicate that the uptake of 137Cs by soil
microorganisms is less important for retention of mobile
137
Cs in the forest surface soils than ion-exchange adsorption
by abiotic components. We are now conducting a similar
experiment on litter and soil samples collected under different
meteorological conditions to fully understand the role of soil
microorganisms in controlling the mobility of 137Cs in natural
biotic systems.
The present study was sponsored in part by the Ministry ofEducation, Culture, Sports, Science and Technology of Japan(MEXT).

Reference
Koarashi, J. et al., Retention of Potentially Mobile Radiocesium in Forest Surface Soils Affected by the Fukushima Nuclear Accident, Scientific Reports,
vol.2, no.1005, 2012, p.01005-1-01005-5.
JAEA R&D Review 201319