Tuesday, December 25, 2012

Fukushima Cesium in Forest Surface Soils

Majia here: This is a very technical article examining microbial retention of cesium from Fukushima sampled in the soil approximately one year after the accident. 
What I find interesting in the article are the detections of cesium and their transferability to "other forest components," which I presume include flora and fauna:
Koarashi J, Moriya K, Atarashi-Andoh M, Matsunaga T, Fujita H, Nagaoka M. Retention of potentially mobile radiocesium in forest surface soils affected by the Fukushima nuclear accident.Sci Rep. 2012;2:1005. doi: 10.1038/srep01005. Epub 2012 Dec 19. http://www.ncbi.nlm.nih.gov/pubmed/23256039


Nuclear Science and Engineering Directorate, Japan Atomic Energy Agency , Ibaraki 319-1195, Japan.


[complete abstract excerpted] The fate of (137)Cs derived from the Fukushima nuclear accident fallout and associated radiological hazards are largely dependent on its mobility in the surface soils of forest ecosystems. Thus, we quantified microbial and adsorptive retentions of (137)Cs in forest surface (0-3 cm) soils. 

The K(2)SO(4) extraction process liberated 2.1%-12.8% of the total (137)Cs from the soils. Two soils with a higher content of clay- and silt-sized particles, organic carbon content, and cation exchange capacity showed higher (137)Cs extractability. Microbial biomass was observed in all of the soils. However, the (137)Cs extractability did not increase after destruction of the microbial biomass by chloroform fumigation, providing no evidence for microbial retention of the Fukushima-fallout (137)Cs. The results indicate that uptake of (137)Cs by soil microorganisms is less important for retention of potentially mobile (137)Cs in the forest surface soils compared to ion-exchange adsorption on non-specific sites provided by abiotic components.

EXCERPTS FROM ARTICLE (available here)

[excerpted] The fate of 137Cs deposited in forest ecosystems, and the associated radiation risks, are largely dependent on the mobility of the 137Cs in the topsoils. It is generally accepted that 137Cs can be strongly, almost irreversibly, fixed by clay minerals in the mineral soil layers8,9, resulting in a low migration rate and reduced availability for uptake by plants. 

However, there is evidence that a considerable percentage of 137Cs remains in mineral soil layers as an easily exchangeable (loosely bound) form over a prolonged period of time10,11,12. It has been reported that even one decade after the Chernobyl fallout, high 137Cs concentrations were still observed in the bottom layer of the O horizon and in the upper (transient) layer of mineral soils in Russian forests3. It was also found that the amount of exchangeable 137Cs in the soils is one of the key factors governing the availability of 137Cs for transfer to other forest components3.

Microorganisms in soils are believed to influence the cycling of 137Cs in forest ecosystems13,14, because they are able to mobilize and subsequently accumulate 137Cs (ref. 14, 15, 16, 17, 18). Therefore, it is conceivable that soil microorganisms are responsible for not only the transfer of 137Cs to plants and deeper soil layers but also the retention of 137Cs in the upper soil layers19. Nevertheless, the microbial involvement in the retention process of the mobile 137Cs in the natural soil environment is still poorly understood18,19,20,21.

This study presents the first investigation into the microbial retention of the Fukushima-fallout radiocesium in soils. Surface soil samples were collected from five forest sites (FR-1 to FR-5) in the city of Fukushima with different vegetation and soil properties approximately one year after the accident.

...The activity concentration of 137Cs in the top soil was relatively similar between the sites, and ranged from 3380 to 4390 Bq kg−1-dry soil (see Table S1)....


Water-extractable C and 137Cs

...The water extraction liberated similar, but smaller, amounts of C from the non-fumigated soils as compared to that obtained via K2SO4 extraction (Fig. 1b)....

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