Transfer and Accumulation of Tritium and Radiocarbon in Terrestrial Ecosystems
Takashi TANI, Masaru NAGAI, Masanori FUJII, Yuichi TAKAKU, Shun’ichi HISAMATSU
Department of Radioecology
Abstract
The operation of the spent nuclear fuel reprocessing plant in Rokkasho, Japan, is accompanied by thedischarge of a small amount of tritium (T) and 14C mainly in the form of HTO and 14CO2. In terrestrial
ecosystems around the reprocessing plant, both radionuclides are incorporated into organic compounds inplants mainly due to photosynthesis, followed by their supply to soil such as via dead leaves and roots. Thisraises the concern about accumulation of those radionuclides in soil, because soil organic matter is recognized
as the largest carbon pool in terrestrial ecosystems. In order to predict the accumulation of those radionuclides
in terrestrial ecosystems, simulation models to describe the dynamics of 3H and 14C in organic matter and HTO
concentration in soil and plant are required. We selected a meadow and a forest of Japanese black pine (Pinusthunbergii), which are common around the reprocessing plant, as our target. To construct those models, weare measuring many parameters, which will be used in the models, through field observations and laboratory
experiments.
In FY 2015, a 50  50 m quadrat in a black pine forest was established ~5 km east of the reprocessing
plant. In FY 2016, root-ingrowth cores and dendrometers were installed in the forest to determine the growth
rates of fine roots and trunk, respectively. Fallout rates of above-ground litter were also measured monthly
from July 2015, for obtaining input rate of dead plant matter to the forest floor. Temporal changes in waterequivalent of snow were observed, and soil properties affecting water percolation were determined in theforest and also in an experimental meadow described next.
We established an experimental meadow in FY 2015 on the grounds of our institute. For determining
the parameters of the photosynthetic model, the growth data of leaves, stems, and roots of grasses germinatedin autumn of 2015 were obtained from April to November 2016. In grass plants, carbohydrates accumulated
in the stem base are redistributed to newly produced leaves and stems after harvesting. To determine thecontribution of carbohydrate reserves to newly produced leaves and stems, part of which are supplied to soil
as dead plant parts at the time of harvest, the concentration of 13C in newly produced leaves and stems of thesecond- and third-harvested grasses were determined using potted grass plants exposed to 13CO2-enriched air
during the growth periods of the first- and second-harvested grasses, respectively. The experimental results
showed that reserve carbon contributed less than 3% to total carbon in the newly produced leaves and stems
at their harvest time.

Fig. 1 The concentrations of 13C in leaf and stem

-1C mol C)

13C (mmol
13

The concentration of-1C mol C)

13C (mmol13

The concentration of80
Leaf and stem
60
40
20010203080 0Stembase
60
40
20010203080 0Root
60
40
2000
102030Days after the first cutting（d）

80
60 Leaf and stem
40
20080 0 10 20 30 40 50 60
60 Stem base40
20080 0 10 20 30 40 50 6060 Root40
2000 10 20 30 40 50 60
Days after the second cutting（d）

Fig. 2The concentrations of 13C in leaf and stem

cut at 5 cm above ground level, stem base,

cut at 5 cm above ground level, stem base,and root after the first cutting of grass

and root after the first cutting of grass

plants exposed to13

CO2-enriched air

plants exposed to13

CO2-enriched air

during the growth of second grass.

during the growth of first grass.
Vertical bars indicate standard
deviation of six samples. Dashed lines

Verticalbars

indicate

standard

deviation of six samples. Dashed lines

show the background concentration of C

show the background concentration of 13C(10.8 mmol 13C mol-1 C).

(10.8 mmol 13C mol-1 C).

13