A Model for Carbon Transfer from the Atmosphere to Crops
Takashi TANI, Ryuji ARAI, Yasuhiro TAKO and Yuji NAKAMURA
Department of Environmental Simulation
Abstract
Estimation of activity concentration of14

C in edible parts of crops is necessary to assess the local

14

radiological impact of C released from the spent nuclear fuel reprocessing plant in Rokkasho, Japan. In thisstudy, carbon transfer in rice plant was described in a three-compartment model consisting of ear, and themobile and immobile carbon pools of shoot. Photosynthetically fixed carbon moves into the ear and mobile
carbon pool, and these two compartments release a part of the carbon into the atmosphere by respiration.
Carbon accumulated in the mobile carbon pool is redistributed to the ear. Carbon transferred into theimmobile carbon pool from the mobile one is accumulated there until harvest. Transfer coefficients ofcarbon going into and coming from these compartments were estimated by fitting the model to observed
ratios of carbon fixed at each growth stage to the total carbon in the edible part at harvest (RP(t)). The model
was validated by measurement of δ13C in the ear of rice plant exposed to atmospheric CO2 with temporally
changing 13C concentration. The estimate of δ13C value in the ear by the model was closer to the observed
one, compared to that calculated by the specific activity method. This result implies that the newly
developed model is applicable to estimate properly radiation dose to the neighboring population of nuclear
facilities due to dynamic release of 14C from the facilities.

PE(t)

250

kEAME(t)Ear

200

Photosynthesis

δ13δC13airC(‰)(ä )ME(t)airkME(t)MM(t)ShootMobile carbon
PS(t)Immobile carbon

MM(t)kMIMM(t)150
1st100

2nd50

3rdMI(t)0kMAMM(t)

-50

050
100
150
Days after cultivation

200Fig. 1 Conceptual diagram of the dynamic compartment

Fig. 2 Temporal changes in δ13C in the air duringmodel. The dotted lines enclose the shoot part of ricecultivation of rice plant. Plants in this study

plant.

were cultivated during three different periods(1st, 2nd, 3rd). The horizontal lines indicate thecultivation period from seeding to harvest foreach of the three groups.
80

0.05
Shoot

RP(t)

0.04

60
Observed
Model

0.03
0.02

EarObserved value （ ± SD）

40
Specific activity method
20Compartment model
0background0.010.00

δ13C (‰)

Ear-2004080
Days after seeding

04080120
Days after seeding-40

1st2nd3rdCultivation group

Fig. 3 The observed ratio and model estimate of carbon fixed

Fig. 4 The observed δ13C and model estimates in ear ofat different times after seeding to the total carbon at therice plant exposed to atmospheric CO2 withtime of harvest (RP(t)) in ear or shoot. Each pointchanging 13C concentration. The three groups

indicates the observed value (mean ± SD, n = 4). The

correspond to those in Fig. 2. Each open column

solid lines indicate the model estimate calculated byindicates the observed value (mean ± SD, n =the dynamic compartment model (Fig. 1). The arrows10). In every stage (1st, 2nd, 3rd), the estimatesin the graph indicate the time of heading (74 days after

by the compartment model (black column) areseeding).

closer to the observed values than the predictions
by the specific activity method (gray column).