Carbon Transfer and Accumulation in Forests, Wetlands and Farmlands
Shizuo SUZUKI, Masaru NAGAI, Yasuhiro TAKO, 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 14C mainly in the form of 14CO2, which is transferred into terrestrial
ecosystems and accumulated in them. In order to predict the fate of this discharged 14C, it is necessary todevelop transfer and accumulation models of carbon in terrestrial ecosystems such as forests, wetlands,paddy fields, farmlands and pastures, which are found around the reprocessing plant.
We selected the following as target forests and plantations in the study: a forest co-dominated bybeech (Fagus crenata) and hiba (Thujopsis dolabrata) trees, a deciduous broad-leaved forest dominated byoak (Quercus crispula) trees, and two tree plantations of 22-y-old and 68-y-old Japanese cedar (Cryptomeria
japonica) trees. In FY 2014, we continued various studies to get data for constructing the models andoptimized parameters in the models by using all available data.
The net primary productivities at the target forests were obtained as the sum of the growth rate of atree type and litter fall by field observations and the allometry equation of the tree. To obtain thedecomposition rates of soil organic matter and their temperature dependency, soil samples from the forests,wetlands, paddy fields, farmlands and pastures were incubated in the laboratory at different temperatures (10,20, and 30°C), and respired CO2 amounts from the soil samples were measured during the periods for up to~700 d. The decomposition rate of labile and resistant soil organic matter was approximated by an
exponential decay function. The rate constants of decay functions for the labile and resistant soil organic
matter were fitted to the Arrehenius equations to obtain the rate constants depending on temperature. To
investigate soil microbial communities during the incubation, incubated soils were analyzed using thedenaturing gradient gel electrophoresis (DGGE) method. The results showed no major changes in microbial
species composition for each incubated period of each soil sample.
To investigate the decomposition rates of plant residues and transfer of decomposed products to soilorganic matter in paddy fields, farmlands and pastures, mixtures of 13C-labelled plant material and soil ofeach study field were packed into glass-fiber filter bags, and buried in the respective study fields. Amountsof 13C in the glass-fiber filter bags were measured at predetermined intervals during periods for up to ~1000
d after burying. The decomposition rate of plant residue or litter in each field was approximated by an
exponential decay function. The dissolved organic carbon drained to the lower layer of each field wereevaluated by lysimeters buried in each study field.
We optimized the parameters of the model programs coded until FY 2013 for the 14C transfer andaccumulation in the target fields by using data obtained, and confirmed reasonable output from them.

k4γ4k4(1‐γ4)k4γ4τC4C6

CO2
C6 : Labile
SOC

k6
ξ6k7
k4γ4τC4C7

C7 : Resistant
SOC
C8 : Hardly
decomposable SOC

CO2

ξ7

k8
k4γ4(1‐τC4C6‐τC4C7)

CO2

CO2

ξ8

ガラス繊維濾紙内容物の13C 量13RemainingC (mg)(mg-13C)

C4 : Litter
OC

110
100
90807060
0200

4006008001000

埋設からの経過日数Days
after burying

Fig. 1 Scheme of a compartment model for
estimating decomposition of litter organiccarbon (OC) and soil organic carbon (SOC).

Fig. 2

Remaining 13C of plant residue and soilorganic matter in glass-fiber filter bags buried
in a pasture field. Open circles and the line,
respectively, indicate observed values and themodel estimation using soil temperature.