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 21-y-old and 67-y-old Japanese cedar (Cryptomeria
japonica) trees. The net primary productivities at the target forests were obtained as the sum of the growthrate of tree and litter fall by field observations and the allometry equation of the tree.
To obtain the decomposition 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 ~500 d. The decomposition rate of labile and resistant soil organic matter wasapproximated by an exponential decay functions. The rate constants of decay functions for the labile andresistant soil organic matter were fitted to the Arrehenius equations to obtain the rate constants depending ontemperature. To investigate soil microbial communities during the incubation, incubated soils were analyzed
using the denaturing gradient gel electrophoresis (DGGE) method during the periods for up to ~280 d. 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 each study field. Amounts of 13C in
the glass-fiber filter bags were measured at predetermined intervals during the periods for up to ~590 d afterburying. The decomposition rate of plant residue or litter in each field was approximated by exponential
decay functions.
A model program for the framework of the 14C transfer and accumulation model in farmlands wasconstructed and successfully tested for its basic input/output function.

(a) Parameter optimization by litterbag(b) Parameter optimization by glass-fiber(c) Parameter optimization by glass-fiberand soil incubation methods

filter bag method in farmlands

filter bag method in forests and wetlands

C4：PR
k4

1-γ4

CO2

By soilincubation
method

C4：PR
k4

1-γ4

C4：PR
k4

CO2

γ4γ4

CO2

By plant residue
fraction

γ4

c4c6

C6：LSOM
k6

CO2

c4c6

C6：LSOM
k6

CO2

CO2

c4c7

C7：RSOMk7

CO2

c4c7

C7：RSOMk7

CO2

CO2

1c4c6c4c7

C8：HSOM
k8

CO21c4c6c4c7

C8：HSOM
k8

CO2

c4c6

C6：LSOM
k6

CO2

c4c7

C7：RSOMk7

1c4c6c4c7

C8：HSOM
k8

By litterbag
method1-γ4

By soilfraction

20℃

30℃

13Soil organic carbon(mg-C g-1-dry soil)

10℃

C content per glass-fiber
filter bag (mg-13C)

Fig. 1 Schematics of the litter and soil carbon transfer and accumulation model.
PR, LSOM, RSOM, and HSOM indicate plant residue, labile, resistant, and highly resistant soil organic matter, respectively.

Decomposition time (d)Incubation time (d)Fig. 2Decrease in the organic carbon of incubated soils
collected from the 21-yr-old Japanese cedar(Cryptomeria japonica) plantation.

Fig. 3

Decrease in 13C content per glass-fiber filter bag
recovered from a carrot field on farmland.