Carbon Transfer and Accumulation in Forests, Wetlands and Farmlands
Masaru NAGAI, Shizuo SUZUKI, Ryuji ARAI, Osamu KOMATSUBARA,Hiroshi SAKATA, 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 14C discharged from the reprocessing
plant, it is necessary to develop a transfer and accumulation model of carbon in terrestrial ecosystems,including the processes of photosynthetic fixation of carbon and the decomposition of organic matter in soil.
Various ecosystems, such as forests, wetlands, paddy fields, farmlands and pastures are found around thereprocessing plant. In FY 2012, we investigated: 1) the net primary productivity (NPP) in forests and tree
plantations, 2) the gross primary productivities (GPPs) of carrot using the Closed Plant Experiment Facility,and 3) the decomposition rates of organic matter in soils by field and room experiments.
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 20-y-old and 66-y-old Japanese cedar (Cryptomeriajaponica) trees. The NPPs at the target fields were obtained as the sum of growth rate of the ground partbiomass from 2010 to 2012, annual fine litter fall rate in 2012, and fine root growth rate in 2012. Although
the obtained results were in the range reported in the literature, the estimation errors were large in the forests,
and further study was required for getting precise results.
The GPPs of carrot were measured at three growth stages of the plants by pot cultivation experiments
using soil from actual farmland. The air and soil temperatures, soil moisture, and light intensity werecontrolled to simulate conditions of the actual farmland. Both maximum GPPs and the initial slopes forphotosynthesis capability of carrot decreased with the increasing number of days after sowing.
To investigate the decomposition rate of soil organic matter in forests, wetlands, paddy fields,farmlands and pastures, mixtures of 13C-labelled plant powder and soil of each study field were packed into
glass-fiber filter bags, and buried in each study field in FY 2011. Concentrations of 13C in the bags weremeasured several times during the period to ~200 d after burying. The decomposition rates of soil organic
matter were calculated using the measurement results; however, the obtained values were generally higher
than those in the literature. Since coarsely cut plant matter was generally used in the previous studies, whichdiffered from the present buried plant shape, this may be one of the reasons for the higher values of thepresent study.
To obtain temperature dependency of the decomposition rate of organic matter in soil, soil samples
from 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 to obtain
the decomposition rates. The organic carbon decomposition was approximated by an exponential decayfunction with a constant term representing the hardly decomposed fraction. The rate constant of the decayfunction was fitted to the Arrehenius equation to obtain the rate constant at any temperature around theexamined temperatures.

Table 1

NPPs of tree plantations and forests fromfield observations
NPP(kg-dry m-2 y-1)

Vegetation typeC. japonica tree plantations
20-y-old tree plantation
66-y-old tree plantation
F. crenata- T. dolabrataco-dominated forest
Q. crispula-dominated forest

1.6 ± 0.6
1.4 ± 0.2
1.3 ± 2.20.8 ± 1.0

NPP: net primary productivity

Incubation time (d)Fig. 1

Decrease in soil organic carbon incubated in oak (Q.
crispula) tree-dominated forest soil at 10ºC (〇), 20ºC(△), and 30ºC (□).
Lines show estimated amounts of soil organic carbon by
the model equation obtained in this study.