Carbon Transfer and Accumulation in Forests, Wetlands and Farmlands
Shizuo SUZUKI, Masaru NAGAI, Hiroshi SAKATA, Ryuji ARAI,Osamu KOMATSUBARA, Yasuhiro TAKO and Yuji NAKAMURA
Department of Environmental Simulation
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 will transfer into terrestrial
ecosystems and will be accumulated in them. In order to predict the fate of 14C discharged from thereprocessing 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. Various ecosystems, such as forests, wetlands, paddy fields, farmlands and pastures are found aroundthe reprocessing plant. To estimate carbon transfer rates from the atmosphere to plants in forests and tree
plantations which are the most widespread, we investigated the above-ground net primary productivity(ANPP) in actual fields. For the estimation of carbon fixation rates in tillage, we studied the gross primaryproductivities (GPPs) of representative crops and vegetation in paddy fields, farmlands and pastures using
the Closed Plant Experiment Facility. The decomposition rates of organic matter were also measured in soilsof those ecosystems to get carbon balance in the target fields.
The ANPPs at a Fagus crenata-dominated forest and a Quercus crispula-dominated deciduous
broad-leaved forest, and those at a 19-y-old and a 65-y-old Cryptomeria japonica tree plantation wereestimated as 0.9, 0.9, 2.1, and 6.0 kg-dry m-2 y-1, respectively, based on the summation of annual fine litter
fall and annual wood increment in the above-ground biomass from 2010 to 2011.
The GPPs of timothy grass, rice and Japanese radish plants were measured at six growth stages ofthese plants by pot cultivation experiments using soils in actual fields. The air and soil temperatures, soil
moisture, and light intensity were controlled to simulate those in actual cultivation fields. While similar
maximum GPPs at the saturated light level were found for timothy grass over the entire growth period, thoseof rice and Japanese radish plants decreased with the increased number of days after sowing.
To investigate the decomposition rate of soil organic matter in wetlands, mixtures of 13C-labelledplants and wetland soil were packed into glass-fiber filter bags, and buried both in an actual wetland and asimulated wetland in the Closed Geosphere Experiment Facilities in FY 2010. In FY 2011, similar
glass-fiber filter paper bags containing mixtures of 13C-labelled plants and forest, paddy field, farmland or
pasture soil were buried in each type of field. Concentrations of 13C in fractions separated chemically fromthe materials in the bags in wetlands were measured at 14 d and 162 d after the burying. The 13C
concentration decreased in the order of base-soluble > acid-insoluble > acid-soluble > base-insoluble
fractions.
To obtain temperature dependency of the decomposition rate of organic materials 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 were measured for respired CO2. All soil samples showed an increase
in respired CO2 with temperature. The incubation experimental study will continue until the end of FY 2012.

(mgC g-1-dry h-1)

GPP per unit leaf mass

PPFD (mmolm-2 h-1)

Fig. 1

Relationship between photosynthetic photon flux
density (PPFD) and gross primary productivity (GPP)per unit leaf mass of rice plant at 74 d after sowing.