Carbon Transfer in Agricultural Soil Containing Plant Biomass
Masaru NAGAI, Hiroshi SAKATA, Yasuhiro TAKO, Yuji NAKAMURA
Department of Environmental Simulation
Abstract
C-14 released into the atmosphere from a spent nuclear fuel reprocessing plant is partly taken up byrice plants through photosynthesis. In Aomori Prefecture, the prefectural government has recommended thatrice straw be buried in paddy fields after harvest for the purpose of soil improvement and/or for theprevention of air pollution by burning the straw. The purpose of this study is to model the movement ofcarbon through the atmosphere, plants and soil in agricultural fields, and to clarify possible long-term
accumulation of 14C in soil, which would be introduced in plant biomass by burying of rice straw. In thisexperiment, a stable isotope of carbon (13C) was used as a substitute tracer for 14C.
In April 2008, the 13C-enriched rice straw was cut into pieces of 10 cm in length and buried ascompost in paddy soil with the concentration 0.6 kg (dry weight)/m2. The total amount of carbon released as
CO2 during the period from April to November, was 261 gC/m2 from the soil with compost, compared to175 gC/m2 from the soil without compost. The highest 13C concentration in CO2 released from the soil wasobserved in June and July, which suggested that active degradation of rice straw buried in paddy soil
occurred in early summer. The result of the analysis of carbon in the soil showed that about 49% of thecarbon originally contained in the straw remained in the soil in November.
The compost consisting of 13C-enriched rice straw and stubble was also spread onto a carrot field inJuly 2008. The total amount of carbon released during the period from July to November was 110 gC/m2 and65.4 gC/m2 from the soils with and without compost, respectively. The concentration of 13C in CO2 released
from the soil was the highest immediately after introducing the compost, and then decreased. It was alsosuggested that about 59% of the carbon in the straw remained in the soil in November, 4 months after adding
it to the soil.
g13C/m2

g13C/m28

3.5

7

3

6

2.5

5

24
1.5

3Fig. 1 The excess 13C amount of the paddy soil
buried with 13C enriched straw

Fig. 2

2008.11.25

2008.11.11

2008.10.28

2008.10.14

2008.9.30

2008.9.16

2008.9.2

2008.8.19

2008.7.22

2008.10.30

2008.10.2

2008.10.16

2008.9.4

2008.9.18

2008.8.7

2008.8.21

2008.7.24

2008.7.10

2008.6.26

2008.6.12

2008.5.29

02008.5.1

02008.5.15

0.5

2008.4.17

1

2008.8.5

1

2

The excess of 13C amount of the carrot
field spread with 13C enriched compost

Fig. 3from the paddy soil
The 13C concentration of gases released

1Fig. 4from the carrot field

2008.11.14

2008.10.31

2008.10.172008.10.3

2008.9.19

2008.9.5

2008.8.22

1.8

2008.8.8

2008.7.25

2008.7.11

2008.6.27

2008.6.13

2008.5.30

1.5

2008.5.2

2

2008.5.16

2.5

13C concentration (atom%)with 13C –enriched straw
13C施用区
施用区
with straw
非施用区
without straw

2008.4.18

2008.12.4

2008.11.20

2008.11.6

2008.10.23

2008.10.9

2008.9.25

2008.9.11

2008.8.28

2008.8.14

3

2008.7.31

2008.7.17

2008.7.3

2008.6.19

2008.6.5

2008.5.22

2008.5.8

2008.4.24

2008.4.10

2008.3.27

13C concentration (atom%)
3.5

2with 13C –enriched compost
13C施用区
施用区
with compost
非施用区
without compost

1.6

1.4

1.2

1

The 13C concentration of gases released