Organic
Geochemistry
Organic Geochemistry 36 (2005) 813–823
www.elsevier.com/locate/orggeochem
Changes in concentration and d13C value of dissolved
CH4, CO2 and organic carbon in rice paddies under
ambient and elevated concentrations of atmospheric CO2
Weiguo Cheng a,*, Kazuyuki Yagi a, Hidemitsu Sakai a,
Hua Xu b, Kazuhiko Kobayashi c
a
c
Greenhouse Gas Emission Team, Department of Global Resources, National Institute for Agro-Environmental Sciences,
3-1-3 Kannondai, Tsukuba, Ibaraki 305-8604, Japan
b
Institute of Soil Science, The Chinese Academy of Sciences, Nanjing 210008, China
Graduate School of Agricultural and Life Sciences, The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan
Received 15 March 2004; accepted 25 January 2005
(returned to author for revision 16 June 2004)
Available online 31 March 2005
Abstract
Changes in concentration and d13C value of dissolved CH4, CO2 and organic carbon (DOC) in floodwater and soil
solution from a Japanese rice paddy were studied under ambient and elevated concentrations of atmospheric CO2 in
controlled environment chambers. The concentrations of dissolved CH4 in floodwater increased with rice growth (with
some fluctuation), while the concentrations of CO2 remained between 2.9 to 4.4 and 4.2 to 5.8 lg C mL1 under conditions of ambient and elevated CO2 concentration, respectively. The amount of CH4 dissolved in soil solution under
elevated CO2 levels was significantly lower than under ambient CO2 in the tillering stage, implying that the elevated CO2
treatment accelerated CH4 oxidation during the early stage of growth. However, during later stages of growth, production of CH4 increased and the amount of CH4 dissolved in soil solution under elevated CO2 levels was, on average,
greater than that under ambient CO2 conditions. Significant correlation existed among the d13C values of dissolved
CH4, CO2, and DOC in floodwater (except for the samples taken immediately after pulse feeding with 13C enriched
CO2), indicating that the origins and cycling of CH4, CO2 and DOC were related. There were also significant correlations among the d13C values of CH4, CO2 and DOC in the soil solution. The turnover rate of CO2 in soil solution was
most rapid in the panicle formation stage of rice growth and that of CH4 fastest in the grain filling stage.
2005 Elsevier Ltd. All rights reserved.
1. Introduction
*
Corresponding author. Tel.: +81 29 838 8231; fax: +81 29
838 8199.
E-mail address: [email protected]ffrc.go.jp (W. Cheng).
Recent anthropogenic emissions of key atmospheric
trace gases (the so-called greenhouse gases, e.g., CO2,
CH4, N2O, and CFCs) that absorb infrared radiation
may lead to an increase in global mean surface temperature and may thus have the potential to bring about
0146-6380/$ - see front matter 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.orggeochem.2005.01.009
814
W. Cheng et al. / Organic Geochemistry 36 (2005) 813–823
changes in climate (IPCC, 2001). Long term records
demonstrate a steady rise in atmospheric CO2 since the
pre-industrial era, with an accelerated rate of increase
during recent decades, and it is thought that the concentration of CO2 in the atmosphere may double by the
middle of the 21st century (IPCC, 2001). After CO2
(and H2O), methane (CH4) is the most important greenhouse gas and is considered to have been responsible for
approximately 20% of anthropogenic forcing of climate
since the onset of the Industrial Revolution. Rice paddy
soils account for a large fraction of global wetland ecosystems and provide a staple food for a large portion of
the worldÕs population, especially in Asia. Methane from
rice paddies accounts for ca. 17% of emissions from
anthropogenic sources (IPCC, 2001).
Many studies have demonstrated that elevated concentrations of CO2 have a positive effect on rice biomass
production (above and below ground) and on grain yield
(Baker and Allen, 1993; Ziska et al., 1997; Sakai et al.,
2001; Kim et al., 2001, 2003). Elevated CO2 also increases the amount of soil microbial C and accelerates
the turnover rate of soil organic C during the middle
and later stages of the rice growing season (Cheng
et al., 2001; Hoque et al., 2001). The direct effect of elevated CO2 (600–700 lmol mol1) on rice root biomass
and tiller number indirectly increases CH4 emissions
from rice fields by 50–60% (Ziska et al., 1998; Inubushi
et al., 2003).
Methane and CO2 (with the exception of N2) are the
two main constituents of gas in flooded rice paddy soil
(Uzaki et al., 1991; Chidthaisong and Watanabe,
1997). Carbon dioxide is produced via decomposition
and fermentation of organic matter and also from respiration of soil organisms and rice roots. Methane is a terminal product of the anaerobic decomposition of
organic matter and is produced primarily via fermentation of acetate and reduction of CO2 (Takai, 1970).
The major sources of organic matter in flooded rice paddy soil are old organic matter (organic matter of the native soil and incorporated organic material, such as
straw and manure) and recent organic matter arising
from plant growth (root exudates and plant debris).
The decomposition of organic carbon in rice paddy soil
can be divided into two steps: organic carbon is initially
decomposed into water soluble or dissolved organic carbon (DOC), which is then converted into CO2 and CH4,
which are entrapped by the soil or lost to the atmosphere
(Cheng et al., 2001).
Methane entrapped in flooded rice paddy soil may be
(1) oxidized to CO2 in the rice plant rhizosphere or at the
water–soil interface as it diffuses upwards, (2) released
via ebullition (bubbles), or (3) emitted to the atmosphere
through the rice plant. Numerous studies have been conducted on CH4 dynamics by measuring the CH4, CO2,
and DOC in soil solution (Minoda et al., 1996; Ziska
et al., 1998; Alberto et al., 2000; King and Reeburgh,
2002). However, there have been no reports of CH4,
CO2, and DOC dynamics in floodwater, despite carbon
cycling at the interface between the ground surface and
floodwater generally being considered more active than
in the subsurface.
The objectives of this study were to (1) determine
how elevated concentrations of atmospheric CO2 influence the concentrations and d13C values of CH4 and
CO2 dissolved in floodwater and soil solution in rice
paddies and (2) understand better the origins and relationships amongst CH4, CO2, and DOC by determining
their d13C values.
2. Materials and methods
2.1. Controlled environment chambers and experimental
design
The study was conducted at NIAES (National Institute for Agro-Environmental Sciences, Tsukuba, Japan)
in a growth chamber system (Climatron; Shimadzu,
Kyoto) which consisted of six 4 · 3 · 2 m (L · W · H)
controlled environment chambers. The space for plant
growth in each chamber was 4 · 2 · 2 m. Each chamber
housed two stainless steel containers (1.5 · 1.5 · 0.3 m;
L · W · D) filled with paddy soil. The frames, rear
(north) walls and floor of each chamber were made of
stainless steel and were glazed with 5 mm thick tempered
glass, which had a visible light transmittance of >80%.
The air temperature and relative humidity in each chamber were controlled by electrical resistive heaters (using a
bubbling system for humidification) and cold water heat
exchangers operated via PID (Proportional + Integral + Derivative) controllers (DB1000; Chino, Tokyo).
Air temperature and relative humidity were measured
with temperature–humidity sensors (HN-Q500-1;
Chino, Tokyo) that were mounted above the rice canopy
and shielded against direct solar radiation. Air temperature was controlled to follow ambient air temperature.
The seasonal mean temperature was 24.0 C. Relative
humidity was maintained at 71 ± 2.9 % (average ± SD).
The seasonal mean photosynthetically active radiation
(PAR) was 22.0 mol m2 day1. Throughout the growing season, the CO2 concentration was maintained at
383 ± 11 lmol mol1 (day) and 446 ± 40 lmol mol1
(night) in the three ambient CO2 chambers and
706 ± 13 lmol mol1 (day) and 780 ± 76 lmol mol1
(night) in the three elevated CO2 chambers. During the
day, CO2 levels were maintained by a computer controlled injection system that released pure CO2
(d13C = 31.5&) to compensate for CO2 uptake by
the rice plants. During the night, CO2 levels increased
because of plant respiration, but ambient concentrations
were maintained within 100 lmol mol1 of daytime levels using a computer controlled air ventilation system,
W. Cheng et al. / Organic Geochemistry 36 (2005) 813–823
which introduced ambient air to reduce CO2 levels (Sakai et al., 2001).
2.2. Experimental soil, rice cultivar, agricultural
practices, and enriched 13C CO2 feeding assay
Bulk soil was collected from the plough layer (top 20
cm) of a rice field in Yawara, Ibaraki Prefecture, Japan.
Relevant soil properties are provided in Table 1. Visible
plant residues were absent because the soil was stored
outdoors in a pile for two years. The rice cultivar was
Nipponbare, a variety popular with Japanese farmers,
and the agricultural practices employed were the same
as those described by Sakai et al. (2001). Briefly, rice
was transplanted on May 15, 2002. One day before
transplanting, basal fertilizers (N:P2O5:K2O = 50:150:
150 kg ha1) and rice straw (chopped into approximately 3 cm lengths) were added at an application rate
of 1500 kg ha1 and mixed thoroughly with the soil.
Key properties of the incorporated rice straw are provided in Table 1. Top dressing with NH4Cl at an application rate of 30 kg N ha1 was undertaken between the
stages of maximum tillering and panicle initiation.
Floodwater was maintained at a depth of ca. 3 to 5
cm throughout the cultivation period. The rice was harvested on September 30, 2002.
Table 1
Major properties of experimental paddy soil and incorporated
rice straw
Soil type
Clay content of soil (%)
Organic C (mg g1 DW)
Total N (mg g1 DW)
C/N
d13C (&)
Soil
Straw
Alluvial
35.0
30.12
2.63
11.50
24.73
345.41
5.19
66.53
27.20
815
On a regular basis throughout the growth season
(June 11–13, July 23–25, and August 26–28) rice plants
in the chambers were fed 13C enriched CO2 to investigate
photosynthetic carbon allocation and transformation at
the tiller, panicle formation, and grain filling stages of
rice growth. Addition of 13C enriched CO2 (Shoko Co.
Ltd., Tokyo) into the chambers was performed via the
computer controlled CO2 injection system described
above. The d13C value of the enriched CO2 was 414&.
2.3. Floodwater and soil solution sampling
Sampling was carried out eight times: June 10 (1st)
and 14 (2nd), July 22 (3rd) and 26 (4th), August 21
(5th) and 29 (6th), and September 5 (7th) and 18 (8th).
All samples were collected in the morning between
10:00 a.m. and 12:00 p.m. The 2nd, 4th, and 6th sampling periods occurred shortly after feeding with 13C enriched CO2 (Fig. 1).
At each sampling date, ca. 40 mL of floodwater was
collected into a 50 mL plastic syringe (without needle).
A needle was then attached and, with the syringe held
vertically, part of the water was transferred into a 60
mL semi-vacuum bottle (filled with pure N2 gas at 0.5
atm) fitted with a rubber stopper and screw cap.
Approximately 30 mL of floodwater were drawn into
the bottle by the time the pressure in the bottle reached
1 atm. The amount of floodwater collected and the headspace volume were determined by weighing the bottle
before and after sampling. Floodwater was added to
the environmental chambers at least 4 days before sampling to avoid any possible influence from new irrigation
water.
The soil solution in each chamber was sampled using
a Rhizon soil solution sampler (10 RHIZON SMSMOM; Eijkelkamp Agrisearch Equipment, Giesbeek,
The Netherlands), which consisted of a 10 cm long
microporous polymer tube (2.5 mm o.d. · 1.5 mm i.d.)
Fig. 1. Schedule of pulse-feeding with 13C enriched CO2 and sampling times while rice plants were growing in controlled environment
chambers.
816
W. Cheng et al. / Organic Geochemistry 36 (2005) 813–823
and a 50 cm long PVC tube (2.7 mm o.d. · 1.0 mm i.d.).
The microporous polymer tube was buried horizontally
under 10 cm of soil before the rice was transplanted.
Prior to sampling, about 5 mL of soil solution were extracted using a vacutainer to remove impurities from the
tube. Then a 60 mL semi-vacuum bottle (filled with pure
N2 gas at 0.5 atm) fitted with a rubber stopper and screw
cap was connected to the sampler. Approximately 30
mL of soil solution were drawn into the bottle by the
time the pressure in the bottle had reached 1 atm. The
amount of soil solution collected and the headspace volume were determined by weighing the bottle before and
after sampling.
13
2.4. Measurements of concentrations and d C values of
CO2, CH4, and DOC
After collection, samples were taken to the laboratory where the concentrations and d13C values of CO2
and CH4 in the headspace of sampling bottles were
immediately analyzed. Concentrations of CO2 and
CH4 were determined using gas chromatography (GC7A; Shimadzu, Kyoto) via thermal conductivity (TCD)
and flame ionization (FID) detectors, respectively. The
injected sample consisted of 0.5 mL of gas taken from
the headspace of each sampling bottle. After measurement of CO2 and CH4 concentrations, d13C values were
determined using an isotope ratio mass spectrometer
(MAT252; ThermoFinnigan) interfaced to a gas chromatograph (HP 5890) by a continuous flow system
(Conflo; ThermoFinnigan; GC/C/IRMS). An automated pre-concentration device (Conflo; ThermoFinnigan; PreCon/GC/C/IRMS) was used to process
samples that contained a low concentration of CH4
(e.g., the floodwater samples). There are no replicate
analyses for d13C measurements of CH4 dissolved in
floodwater from June to July because the 3 replicates
were mixed to create a single sample.
After determination of the d13C values of headspace
CO2 and CH4, all samples were stored in a freezer at
18 C prior to analysis of DOC in the floodwater
and soil solution using a TOC analyzer (TOC–5000; Shimadzu). Inorganic carbon in floodwater and soil solution were removed by adding three drops of 10%
hydrochloric acid in 5 mL subsamples. After freeze drying the water samples, the d13C value of DOC was measured using the same isotope ratio mass spectrometer
connected to an elemental analyzer (EA1108; Fisons)
via a continuous flow interface (Conflo II; ThermoFinnigan; EA/IRMS).
2.5. Calculation and statistical analysis
Concentrations of CO2 and CH4 in floodwater and soil
solution were estimated from the gaseous concentrations
in syringe headspaces using HenryÕs Law coefficients
C L ¼ C A ðV A þ BV L Þ=V L ;
ð1Þ
where CL is the concentration of CH4 (ng C mL1) or
CO2 (lg C mL1), CA is the concentration of CH4
(ng C mL1) or CO2 (lg C mL1) in the headspace, VA
and VL are the volumes (mL) of headspace and solution
and B = 0.0329 and 0.828 for CH4 and CO2 at 25 C,
respectively.
The d13C values are expressed in the conventional d
notation in parts per thousand (&) relative to the international standard Vienna Peedee Belemnite (VPDB)
d13 C ¼ ½ðRsa RVPDB Þ=RVPDB 1000ð‰Þ;
13
ð2Þ
12
where Rsa is the C/ C ratio of the sample and RVPDB is
the 13C/12C ratio of VPDB.
Significant differences for all parameters between the
two treatments (ambient versus elevated CO2) were
determined using paired t-tests. The correlation coefficient (r) was used to assess the significance of relationships between CH4, CO2, and DOC in floodwater and
soil solution. The r values marked with ** and * indicate
significant correlation at P < 0.01 and P < 0.05,
respectively.
3. Results
3.1. Changes in concentrations and d13C values of CH4
dissolved in floodwater and soil solution
The concentration of CH4 dissolved in floodwater
collected on June 10 and 14 ranged between 0.8 and
0.9 ng C mL1. There were no significant differences in
CH4 concentration between the ambient and elevated
CO2 treatments. The concentration of CH4 increased
in the samples collected in July. On average, the abundance of dissolved CH4 was greater under elevated
CO2 conditions than under ambient CO2 levels (Fig.
2a). The three pulse-feeding episodes involving injection
of 13CO2 into the environmental chambers increased
d13C values of dissolved CH4 in floodwater by 8.9&,
21.9& and 27.4& under ambient CO2 levels and
13.8&, 40.0& and 32.4& under elevated CO2 conditions (Fig. 2(b)).
The concentration of CH4 in soil solution was lower
during early stages of rice growth in June and July (<150
ng C mL1) than during later growth stages in August
and September (>700 ng C mL1). In the early stages
of rice growth (June 10 and 14 samplings) the concentration of CH4 dissolved in soil solution under ambient
CO2 conditions was significantly larger than under elevated CO2 conditions. Conversely, during later stages
of growth (August and September), the concentration
of CH4 dissolved in soil solution increased on average
more under elevated than ambient CO2 conditions
(Fig. 2c). The d13C values of CH4 in soil solution
0
-10
-20
-30
-40
-50
-60
-70
-80
(b)
EL
* *
100
0
0
-10
-20
-30
-40
-50
-60
-70
-80
* *
(d)
In flood water
817
Aug.21
Aug.29
Sep.05
Sep.18
(c)
AM
Jul.22
Jul.26
Aug.21
Aug.29
Sep.05
Sep.18
Jun.10
Jun.14
*
3000
2500
2000
1500
1000
500
Jun.10
Jun.14
EL
δ 13C-CH4 (‰)
δ 13C-CH4 (‰)
(a)
AM
Jul.22
Jul.26
CH 4 (ng C mL-1)
16
14
12
10
8
6
4
2
0
CH4 (ng C mL-1)
W. Cheng et al. / Organic Geochemistry 36 (2005) 813–823
In soil solution
Fig. 2. Changes in concentration and d13C value of CH4 dissolved in floodwater (a and b) and soil solution (c and d) in paddies where
rice plants were grown under ambient (AM) and elevated (EL) CO2 conditions. Pulse-feedings were carried out on June 11–13, July 23–
25, and August 26–28. Bars indicate standard deviation (no replicates for CH4 dissolved in floodwater on June 11 and 14, or July 22
and 26). Asterisks (*) indicate a significant difference (P < 0.05) between ambient and elevated CO2 treatments (paired t-tests).
changed slightly after the first 13CO2 pulse-feeding in
June (+1.3& for ambient and +2.8& for elevated CO2
conditions) and more significantly after the third pulsefeeding in August (+22.6& for ambient and +28.1&
for elevated CO2 conditions; Fig. 2d).
3.2. Changes in concentrations and d13C values of CO2
dissolved in floodwater and soil solution
During all growth periods, the concentration of CO2
in floodwater ranged from 2.9 to 4.4 lg C mL1 and 4.2
to 5.8 lg C mL1 under ambient and elevated CO2 conditions, respectively (Fig. 3a). The concentration of CO2
in floodwater was greater under elevated CO2 levels for
all samples. The d13C values of CO2 in floodwater changed after 13CO2 pulse-feeding under both ambient and
elevated CO2 conditions. Significant differences between
ambient and elevated CO2 treatments were observed in
the June 10 and 14, and July 22 samplings (Fig. 3b).
The concentration of CO2 in soil solution ranged
from 80.1 to 199.3 lg C mL1 during all growth periods
and there were no significant differences between ambient
(b)
2
50
40
30
20
10
0
-10
-20
-30
Aug.21
Aug.29
Sep.05
Sep.18
Jul.22
Jul.26
Jun.10
Jun.14
0
(c)
EL
200
100
0
Aug.21
Aug.29
Sep.05
Sep.18
*
4
AM
Jul.22
Jul.26
6
Jun.10
Jun.14
EL
0
δ13C-CO2 (‰)
δ13C-CO2 (‰)
(a)
300
AM
CO2 (µg C mL-1)
CO2 (µg C mL-1)
8
*
*
*
In flood water
-10
-20
-30
(d)
In soil solution
Fig. 3. Changes in concentrations and d13C values of CO2 in floodwater (a and b) and soil solution (c and d) in paddies where rice
plants were grown under ambient (AM) and elevated (EL) CO2 conditions. Pulse-feedings were carried out on June 11–13, July 23–25,
and August 26–28. Bars indicate standard deviation. Asterisks (*) indicate a significant difference (P < 0.05) between ambient and
elevated CO2 treatments (paired t-tests).
W. Cheng et al. / Organic Geochemistry 36 (2005) 813–823
DOC (µg C mL-1)
250
30
20
10
Aug.21
Aug.29
Sep.05
Sep.18
0
100
50
0
(c)
0
δ13 C-DOC (‰)
δ13C-DOC (‰)
EL
150
20
(b)
AM
200
Aug.21
Aug.29
Sep.05
Sep.18
EL
Jun.10
Jun.14
AM
Jul.22
Jul.26
(a)
40
Jun.10
Jun.14
DOC (µg C mL-1)
50
Jul.22
Jul.26
818
10
0
-10
-20
-30
-10
-20
-30
(d)
In flood water
In soil solution
Fig. 4. Changes in concentration and d13C value of dissolved organic carbon (DOC) in floodwater (a and b) and soil solution (c and d)
in paddies where rice plants were grown under ambient (AM) and elevated (EL) CO2 conditions. Pulse-feedings were carried out on
June 11–13, July 23–25, and August 26–28. Bars indicate standard deviation.
and elevated CO2 treatments (Fig. 3c). The concentration of CO2 increased to a maximum level on September
5 for both the ambient and elevated CO2 treatments and
then decreased in the last sampling. The d13C values of
CO2 dissolved in soil solution increased more significantly after the second pulse-feeding in July (+13.6&
for ambient and +12.3& for elevated CO2 conditions)
than after the first 13CO2 treatment in June (+4.2& for
δ13C-CH4 (‰) in flood water
2
20
40
(b) 30
20
6
10
10
0
Ambient
3
-20
0
7
8
-10
-30
Elevated
3
ra= 0.993**
1
(c)
Ambient
7
8
5
-40
20
δ13C-DOC (‰) in flood water
4
2
-10
-20
5
re =0.999**
1
Elevated
10
6
6
4
0
0
4
8
7
-10
-30
-40
20
(d)
10
-20
-30
1
3
8
2
2 5
7
-10
5
13
ra= 0.955**
-20
re = 0.950**
-40
-80 -70 -60 -50 -40 -30 -20 -10 -80 -70 -60 -50 -40 -30 -20 -10
13C-CH
4
δ13C-CO2 (‰) in flood water
(a)
6
4
30
0
-30
-40
δ13C-DOC (‰) in flood water
δ13C-CO2 (‰) in flood water
-80 -70 -60 -50 -40 -30 -20 -10 -80 -70 -60 -50 -40 -30 -20 -10
40
0
(‰) in flood water
Fig. 5. Correlations between d13C values of CH4 and CO2 (a and b) and CH4 and DOC (c and d) in floodwater of paddies where rice
plants were grown under ambient and elevated CO2 conditions. Numbers indicate sampling times.
W. Cheng et al. / Organic Geochemistry 36 (2005) 813–823
ambient and +8.2& for elevated CO2 conditions) or the
third pulse-feeding in August (+7.1& for ambient and
+7.9& for elevated CO2 conditions; Fig. 3d).
3.3. Changes in concentrations and d13C values of DOC
dissolved in floodwater and soil solution
There were no significant differences in the concentrations and d13C values of DOC in floodwater and soil
solution between the ambient and elevated CO2 treatments. The concentration of DOC in floodwater ranged
from 11.5 to 14.5 lg C mL1 during the early stages of
rice growth in June and July and increased to between
27.1 and 42.5 lg C mL1 during later growth stages
(Fig. 4a). The d13C values of DOC in floodwater increased more significantly after 13CO2 pulse-feeding in
July (+16.8& for ambient and +14.3& for elevated
CO2 conditions) and August (+15.1& for ambient and
20.4& for elevated CO2 conditions) than after the
13
CO2 treatment in June (+7.1& for ambient and
+6.7& for elevated CO2 conditions). The d13C values
of DOC in floodwater decreased rapidly by September
5, one week after the third and final 13CO2 pulse-feeding
(Fig. 4b).
The concentration of DOC in soil solution ranged
from 26.5 to 28.2 lg C mL1 in the June 10 and 14
819
samples and from 49.4 to 52.8 lg C mL1 in the July
22 and 26 samples. The abundance of soil solution
DOC then increased significantly to concentrations between 163.5 and 195.4 lg C mL1 during the later
stages of growth (Fig. 4c). The d13C values of DOC
in soil solution increased more significantly after the
third 13CO2 pulse-feeding in August (+7.8& for ambient and +7.7& for elevated CO2 conditions) than after
13
CO2 treatment in June (+2.9& for ambient and
4.9& for elevated CO2 conditions) and July (+5.0&
for ambient and +4.2& for elevated CO2 conditions;
Fig. 4d).
3.4. Correlations between d13C values of dissolved CH4,
CO2 and DOC in floodwater and soil solution
If samples taken directly after 13CO2 pulse-feeding
(the 2nd, 4th, and 6th sampling periods) are excluded,
then significant correlations exist between d13C values
of CH4 and CO2 dissolved in floodwater under both
ambient and elevated CO2 conditions (r = 0.993** for
ambient and r = 0.999** for elevated CO2 conditions;
Figs. 5a and b). There are also significant correlations
between d13C values of dissolved CH4 and DOC in
floodwater under both conditions (r = 0.955** for ambient and r = 0.950** for elevated CO2 conditions; Figs. 5c
5
8
-20
-30
4
6
-10
7
-10
6
8
5
2
0
(b)
Elevated
7
-20
2
3
1
3
1
-30
re = 0.794*
ra= 0.909**
δ13C-DOC (‰) in soil solution
-40
0
-40
0
(c)
Ambient
-10
6
8
5
2
6
8
-10
7
5
-20
2
4
1
(d)
Elevated
7
-20
-30
δ13C-CO2 (‰) in soil solution
(a)
Ambient
4
0
4
3
3
1
ra= 0.878**
-30
re = 0.797*
-40
-80 -70 -60 -50 -40 -30 -20 -10 -80 -70 -60 -50 -40 -30 -20 -10
δ13C-DOC (‰) in soil solution
δ13C-CO2 (‰) in soil solution
δ13C-CH4 (‰) in soil solution
-80 -70 -60 -50 -40 -30 -20 -10 -80 -70 -60 -50 -40 -30 -20 -10
0
-40
0
δ13C-CH4 (‰) in soil solution
Fig. 6. Correlations between d13C values of CH4 and CO2 (a and b) and CH4 and DOC (c and d) in soil solution of paddies where rice
plants were grown under ambient and elevated CO2 conditions. Numbers indicate sampling times.
820
W. Cheng et al. / Organic Geochemistry 36 (2005) 813–823
and d). These results suggest that CH4, CO2, and DOC
in floodwater originated from the same carbon source,
except when 13CO2 from pulse-feeding contaminated
the irrigation water.
Significant correlations also exist between d13C values of CH4 and CO2, and between CH4 and DOC in soil
solution under both ambient and elevated CO2 conditions. Correlation coefficients (r) are 0.909** and
0.794* for CH4 and CO2 under ambient and elevated
CO2 conditions, respectively (Figs. 6a and b), and
0.878** and 0.797* for CH4 and DOC under ambient
and elevated CO2 levels, respectively (Figs. 6c and d).
The correlation coefficients for relationships between
dissolved carbon species in the soil solution are lower
than those for floodwater.
There are also significant correlations between d13C
values of CO2 and DOC in floodwater and soil solution under both ambient and elevated CO2 conditions
(Fig. 7). Excluding samples collected directly after
13
CO2 pulse-feeding, the correlation coefficients are
0.959** and 0.982** for CO2 and DOC in floodwater
under ambient and elevated CO2 conditions, respectively. The correlation coefficients are 0.854** for
CO2 and DOC in soil solution under ambient CO2
conditions and 0.827* under elevated CO2 levels.
4. Discussion
4.1. CH4 and CO2 dissolved in floodwater
If the concentrations of CH4 and CO2 in ambient air
are 1.7 and 360 ppmv, respectively, then the dissolved
concentrations in aqueous solution at 25 C will be
CH4 0.0274 ng C mL1and CO2 0.146 lg C mL1.
The levels of CH4 dissolved in floodwater in June (0.8
to 0.9 ng C mL1) were the lowest of the entire growth
period, but were still 30 times that for water in equilibrium with CH4 in ambient air (Fig. 2a). This indicates
that the dissolved CH4 in floodwater was produced in
the rice paddy and that it was controlled by the production, oxidation, and diffusion of CH4 from paddy soil.
The concentrations of dissolved CO2 in floodwater during the growth period ranged from 2.9 to 4.4 lg C mL1
for ambient CO2 levels and 4.2 to 5.8 lg C mL1 for elevated CO2 conditions. These values are 20 to 30 and 15
to 20 times, respectively, those expected for water in
equilibrium with background levels of atmospheric
CO2 (Fig. 3a) because the concentration of dissolved
CO2 in rice paddy floodwater is controlled by respiration
of organisms in the water and underlying soil and consumption by photosynthetic organisms (Koizumi et al.,
δ13C-DOC (‰) in flood water
-10
0
10
-30
4
-10
0
10
20
40
6
4
2
2
20
-20
(b)
30
20
6
10
10
Ambient
0
Elevated
7
-10
-20
1
3
-20
5
1
ra= 0.959**
-30
-10
8
5
3
0
7
8
re = 0.982**
-30
δ13C-CO2 (‰) in soil solution
-40
0
δ13C-CO2 (‰) in flood water
-20
(a)
30
-40
0
Ambient
4
(c)
Elevated
(d)
1
1:
1
1:
6
-10
-10
6
4
7
8
8
7
2
-20
5
3
2
ra= 0.854**
1
-30
-30
-20
5
3
re = 0.827*
1
-20
-10
-30
δ13C-CO2 (‰) in soil solution
δ13C-CO2 (‰) in flood water
-30
40
-30
-20
-10
0
δ13C-DOC (‰) in soil solution
Fig. 7. Correlation between d13C values of CO2 and DOC in floodwater (a and b), and soil solution (c and d) of paddies where rice
plants were grown under ambient and elevated CO2 conditions. Numbers indicate sampling times. Dashed lines (c and d) represent 1:1
relationships for d13C values of CO2 and DOC in soil solution.
W. Cheng et al. / Organic Geochemistry 36 (2005) 813–823
2001; Kirk, 2004). In all samples, the abundance of CO2
dissolved in floodwater was greater under elevated CO2
conditions than ambient CO2 levels, suggesting that the
difference between respiration and absorption of CO2 in
floodwater was larger under elevated CO2 conditions.
Significant differences also existed between d13C values
of dissolved CO2 in floodwater under ambient and elevated CO2 conditions during the June 10 and 14, and
July 22 samplings (Fig. 3b), indicating that enhanced
levels of CO2 in the environmental chambers may have
accelerated CO2 turnover rates.
4.2. CH4 dissolved in soil solution
Most CH4 produced and emitted during early stages
of rice growth is derived from soil organic matter and
incorporated rice straw, while during later stages of
growth, root exudates and other recent plant photosynthates contribute the majority of carbon to methanogenesis (Yagi and Minami, 1990; Minoda et al., 1996;
Chidthaisong and Watanabe, 1997). In a previous study,
it was demonstrated that CH4 emissions from experimental rice paddy soils are greatest in August and September during later stages of rice growth (Yagi et al.,
2000). Similar to this study, rice straw was applied to
the paddy soil at a rate of 1500 kg ha1 (Yagi et al.,
2000). The present results confirm that CH4 concentrations in soil solution are much greater during later stages
of rice growth in August and September (>700
ng C mL1) than during early stages in June and July
(<150 ng C mL1; Fig. 2c).
During early stages of growth (June 10 and 14) the
amount of CH4 dissolved in soil solution was significantly greater under ambient CO2 levels compared to
elevated CO2 conditions. However, CH4 emissions were
not detected from either the ambient or elevated CO2
treatments (data not shown), suggesting that oxidation
of CH4 may have been enhanced by the higher CO2 levels. Accelerated CH4 oxidation likely would have resulted from enhanced root growth, which would have
increased rates of rhizosphere oxidation because root
formation during early stages of rice growth plays an
important role in transferring O2 from the atmosphere
to submerged soil.
Methane oxidation in the rhizosphere of rice plants
is considered to be the main process responsible for
CH4 consumption in paddy soils (Denier van der
Gon and Neue, 1996). Root biomass is known to increase under elevated CO2 conditions (Inubushi et al.,
2003) and the rate of CO2 production from root respiration is greatest during early stages of rice growth
(Yamagishi, 1984). The d13C values of dissolved CH4
under elevated CO2 conditions were 4.7& and 6.2&
heavier than under ambient CO2 conditions in the
June 10 and 14 samplings, respectively, supporting
the suggestion that CH4 was more oxidized under ele-
821
vated CO2 levels during early stages of rice growth
(Fig. 2d).
Stable carbon isotope values can be used to estimate
the fraction of CH4 oxidized in the rhizosphere according to the following equation (Tyler et al., 1997):
F ¼ ½d13 C–CH4 ðoriginalÞ
d13 C–CH4 ðoxidizedÞ=½ðd13 C–CH4 ðoxidizedÞ
þ 1000Þ ðð1=aÞ 1Þ;
ð3Þ
where d13C–CH4 (original) is the composition of primary, unaltered methane, d13C–CH4 (oxidized) is the
composition of residual CH4 remaining after methanotrophy, and a is the isotope fractionation factor
(1.025) which describes preferential consumption of
12
CH4 by methane oxidizing bacteria. It is assumed that
all CH4 produced during the early stages of rice growth
is derived from incorporated rice straw and soil organic
matter. Consequently, d13C–CH4 (original) values during this stage of growth can be determined by measuring
d13C values of CH4 produced via anaerobic incubation
of soil and rice straw in the absence of living rice plants.
The d13C–CH4 (oxidized) values are determined from
measurements of d13C values of CH4 dissolved in soil
solution. Based on this approach, it is calculated that
the fractions of methane oxidized were 0.35 for the
ambient CO2 treatments and 0.55 for elevated CO2
treatments. Therefore, elevated CO2 levels in the environmental chambers appears to have increased rates of
methanotrophy in the June samples by 57%.
4.3. Correlations between d13C values of dissolved CH4,
CO2 and DOC in floodwater and soil solution
Stable isotope ratios provide clues about the origins
and transformations of organic matter in natural and
anthropogenic systems (Fry and Sherr, 1989). Measurement of stable carbon isotope values in this study has
yielded useful information about relationships between
dissolved CH4, CO2 and DOC in floodwater and soil
solution.
Excluding the effects of 13CO2 pulse-feeding observed
in the 2nd, 4th, and 6th samplings, significant correlations existed between d13C values of CH4 and CO2 (Figs.
5a and b), CH4 and DOC (Figs. 5c and d), and CO2 and
DOC (Figs. 7a and b) in floodwater under both ambient
and elevated CO2 conditions. These correlations suggest
that CH4, CO2 and DOC in floodwater originated from
the same carbon source. Although significant correlations also existed between d13C values of CH4 and
CO2 (Figs. 6a and b), CH4 and DOC (Figs. 6c and d),
and CO2 and DOC (Figs. 7c and d) in soil solution under both ambient and elevated CO2 conditions, the coefficients of correlation for relationships between these
822
W. Cheng et al. / Organic Geochemistry 36 (2005) 813–823
(a)
0
δ13C (‰)
-20
-40
CO2
CH4
-60
Ambient
δ13C (‰)
(b)
Aug.21
Aug.29
Sep.05
Sep.18
Jul.22
Jul.26
Jun.10
Jun.14
-80
from root exudates at the panicle formation stage of rice
growth. Yamagishi (1984) measured respiration of rice
roots over all periods of rice growth and reported that
root respiration peaks at panicle formation and then decreases towards harvest. The d13C values of CO2 were
more positive than those of DOC at the 2nd sampling
(after the first 13CO2 pulse-feeding) under elevated
CO2 conditions, but not under ambient conditions, indicating that root respiration was more rapid under elevated CO2 conditions than under ambient CO2 levels.
0
Acknowledgements
-20
-40
CO2
CH4
-60
-80
Elevated
Fig. 8. Average d13C values of CO2 and CH4 in soil solution at
the 8 sampling times: (a) ambient, and (b) elevated CO2
conditions.
dissolved carbon species were lower for soil solution
than for floodwater. The slight disagreement between
floodwater and soil solution may have occurred because
turnover rates of CH4, CO2 and DOC were more consistent than in floodwater. For example, if the difference
between d13C values of dissolved CH4 and CO2 in soil
solution before and after application of 13CO2 are compared for both ambient and elevated CO2 treatments
(Fig. 8), it is obvious that the d13C values of CO2 have
changed more than those for CH4 after the first and second pulse-feedings during early stages of rice growth
(for both ambient and elevated CO2 treatments). Conversely, the changes in d13C value were larger for CH4
than CO2 after the third pulse-feeding and 1 and 3 weeks
after pulse-feeding during later stages of rice growth for
both ambient and elevated CO2 treatments. The large
changes in CO2 in soil solution were caused by root respiration during early stages of rice growth, while
changes in CH4 in soil solution are thought to have resulted from high rates of CH4 production and oxidation
during later stages of rice growth (after rice grain filling).
There were significant correlations between d13C values of CO2 and DOC in soil solution and in general,
d13C–DOC values were similar to those of d13C–CO2
values (except during the 4th sampling period for ambient CO2 treatments and the 2nd and 4th sampling periods for the elevated CO2 treatments; Figs. 7c and d,
dashed 1:1 line). The d13C values of CO2 were greater
than those of DOC at the 4th sampling periods (after
the second 13CO2 pulse-feeding) under both ambient
and elevated CO2 conditions because production of
CO2 via root respiration exceeded production of DOC
This work was supported by Japan Society for the
Promotion of Science in the form of a postdoctoral fellowship to W. Cheng. We thank Drs. Y. Hosen and H.
Chu in JIRCAS for use of their TOC analyzer. We are
also grateful to Professors R. Sass and G. Kirk and to
an anonymous reviewer whose comments improved the
manuscript.
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