Methane formation and emission from flooded rice soil incorporated with 13C-labeled rice straw

Pergamon~ PII:

Soil Biol. Eiochem.Vol. 29, No. 8, pp. I 173- 1I8 I. 1997 0 1997 Elsevier Science Ltd. All rights reserved Printed in Great Britain 8003?34717(97)ooo34-5 0038-0717/97 517.00 + 0.00

M:ETHANE FORMATION AND EMISSION FROM IFLOODED RICE SOIL INCORPORATED WITH 13C-LABELED RICE STRAW AMNAT CHIDTHAISONG* and IWAO WATANABE Laboratory of Soil Science and Plant Nutrition, Faculty of Bioresources, Mie University, Mie 514, Japan (Accepted I2 January 1997) Summary-Pot experiments with or without ‘3C-labeled rice straw and rice plants were made to investigate the contribution of organic sources to the formation and emission of CH4 from flooded rice soil. The 13C abundance in emitted CH4 and contained in soil bubbles peaked at 2S-40 d after flooding (DAF). After 40 DAF, 13C abundance was higher in unplanted pots than in planted pots. A similar pattern was found in the abundance of “C-CO2 in the soil bubbles. Using the 613C values in the soil bubbles of the pots without straw application as the background values, the percentages of the CH4 emitted from soil to the atmosphere or contained in the soil bubbles originating from straw were calculated. These percentages were multiplied by the CH, flux to estimate the quantities of the emitted CH4 from rice stmw. The most active CH4 emission from the straw was found during 40-80 DAF, when the first peak of CH4 emission was observed. The contribution of sources other than added straw to the formation and emission of CH4 increased sharply only in the planted pots after 80 DAF (heading stage), suggestin this increase was due to the release of organic materials from rice plants. The relative contribution of 63C derived from straw in CH4 and CO* was not significantly different, indicating that methanogena did not have any preference for the straw-derived substrates. After the cropping season, ca. 5% of the rice straw carbon remained in the soil and 1% was incorporated into the rice plants. 0 1997 Elsetier Science Ltd

INTRODUCTION

There is concern over increasing concentrations of CH4 in the atmosphere because, on a mole basis, it is 26 times more potent as a “greenhouse gas” than CO* (Duxbury et al., 1993). Flooded rice soil has been recognized as a source of atmospheric CH4 because it provides the suitable conditions for methanogenesis and large areas of land are given over to rice production (Bachelet and Neue, 1993). According to an estimation made by the IPCC (1992), flooded rice soil contributed ca. 12% (60 Tg) of the total global emission (500 Tg) of CH4 in 1992. The CH4 in flooded rice soil is a terminal product of the anaerobic decomposition of organic matter. The major sources of organic matter in flooded rice soil are native soil organic matter, root exudates, plant debris and incorporated organic material such as straw and manure. To obtain a comprehensive estimation of the importance of rice cultivation to the global CH4 budget, the role of these organic materials with respect to CH4 formation and emission should be understood. Incorporation of straw into flooded rice soil, whether or not in combi*Author for correspondence.

nation with mineral fertilizer, has been found to enhance CH4 emission (Sass et al., 1990, 1991; Yagi and Minami, 1990; Watanabe et ul., 1993a; Inubushi et al., 1994). The cumulative amounts of CH4 emission were found to be related to the amount of rice straw applied (Watanabe et al., 1995a,b). On the other hand, the emissions of CH4 from flooded rice soils in various locations share a common pattern of two to three peaks during the entire cultivation period (Yagi and Minami, 1990; Jermsawatdipong et al., 1994; Nugroho et al., 1994). The appearance of these emission peaks is thought to be related to the anaerobic decomposition of organic sources when rice soil is flooded. However, there is little experimental evidence showing the relationship between the decomposition of organic matter and the emission peaks of CH.+ Our purpose was to investigate the role of organic sources in contributing to the peaks of formation and emission of CH4. For this purpose, rice soil was mixed with 13C-labeled rice straw, both in the presence and absence of rice plants, placed in the pots and flooded with water. The 613C values of both CH4 and CO* within the sediment and emitted from the flooded rice soil were then monitored throughout the rice growing period. This allowed the calculation of the proportion of CH4 and CO2

1173

A. Chidthaisang and I. Watanabe

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derived from 13C-labeled rice straw. The importance of soil organic matter and plant-supplied organic material is also discussed with respect to the formation and emission of CH4. MATERIALS AND METHODS

d-i! pump

KOH

a09

Soil and pot preparations

Ichishi soil (Typic Fluvaquent) was used throughout this study. A full description of this soil is given by Watanabe et al. (1995b). The soil was sieved (2 mm) before mixing with 13C-labeled rice straw and fertilized (10.3 mg kg-’ N-PzOs-KzO). The uniformly labeled rice straw (613C = 327.96%0, C = 36.14%, N = 0.88%) was ground to < 1 mm particles and mixed thoroughly before use. This straw was provided by Professor T. Yoneyama of the National Institute of Agro-Environmental Science, Tsukuba, Japan. The results of the straw analysis for 13C can be found in Yoneyama et al. (1989). The final concentration of rice straw was 10 g pot-‘. Three kilograms of treated soil was transferred to 0.02-m2 pots. The pots were flooded with water and three replicates planted (four seedlings per pot) with 20-day-old Japonica-type rice seedlings (Oryza sativa L. cv. Yamahikari) in June of 1994 and 1995. Control pots, also in triplicate, were left unplanted or without rice straw application. The pots were transferred to large containers for maintenance of the water level. The containers were kept in a greenhouse whose temperature varied in the range 17-40°C throughout the experiment. Gas sampling and analysis

Gas sampling and determination of CH4 emission were performed as described by Inubushi et al. (1989). Sampling and analysis of the soil bubbles were performed as described by Uzaki et al. (1991). Sample preparation for isotopic analysis

The gas inside the assay chamber was pumped at 0.4 1 min-’ with a mini-pump (Shibata Co. Ltd, Japan) into a 2-l plastic bag until the bag was full. CH4 was converted to CO2 by a modification of the combustion method of Glascock (1954). The apparatus consisted of two banks of alkali traps and two CuO columns, all connected in series with silicon tubing (Fig. 1). Each bank of alkali traps consisted of four 30-ml tubes containing 0.25 N Ba(OH)z for trapping CO2 (precipitating it as BaCOs). The air sample was drawn through the first bank of alkali traps with a mini-pump at a flow rate of 20 ml min-‘. An additional tube containing 1N KOH solution was connected to the outlet of the pump to trap any remaining COz. Measurement of the gas coming out of each tube showed that most CO2 was trapped by the first tube. Therefore, the other three Ba(OH)z tubes and the KOH tube were

t

r

P ti

BdOW2

WOW2

Fig. 1. Combustion apparatus for conversion of CO2 to CH4.

included as a precaution. The air (containing CH4) leaving the KOH tube was then introduced into the double-CuO columns (1.3 x 20 cm) and heated to over 800°C to convert CH4 to C02. The newly-generated CO2 was trapped and precipitated as BaC03 in the second bank of alkali traps. The BaCOs from the first and second banks was then removed and stored for later isotopic analysis for CO2 and CH4, respectively. Prior to the next sample combustion, the apparatus was purged by introducing pure O2 through the entire system until the gas coming out from the CuO columns contained no detectable COz. For each sampling time, gas samples from six chambers of each treatment (with or without labeled rice straw) were collected (12 1 in total). The Ba(OH)2 solution was prepared with CO*-free water. Before use for trapping, the solution was filtered under a N2 atmosphere to remove any contaminating BaC03. A preliminary experiment showed there was no significant isotopic fractionation when these procedures were used (Table 1). More than 98% of the CH4 was converted to CO2 at the pumping rate of 20 ml min-*. After trapping, the excess Ba(OH)z was eliminated by immediate titration with 1 N HCl using phenopthalein as an indicator. The BaCOs was transferred to a 5-ml Venoject tube (Terumo Co. Ltd, Japan) and dried at 100°C. The tube was sealed, evacuated to vacuum and stored at -20°C until use for isotopic analysis. The BaC03 was converted to CO2 by acidification with the excess amount of 1 N HCl solution when isotopic analysis was performed. The S13C values were determined by a GC/C/ IRMS, with a Finigan MAT delta S/GC isotope ratio mass spectrometer at the Center for Ecological Research, Kyoto University (in 1994) and with a Finigan MAT 252 isotope ratio mass spectrometer at the National Institute of AgroEnvironmental Science in Tsukuba, Japan (in 1995). Results were expressed in the 6 notation in parts

Methane formation and emission from soil with “C-labeled rice straw

1175

Table 1. Isotopicfractionationaffectedby the precipitationof CO2as BaCOJand by the conversion of CH4 to CO2 by passing it through the CuO columns heated to over 800°C 6% values of CO 2 (1) Tank CO? -29.12 -27.58 -21.66 -27.86 -26.68 -27.78 f 0.88

Average + SD

613C values of CH 4

(2) From BaCO! -29.18 -29.13 -28.60 -28.83 -29.55 -29.18+0.49

(I) Tank CHt

(2) From BaCO$

-65.96 -66.16 -61.63 -66.71 -65.83 -66.46 k 0.74

-65.66 -67.69 -65.06 -67.50 -65.53 -66.28 k 1.21

‘Direct injection of pure CO2 or CHd from commercial gas samples (Takachiho Co. Ltd, Japan). %Zommercial gas samples were passed through Ba(OH), solution and precipitated as BaCOs.

per thousand or per mil (‘G) relative to the PBD standard (Craig, 1953).

that [Fig. 2(b)]. In the unplanted pots, the 613C values of CH4 were always higher than those of the planted pots except at the beginning of flooding. 1995 cropping season

RESULTS

1994 cropping seamn Seasonal variations of CH4 flux and its 613C values from the pots incorporated with 13C-labeled rice straw are given in Fig. 2(a). High variations in both planted and unplanted pots were observed with double-peak maxima in the planted pots. The emission from the unplanted pots usually occurred when the soil and chamber temperatures were high (data not shown). The total CH4 emission was 159 f 38 g rnp2 crep-’ in planted pots and 44 f 31 . unplanted pots (average + SD of g mm2crop-’ m three replicates). The heading date of rice plants was on 4 August 1994, or 80 DAF. The 613C value,s of the emitted CH4 from the planted pots were above -20%0 from the beginning until 80 DAF and decreased to below -20% after

CH4 concentrations in the soil bubbles of the planted pots [Fig. 3(a)] and unplanted pots [Fig. 3(b)] gradually increased up to 60 DAF. CH4 concentrations in the planted pots remained below 40% (v/v), while in the unplanted pots the maximum concentration exceeded 70% at the end of the growing season. On the other hand, the concentrations of CO2 increased during the early period and fluctuated during the late period of flooding. Rice plants reduced the concentrations of CO2 and CH4 in the soil bubbles. The concentrations of N2 dropped more sharply in the unplanted pots than in the planted pots. It was observed that the decrease in concentrations of N2 paralleled the increase in the concentrations of CH4. CH,, flux in 1995 is shown in Fig. 4(a) and the 613C values of CH4 and CO2 in the soil bubbles are

140 r

-+

Planted pots

20-

O-

-20 - &

-4O-60, 0

20

40

60 (a)

80

100

120

0

- 1 . 20

Day after flooding (DAF)

1 * 1 . 40

60

I 80

.

1 100

120

(b)

Fig. 2. Seasonal variations of CH4 flux (a) and its S13Cvalues (b) in the pots incorporated with “Clabeled rice straw in 1994.

A. Chidthaisang and I. Watanabe

1176

100

60

40

20

0 0

20

40

60

80

100

120

20

0

40

60

Day after flooding (DAF)

(a)

80

100

120

00

Fig. 3. The composition of the soil bubbles in the planted pots (a) and the unplanted pots (b) in 1995. shown in Fig. 4(b). A pattern of double emission peaks similar to that observed in 1994 was found. The heading date of rice plants was on 8 August 1994, or 84 DAF. The total emissions of CH4 from the planted and unplanted pots are given in Table 3. Application of straw increased the amount of CH4 emission by 42.3% in planted pots and 33.9% in unplanted pots, respectively. The seasonal variations of CH4 flux and its 613C values are shown in

*

Planted pots

+

Unplanted pots

Fig. 4(a,b), respectively. In Fig. 4(b), the higher 6i3C values implied that more rice straw was transformed to CH4 or CO* than from the other sources. Decomposition of incorporated rice straw released CH4 and CO2 as early as the first week of flooding, resulting in higher 613C values in comparison with the pots without 13C-labeled rice straw (-65.91% and -18.86% for CH4 and COz, respectively, Chidthaisong and Watanabe, 1997). In planted

-I w

Planted pot&H4

Plantedpots:q unplalItcd pots :CH4 Unplanted pots :cO,

-50 -f-t_ ,-A0

20

40

60

(a)

go

loo

120

0

20

Day after flooding (DAF)

40

60

80

100

(b)

Fig. 4. Seasonal variations of CH4 flnx (a) and the S”C values of CH, and CO2 in the soil bubbles of the pots incorporated with ‘3C-labeled rice straw (b) in 1995.

120

Methane formation and emission from soil with “C-labeled rice straw pots, the maximum 6’? value of CH4 (124.40?&) and CO2 (152.93!C) occurred in the third and fourth weeks, respectively. Thereafter, the 613C values of both gases continued to decrease toward the end of the growing season. The 613C values of both gases in the planted and unplanted pots were not significantly diRerent during the early period up to 40 DAF, and after that the values in the unplanted pots surpassed those in the planted pots. In the unplanted pots, the maximum values of 613C for CH4 (121.73k) and CO2 (161.35%) were found in the third week. Analysis of the soil and plant samples (above ground biomass) after the end of the cropping season showed that a portion of the straw carbon remained in the soil and was incorporated into the rice plants (Table 2). The remaining carbon accounted for 6% in 1994 and 4% in 1995 of the added rice straw carbon. A small amount of the added rice straw ca.rbon (1.40% in 1994 and 0.76% in 1995) was incorporated into the rice plants.

DISCUSSION

The emission pattern of the double-peak maxima observed in this study was similar to that reported by other author:; (Yagi and Minami, 1990; Jermsawatdipong (?t al., 1994; Nugroho et al., 1994). The first peak of emission was probably related to the application of straw into the soil at the beginning of flooding. The stimulation of an emission peak by straw was possibly due to the release of available substrates from its decomposition processes. Apart from the carbon substrates supplied from the added straw, other organic sources, including soil organic carbon (Denier van der Gon et al., 1993), readily mineralizable carbon (Yagi et al., 1994) and photosynthetically-produced carbohydrates by rice plants (Sass et al., 1991), were also possibly utilized for CH4 formation in the flooded rice soil. Measurement of the 613 values of emitted CH4 and CO* in the soil bubbles allowed us to calculate

1177

the relative importance of added rice straw to other organic sources. Measurements of the 613C values of emitted CH,, were performed in the 1994 cropping season. In 1995, the same experiments were performed to confirm the first year’s results. Unfortunately, the 613C values of the CH4 emitted from the pots could not be obtained because of contamination from 13C-enriched sources. Thus, only the results of the soil bubbles in the pots incorporated with the 13C-labeled rice straw were reported for the 1995 cropping season. The 613C values of CH4 in the soil bubbles obtained from pots without straw addition were used as the background values in all cases because the 6°C values in the emitted CH4 were not available. The fraction of straw-derived CH4 can be estimated according to the equation given below: f

=

@3~sample) (a’3cadded

- @‘3cbubble)

maw) -

(@3Cbubble)

1x

SOW

‘Control ‘Labeled

Control Labeled

The 6’k values of soil after harvest Planted Unplanted Planted Unplanted

1994 cropping -24.86 -25.30 -5.20 -4.70

Planted Unplanted Planted Unplanted

1995 cropping season -25.96 f 0.02 -25.83 + 0.02 -11.851t2.15 -12.4Ok 1.80

“Pots without straw application. b f SD of two replicates.

(1)

where f is the fraction of straw-derived CH4 (%), 613Csamp’eis the 613C value of emitted CH4 or CH4 in the soil bubbles of the pots with 13C-labeled rice straw 613Cbubble is the 613C value of CH4 in the soil bubbles of the pots without 13C-labeled rice straw and 613Cad&d Strawis the 613C value of added 13C-labeled rice straw. The amounts of CH4 flux in 1994 [Fig. 2(a)] were multiplied by the calculated fraction of straw-derived CH4 (f) shown in Fig. 5(a), to yield the amount of straw-derived CH4 flux [Fig. 5(b)]. The difference between the total flux and the straw-derived flux is the portion of CH4 that originated from the other sources; namely from soil organic matter or plant-released organic material. By assuming that the relative contribution percentages of the emitted CH4 originating from straw is equal to that in the soil bubbles [Fig. 6(a), calculated from data in Fig. 4(b) using equation (l)], the straw-derived CH4 fluxes in 1995 were also estimated [Fig. 6(b)]. The relative contribution of carbon that originated from straw in the CH4 emitted or contained in the soil bubbles varied with the CH4 formation

Table 2. The 6°C values obtained from soil and plant after harvest in the 1994 and 1995 cropping sea-

Treatments

100,

season f 0.52b f 0.20 f 0.93 + 1.07

The 8°C values of rice straw after harvest -28.15 k 0.73 -23.33 f 0.69

-27.21 f 0.23 -24.76 + 0.13

A. Chidthaisang and I. Watanabe

1178

-t_

Planted pots

,-z-

Unplanted pots

0’ * 20 0 I.

I

40

.

I.

60

I.

1.

80

100

120 O-40

Day after flooding (DAF)

40-80

80-l 16 DAP

(a) Fig. 5. The relative contribution of the emitted CH4 from ‘3C-labeled rice straw (a) and the amount of CH4 flux from straw or from the other sources (b) in 1994 (1, planted pots and 2, unplanted pots).

from soil organic

matter

or plant-supplied

organic

material. As shown in Figs 5(a) and 6(a), the peaks of the relative contribution appeared between 20

and 40 DAF. The appearance of peaks in the relative contribution between 20 and 40 DAF does not mean that the CH4 formation and emission were

80 gx! 3 .5 60

30 --+Y +

Planted:CO2 Unplanted: CH4 Unplanted: CO2

$? 5 %40 8 ‘5 5!

fl ‘E 3 20 S b: d B “0 1

20

10

*$ z d

Day after flooding (DAP) (a)

O-40

40-80

80-116 DAP

(b)

Fig. 6. The relative contribution of CH4 from ‘3C-labeled rice straw in the soil bubbles (a) and the amount of CH.+ flux from straw or from the other sources (b) in 1995 (1, planted pots and 2, unplanted pots).

Methane formation and emission from soil with ‘.‘C-labeled rice straw

the most active lluring this period. As shown in Figs 2(a), 3(a) and 4(a), the amount of CH4 flux or its concentrauon in the soil bubbles during 20-40 DAF was still low. The peaks in the relative contribution of straw found between 20 and 40 DAF probably originated from the decomposition of a small fraction of easily-decomposable forms in the straw, such as water-soluble polysaccharides (Watanabe et al., 1993b). However, the peaks of CH4 emission from straw appeared during 40-80 DAF in both years [Figs 5(b) and 6(b)]. Thus, the major portions of straw were decomposed more slowly and contributed to CH4 emission during 40-80 DAF, when the first peak of CH4 emission was observed. On the other hand, the first peaks of CH4 emission from sources other than added straw also appeared during 40-80 DAF. These sources were likely to be soil organic matter combined with plant-supplied organic material. The straw-derived CH4 flux decreased after 80 DAF in both years, while high emission from the other sources was found only in the planted pots. These results suggested that the high emission after 80 DAF resulted from the release of organic material from the rice plants. Although the contribution of plant-supplied organic material was observed before 80 DAF, its main contribution to emission peaks was found after 80 DAF in both years. Accordingly, it could be concluded that the release and decomposition of organic material from rice plants mainly occurred after 80 DAF during the reproductive sl:age of rice plants or when the second peak of CH4 emission was detected. The difference in the peak heights of the relative contribution of straw carbon to CH4 emission in both years appears to be brought about by a change in CH4 formation from soil organic matter or plant-supplied organic material. The higher contribution of carbon from rice straw in 1995 as compared with 1994 suggested more decomposition of straw in 1995 than in 1994. The lower amount of residual 13C in 1995 supports this view (Table 2). Since the decomposition of soil organic matter occurred during the same period as that of added

1179

rice straw, the decomposition rate of soil organic matter would have affected the calculation of the relative contribution of rice straw to the emitted or produced CH4. If the decomposition of soil organic matter and straw released methanogenic substrates to the same pools and these substrates were utilized by methanogens, the higher flow of the intermediates from soil organic matter would have diluted the 13C that originated from the straw. As a result, the produced or emitted CH4 would be diluted, leading to an underestimate of the relative contribution of rice straw. This is one possible reason for the higher contribution of straw in 1995 than 1994. However, this dilution effect could not be the sole reason for the lower 13C-CH4 contribution in 1994 because more 13C remained in the soil. The net amount of CH4 emission stimulated by straw application from the pots with or without rice straw and in the presence or absence of rice plants was calculated (Table 3). The application of rice straw increased the emission of CH4 by, 28.5 g m-* crop-’ in planted pots and 12.2 g m-* crop-’ in unplanted pots. However, the total amount of CH4 derived from 13C-labeled rice straw [the sum of CH4 emission from I3C-labeled rice straw shown in Fig. 6(b)] was 10.6 g m-* crop-’ in planted pots and 8.2 g m-* crop-’ in unplanted pots. Thus, not only did the application of straw directly stimulate CH4 emission, but also stimulated the methane emission from sources other than the incorporated straw (indirect effect). Indirect effects may include a stimulation of reduced conditions, the stimulation of growth of microbial communities which promote the proliferation of methanogens by supplying their required substrates and the stimulation of decomposition of native soil organic matter which would be resulted in the release of substrates that could be used for CH4 production. It was noted that the relative contribution of 13C derived from straw to CH4 and CO2 [Fig. 6(a)] was not significantly different in the planted and unplanted pots. If methanogens used carbon substrates released from the decomposition of straw more preferentially than from other sources, the

Table 3. Contribution of added rice straw to emitted CH4 in 1995 CH4 from straw Pot

Total flux (g mm2crop-‘)

With straw Without straw

67.0 38.5

With straw Without straw

36.0 23.8

% CH4 from straw

(a)

(b)

(a)

(b)

Planted 28.5

10.6

42.3

15.8

Unplanted 12.2

8.2

33.9

22.8

(a) Cakulated from the difference in total flux between ,{ots with and without straw. (b) Summation of the amount of CH4 emission from C-labeled rice straw throughout season as shown in Fig. 6(b).

the cropping

A. Chidthaisang and I. Watanabe

1180

relative contribution of 13C to 13C-CH4 should be higher than that to 13C-COz. However, the contributions of t3C to CH4 and COz were nearly equal. Thus, it could be said that methanogenic bacteria did not have any preference for straw-derived sub-

strates. In conclusion, our study has shown that the degradation of easily-decomposable portions of straw occurred in the period during 2040 DAF, leading to the appearance of 13C abundance peaks in the emitted CH4 (1994 experiment) and in the soil bubble CH4 (1995 experiment). The majority of the emitted CH4 peaked at 40-80 DAF in both years. Therefore, the rice straw was decomposed and CH4 formation and emission from the straw occurred during this period. The peaks of CH4 emission from soil organic matter, as well as partly from rice plants, also appeared in the same period. The decomposition of these organic sources resulted in the appearance of the emission peaks before 80 DAF. After 80 DAF, sources other than added rice straw that contributed to the peak were found only in the planted pots. This peak was, therefore, attributed to the release of organic material from rice plants. Apart from its direct stimulation of CH4 production and emission, the application of straw also indirectly enhanced the amount of CH4 emission. The application of straw would stimulate the development of conditions which lead to the stimulation of CH4 production and, hence, its emission. In addition, it was noted that methanogenic bacteria did not have any preference for the substrates derived from added rice straw.

Potentials (N. H. Batjes and E. M. Bridges, Eds), pp.

81-92. WISE Report 2, ISRIC, Wageningen. Duxbury J. M., Harper L. A. and Mosier A. R. (1993) Contributions of agroecosystem to global climate change. In Agricultural Ecosystem Effects on Trace Gasses and Global Climate Change (L. A. Harper, A. R. Mosier, J. M. Buxbury, D. E. Rolston, G. A. Peterson, P. S. Baenziger, R. J. Luxmoore, D. M. Kral and J. M. Bartels, Eds), pp. 1-18. American Society of Agronomy, Madison. Glascock R.F. (1954) Isotopic Gas Analysis for Biochemists. Academic Press, New York. Inubushi K., Ahori K., Matsumoto S., Umebayashi M. and Wada H. (1989) Methane emission from the flooded paddy soils to the atmosphere through rice plant. Japanese Journal of Soil Science and Plant Nutrition 60, 318-324 (in Japanese with English summary).

Inubushi K., Muramatsu M. and Umebayashi M. (1994) Effect of incorporation-timing of rice straw on methane emission from paddy soil. Japanese Journal of Soil Science and Plant Nutrition 65, 22-26 (in Japanese with English summary). Intergovernmental Panel on Climate Change (1992) Climate change. In The Supplementary Report to the IPCC Scientific Assessment (J. T. Houghton, B. A. Callender and S. K. Varney, Eds). Cambridge University Press, Cambridge. Jermsawatdipong P., Murase J., Prabuddham P., Hasathorn Y., Khomthong N., Naklang K., Watanabe A., Haraguchi H. and Kimura M. (1994) Methane emission from plots with differences in fertilizer application in Thai paddy fields. Soil Science and Plant Nutrition 40, 63-71.

Nugroho S. G., Lumbanraja J., Suprapto H., Sunyoto, Ardjasa W. S., Haraguchi H. and Kimura M. (1994) Methane emission from an Indonesian paddy field subjected to several fertilizer treatments. Soil Science and Plant Nutrition 40, 275-281.

Sass R. L., Fisher F. M. and Harcombe P. A. (1990) Methane production and emission in a Texas rice field. Global Biogeochemical Cycles 4, 47-68.

Acknowledgements-We

thank Professor E. Wada and Dr

T. Miyajima of the Center for Ecological Research, Kyoto University and K. Yagi of the National Institute of AgroEnvironmental Science for the use of the GCICIIRMS. The ‘3C-labeled rice straw was provided and analyzed by Professor T. Yoneyama of the National Institute of AgroEnvironmental Science. We thank N. Yasuda of Mie Agricultural Research Center for the analysis of total carbon and nitrogen of the soil and plant samples. The soil used in this study was provided by Dr H. Obata of the Faculty of Bioresources, Mie University. This work was supported partly by the Ministry of Agriculture, Forest and Fisheries Research Fund “Managing the Agro-Forest and Marine Ecosystems to Control Global Change”.

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Craig H. (1953) The geochemistry of the stable carbon isotope. Geochimica ei Cosmochimica Acta 3, 53-92. Chidthaisona A. and Watanabe I. (1997) Chanaes in concentration- and 613C values of boil-trapped-CH4 and COz in flooded rice soil. Biology and Fertility of Soils 24,70-75.

Denier van der Gon H. A. C., Neue H. U., Lantin R. S., Wassmann R., Albert0 M. C., Aduna J. B. and Tan M. J. P. (1993) Controlling factors of methane emission from rice fields. In World Inventory of Soil Emission

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Uzaki M., Mizutani H. and Wada E. (1991) Carbon isotope composition of methane from rice paddies in Japan. Biogeochemistry 13, 1599175. Watanabe A., Katoh K. and Kimura M. (1993a) Effect of rice straw application on CH4 emission from paddy fields. I. Effect of weathering of rice straw in the field during off-crop season. Soil Science and Plant Nutrition 39, 701-706.

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Yagi K. and Minami K. (1990) Effect of organic matter application on methane emission from some Japanese paddy fields. Soil Science and Plant Nutrition 36, 599610.

Yagi K., Tsuruta H., Minami K., Chairoj P. and Chotikul W. (1994) Methane emission from Japanese and Thai

Methane formation and emission from soil with ‘3C-labeled rice straw paddy fields. In (X4 and N20: Global Emissions and Controls from Rice Fields and Other Agricultural and Industrial Sources (K. Minami, A. Mosier and R. Sass, Eds), pp. 41-54. National Institute of AgroEnvironmental Sciences, Tsukuba.

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Yoneyama T., Fukuda M. and Kouchi H. (1989) Partitioning of carbon, nitrogen, phosphorus, potassium, calcium and magnesium in a semidwarf high-yielding rice variety: comparison with a conventional Japonica variety. Soil Science and Plant Nutrition 35,43-54.