Methane emissions from five paddy fields with different amounts of rice straw application in central Hokkaido, Japan

Abstract

Rice paddy fields are a major source of methane (CH₄) emissions, a potent greenhouse gas. We assessed CH₄ emissions from five existing paddy fields farmed in a snowy temperate region in central Hokkaido, Japan. All fields had continuous flooding and a paddy–fallow–paddy (rice) crop rotation system, but with different amounts of rice straw application. The rice straw application rate in the fields ranged from 0 to 219 g dry matter m⁻². CH₄ emission increased with increasing amounts of rice straw. A peak in CH₄ emission at the end of the reproductive stage was observed in all fields receiving rice straw. When continuous flooding was interrupted by drainage for harvesting, emissions from all fields also dropped quickly. Total CH₄ emissions ranged from 4.04 to 40.8 g CH₄-C m⁻² per growing season. We found that CH₄ emissions (g CH₄-C m⁻² per g dry matter) as per unit (dry matter) of rice straw applied in this study were significantly (P < 0.05) higher than those of calculated reported values, presumably because of the retardation of straw decomposition rates during the winter fallow. There was a significant correlation between rice straw carbon application (SCA) rate and total CH₄ emission in continuously flooded fields (CH₄ emission [g C m⁻² per growing period] = 0.486 × SCA [g C m⁻²] − 1.644, r ² = 0.884, P < 0.05), and emissions were 2–10-fold greater than from fields with no rice straw. The results indicate that the presence of rice straw has a significant influence on CH₄ emissions from paddy fields.

Key words:

• continuous flooding
• methane emission
• rice straw
• temperate region

INTRODUCTION

Rice farming is now believed to be the leading anthropogenic source of methane (CH₄) (Jacobson 2005). CH₄ is an important greenhouse gas, with a 23-fold higher global warming potential (GWP) than carbon dioxide (CO₂) over a 100-year time horizon (Intergovernmental Panel on Climate Change 2001). CH₄ has been reported to account for 95% of total CO₂-equivalent emissions from paddy fields (Naser et al. 2005). Recent studies have reported global annual CH₄ emissions from paddy fields to be 53 Tg CH₄ (Cao et al. 1998), 25–54 Tg CH₄ (Mosier et al. 1998) and 33–49 Tg CH₄ (Neue and Sass 1998), accounting for 4–9% of the total emission of 598 Tg CH₄ (Intergovernmental Panel on Climate Change 2001). This amount would be required to be reduced stabilizing the concentration of atmospheric CH₄ for (Intergovernmental Panel on Climate Change 1990; 1992). Irrigated rice is one of the few major CH₄ sources that is manageable and is, therefore, likely to be a critical focus of mitigation efforts (Sass and Fisher 1997).

CH₄ emissions from paddy fields are regulated by a complex set of biogeochemical characteristics in flooded soils, and are influenced by agricultural management practices (Neue 1997; Neue et al. 1990; Sass et al. 1992; Wassmann et al. 2000b; Yagi et al. 1996). It has also been reported that the addition of plant residues/straw to highly reduced soils enhances CH₄ emissions (Bossio et al. 1999; Cicerone et al. 1992; Liou et al. 2003; Sass et al. 1991; Wassmann et al. 1996a; Yagi and Minami 1990). Using experimental data collected from different areas of the world, Denier van der Gon and Neue (1995) have developed a relationship between CH₄ emissions and added organic matter.

Table 1 Some physical and chemical characteristics of the investigated paddy field soils (0–10 cm depth) in Mikasa, central Hokkaido, Japan

Site	Soil type	Field area (10^4 m^2)	Soil pH (H2O)	Particle size distribution (%)			Soil texture	Total-C (g kg^−1)	Total-N (g kg^−1)
				Sand	Silt	Clay
S1	GL	0.26	5.78	15.2	51.0	33.8	SiC	2.26	28.5
S2	GLP	0.26	6.03	17.3	46.8	35.9	SiC	3.44	47.0
S3	GL	0.26	5.78	15.2	51.0	33.8	SiC	2.26	28.5
S4	PG	0.20	5.39	28.1	41.4	30.5	LiC	1.67	17.1
S5	BL	0.32	5.40	13.1	50.5	36.4	SiC	2.87	34.9
GL, Gray Lowland soil; GLP, Gley Lowland soil with burying peat soil; PG, Pseudogley; BL, Brown Lowland soil.

The practice in central Hokkaido is, in general, to leave rice straw on paddy fields after harvest in autumn and to incorporate the straw into the soil in the following spring by plowing. Rice straw is also applied on purpose as an organic fertilizer (Watanabe et al. 1995). Central Hokkaido has a cold climate with a long period of snow cover from late November to early April. During the winter fallow (October to April), rice straw is generally left on the unplowed, snow-covered fields. Information is lacking on the release of CH₄ during the rice growing period under these conditions, that is, variations in agricultural management, decomposing organic materials and a temperate snowy climate. Thus, field investigations were carried out to assess the CH₄ emissions from paddy fields of central Hokkaido under continuously flooding conditions, with an emphasis on rice straw management practices.

MATERIALS AND METHODS

Site description and field management schemes

Field investigations were carried out from May to September 2004 at Mikasa (43°14′ N, 141°49′ E), located in central Hokkaido, a major rice growing area of Japan. We selected five fields, giving emphasis to the influence of added carbon (C) in the form of rice straw under continuous flooding conditions. The fields in Mikasa contained various types of mineral soils (Gray Lowland soil, Gley Lowland soil with burying peat, Pseudogleys and Brown Lowland soil). All of the fields were managed under single cropping in annual and had a paddy–fallow–paddy crop rotation system. Combine harvesters left short pieces of rice straw on the soil surface as spreading, which was incorporated into the soil the following spring (early May). We observed that a variable amount of rice straw was leftover on the fields. Differences in the amount of rice straw left on the fields from the previous year's rice crop resulted from ease of removal. Part or all of the rice straw of previous crops was taken out from the fields and used as bedding materials or feed for livestock. Around Mikasa, there are livestock farms and rice paddy farmers carried out rice straw to sell to livestock farmers, but left some rice straw when rainfall made transport difficult. Rice straw was present on fields S2 to S5 in differing amounts, while field S1 served as a no rice straw control. All fields were continuously flooded. All fields were drained for harvest at the end of the growing season. Physical and chemical properties of the studied fields are presented in Table 1. Detailed information on the amount of straw on the fields and other management practices are presented in Table 2.

CH₄ emission determination

A closed-chamber method (Yagi et al. 1991) was used to collect gas from the experimental fields. Immediately after transplantation, four plastic plates (the base of a gas chamber) were installed in the waterlogged soil for the chambers to be set on. Transparent, rectangular gas-sampling chambers (60 cm × 30 cm × 100 cm) were constructed using 5-mm-thick acrylic sheets and placed over the rice plants, covering four hills in the paddy fields in each chamber site. To allow pressure adjustments in the chamber during gas sampling, a lightweight plastic bag was affixed inside. To measure the inside temperature, a digital thermometer was attached inside the chamber. A silicon tube with a three-way stopcock was also attached to each chamber for gas sampling. To avoid soil disturbance during gas collection, all observations were made from boardwalks, which were constructed across each sampling site. Sampling was carried out between 10:00 to 15:00 hours. For each field, sampling was replicated at three sites. At each sampling time, gas was sampled at 0, 10 and 20 min using a 25 mL polypropylene syringe and transferred into a 20 mL vacuum glass vial. CH₄ concentrations in the collected samples were analyzed using a gas chromatograph equipped with a flame ionization detector (GC-8A, Shimadzu Corporation, Kyoto, Japan). Soil temperature was also measured at a depth of 3 cm.

Table 2 Summary of management practices for paddy fields in Mikasa, central Hokkaido, Japan

Site	Water regime	Straw leftover/applied				N Fertilizer (kg ha^−1)	Rice variety	Dates of
		Dry wt (g m^−2)	C conc. (%)	C amount (g C m^−2)	N conc. (%)			Trans- planting	Final drainage	Harvest
S1	CF	–	–	–	–	80	Nanatsuboshi	22-May	27-Jul.	27-Sep.
S2	CF	80	36.4	29.1	0.93	77	Kirara 397	22-May	27-Jul.	10-Sep.
S3	CF	105	38.7	40.5	0.71	80	Kirara 397	22-May	27-Jul.	15-Sep.
S4	CF	190	38.2	72.7	0.77	81	Kirara 397	28-May	3-Aug.	17-Sep.
S5	CF	219	39.2	85.9	1.15	81	Kirara 397	29-May	3-Aug.	18-Sep.
CF, continuously flooded; conc., concentration; wt, weight.

CH₄ emissions were calculated from the increase or decrease of gas concentration in the gas-sampling chamber over time using the following equation:

where F is the gas emission; ρ is the density of gas at the standard condition (CH₄ = 0.716 kg m⁻³); V (m³) and A (m²) are the volume and bottom area of the chamber, respectively; Δc/Δt (10⁻⁶ m³ m⁻³ h⁻¹) is the gas concentration change in the chamber during a given period; T is the absolute temperature (K); and α is the conversion factor for CH₄ to C (12/16). Negative values indicate CH₄ uptake from the atmosphere. Total CH₄ emission during the cropping season was calculated by successive linear interpolation of average gas emissions on the sampling days, assuming that gas emissions followed a linear trend during the periods when no sample was taken:

where, R_i is the mean gas emission (mg m⁻² day⁻¹) of the two sampling times, D_i is the number of days in the sampling interval, and n is the number of sampling times.

Soil and plant sample analysis

Initial soils from 0–10 cm depth in each field were sampled using a stainless steel auger, air dried for more than 3 weeks in the laboratory, and then passed through a 2 mm sieve to remove coarse materials. Particle size distributions were measured using the pipette method. Soil pH was measured using a glass electrode pH meter (HORIBA pH meter F-8, Kyoto, Japan) in a supernatant suspension of 1:2.5 soil : water mixture. To record the amounts of residues from the previous year's crop, rice straw for each field was collected by hand from three 1-m² quadrats and dried in an oven at 70°C for 3 days. Straw was considered as the above ground harvested parts of the rice plant excluding grain; however, intact stubble 2–3 cm was not considered. Dried soil and plant samples from each field were ground by hand with a mortar and pestle to determine total carbon (C) and nitrogen (N) concentrations with a C–N analyzer (vario MAX CNS, Elemental, Hanau, Germany).

Statistical analysis

Statistical differences were examined using Tukey's multiple comparison tests and simple linear regression analyses were done using Excel Statistics version 4.0 (Esumi Company, Tokyo, Japan). To compare the amount of CH₄ emissions per unit of rice straw applied in this study with reported calculated values, t-tests for unpaired comparisons was done using KyPlot version 4.0 (KyensLab Incorporated, Tokyo, Japan).

RESULTS

In Mikasa, the average air temperature after harvest to before the first snowfall (October–November) was 8.8°C (range: 1.2–15.3°C) (Fig. 1a). During the snowy period (late November–late April) the average air temperature was −1.1°C (range: −9.8 to 10.8°C) (Fig. 1a). Snow depth averaged 67 cm (range: 0–131 cm). During the rice growing period (May–September), the total precipitation was 591 mm (Fig. 1b), accounting for 49% of the annual total precipitation (Sapporo District Meteorological Observatory 2005). The mean air temperature during the rice growing period was 17.8°C (range: 8–27°C), which was 5.2°C lower than the average soil temperature at a depth of 3 cm.

Seasonal variations in CH₄ emissions from the paddy fields are shown in Fig. 2. The incorporation of rice straw increased CH₄ emissions during the rice growing season and emissions increased with increasing rice straw application. Within 1 week of transplantation, CH₄ emissions from fields S4 and S5, which received higher amounts of rice straw, were 5.75 and 4.68 mg CH₄-C m⁻² h⁻¹, respectively. These emissions were higher than those of 0.15 and 0.41 mg CH₄-C m⁻² h⁻¹ in fields S2 and S3, respectively, which received lower amounts of rice straw. A late-season peak in CH₄ emissions at the end of the reproductive stage was observed from all

Figure 1  Climate conditions of study area during winter-fallow and growing period.

Figure 2  Seasonal variation in CH4 emission rates from paddy fields in Mikasa during growing season.

fields receiving rice straw. When continuous flooding was interrupted by drainage for harvesting, the emissions from all fields also dropped quickly.

Total CH₄ emissions in the fields containing rice straw ranged from 243% to 1000% of the field with no rice straw application (S1). The total CH₄ emissions in S4 and S5, with higher amounts of rice straw, were 38.9 and 40.8 g CH₄-C m⁻², respectively, which were significantly (P < 0.01) greater than those in the other fields (Table 3). We compared the amount of CH₄ emissions as per unit (dry matter) of rice straw applied in rice fields in the present study with previously reported values (Table 4). The average value of CH₄ emissions in the present study was 2.4-fold higher than the reported average value (P < 0.05). We carried out regression analyses for rice straw C application (SCA) rate and CH₄ emissions. Total CH₄ emission was significantly correlated with organic C application rate (y = 0.486x − 1.644, r ² = 0.884, P < 0.05) (Fig. 3).

DISCUSSION

CH₄ emissions increased with increasing amounts of rice straw, and were 2–10-fold higher than in fields with no residual rice straw. Similar trends have also been observed in rice paddy fields in Italy (Schütz et al. 1989), Texas, USA (Sass et al. 1990), Japan (Yagi and Minami 1990), the Philippines (Wassmann et al. 1996b) and Louisiana, USA (Kongchum et al. 2006). Moreover, Wang et al. (1992) found that incorporating rice straw (500–1,200 g dry matter m⁻²) into paddy fields increased CH₄ emissions two–ninefold, showing a linear relationship with the amount of straw incorporated. In our study, a gradual increase of CH₄ emissions resulting from the presence of rice straw (S4 and S5) was observed, with no early peaks. We found a fivefold increase in total CH₄ emissions with the addition of one-fifth the amount of straw compared with those of Bossio et al. (1999) in California, USA, where rice straw

Table 3 Daily average (± standard deviation) fluxes and total seasonal (± standard deviation) CH4 emissions as affected by the addition of rice straw

Site	Water regime	Straw leftover/ applied (g m^−2)	Methane emissions		CH4 emission increment (%) compared with S1
			Daily average* (mg CH4–C m^−2 day^−1)	Total seasonal* (g CH4–C m^−2)
S1	CF	–	31.2 ± 36.2a	4.04 ± 1.85a	–
S2	CF	80	131 ± 109a	9.84 ± 4.69a	144
S3	CF	105	125 ± 117a	9.09 ± 2.30a	125
S4	CF	190	445 ± 430b	38.9 ± 13.0b	863
S5	CF	219	456 ± 303b	40.8 ± 13.7b	910
*Values in a column followed by the same letter are not significantly different at P < 0.01. CF, continuously flooded; –.

Table 4 Comparison of CH4 emissions obtained per unit of rice straw application under continuous flooding conditions from paddy fields in Mikasa, central Hokkaido, Japan, with those reported from previous studies in various locations

Place	Location		Country	Rice straw applied		Total seasonal CH4 emission (g C m^−2)	CH4 emission obtained per unit of rice straw application (g C m^−2 g dry matter^−1)	Source^†
	Lat.	Long.		Season*	Rate (g m^−2)
Texas	29°57′N	94°30′W	USA	on_	1,200	23.3	0.019	1
Varanasi	25°3′N	83°05′E	India	on_	1,000	10.2	0.010	2
Lu Tsao, Chia-Yi	23°42′N	120°27′E	Taiwan	on_	980	3.95	0.004	3
Maligaya, Nueva-Ecija	15°67′N	120°88′E	Philippines	on_	400	71.4	0.179	4
Surin	14°94′N	103°73′E	Thailand	on_	310	21.4	0.069	5
Iksan	35°94′N	126°95′E	South Korea	on_	500	21.3	0.043	6
Yamagata	38°3′N	140°5′E	Japan	on_	600	25.4	0.042	7
Kamikawa, Hokkaido	43°52′N	142°28′E	Japan	on_	300	37.7	0.126	8
Mikasa, Hokkaido	43°14′N	141°49′E	Japan	–	–	4.04	–	TS
Mikasa, Hokkaido	43°14′N	141°49′E	Japan	off_	80	9.84	0.123	TS
Mikasa, Hokkaido	43°14′N	141°49′E	Japan	off_	105	9.09	0.087	TS
Mikasa, Hokkaido	43°14′N	141°49′E	Japan	off_	190	38.9	0.205	TS
Mikasa, Hokkaido	43°14′N	141°49′E	Japan	off_	219	40.8	0.186	TS
^†1, Sass and Fisher (1997); 2, Singh et al. (1996); 3, Liou et al. (2003); 4, Corton et al. (2000); 5, Jermsawatdipong et al. (1994); 6, Lee et al. (1997); 7, Kumagai and Konno (1998); 8. Goto et al. (2004). TS, this study;
*on_, rice straw application in spring just before transplanting; off_, rice straw application in autumn just after the harvest of the previous crop; –.

was chopped and incorporated into the soil to a depth of 15 cm 6 months before the growing season. This difference might result from differences in straw decomposition rates during the winter fallow, methods of addition or variations in climate. Bossio et al. (1999) also suggested that autumn (October) incorporation and over-winter decomposition of the straw, despite low temperatures, is likely to be responsible for the lack of an early season peak in CH₄ emissions during the growing season (May–September). The amount of CH₄ emissions (g CH₄-C m⁻² g dry matter⁻¹) as per unit (dry matter) of rice straw applied in this study were significantly (P < 0.05) higher than those of calculated reported values (Table 4). This is presumably because of the deep snow cover, low temperature and unplowed conditions, which may have retarded the decomposition of rice straw over the winter fallow. With rice straw applied during the off-season, Kondo and Yasuda (2003) found a lower rate of decomposition (26%) under cool temperate conditions. Retardation of straw decomposition rates at low temperatures and unplowed conditions have previously been reported (Kisselle et al. 2001; Sain and Broadbent 1997). The relationship between the amount of rice straw and seasonal total CH₄ emissions under continuously flooded conditions reported in the present study is supported by many other researchers (Wang et al. 1992; Watanabe et al. 1995; Yagi and Minami 1990). Our results suggest that the environmental conditions of central Hokkaido did not limit, but rather favored, CH₄ emissions during the rice growing season because of the presence of rice straw.

Figure 3  Relationship between the amount of organic C applied and CH4 emission in paddy fields during growing season.

Conclusions

Rice straw application had a significant influence on CH₄ emissions in continuously flooded fields in central Hokkaido, Japan. There was a significant correlation between rice straw C application rate and total CH₄ emission during the rice growing season. Our results suggest that the presence of rice straw has a significant influence on CH₄ emissions from paddy fields in a snowy, temperate region. Comprehensive field studies are needed to establish CH₄ mitigation strategies for paddy fields in this climate.

ACKNOWLEDGMENTS

his study was partly supported by the Global Environment Research Program of the Ministry of the Environment of Japan (No. S3–3a).