Forage rice varieties Fukuhibiki and Tachisuzuka emit larger CH₄ than edible rice Haenuki

ABSTRACT

To determine methane (CH₄) emission differences between edible and forage rice cultivars, we conducted a pot experiment in Yamagata, Japan to grow edible rice Haenuki, and forage rice Fukuhibiki (for feed rice) and Tachisuzuka (for whole-crop silage (WCS) rice) under similar soil and meteorological conditions. The total amounts of N, P, and K fertilizers applied for Fukuhibiki and Tachisuzuka were 1.7, 1.3, and 1.3 times, respectively, higher than those of Haenuki. CH₄ fluxes, and rice plant night respirations were measured once weekly or fortnightly. As per the results, for the whole growth period, shoot height, maximum and productive tiller numbers, and plant biomass were significantly different among the three rice varieties. The rice growth period for Haenuki and Fukuhibiki was 107 days after transplanting (DAT), while that for Tachisuzuka was 135 DAT. The highest peak of CH₄ flux occurred around the heading stage for the three varieties. Consistently significant (P < 0.05) or obvious (P < 0.1) positive correlations between CH₄ flux and night respiration among the varieties were observed from 9 weeks after rice transplanting to harvest, indicating that much of the CH₄ flux was from newly produced root exudates and plant debris through plant photosynthesis. The cumulated CH₄ emissions during the same growth period, 106 DAT, from Haenuki, Fukuhibiki, and Tachisuzuka were 55.36, 77.46, and 78.40 g C m⁻², respectively. Additionally, Tachisuzuka emitted 25.11 g C m⁻² more CH₄ between 106–134 DAT. The final cumulated CH₄ emissions from Fukuhibiki and Tachisuzuka were 39.9% and 87.0% higher than that from Haenuki, respectively, throughout their growth period.

KEY WORDS:

• Forage rice
• methane emission
• night respiration
• rice variety
• whole-crop silage (WCS)

1. Introduction

Rice (Oryza sativa L.) is a staple of the Japanese diet, and rice production is important to the food supply in Japan. However, occasioned by changes in the Japanese diet with people consuming more milk, meat, and bread than rice (Smil and Kobayashi 2012), rice cultivation area in Japan decreased from 3.3 million ha in the 1960s to 1.6 million ha in 2015 leading to an increase in the area of abandoned agricultural land (Fig. S1). Rice consumption decreased in Japan from the 118 kg per person per year peak in 1962 to 55 kg in 2015 (MAFF 2017). On the other hand, the percentage of self-sufficient food supply was about 40% on a calorie basis in Japan during the recent two decades. About 75% of the domestic demand of feed for livestock is dependent on import from overseas (Kato 2008; Ozawa and Yoshida 2009). To increase the percentage of self-sufficient food supply and improve the domestic demand of livestock feed in Japan, production of forage rice is increasing in Japan quickly since the last decade (Fig. S2). The harvest area of forage rice was increased to 131 kha in 2016, and this was about 8.2% of the total rice harvest area (1.6 Mha) in Japan. Currently, forage rice production is being promoted by the government in Japan as it has the potential to, firstly stop the increase of abandoned agricultural land since 1970s, which if not addressed will prove a serious menace to the basis of Japanese agriculture and farm structure, secondly improve self-sufficiency in food, which is a more important issue for the national strategy. The production of forage rice will be increasing continually in the future (MAFF 2017).

Normally, there are two types of forage rice, one is feed rice whose cultivars are harvested as edible rice and only the grain is used to feed domestic fowl and animal as concentrate feed, the other is whole-crop silage (WCS) rice, whose cultivars are harvested at the yellow ripening stage, and the whole rice plant including panicles, leaves, and stems are conditioned into silage, and used to feed cattle (Kato 2008; Ozawa and Yoshida 2009; Matsushita et al. 2011). As shown in Fig. S2, the harvest area of feed rice is increasing more quickly than that of WCS rice in recent years. In 2015, 61.8%, 28.3%, and 9.9% of feed rice grains were for fowls, pigs, and cows, respectively, as concentrate feed in Japan (MAFF 2017). Since the forage rice is used to feed livestock, the grain weight and total above ground biomass of feed and WCS rice, respectively, are larger than those of edible rice. The fertilization rates of forage rice cultivars are also higher than those for edible rice.

After carbon dioxide (CO₂), methane (CH₄) is the most important greenhouse gas, responsible for approximately 20% of the anthropogenic global warming effect. Most of the atmospheric CH₄ is produced by the bacterial activities in extremely anaerobic ecosystems such as natural and cultivated wetlands, sediments, sewage, landfills, and the rumen of herbivorous animals. Rice paddy fields are considered as one of the most important sources of CH₄. During the past decades, there were many studies carried out around the world, especially in Japan to understand the CH₄ dynamics in rice paddies (Yagi and Minami 1990; Watanabe and Kimura 1998; Inubushi et al. 2003; Cheng et al. 2008; Itoh et al. 2011; Hayano et al. 2013; Win et al. 2016; Cheng et al. 2016; Nakajima et al. 2016). It is well known that there are different CH₄ emission potentials among different rice varieties (Huang et al. 1997; Wassmann and Aulakh 2000; Yao et al. 2000; Aulakh et al. 2002; Jia et al. 2006; Lou et al. 2008; Gutierreza et al. 2013; Zhang et al. 2015). However, the previous studies were mainly focused on the edible rice varieties. As forage rice has different morphological and physiological properties, cultivated practice, and usage compared to the edible rice, the CH₄ production, oxidation, and emission in rice paddies should be different. Recently, some studies were carried out in Japan to measure CH₄ emission from WCS rice with biogas slurry application (Riya et al. 2012; Win et al. 2014; 2016). However, there was no research simultaneously comparing the differences of CH₄ emission between forage and edible rice varieties.

The objective of the present study is to determine the CH₄ emissions from three types of rice varieties (edible rice, forage rice for feed and forage rice for WCS) under similar soil and meteorological conditions.

2. Materials and methods

2.1. Experimental site, cultural practice, rice varieties, and soil

This research was conducted in 2016 at the Experiment Farm, Faculty of Agriculture, Yamagata University, Tsuruoka, Yamagata Prefecture, located in northeastern Japan (38° 44ʹ N, 139° 50ʹ E, 16 m elevation). According to the Japan Meteorological Agency database for Tsuruoka Meteorological Observatory (http://www.data.jma.go.jp/obd/stats/etrn/index.php), which is located inside of the Experiment Farm, Faculty of Agriculture, Yamagata University, the climate condition during rice growth season (June–October) in 2016 was hotter and sunnier than the historic average of 1981–2010 (Fig. 1). The daily average air temperature and sunshine times were 22.75°C and 5.52 h than those of historic average of 1981–2010 at 21.65°C and 5.46 h, respectively.

Figure 1. Daily sunshine time (■) and average air temperature (●) during the experiment period from 31 May to 13 October 2016 in Tsuruoka, Japan. The bold line and dish line crossed Air Temp and Sunshine time are the average values for 1981–2010 (Av. 30 years). Data was from Japan Meteorological Agency.

Three types of rice varieties, Haenuki, Fukuhibiki, and Tachisuzuka were used in this study. Haenuki is one popular edible variety grown widely in the local area, Yamagata Prefecture. Fukuhibiki is one Japonica type high-yield variety which was recommended as feed rice grown in Tohoku region (Fukushima 2012; Morita et al. 2012). Fukuhibiki is cultivated widely for piggery in Shonai area, Yamagata (Ozawa and Yoshida 2009). Tachisuzuka is a new rice cultivar with high straw yield and high sugar content for WCS use (Matsushita et al. 2011).

On 31 May 2016, seedlings of 3 types of rice varieties were transplanted to 12 plastic pots (19.5-cm inside diameter, 27-cm height, 0.2-cm thickness) filled with 6.0 kg alluvial soil (4.5 kg dry soil equivalent, water content was 25% in w/t), which was collected from rice field at the University Farm and light air-dried for sieving at 5-mm mesh size. The soil had been piled up outdoors for 2 years before being used, visible plant residues were absent. Each variety had four replications. The soil contained 28.5 g kg^–1 organic C and 2.90 g kg^–1 total N with a pH value of 6.3. Soil was air dried, sieved (5-mm mesh size) and mixed well before use. Before rice transplanting, 0.87 g CO(NH₂)₂ and 0.87 g KH₂PO₄ were mixed to the soil in each pot as basal fertilizers for Haenuki. The amounts of nitrogen (N), phosphorus (P), and potassium (K) of the basal fertilizers were 0.40, 0.20, 0.25 g pot^–1, respectively. While, the basal fertilizers for forage rice of Fukuhibiki and Tachisuzuka were 1.5 time higher than Haenuki. At 50 days after transplanting (DAT), we applied first added fertilization with 0.20 g N, 0.10 g P, and 0.13 g K pot^–1 for all varieties by 0.43 g CO(NH₂)₂ and 0.43 g KH₂PO₄. The second added fertilization (0.43 g CO(NH₂)₂ only) was carried out at 78 DAT for Fukuhibiki and at 92 DAT for Tachisuzuka. The total amounts of N, P, and K fertilizers applied for Fukuhibiki and Tachisuzuka were 1.7, 1.3, and 1.3 times, respectively, higher than those of Haenuki. The flooding water was maintained at about 5-cm depth throughout the entire experimental period.

2.2. Daily CH₄ flux and night respiration (CO₂ flux) measurement

CH₄ and CO₂ fluxes were collected with a cylindrical, transparent, acrylic, closed-top chamber (20.5-cm inside diameter, 100-cm height, and 0.3-cm thickness). The frequency of flux measurement is once weekly or fortnightly. During the sampling time of 30 min, we covered each pot with the chamber to capture gas exchanged between the pots and the atmosphere. At 0, 15, and 30 min after the chamber was placed, a gas sample of about 30 ml was drawn with a 30-ml plastic syringe through a capillary tube at the top of the chamber and injected into a 19-ml vacuum bottle with a rubber stopper and screw cap. The bottles were taken back to the laboratory, where the amounts of CO₂ and CH₄ were analyzed using an automated analysis system for three gases of CO₂, CH₄, and N₂O. This system consists of two gas chromatographs (GC-14B, Shimadzu, Kyoto, Japan), of which one has both a thermal conductivity detector (TCD) and a flame ionization detector (FID), and the other has an electron capture detector (ECD). This system can analyze 40 samples consecutively with a modified automated headspace sampler (HSS-2B, Shimadzu, Kyoto, Japan) (Sudo 2006). The flux was calculated from the increase in the gas concentration inside the chamber per square meter per hour (Cheng et al. 2006; 2008). We measured CH₄ fluxes at night time about 20:00–22:00, because the average of daily CH₄ flux from rice paddy was similar to the flux at day time 8:00–10:00 and night time 20:00–22:00 (Cheng et al. 2008; Minamikawa et al. 2015). The increase in CO₂ concentration in the chamber in the evening samples was used to calculate plant night respiration. The N₂O fluxes were detected at first gas sampling on 15 June (about 8 μg N m⁻² h⁻¹) only from denitrification process of initial nitrate after the moisture soil was flooded. After that, N₂O flux was not detected, so we did not include N₂O emission data in the paper.

2.3. Growth investigation, destructive sampling, and dry matter measurements

Plant height and tiller numbers of each rice hill (or pot) were measured once a week beginning 8 days after transplanting (DAT). At that time, top rice leaf greenness (SPAD value, indicating the amount of chlorophyll present) was measured using a SPAD-502 Plus chlorophyll meter (Konica Minolta Inc., Tokyo, Japan). Haenuki and Fukuhibiki were harvested at 107 DAT (on 15 September), and Tachisuzuka was harvested at 135 DAT (on 13 October 2016), respectively, depending on the physiological maturity. At harvest, we collected plants from the pots, carefully washed the soil from the roots in running water, and then separated the plants into stems, leaves, roots, and ears. The parts were then oven-dried at 70°C for 2 days and weighed (Cheng et al. 2009). For Haenuki and Fukuhibiki, much of the ears were lost to the field mouse invasion, so only the ears dry weight of Tachisuzuka was shown in this paper.

2.4. Statistical analyses

Analysis of variance (ANOVA) using SPSS 19 (SPSS Inc., Chicago, IL, USA) was used to determine the statistical significance among the three varieties. Least significant difference (LSD) values were calculated for each parameter for which significant differences were found at P < 0.05.

3. Results

3.1. Changes in rice plant height, tiller number, and SPAD value during entire experimental period

Rice growth period from transplanting to harvesting was about 107 DAT for both Haenuki and Fukuhibiki, and 135 DAT for Tachisuzaka. Tachisuzaka was 4 weeks late compared to Haenuki and Fukuhibiki. Plant height increased steadily until the grain-filling stage at about 85 DAT in Haenuki and Fukuhibiki and 113 DAT in Tachisuzuka (Fig. 2A). At harvest, the shoot height was 78.0, 82.2, and 111.7 cm for Haenuki, Fukuhibiki and Tachisuzuka, respectively. There was a significant difference in rice plant height among the three varieties (Table 1).

Table 1. The main rice growth parameters, dry weight of rice plant at harvest and the accumulated CH₄ emission during the experimental period among three rice varieties.

				Dry weight at harvest				Total CH4 emission
	Maximum tiller	Productive tiller	Shoot height at harvest	Ear	Leaf	Stem	Root	Same period	All period^a
Varieties	(no. hill^−1)		(cm)	(g hill^−1)				(g C m^−2)
Haenuki	50.8ab*	39.5a	78.0c	–^b	9.36c	57.11c	9.75c	55.36b	55.36c
Fukuhibiki	55.8a	41.5a	82.2b	–	13.75b	65.39b	15.24b	77.46a	77.46b
Tachisuzuka	46.5b	30.5b	111.7a	14.96	28.95a	162.3a	23.24a	78.40a	103.51a

^aTachisuzaka was harvested 4 weeks later than Haenuki and Fukuhibiki.

^bThe ear dry weight of Haenuki and Fukuhibiki were lost due to the damage by field mouse.

*Values within each column followed by the different letters indicated significantly different among the varieties (P < 0.05).

Figure 2. Changes in plant heights (A), tiller numbers of rice plants (B) and leaf color in SPAD values (C) among three rice varieties throughout the experiment. Bars indicate standard deviation (n = 4). Arrows indicate the days of fertilizer addition.

Rice tiller numbers reached a maximum at 43 DAT for Haenuki and Fukuhibiki, and at 50 DAT for Tachisuzuka (Fig. 2B). The maximum tiller numbers per hill were 50.8 for Haenuki, 55.8 for Fukuhibiki, and 46.5 for Tachisuzuka. At harvest, the productive tiller numbers per hill were 39.5 for Haenuki, 41.5 for Fukuhibiki, and 30.5 for Tachisuzuka (Table 1).

Top rice leaf greenness (SPAD value) increased to maximum value until 22–29 DAT, then declined sharply for Tachisuzuka, but slowly for Haenuki and Fukuhibiki. Later, the leaf greenness was improved by first added fertilization on 49 DAT for all varieties and second added fertilization on 78 DAT for Fukuhibiki and 92 DAT for Tachisuzuka (Fig. 2C). During the entire rice growth period, SPAD value of Tachisuzuka was mostly darker than that of Haenuki and Fukuhibiki. SPAD values of Tachisuzuka were larger than 30 during the entire rice growth period, even at the harvest stage (Fig. 2C),

3.2. Dry weight of rice plants at harvest

Though we lost the measure of ear dry weight of Haenuki and Fukuhibiki due to damage by field mouse, the data showed that there were significant differences of leaf, stem, and root dry weights among the three varieties (P < 0.05). Leaf, stem, and root dry weights of Tachisuzuka were significantly larger than those of Fukuhibiki. And those of Fukuhibiki were significantly larger than those of Haenuki (Table 1). The leaf biomass was 9.36, 13.75, and 28.95 g per hill for Haenuki, Fukuhibiki, and Tachisuzuka, respectively. Fukuhibiki and Tachisuzuka were 46.9% and 209.3% higher than Haenuki. The stem biomass was 57.11, 65.39, and 162.30 g per hill for Haenuki, Fukuhibiki, and Tachisuzuka, respectively. Fukuhibiki and Tachisuzuka were 14.5% and 184.2% higher than Haenuki. The root biomass was 9.75, 15.24, and 23.24 g per hill for Haenuki, Fukuhibiki and Tachisuzuka, respectively. Fukuhibiki and Tachisuzuka were 56.3% and 138.4% higher than Haenuki (Table 1).

3.3. Changes in daily CH₄ fluxes and accumulated emissions

The changes in daily CH₄ fluxes and the daily maximum temperature on the sampling day and night temperature at sampling time during the entire experimental period were shown in Fig. 3. CH₄ fluxes were not detected or were relatively low during the early period of rice growth (until 6 weeks after flooding or rice transplanting), but increased quickly to the highest peak on the 9–10th weeks after rice transplanting from Haenuki and Fukuhibiki pots. The highest peak of Tachisuzuka was detected on the 14th week after rice transplanting. The largest fluxes coincided with the heading stage for all varieties (Fig. 3A). There was no relationship between the daily CH₄ flux and daily maximum temperature or the night temperature at sampling time (Fig. 3A and B). The cumulated CH₄ emissions during the same growth period until 106 DAT were 55.36, 77.46, and 78.40 g C m⁻² from Haenuki, Fukuhibiki, and Tachisuzuka pots, respectively. Fukuhibiki and Tachisuzuka were significantly higher than Haenuki (Table 1, P < 0.05). Since Tachisuzuka growth period was 4 weeks longer than Fukuhibiki, the accumulated CH₄ emission from Tachisuzuka pots increased to 103.51 g C m⁻², which was also significantly larger than that of Fukuhibiki (Table 1, P < 0.05). Total CH₄ emissions from Fukuhibiki and Tachisuzuka pots were 39.9% and 87.0% higher than that of Haenuki throughout their entire growth season (Table 1 and Fig. S3).

Figure 3. Changes in the daily CH₄ flux (A) from the pots planted among three rice varieties and the daily maximum temperature and the night temperature at sampling time (B) throughout the experiment period. Bars indicate standard deviation (n = 4). Bold arrows in (A) indicate the heading days for each variety.

3.4. Changes in daily night respiration

Night respiration (the CO₂ mostly emitted from rice plant) ranged from 90.4 to 1034.1 mg C m^–2 h^–1 during the entire experimental period, and this parameter varied with growth stage. The highest peak of nighttime CO₂ respiration was found at the 10th week after rice transplanting for all the three varieties (Fig. 4A). Obviously, the small peaks before and after the highest one were as a result of the daily sunshine time (Fig. 4B). Nighttime CO₂ respiration was always significantly higher from Fukuhibiki and Tachisuzuka than that from Haenuki (P < 0.01). The average values of nighttime CO₂ respiration during entire experimental period were 412.6, 547.4, and 452.1 mg C m^–2 h^–1 from Haenuki, Fukuhibiki, and Tachisuzuka plants, respectively.

Figure 4. Changes in night respiration (CO₂ emission) of rice plants grown in the pots among three rice varieties throughout the experiment period (A). Bars indicate standard deviation (n = 4). The sunshine time on day for CO₂ and CH₄ fluxes sampling (B).

3.5. Relationships between CH₄ flux and night respiration

Except the first 4 times gas sampling, significant (P < 0.05) or obvious (P < 0.1) positive correlations were found between CH₄ flux and night respiration among the three varieties throughout after 9 weeks of rice transplanting (Fig. 5). The samples for Haenuki, Fukuhibiki, and Tachisuzuka were 6, 6, and10 times, respectively. Although the P values were 0.015, 0.056, and 0.073 for Haenuki, Fukuhibiki, and Tachisuzuka, respectively, the P value was 0.004 for the three varieties combined together.

Figure 5. Relationship between daily CH₄ flux and night respiration (CO₂ emission) among three rice varieties throughout after 9 weeks of rice transplanting. The samples for Haenuki, Fukuhibiki and Tachisuzuka were 6, 6, and 10, respectively.

4. Discussion

For cultivating forage rice, 1.5–2 times of conventional N fertilizer applications for edible rice have been recommended. In this pot experiment, we applied 1.5 times of basal fertilizers for forage rice Fukuhibiki and Tachisuzuka compared to Haenuki. Further, Fukuhibiki and Tachisuzuka had an extra one time added N fertilization. The total amounts of N, P, and K fertilizers applied for Fukuhibiki and Tachisuzuka were 1.7, 1.3, and 1.3 times, respectively, higher than those of Haenuki. As the results, the rice growth periods of Fukuhibiki and Haenuki were similar to those of field cultivations in Tsuruoka,Yamagata as reported by Morita et al. (2012), while the growth parameters of Tachisuzuka, such as shoot height, heading day, and total growth period, were also similar to Matsushita et al. (2011), for rice plants grown in Kanto and Shikoku regions, Japan. Therefore, the results obtained from our pots experiment could be representative of the rice grown under field condition. In this experiment, flooding water depth was maintained for the entire rice growth period until harvest, so total CH₄ emission from this pots experiment should be higher than those from the field condition where the middle and before harvest drainage are normally carried out (Itoh et al. 2011; Ma et al. 2013). However, the different CH₄ emission rates between the edible and forage rice varieties from this experiment could be of importance to explain the emission differences under field conditions.

Most CH₄ released from flooding paddies is released through the rice plants (Nouchi et al. 1990; Cheng et al. 2006). The sources of CH₄ production are from both older matter (e.g., native soil organic matter, incorporated organic material such as straw and manure) (Chidthaisong and Watanabe 1997; Nakajima et al. 2017) and new matter from plant growth (e.g., root exudates and plant debris) (Watanabe et al. 1999; Tokida et al. 2011). In this pot experiment, we used light air-dried soil (at 25% w/t moisture) without any visible plant residues for cultivating the three rice varieties, no obvious CH₄ flux was detected during early rice growth period until 43 DAT (Fig. 3A). The CH₄ flux increased sharply after 43 DAT and reached to the highest peaks at heading stage for Haenuki and Fukuhibiki, and the second highest peak for Tachisuzuka around the 9–10th weeks after rice transplanting. During the same period, the plant night respirations also recorded the highest peaks for the three rice varieties (Figs. 3 and 4). Also significant (P < 0.05) or obvious (P < 0.1) positive correlations were found between CH₄ flux and plant night respiration among the three varieties throughout 9 weeks after rice transplanting (Fig. 5). Since there were significant relationships between night respiration and gross or net rice photosynthesis rates (Sakai et al. 2001), the plant night respiration could be representative of the plant photosynthesis potential in day-time, which can become the carbon source to soil for CH₄ production (Cheng et al. 2008). Thus, our results implied that the source of CH₄ emissions from the three varieties was mostly from new matter through root exudates and plant debris, and not from older matter, such as native soil organic matter and incorporated organic material, which is consistent with our previous results (Cheng et al. 2006; 2008; Lou et al. 2008). Compared to Haenuki and Fukuhibiki, Tachisuzuka had multiple CH₄ flux peaks from the longer growth period under continuous flooded condition (Fig. 4). Though the highest peak of CH₄ flux for Tachisuzuka also occurred around the heading stage at 99 DAT, the plant night respiration on the same day was 506.4 mg C m⁻² h⁻¹, which was lower than the largest value (1034.1 mg C m⁻² h⁻¹) at 71 DAT, when the second highest peak of CH₄ flux occurred. The two largest CH₄ peaks at 64 and 99 DATs for Tachisuzuka could partly be attributed to, among other factors, the differences in CH₄ flux pattern and growth periods between Tachisuzuka and Fukuhibiki or Haenuki. However, we have no clear reference to support our observation.

In this experiment, we also found that there was a high relationship between dry weight of root biomass at harvest and accumulated CH₄ emission during the entire experimental period from the three varieties with four replicates (Fig. S4). The net CH₄ emission is determined by the balance between its production and oxidation in rice plant and paddy soil ecosystems (Inubushi et al. 2003). Large root biomass of rice can affect CH₄ production and oxidation simultaneously. It could be presumed that large root biomass increased CH₄ production by root exudates and increased CH₄ oxidation through delivery of oxygen from atmosphere to the rhizosphere (Cheng et al. 2008). The root biomass of forage rice, Fukuhibiki and Tachisuzuka were 56.3% and 138.4% higher than edible rice Haenuki at harvest. The cumulated CH₄ emissions were 39.9% and 87.0% higher from Fukuhibiki and Tachisuzuka pots than that of Haenuki in this study (Table 1). Though we had no effective method to detect the CH₄ oxidation among the three rice varieties, the increasing rate of root biomass, and the cumulated CH₄ emissions by forage rice shown above implied that large root biomass increased the net CH₄ emission (gross CH₄ production minus CH₄ oxidation). Since this pot experiment was conducted under flooded conditions throughout the entire experimental period, the balance between CH₄ production and oxidation would be different to the real forage rice cultivation in the fields. More measurements are needed to carry out under field condition in future.

In addition, developing domestic feed production can decrease atmospheric CO₂ emission from the transportation sector, though the cost of domestic forage crop production is higher than the importation cost in some cases. However, the CH₄ emission from forage rice production is still not considered by scientists and policy makers. To our knowledge, this study is the first to compare the CH₄ flux and emission from edible and forage rice, including feed rice and WSC varieties together. Our results showed that forage rice Fukuhibiki (for concentrate feed) and Tachisuzuka (for roughage feed as WCS) emitted larger CH₄ than edible rice Haenuki when they were grown under similar soil and meteorological conditions and managements, except the heavier fertilizations during the rice growth period (Fig. 3). The cumulated CH₄ emissions from Fukuhibiki and Tachisuzuka were 39.9% and 87.0% higher than that from Haenuki throughout their growth period (Table 1). This implied that promoting the forage rice production would increase more CH₄ emission from Japanese agriculture sector. But the comprehensive effect from decreasing CO₂ emission from transportation sector and possibly decreasing soil C loss from rice paddies should be considered together by life-cycle inventory analysis (Harada et al. 2007) and reflect into National Greenhouse Gas Inventory Report of Japan (NIES 2016).

5. Conclusions

Edible rice Haenuki, forage rice Fukuhibiki (for concentrate feed), and Tachisuzuka (for roughage feed as WCS) were cultivated under similar soil and meteorological conditions to compare the CH₄ emissions in pots experiment. The rice growth period was 107 DAT for Haenuki and Fukuhibiki, and 135 DAT for Tachisuzuka, which was 4 weeks later than Haenuki and Fukuhibiki. The highest peak of CH₄ flux occurred around the heading stage for the three varieties, while Tachisuzuka had multiple peaks of CH₄ fluxes from the longer growth period Since the soil used for the pot experiment had been piled up outdoors for 2 years before being used, the C source for CH₄ production was mostly from new matter through rice plant photosynthesis in the current-season. Our results indicated that forage rice Fukuhibiki (for concentrate feed) and Tachisuzuka (for roughage feed as WCS) emitted larger CH₄ than edible rice Haenuki. The cumulated CH₄ emissions were 39.9% and 87.0% higher from Fukuhibiki and Tachisuzuka, respectively, than that from Haenuki throughout their growth seasons. It should be noted that our results are based on pot experiment, field studies should be carried out in future.

Acknowledgments

Second author Samuel Munyaka Kimani would like to appreciate the African Business Education Initiative program for supporting his study in Japan.

Supplemental material

Supplemental data for this article can be accessed here.

Additional information

Funding

This work was partly supported by JSPS Grant-in-Aid for Scientific Research Grant Number (B) 26310304.