Rice plants reduce methane emissions in high-emitting paddies

Introduction

The role of rice in methane (CH₄) emissions changes according to emission levels. Rice performs three key functions related to CH₄ emissions: i) providing a CH₄ pathway through a well-developed system of intercellular air spaces (aerenchyma), ii) providing a substrate for methanogenesis, and iii) oxidizing CH₄ in rhizosphere by supporting O₂ counter-transport through aerenchyma system^1–6. The level of contribution of these functions varies with the overall emissions, and the total amount of CH₄ emitted to the atmosphere is thus a balance between CH₄ production and oxidation⁶. To the best of our knowledge, rice enhances overall CH₄ emissions from paddy fields. In addition, previous studies have mostly disregarded a potential impact of overall emission levels on the role of rice in enhancing or suppressing CH₄ emissions.

Cicerone and Shetter measured CH₄ emissions with a closed chamber on the water surface of a paddy field, and found that 4 hours after starting measurement, CH₄ concentration was 290 ppm over rice plants and only 4 ppm over open water¹. Further studies have revealed that CH₄ produced under methanogenesis diffuses through the soil, which is oxidized by the surface barrier before reaching the atmosphere^2,7. Rice absorbs diffused CH₄ from its roots and emits CH₄ through aerenchyma^3,5,7. Therefore, an established theory has emerged that CH₄ is not emitted from the soil without rice. Other recent studies have provided additional evidence that the primary source of CH₄ is current-season photosynthates—specifically, root exudates or decaying tissues^8–11. This results in CH₄ emissions that peak during the late stage of rice growth. Thus, the presence of rice plants has been determined to be the cause of CH₄ emissions in paddy fields.

Wassmann et al.¹² measured CH₄ emissions on the water surface of a paddy field amended with organic matter. They found that organic matter incorporation increased total CH₄ emission levels from 27–90 to 160–240 kg CH₄ ha⁻¹ crop⁻¹, and ebullition increased from 15–23% to 35–62%, respectively¹². Since it is known that organic matter incorporation causes CH₄ emissions to peak during the early stages of rice growth, when the rice is still small and the aerenchyma is not well developed, the results of Wassmann et al.¹² should be closely examined to determine whether ebullition increased with total emissions. According to the 2006 Intergovernmental Panel on Climate Change (IPCC) guidelines, CH₄ emissions for 100 days of rice cropping are 130 kg CH₄ ha⁻¹ crop⁻¹; however, emissions were observed at almost twice this value by Wassmann et al.¹² Average CH₄ emissions from rice paddies in Asia without and with organic matter incorporation ranged from 16 to 200 and 250 to 500, respectively¹³. Although ebullition has been little studied¹⁴, ebullition must occur at high emission levels. Furthermore, in the study by Wassmann et al.¹², twin CH₄ emissions peaks appeared, with an early peak corresponding to the organic matter amendment and a later peak corresponding to rice-originated substrate¹². An alternate interpretation of these results is that the twin peaks could reflect the oxidation performance of rice, since CH₄ oxidation is known to increase with rice growth, up to the maximum tillering stage, and then decrease¹⁵.

We monitored CH₄ emissions in the paddy fields for 5 years (total 15 crops) in triple rice cropping fields in the Mekong Delta, Vietnam. The CH₄ emission level was an order of magnitude higher than IPCC standards. These high CH₄ emissions suggest that ebullition must have been occurring. We thought there was a possibility that rice plants decrease overall methane emission based on the extremely high emission levels. It will be epochal if present the fact that rice plants decrease overall methane emission in paddy fields.

Results and discussion

We compared CH₄ emissions with (Rice) and without rice (No Rice) on the water (soil) surface of paddy fields. Our results showed that rice decreased CH₄ emissions by half of No Rice. The effect of rice was large even in the early growth stage because of the high plant density (230 kg ha⁻¹ in dry weight; approximately 3 cm interval) and the rapid growth in the tropical climate (see Figure 1, Figure 2). Although the high CH₄ emission is shown in the first week, it is considered to be caused by the erratic character of “ebullition”. In fact, there was no difference in the emission between Rice and No Rice on both the 7th and 9th day (see Figure 1). Complete, unprocessed data are available on figshare¹⁶. There was no marked CH₄ emissions peak in the late-stage of the rice mentioned in previous studies^8–11. This suggests that the amount of methanogenesis from the rice-derived substrate is relatively small. Note the high emission levels (500–1400 kg CH₄ ha⁻¹ crop⁻¹). These findings suggest that total CH₄ emissions are reduced by oxidation or methanogenesis inhibition associated with growing the rice plant.

Figure 1. Seasonal CH₄ emissions in paddy fields.

Average CH₄ emissions of rice-planted areas and no-rice-planted areas in triple cropping rice fields in the Mekong Delta. Winter–Spring season (2017). The paddy fields did not receive rice straw incorporation. Error bars are s.d. (n = 3).

Figure 2. Cumulative CH₄ emissions in paddy fields.

Cumulated CH₄ emissions of rice-planted areas and no-rice-planted areas in triple cropping rice fields in the Mekong Delta. The number in the legend relates to the fields.

We also found that the reduction rate of CH₄ emissions increased with the growth of the rice plant. The CH₄ reduction rate was calculated using a moving average of five values by the following formula.

The rate peaked at maximum tillering stage, then bottomed at after heading stage, and then recovered (see Figure 3). The decrease around the heading stage was caused partially by an increase of emissions in rice-planted areas, and mainly by the erratic emissions in the unplanted area.

Figure 3. Reduction rate of CH₄ emissions.

The CH₄ reduction rate was calculated by (No Rice – Rice)/No Rice.

No consensus has yet been reached on the extent to which methanotrophs or rice roots attenuate CH₄ emissions¹⁷. Using the N₂ atmosphere technique, CH₄ oxidation ratios have been found to be around 40% on average and were relatively stable throughout the rice growing season¹⁷. However, genuine CH₄ oxidation, as measured using inhibitors, tend to decrease with rice growth, and the reduction rate for total CH₄ emissions can reach up to 20%¹⁷. Although, most of those studies assume plant-mediated transportation¹⁷, in general, our results roughly matched the results using the N₂ atmosphere technique. This suggests that the reduction in CH₄ emissions is not due to genuine oxidation and is more likely to be due to methanogenesis inhibition by oxygen from aerenchyma, which lasts until harvesting¹⁸.

We found high CH₄ emissions in unplanted paddy fields of which not incorporate organic materials. Despite this lower input of methanogenesis substrate, CH₄ emission levels were 12 times higher than the IPCC guidelines. The emission levels remained almost stable after reaching a maximum. This suggests that methanogenesis mainly depends on soil organic matter that has been accumulated from past rice crops. Prior studies have suggested that the contribution of soil organic matter to methanogenesis is small; however, these studies also indicated that higher emission levels tend to be associated with higher contribution rates of soil organic matter^8–11,19. Furthermore, a recent study found that large rice plants reduce CH₄ emissions compared to small rice plants in paddy fields with high soil C contents; instead, they show the opposite effect in paddies with low soil C contents²⁰. Thus, our results are consistent with prior studies that assume that emission levels are proportional to the amount of soil organic matter which can be a methanogenesis substrate.

High-emitting paddies of CH₄ emissions, which exist widely across tropical Asia, would have huge soil organic matter stock formed by sequential rice cropping under flooded conditions. For instance, the use of a rice variety which has better performance of methanogenesis inhibition in high-emitting paddies is very effective; the 10 % reduction is equivalent to 100% of methane emission in standard paddy fields. On the other hand, without rice plants, the methane emission from the existing soil organic matter stock will double by ebullition; the 100% increase is equivalent to 1000% of the standard paddies. This suggests that the future study for the soil organic matter stock map is critical.

Conclusion

Most studies of CH₄ emissions in paddy fields have been conducted in fields with low overall emission levels. Since the role of rice in CH₄ emissions varies according to the overall emission levels, these results cannot be appropriately generalized to rice paddies with high emission levels. The results of our study suggest that rice reduces emissions in high-emitting paddies. The emission levels are related to the amount of soil organic matter which can be a methanogenesis substrate; this suggests that the future study for the soil organic matter stock map is critical.

Methods

Study site

Experimental fields were in Tan Loi 2 Hamlet, Thuan Hung village, Thot Not district, Can Tho city, Vietnam. Farmers conduct triple rice cropping by direct seeding and full flooding. This district receives almost 2 months of a flood annually from the Mekong River. The flood decomposes rice straw underwater to the extent that it is no obstacle for seeding. Therefore, farmers start the rice cropping by leveling the fields, without incorporating rice straw. We observed CH₄ emissions in 18 paddy fields (26 × 17 m each) under several conditions for 5 years from September 2011. A preliminary study was conducted with the rice variety OM501 (suitable for the season) in the Summer-Autumn season of 2016 by the same methods and paddies of the present study (see Figure 4). In the present study, we used three fields for replication. We conducted the present study after the annual flood (4 November 2016–12 February 2017); these fields did not incorporate rice straw because of the period was after the annual flood.

Figure 4. Seasonal CH₄ emission in paddy fields.

Average CH₄ emissions of rice-planted areas and no-rice-planted areas in triple cropping rice fields in Mekong Delta. Summer–Autumn season (2016). The paddy fields received incorporated rice straw of the previous crop. Error bars are s.d. (n = 3).

Data availability

Raw data of this article is available from figshare: https://doi.org/10.6084/m9.figshare.6916277.v1¹⁶. Data are available under the terms of the Creative Commons Zero “No rights reserved” data waiver (CC0 1.0 Public domain dedication).