Rice cultivation reduces 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) oxidising 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, all studies have concluded that rice enhances overall CH₄ emissions from paddy fields; however, these studies have mostly disregarded overall emission levels and their potential impact 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 oxidised 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.

Results and discussion

In the present study, We compared CH₄ emissions on the water (soil) surface of paddy fields of with and without rice in paddy fields. Our results showed that rice decreased CH₄ emissions by half relative to paddy fields without rice (see Figure 1, Figure 2). Complete, unprocessed data are available on figshare¹⁶. There was no marked CH₄ emissions peak in the late-stage of the rice field. This suggests that the amount of methanogenesis from rice-providing substrate was 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 a triple cropping rice field in 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 a triple cropping rice field in Mekong Delta. The number in the legend relates to the plot.

We also found that the reduction rate of CH₄ emissions increased up to 60%, at maximum tillering stage, then decreased to 20% after the heading stage, and then finally recovered to 60% (see Figure 3). The decrease around the heading stage was caused partially by an increase of emissions in rice-planted fields, and mainly by the erratic emissions in the unplanted area (see Figure 1).

Figure 3. Reduction rate of CH₄ emissions.

Difference in CH₄ emissions between rice-planted areas and no-rice-planted areas, calculated by a moving average of five values. 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 was not due to genuine oxidation and was more likely to be due to methanogenesis inhibition by oxygen from aerenchyma, which lasts until harvesting¹⁸.

We found high CH₄ emissions in paddy fields that were not planted with rice and did not incorporate rice straw or other organic material. 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 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. Therefore, 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. Hotspots of CH₄ emissions, which exist widely across tropical Asia, would have huge soil organic matter stock formed by sequential rice cropping under flooded conditions.

To summarise, 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 generalised to rice paddies with high emission levels. The results of our study suggest that rice reduces CH₄ emissions in hotspot paddy fields.

Methods

Study site

Experimental fields were in Tan Loi 2 Hamlet, Thuan Hung village, Thot Not district, Can Tho city, Vietnam. Farmers conducted triple rice cropping by direct seeding and full flooding. This area receives almost 2 months of floods 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 levelling 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 main study (see Figure 4). In the present study, we used three such fields (managed with straw return under flooded conditions) for replication. We conducted the main experiment after the annual floods (4 November 2016–12 February 2017); these fields did not incorporate rice straw.

Figure 4. Seasonal CH₄ emission in paddy fields.

Average CH₄ emissions of rice-planted areas and no-rice-planted areas in a triple cropping rice field 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).