Although bubble ebullition through water in rice paddy fields dominates direct methane (CH4) emissions from paddy soil to the atmosphere in tropical regions, the temporal changes and regulating factors of this ebullition are poorly understood. Bubbles in a submerged paddy soil also contain high concentrations of carbon dioxide (CO2), implying that CO2 ebullition may occur in addition to CH4 ebullition. We investigated the dynamics of CH4 and CO2 ebullition in tropical rice paddy fields using an automated closed chamber installed between rice plants. Abrupt increases in CH4 concentrations occurred by bubble ebullition. The CO2 concentration in the chamber air suddenly increased at the same time, which indicated that CO2 ebullition was also occurring. The CH4 and CO2 emissions by bubble ebullition were correlated with falling atmospheric pressure and increasing soil surface temperature. The relative contribution of CH4 and CO2 ebullitions to the daily total emissions was 95–97% and 13–35%, respectively.
Understanding the dynamics of methane (CH4) and carbon dioxide (CO2) fluxes in rice paddy fields is crucial for improving the accuracy of estimating CH4 and CO2 emissions from global rice paddy fields. In particular, flooded rice paddies are considered to be a major source of anthropogenic CH4. Methane emissions from rice paddies in tropical Asian countries account for 90% of global annual CH4 emissions from rice paddies [
Methane produced in an anaerobic-flooded paddy soil is mainly transported to the atmosphere through the aerenchyma of rice plants [
Also, some of the CH4 and CO2 produced in rice field soil is directly emitted to the atmosphere through paddy water. In one study, when rice straw was applied to a paddy field, CH4 emissions via bubble ebullition from the soil accounted for 35–62% of total CH4 emissions [
Methane in paddy soil is transported to the atmosphere through paddy water by two pathways: (
Methane ebullition from submerged peatlands, which are similar to flooded paddy soil in that they contain many bubbles, is controlled by atmospheric pressure, soil temperature, and water table level [
In contrast, CO2 exchange through paddy water is the result of photosynthesis of aquatic plants and respiration of both the plants and the soil microorganisms [
Therefore, in this paper, we examined the dynamics of both CH4 and CO2 ebullition in tropical rice paddy fields in Thailand using an automatically closing chamber method.
Gas field measurements were conducted on September 20th and 21st, 2014, in a rice field of Kasetsart University, Kamphaeng Saen campus (14°00′33′′N, 99°59′03′′E) located in Nakhon Pathom Province, Thailand. The soil had a clay texture (65.7% clay, 23.30% silt, and 11.0% sand) with a dry bulk density of 1.69 g m−3. The soil was sampled on September 17 and had a pH of 6.0 (1 : 1 for soil : water), 4.32% organic matter, 1.81% total carbon, and 1.85% total nitrogen. Seedlings of the rice variety “Homcholasit” were transplanted on June 30 at
The CH4 and CO2 fluxes were measured using the automatic closed chamber method. A customized-bottomless polycarbonate chamber (
Schematic diagram of an automatic closed chamber placed between the rows of rice plants.
Temporal changes in CH4 concentration in the chamber during a measurement cycle were categorized into either a sudden increase (Figures
Examples of the changes in CH4, CO2 concentrations (7-point running average) in the closed chamber measured at 2:50 p.m. on September 20 ((a), (b)), at 2:50 a.m. on September 21 ((c), (d)), and at 4:50 p.m. on September 21 ((e), (f)). The solid line denotes the best fitting line for each emission/uptake. The white circle with black edge indicates the event starting point. The dashed lines denote the tangent lines at the local maximum or minimum points for CH4, CO2 emission/uptake rates, before respective increase or decrease events.
20/9 2:50 p.m.
20/9 2:50 p.m.
21/9 2:50 a.m.
21/9 2:50 a.m.
21/9 4:50 p.m.
21/9 4:50 p.m.
Changes in CO2 concentration in the chamber showed either an episodic increase accompanied by CH4 ebullition events (Figures
Since CH4 and CO2 concentrations in the chamber often changed episodically with time due to bubble ebullition events (Figures
Atmospheric pressure and air temperature were measured with a barometer (MPXAZ6115A and MPXV7007DP, Freescale Inc., TX, USA) and a thermometer (HMP45A, Vaisala Inc., Helsinki, Finland), respectively. Water depth in the rice field was measured with a water level sensor (eTape Continuous Fluid Level Sensor, Milone Technologies Inc., NJ, USA). Soil surface temperature was measured with a type T thermocouple.
Bubbles in soil were collected directly with a syringe by disturbing the topsoil at 3 p.m. local time on September 20. The CH4 and CO2 concentrations in the bubbles were measured using the CH4/CO2 gas analyzer after the sampled air was diluted 101 times with high-purity nitrogen gas.
Episodic and rapid increases in CH4 concentration were identified in 21 out of the 46 measurements (Figures
The large CH4 emissions via bubble ebullition mainly occurred between 10:00 a.m. and 5:00 p.m. local time (Figure
Temporal changes on September 20 and 21 in CH4 and CO2 fluxes measured with the automatic closed chamber method (a) and atmospheric pressure and soil surface temperature (b).
Relationship between CH4 emission by bubble ebullition and change of atmospheric pressure (a) or soil surface temperature (b). Relationship between CO2 emission by bubble ebullition and change of atmospheric pressure (c) or soil surface temperature (d). The change in atmospheric pressure was determined as the difference between the local maximum or minimum value and the value closest to the time when the CH4 or CO2 ebullition occurred.
In peatlands, air pressure reduction expands bubble volume and thereby enhances bubble buoyancy which causes the bubbles to rise to the water surface [
Rising soil temperature also increases the buoyancy and CH4 concentration of bubbles as barometric pressure decreases [
CH4 emission via bubble ebullition (546–617 mg m−2 d−1) contributed 95-96% of total daily CH4 emission (567–647 mg m−2 d−1) through flooded water (Table
Cumulative CH4 emissions and relative contribution of bubble ebullition and diffusion processes to total emissions.
Date | CH4 ebullition |
Via CH4 ebullition |
CH4 diffusion |
Via CH4 diffusion |
Total CH4 emission |
---|---|---|---|---|---|
Sep. 20 | 617.4 | 95.3 | 30.3 | 4.7 | 647.7 |
Sep. 21 | 546.2 | 96.3 | 20.9 | 3.7 | 567.1 |
Episodic increases in CO2 concentration were found in 14 of the 21 measurements when CH4 ebullition events occurred. During these 14 chamber closure periods, the CO2 concentration in the chamber air increased abruptly (Figures
CO2 uptake via the photosynthetic activities of the aquatic plants was also observed in these measurements. In the other 25 measurements, there was a transfer of CO2 from flooded water to the atmosphere by diffusion, likely due to the gradient in CO2 concentration at the interface between the flooded water and the atmosphere and also due to respiration of the aquatic plants [
On September 20, most of the CO2 fluxes were outgoing emissions due to bubble ebullitions. The highest CO2 emission (196.2 mg m−2 h−1) occurred at 2:50 p.m. (Figures
During the daytime on September 21, the CO2 fluxes mainly showed negative values even though CO2 ebullition events were observed. Therefore, this indicates that CO2 assimilation by the aquatic plants dominated CO2 fluxes on that day.
The log10-CO2 emissions by bubble ebullitions, omitting measurements with evidence of absorption by plant photosynthesis, were significantly correlated to changes in atmospheric pressure (
CO2 emission by bubble ebullition, accounted for only 13–35% of total CO2 emissions, compared with 65–87% from CO2 diffusion (Table
Cumulative CO2 emissions and relative contributions of bubble ebullition and diffusion processes to total emissions.
Date | CO2 ebullition |
Via CO2 ebullition |
CO2 diffusion |
Via CO2 diffusion |
Total CO2 emission |
---|---|---|---|---|---|
Sep. 20 | 648.2 | 35.0 | 1203.8 | 65.0 | 1852.0 |
Sep. 21 | 159.7 | 13.3 | 1040.4 | 86.7 | 1200.1 |
Our study found that daytime CH4 ebullition events in tropical rice paddy fields occurred due to falling atmospheric pressure and increasing soil surface temperature. At nighttime, the drop in atmospheric pressure predominately triggered the CH4 ebullition because soil temperature was low compared with that in the daytime. The fact that CH4 and CO2 concentrations in the chamber air increased abruptly when bubbles were released suggests that bubble ebullition events caused not only CH4 emission but also CO2 emission. The CO2 ebullition events were also controlled by decreases in air pressure and increases in soil temperature. Therefore, diurnal changes in atmospheric pressure and soil temperature play major roles in regulating CH4 and CO2 ebullitions in tropical rice paddy fields.
We also found that CH4 emission was predominant due to daytime ebullition, whereas only a small proportion of CO2 emissions was due to daytime ebullition. The low CO2 ebullition throughout the day was due to CO2 photosynthesis and respiration by aquatic plants, meaning that CO2 emission was mainly by diffusion between flooded water and the atmosphere.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This research was partly supported by Grant-in-Aid for Scientific Research (A) (25252044, PI: K. Noborio), a JSPS Fellowship (DC1, 12J10924, for S. Komiya) by the Japan Society for the Promotion of Science, and a Program for Establishing Strategic Research Foundations in Private Universities (S0901028, PI: K. Noborio) by MEXT of Japan. The authors are grateful to Dr. Jonaliza Siangliw (BIOTEC) and Ms. Rungthip Kohkhoo for their support at our experimental field, to Dr. Fumiyoshi Kondo (National Institute for Environmental Studies), Dr. Takeshi Tokida, and Dr. Seiichiro Yonemura (National Institute for Agro-Environmental Sciences) for their valuable comments, to Dr. Iain McTaggart (Meiji University) for reviewing a draft, to Dr. Masaru Mizoguchi (University of Tokyo) for making the water level sensor, to Mr. Ryoji Taniyama (Takumi Technical Laboratory Inc., Japan) for assisting with data analysis, to Mr. Ryo Higuchi for making the sensors, and to Mr. Shinsuke Aoki, Mr. Naoto Sato, and Mr. Ryuta Honda for analyzing soil samples. Experimental information and data are available on request to K. Noborio.