Effect of Elevated [CO₂] on Soil Bubble and CH₄ Emission from a Rice Paddy: A Test by ¹³C Pulse-Labeling under Free-Air CO₂ Enrichment

Abstract

A ¹³ C pulse-labeling experiment was conducted under ambient CO₂ and free-air CO₂ enrichment (FACE) conditions to understand how elevated CO₂ affects the CH₄ in soil bubbles and CH₄ emissions from rice paddies. Two hills of rice plants enclosed by frames in ambient CO₂ and FACE plots were labeled with ¹³ CO₂ at the panicle formation stage, and plants and soil bubbles were collected 2 and 51 days after the pulse labeling. CH₄ and ¹³ CH₄ emissions were also measured by the closed-chamber method between the 2 bubble samplings. Conversion of photosynthate carbon to CH₄ entrapped in soil bubbles and CH₄ emission to the atmosphere occurred quickly, suggesting that recent photosynthates can be an important source of carbon for methanogenesis. About 1% of the ¹³ C-labeled C taken up through photosynthesis was emitted as ¹³ CH₄ during the 7 weeks of measurement, most during the first 3 weeks after labeling. Total CH₄ and ¹³ CH₄ emissions did not differ between the ambient CO₂ and FACE treatments, because the entrapped CH₄ in the soil and the root biomass were not affected by elevated CO₂ levels in this study.

Keywords:

• ¹³C pulse labeling
• bubble
• free-air CO₂ enrichment (FACE)
• methane
• rice paddy

Acknowledgments

This research was funded in part by the Global Environment Research Program, Ministry of the Environment, Japan. We thank T. Mizuno and M. Murakami of Chiba University for their help in the bubble sampling and ¹³C analysis, and H. Nakamura of NARCT for field management. Present address for M. Hoque is Department of Food and Tea Technology, Shahjalal University of Science & Technology, Sylhet 3114, Bangladesh.