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

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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.