Stable Isotope and Radiocarbon Compositions of Methane Emitted from Tropical Rice Paddies and Swamps in Southern Thailand

Stable isotopes (δ 13C,δ D) and radiocarbon were measured in methane bubbles emitted from rice paddies and swamps in southern Thailand. Methane emitted from the Thai rice paddies was enriched in 13C (mean δ 13C; -51.5 ± 7.1‰ and -56.5 ± 4.6‰ for mineral soil and peat soil paddies, respectively) relative to the reported mean value of methane from temperate rice paddies (- 63 ± 5‰). Large seasonal variation was observed in δ 13C(∼32‰) in the rice paddies, whereas variation in δD was much more smaller (∼ 20‰), indicating that variation in δ 13C is due mainly to changes in methane production pathways. Values of δ 13C were lower in swamps (-66.1 ± 5.1‰) than in rice paddies. The calculated contribution of acetate fermentation from δ 13C value was greater in rice paddies (mineral soils: 62-81%, peat soils: 57-73%) than in swamps (27-42%). δD in methane from Thai rice paddies (-324± 7‰ (n=46)) is relatively higher than those from 14 stations in Japanese rice paddies ranging from -362 ± 5‰ (Mito: n=2) to -322 ± 8‰ (Okinawa: n=3), due to higher δD in floodwaters. 14C content in methane produced from Thai rice paddies (127±1 pMC) show higher 14C activity compared with previous work in paddy fields and those from Thai swamps (110±2 pMC).