Stable isotope and radiocarbon compositions of methane emitted from tropical rice paddies and swamps in Southern Thailand

Stable isotopes (delta(13)C, deltaD) 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 C-13 (mean delta(13)C; -51.5 +/- 7.1parts per thousand and -56.5 +/- 4.6parts per thousand for mineral soil and peat soil paddies, respectively) relative to the reported mean value of methane from temperate rice paddies (- 63 +/- 5parts per thousand). Large seasonal variation was observed in delta(13)C(similar to32parts per thousand) in the rice paddies, whereas variation in deltaD was much more smaller (similar to20parts per thousand), indicating that variation in delta(13)C is due mainly to changes in methane production pathways. Values of delta(13)C were lower in swamps (-66.1 +/- 5.1parts per thousand) than in rice paddies. The calculated contribution of acetate fermentation from delta(13)C value was greater in rice paddies (mineral soils: 62-81%, peat soils: 57-73%) than in swamps (27-42%). deltaD in methane from Thai rice paddies (-324 +/- 7parts per thousand (n=46)) is relatively higher than those from 14 stations in Japanese rice paddies ranging from -362 +/- 5parts per thousand (Mito: n=2) to -322 +/- 8parts per thousand (Okinawa: n=3), due to higher deltaD in floodwaters. C-14 content in methane produced from Thai rice paddies (127 +/- 1 pMC) show higher C-14 activity compared with previous work in paddy fields and those from Thai swamps (110 +/- 2 pMC).