Stable Carbon Isotope Studies of CH₄ Dynamics Via Water and Plant Pathways in a Tropical Thai Paddy: Insights Into Diel CH₄ Transportation

Abstract

Abstract Separate evaluation of methane (CH 4 ) emission dynamics (e.g., oxidation, production, and transportation) at the soil‐plant‐atmosphere and soil‐water‐atmosphere interfaces has been limited in tropical rice paddies, but it is crucial for comprehending the entire CH 4 cycles. We investigated CH 4 oxidation, production, and transportation through plant and water pathways during the reproductive stage in a tropical Thailand rice paddy field using natural abundance carbon stable isotope ratios (δ 13 CH 4 and δ 13 CO 2 ). Mass balance equations using δ 13 CH 4 and δ 13 CO 2 in soil gases indicated that CH 4 oxidation in the planted soil exceeded those in the interrow soil due to oxygen supply through rice roots. In addition, at 1–11 cm depth acetate fermentation was the dominant process in the planted soil, whereas in the interrow soil the dominant process was H 2 /CO 2 reduction. The water pathway showed a significant negative correlation between CH 4 flux and released δ 13 CH 4 over 24 hr, driven by a diel change in episodic ebullition, steady ebullition, and diffusion, all due to diel changes in soil temperature and atmospheric pressure. In contrast, the plant pathway showed a significant positive relationship between CH 4 flux and emitted δ 13 CH 4 throughout one day. A comparison of the diel change in emitted δ 13 CH 4 between the water and plant pathways showed that the rice plants transported CH 4 in soil bubbles without any large isotopic fractionation. The diel change in the plant‐mediated CH 4 transportation was mainly controlled by diel changes in soil bubble expansion and CH 4 diffusion through plants, which were probably regulated by diel changes in soil temperature and atmospheric pressure.