CH4, CO2 Exchanges at Plant-Atmosphere and Water-Atmosphere Interfaces in Tropical Rice Paddy Fields.

The comprehensive studies on methane (CH₄) and carbon dioxide (CO₂) exchanges between interfaces in rice paddy fields will gain the better knowledge of CH₄ and CO₂ balance in the globe. Using stable isotopes is a useful method for understanding the mechanisms of CH₄ and CO₂ exchanges in the ecosystem. Although CH₄ and CO₂ directly exchange between plants and the atmosphere or between the water surface and the atmosphere in rice paddy fields, research on exploring those exchanges at each interface has been limited. Here, we report the results of our field experiments on the CH₄ and CO₂ exchanges between the interfaces using stable isotope techniques. Research was conducted from September 18 to 27, 2014 in a farming rice paddy field of Kasetart University, Kamphaeng Saen campus in Thailand. The CH₄ and CO₂ fluxes were measured every hour using two automated closed chambers: one for the plant-atmosphere interface covering a rice hill, and the other for the water-atmosphere interface covering the water surface between rice plants. The CH₄/CO₂ concentrations, δ¹³C-CH₄ andδ¹³C-CO₂ in the chamber headspace air were analyzed using a wavelength-scanned cavity ring-down spectroscopy CH₄/CO₂ analyzer (G2201-i, Picarro Inc., Santa Clara, CA, USA). Diurnal changes in CH₄ emission at the plant-atmosphere interface seemed to correspond to soil temperature. The CH₄ emission at the water-atmosphere interface was dominated by bubble ebullition in the daytime and by molecular diffusion in the nighttime. The CH₄ ebullitions were most likely controlled by decreases in atmospheric pressure and increases in soil temperature. The δ¹³C-CH₄ values from the rice plant increased with increasing CH₄ emission rates in daytime. The δ¹³C-CH₄ values from paddy water was lower in the daytime than in the nighttime. The CO₂ dynamics in paddy fields will be discussed as well.