Seasonal Changes in Methane Emissions via Different Pathways from a Rice Paddy Field

Abstract Rice paddies are significant sources of methane (CH₄) emissions, with CH₄ released into the atmosphere through three pathways: molecular diffusion of dissolved methane across the atmosphere-water boundary, ebullition of gas bubbles, and diffusive transport through the aerenchyma tissue of rice plants. This study aimed to analyze seasonal variations of CH₄ fluxes separately via ebullition and rice plants, and to explore the potential relationship between in-situ gas volume and CH₄ emissions. Field monitoring of CH₄ emissions, gas-filled porosity (i.e., bubble volume), and soil temperature was conducted in a rice paddy with four different treatments: plots with/without rice plant (Oryza sativa, cv Koshihikari) and with/without straw application. Results indicated that both total CH₄ flux and CH₄ ebullition were higher during the ripening stage for plots without straw application, where over 60% of total CH₄ flux was attributed to ebullition. Rice straw application enhanced both fluxes during the early vegetative stage. Seasonal trends of total CH₄ flux corresponded to those of CH₄ flux via ebullition. Gas-filled porosity increased during the vegetative stage particularly in plots with straw application, reaching almost maximum values during the late reproductive stage. Methane flux via rice plant and ebullition were correlated well with gas-filled porosity during the vegetative and reproductive stages. This study suggests that the size of gaseous CH₄ pool is a good measure for estimating flux intensities of both ebullition and rice-mediated pathways.