Diurnal variation in methane emission from a rice paddy due to ebullition

Abstract

The diurnal variation in methane (CH4) emission commonly observed in rice paddies has been linked to changes in air and soil temperatures. The temperature dependence is not simple, however, and the mechanisms underlying it in relation to the diurnal variation are still poorly understood. Here, we investigated the diurnal variation of CH4 flux and its relationship with air temperature in a Japanese paddy by dividing the total flux into two major pathways, plant-mediated and bubbling emission. The diurnal variation of total flux was minimal at the early parts of reproductive growth (panicle formation and booting stages), when plant-mediated emission was dominant. At a later stage (heading period), ebullition accounted for 43–70% of the total emission, and CH4 emission via both pathways varied diurnally; however, the diurnal range was much greater for bubbling emission (ratio of max. to min. = 3.5) than for plant-mediated emission (ratio of max. to min. = 1.4). The Bubbling emission depended on the temperature, but the emission was greater during the flux-increasing phase in the morning than the flux-decreasing phase in the afternoon even at the same temperature, forming a hysteresis in the flux versus temperature relationship. These results indicate that abiotic processes, such as a change in the solubility of gases and the temperature dependence of the gas-phase volume (Charles’s law), may play a central role in shaping the diurnal pattern of the CH4 ebullition.