The dependence of methane transport in rice plants on the root zone temperature

Large diurnal and seasonal variations in methane flux from rice paddies have been found in many studies. Although these variations are considered to result from changes in methane formation rates in the soil and the transport capacity (e.g. biomass, physiological activities, and so on) of rice plants, the real reasons for such variations are as yet unclear. This study was conducted to clarify the effects of temperature on the rate of methane transport from the root zone to the atmosphere using hydroponically grown rice plants. Methane emission rates from the top of the rice plants whose roots were soaked in a solution with a high methane concentration were measured using a flow-through chamber method with the top or root of the rice plants being kept at various temperatures. The methane emission rates and methane concentrations in solution were analyzed using a diffusion model which assumes that the methane emission from a rice paddy is driven by molecular diffusion through rice plants by a concentration gradient. In the experiment where the temperature around the root was changed, the conductance for methane diffusion was typically 2.0-2.2 times larger when the solution temperature was changed from 15 to 30 °C. When the air temperature surrounding the top of the rice plant was changed, the change in conductance was much less. In addition, from measurements of methane flux and methane concentration in soil water in a lysimeter rice paddy during the 2 growing seasons of rice, it was found that the conductance for methane transport was correlated with the soil temperature at 5 cm depth. These results suggest that the temperature around the root greatly affects the methane transport process in rice plants, and that the process of passing through the root is important in determining the rate of methane transport through rice plants.