Diurnal variations in methane emission from rice plants

A greenhouse study was conducted to investigate the mechanisms causing diurnal variations in methane emission from rice plants (Oryza sativa L.). Methane emission was measured using a closed chamber system on individual rice plants at five stages of development. The role of the rice plant as the primary methane transport component was examined by comparing emission from intact plants to plants severed above and below the water. No diurnal variations were present in the severed plants and the emission was greatly reduced when compared to the intact plant. Results from the vascular transport experiment showed that transpiration is a major factor in methane emission. Emission dependence on soil temperature was examined to test the hypothesis that soil temperature affects emission. With some plants, soil temperature was held constant using a water bath, otherwise the soil temperature was allowed to vary with environmental conditions in the greenhouse. Diurnal variations in emissions were higher for plants with uncontrolled soil temperature than for plants with controlled soil temperature. Soil temperature at a 5 cm depth explained 46% of the emission variation. Soil temperature affects the source of methane in the soil while transpiration promotes the uptake of water and subsequently the emission of methane. Methane emission was negatively correlated with biomass, probably due to effects of root biomass on soil water methane concentration. Methane concentration in soil water was negatively correlated with root biomass, most likely due to increases in soil oxidation with increasing biomass in a fixed soil volume, and change in root conductance with age.