Pathway-specific flux and dissolved CH<sub>4</sub> pool in the soil across 22 rice cultivars

Methane (CH4) produced in rice paddy soil is transported to the atmosphere mainly via the rice plants and partly by bubbling events (ebullition). Recent studies have shown that ebullition is more significant than previously thought in fields planted with the popular Japanese cultivar ‘Koshihikari’. It remains unclear whether the substantial contribution of ebullition is unique to this specific cultivar, as no previous reports have compared plant-derived and bubbling fluxes separately among various cultivars. Therefore, we planted 22 genetically diverse rice cultivars and measured plant-mediated and bubbling fluxes at three growth stages. Both fluxes, as well as the contribution of bubbling to the total flux, differed among the cultivars. The plant-mediated flux in Koshihikari was similar to or less than those in other cultivars, whereas the bubbling flux and its contribution to total flux were larger, especially at the later stage. The absence of a correlation between plant-mediated flux and dissolved CH4 in the soil water at the later stage suggests that varietal differences in CH4 entry from the soil to the plant or gas flow permeability in the plant, rather than the pool size of CH4 in the soil, control the plant-mediated flux. On the other hand, the increase in bubbling flux associated with plant maturation and its close correspondence with dissolved CH4 concentration indicate that bubbling flux was controlled by the size of CH4 pool in the soil, which likely increased with senescence and decay of rice roots. A low correspondence between panicle weight and CH4 emissions points to the potential for breeding high-yielding rice cultivars with low CH4 emissions