Pathway-specific emissions and dissolved CH­4 pool in the soil across 22 rice varieties

Methane (CH ₄ ) produced in rice paddy soil is transported to the atmosphere mostly via the rice plants or by bubbling events (ebullition) with the former believed to be the dominant pathway. However, recent studies have shown that ebullitions occurred more significantly than previously thought in a rice field planted with the popular Japanese inbred variety ‘Koshihikari’. It remains unclear if the substantial contribution of ebullition is unique to this specific variety, as no previous reports have compared plant-derived and bubbling flux separately among various varieties. Therefore, we planted 22 genetically diverse rice varieties and measured plant-mediated and bubbling fluxes at three different growth stages. The results showed that plant-mediated and bubbling fluxes, as well as the bubbling contribution to total flux, differed among the varieties. Koshihikari ehxibited plant-mediated flux similar to or less than the other varieties at all measurement stages, whereas the bubbling flux and its contribution to total flux were larger, especially at the later stage. Plant-mediated flux showed no correlation to the dissolved CH ₄ concentration in soil water at the later stage, suggesting that factors controlling CH ₄ transport, rather than the pool size of CH ₄ in the soil, control this pathway. On the other hand, the increase in bubbling flux associated with plant maturity and its close correspondence with dissolved CH ₄ concentration indicated that bubbling flux was controlled by the size of CH ₄ pool in the soil, which likely increased with senescence and decay of rice roots. A low correspondence between panicle weight and CH ₄ emissions points to the potential for breeding ideal rice varieties that are high- yielding with low CH ₄ emissions.