Rice plant-mediated methane emissions in paddy fields: a review

Methane (CH4) is the second-most significant contributor to global warming after carbon dioxide, and paddy fields represent one of its major agricultural emission sources. CH4 is emitted from paddy fields through two primary pathways: non-plant-mediated processes, including diffusion across the water-soil interface and ebullition, and plant-mediated transport. It is widely recognized that the plant-mediated pathway is the principal pathway for CH4 emissions from rice paddies. Previous studies have demonstrated substantial variation in the efficiency of plant-mediated CH4 transport among rice genotypes, with emission fluxes differing by several fold, highlighting the potential for genetic improvement through breeding. Therefore, understanding the physiological and ecological mechanisms underlying plant-mediated CH4 transport is crucial for mitigating CH4 emissions from rice paddies. This review first outlines the general processes and relative contributions of plant-mediated and non-plant-mediated pathways in CH4 emissions from paddy fields. It then reviews the key processes of plant-mediated CH4 emission, including CH4 diffusion from the rhizosphere into root aerenchyma, its long-distance transport from roots to aboveground tissues, and its final release into the atmosphere through stomata or micropores in stems and leaves. Building upon this, the major factors influencing the different transport processes are further discussed, encompassing anatomical traits (e.g., aerenchyma structures in roots and stems), morphological characteristics (e.g., root system architecture), and environmental variables (e.g., soil properties, temperature, and wind speed). In addition, this review summarizes and evaluates the existing methodologies for measuring CH4 emission fluxes via plant-mediated pathways, highlighting their respective advantages and limitations. Finally, the review proposes that future research should focus on elucidating multi-scale mechanisms of plant-mediated pathways and developing low-emission rice cultivars to enable targeted and sustainable mitigation of CH4 emissions from rice paddies.