As in a natural wetland, flooding a rice field cuts off the oxygen supply from the atmosphere to the soil, which results in anaerobic fermentation of soil organic matter. Methane is a major end product of anaerobic fermentation. It is released from submerged soils to the atmosphere by diffusion and ebullition and through roots and stems of rice plants. Recent global estimates of emission rates from wetland rice fields range from 20 to 100 Tg/yr (IPCC 1992), which corresponds to 6-29% of the total annual anthropogenic methane emission.

https://academic.oup.com/bioscience/article-abstract/43/7/466/249121