How does paddy cultivation cause the emission of methane

Paddy fields (irrigated lowland rice) are estimated to contribute to around 10-15 per cent of global methane emissions. Methanogens in paddy fields are the major producers of methane and the rice plant plays a major role in regulating methane emissions.

Methanogens are archaea (bacteria-like prokaryotes) that are capable of producing methane under oxygen-limiting conditions. Most methanogens are capable of using CO{-2} as their source of carbon, and hydrogen as the reducing agent. A resulting by-product of this energy-generating reaction is methane. A number of methanogens are commonly found in wetlands and also in the marine environment and guts of animals.

Rice contributes to methane emission by providing substrates through root exudates, and dead root tissues. Rice plants also help methane emission by transporting methane from around the roots to the atmosphere through well-developed aerenchyma cells.

Methane emission in rice paddies from the early to mid season is from decomposition of organic matter in the soil. Conversely, methane emission in the late season (during the reproductive phase of rice) is from using root exudates and dead root cells as substrate.

There is some evidence to suggest that management practices can help reduce methane emissions in the early phase. For example, use of certain fertilizers or sulphur-containing compounds has been shown to reduce methane emissions. These are thought to act by promoting bacteria that either compete with methanogens for substrate or utilize the methane produced by methanogens.

Other management practices to reduce methane emission include soil aeration and crop rotation with upland crops.