Methane Emissions from Flooded Rice Fields

Flooded paddy fields emit methane because prolonged waterlogging restricts oxygen entry into the soil, creating strongly reducing conditions in which organic matter is decomposed anaerobically. Methanogenic microorganisms then produce methane as an end product, while the flooding regime determines how long these conditions persist and how much methane is produced, oxidised and released.

Creation of anaerobic conditions

Flooding fills soil pores with water and greatly slows the diffusion of atmospheric oxygen. The remaining oxygen is consumed by plant roots and microorganisms, after which the submerged soil becomes anaerobic and its redox potential declines.

• The standing water is not itself the source of methane; methane formation occurs mainly in the oxygen-depleted soil beneath it.
• Rice straw, soil organic matter, root exudates and other plant residues provide carbon for microbial decomposition.
• Continuous flooding maintains reduced soil conditions for long periods and therefore generally favours methane production.

Role of methanogenic microorganisms

Different anaerobic microorganisms first break complex organic matter into simpler compounds. Methanogens, anaerobic microorganisms belonging to the Archaea, use substrates such as acetate or hydrogen and carbon dioxide to form methane during the final stages of anaerobic decomposition.

• Methanogenesis becomes important under strongly reduced conditions after more favourable electron acceptors have been depleted.
• A portion of the methane is consumed by methane-oxidising microorganisms where oxygen is available, especially near the soil-water interface and around oxygenated parts of rice roots.
• The methane that escapes oxidation reaches the atmosphere mainly through the air spaces of rice plants, and also through bubbles and diffusion across the soil-water interface.

Influence of water and crop-residue management

Water management changes the soil's oxygen status and is therefore a major control over methane emissions from rice cultivation.

• Continuous flooding sustains anaerobic conditions and permits methanogenesis throughout much of the crop period.
• Mid-season drainage and alternate wetting and drying periodically admit oxygen into the soil, interrupting methane production and promoting methane oxidation.
• Incorporating readily decomposable organic material, especially close to flooding, can increase the supply of substrates for methanogens.
• The mitigation achieved through drainage varies with soil properties, climate, duration of aeration, residue management and irrigation practice.
• Periodic aeration can increase the possibility of nitrous oxide formation under suitable nitrogen and moisture conditions, so water management should be assessed using total greenhouse-gas effects as well as crop and water requirements.

How UPSC asks this