Impact of Aquatic Earthworms on Methane Emission Reduction from the Paddy Field Soil in Japan

Methane (CH4) is one of the major greenhouse gases that significantly contributes to global warming. Therefore, substantial efforts are being made to reduce CH4 emissions. Paddy fields make a major contribution to atmospheric CH4 concentration because of their anoxic soil environment. Paddy field is habitat of many aquatic earthworms which can play a crucial role in reducing CH4 emissions from paddy field, because their bioturbation activities influence the soil structure and increase oxygen penetration and hence the activity of methanotrophs. Therefore, it was hypothesized that aquatic earthworms may accelerate oxidation of CH4 through their bioturbation activity. This incubation study evaluated the effects of earthworms on the activity of methanotrophs. Soil was incubated with a full factorial combination of two levels of aquatic earthworm (with and without earthworm) and two levels of fertilizer (with and without urea) for 35 days. The addition of urea increased the earthworm density in paddy soil by nearly doubled of without urea. At 28 days after incubation (DAI) the CH4 flux decreased from 2055 mg m-2 h-1 in only urea received soil to 425 mg m-2 h-1 in soil received both urea and earthworm treatment. Phospholipid fatty acid analysis showed that the presence of aquatic earthworms contributed to an increase in the biomass of methanotrophs. The study implies that aquatic earthworms may play vital role to reduce CH4 emission form paddy fields by creating favorable environment for methanotrophs, even in the soils fertilized with urea.