Measurement of Methane Flux from a Rice Paddy

Abstract

Methane flux from a rice paddy, using a chamber method, was measured at Fukuoka Agricultural Research Center during a cultivation period, from June to October in 1990. Methane flux increased since two weeks of rice-transplanting, and was in the range of 10-20 mg CH₄/ m²/ hr when the paddy was flooded. However, the flux decreased less than 1mg CH₄/ m²/ hr after the first drainage of flooding water at the end of July. Afterwards, the flux stayed low at the same level, because flooding and drainage were alternatively repeated in the rice paddy so that the soil in the paddy was prevented from being anaerobic. Some physical parameters, such as temperatures of the ambient atmosphere, flooding water, paddy soil, and oxidation-reduction potential (Eh) of the paddy soil were measured simultaneously. Especially, Eh showed a strong correlation wi th the observed change of the methane flux, i. e. the flux was high when Eh was lower than -200 mV, which indicated that the soil was in a highly reduced condition. Methane flux did not show any significant diurnal variation.
From the observed results, average methane flux for the cultivating period is calculated as 3.3 mg CH₄/ m²/hr (0.08g CH₄/ m²/ day), and average flux for the flooding period is to be 7.4 mg CH₄/ m²/ hr (0.18g CH₄/ m²/day). The total methane flux from the rice paddy is estimated to be about 9g CH₄/ m²/period by integrating the observed methane flux during the cultivating period.