Determination of soil‐entrapped methane

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Abstract
A sampling method was developed and modified to sample soil from paddy fields for entrapped methane determination. A 25‐cm long plexiglass tube (4.4‐cm i.d.) fitted with gas bag was used to sample soil and entrapped gases to a depth of 15‐cm. The sampling tube was shaken vigorously to release entrapped gases. Headspace gas in sampling tube and gas bag was analyzed for methane. The procedure was verified by doing field sampling weekly at an irrigated ricefield in the IRRI Research Farm on a Maahas clay soil. The modified sampling method gave higher methane concentration because it eliminated gas losses during sampling. The method gave 98% ± 5 recovery of soil‐entrapped methane. Results of field sampling showed that at the early growth stage of the rice plant, entrapped methane remained at low levels with chemical fertilization. Application of organic matter increased entrapped methane 10‐fold. After the vegetative phase, entrapped methane increased irrespective of treatment. This suggests that entrapped methane was primarily derived from fermentation of soil organic matter at the early growth stage. At the latter stage, the rice plant seems to be the major carbon source for methane production.