Development of a gas diffusion probe for the determination of methane concentrations and diffusion characteristics in flooded paddy soil

A probe for the measurement of dissolved CH4 in anoxic methanogenic environments was developed. The probe was based on the diffusion of dissolved CH4 through a silicone membrane into a gas space at the end of the probe. This gas space was flushed with N2 and analyzed gas-chromatographically for CH4. The probe had a spatial resolution of < 1.3 mm, the detection limit was about 20 micromolar CH4 the precision of the measurement was 9%, and consecutive measurements could be made every 4 min. Memory effects after analysis of high CH4 concentrations could be avoided by flushing the probe with N2 between each measurement. The probe was sensitive for water movement and, therefore, was calibrated in an artificial sediment of glass beads (100 micromoles diam.) immersed by aqueous solutions of known CH4 concentrations. Sensitivity of the probe for changes in the sediment's porosity could not presently be excluded. The probe was also used to measure vertical profiles of dissolved CH4 in microcosms of anoxic paddy soil. The vertical CH4 profiles measured with the probe compared fairly well with those measured after an extraction procedure. The profiles clearly showed that CH4 was produced in deeper layers and diffused upwards to be consumed in the oxic top 2 mm soil layers. The probe was also used to determine the diffusion coefficient of CH4 in an inactivated paddy soil microcosm using a set-up which allowed modelling of a measured CH4 concentration profile using Fick's 2nd law.