Biotic and abiotic methane releases from Lake Biwa sediment slurry

To determine the rate and mechanism of CH4 production in Lake Biwa sediment, slurry was prepared and incubated. Surface sediment (sed) slurry (1.5–6 cm) showed a CH4 release rate (4.9–9.5 nmol g-dry-sed−1 day−1) higher than that observed in the 5- to 10-cm sediment slurry (0.2–2 nmol g-dry-sed−1 day−1). Methane release from the surface (1.5–6 cm) sediment slurry was biotic and was inhibited by addition of 2-bromoethanesulfonate (BES, an inhibitor of CH4 production), whereas that from 5- to 10-cm sediment slurry was abiotic. The addition of BES, HNO3, and O2 showed no effect on the CH4 release rate from the 5- to 10-cm sediment slurry. In addition, tracers (NaH13CO3, 13CH3COONa) were not incorporated into the released CH4. However, δ13C of CH4 released from the 5- to 10-cm sediment slurry (−74.0 ± 0.6‰) indicated that this CH4 was produced by bacterial metabolism in the past, stored by adsorption on the surface of clay minerals in the sediment, and then released abiotically by desorption from the sediment slurry as a result of a decrease in hydraulic pressure and CH4 concentration in the pore water. This CH4 stored by adsorption could be extracted by autoclaving. In the sediment below 5 cm, bacterial activity for CH4 production ceased, possibly because of the limitated availability of H2. To clarify the mechanism of CH4 production in the sediment, biotic CH4 production and the abiotic CH4 release found here should be estimated separately.