Contribution of methanol to the production of methane and its 13C-isotopic signature in anoxic rice field soil

Abstract

Conversion of methanol to CH4 has a large isotope effect so that a small contribution of methanol-dependent CH4 production may decrease the δ13CH4 of total CH4 production. Therefore, we investigated the role of methanol for CH4 production. Methanol was not detectable above 10 μM in anoxic methanogenic rice field soil. Nevertheless, addition of 13C-labeled methanol (99% enriched) resulted in immediate accumulation of 13CH4. Addition of 0.1 μM 13C-methanol resulted in increase of the δ13CH4 from −47 to −6‰ within 2 h, followed by a slow decrease. Addition of 1 μM 13C-methanol increased δ13CH4 to +500 ‰ within 4 h, whereas 10 μM increased δ13CH4 to +2500‰ and continued to increase. These results indicate that the methanol concentrations in situ, which diluted the 13C-methanol added, were ≤0.1 μM and that the turnover of methanol contributed only about 2% to total CH4 production at 0.1 μM. However, contribution increased up to 5 and 17% when 1 and 10 μM methanol were added, respectively. Anoxic rice soil that was incubated at different temperatures between 10 and 37 °C exhibited maximally 2–6% methanol-dependent methanogenesis about 1–2 h after addition of 1 μM 13C-methanol. Only at 50 °C, contribution of methanol to CH4 production reached a maximum of 10%. After longer (7–10 h) incubation, however, contribution generally was only 2–4%. Methanol accumulated in the soil when CH4 production was inhibited by chloroform. However, the accumulated methanol accounted for only up to 0.7 and 1.2% of total CH4 production at 37 and 50 °C, respectively. Collectively, our results show that methanol-dependent methanogenesis was operating in anoxic rice field soil but contributed only marginally to total CH4 production and the isotope effect observed at both low and high temperature.