29 November 2017

Methanogenesis in Oxygenated Soils is Important for Wetland Methane Emissions: Implications for Earth System Models

Observed methane dynamics contradict widely-held assumptions

Science

Our porewater and greenhouse-gas flux measurements in a freshwater wetland show clear evidence for methane production in well-oxygenated soils from, contradicting assumptions in most Earth System Models (ESMs). A comparison of oxic to anoxic soils revealed up to ten times greater methane production and nine times more methanogenesis activity in oxygenated soils. Further, metagenomic and metatranscriptomic sequencing recovered the first near complete genomes for a novel methanogen species.

Impact

Our genetic sequencing demonstrated acetoclastic production from a novel methanogen (Candidatus Methanothrix paradoxum), which appears to be prevalent across terrestrial methane emitting ecosystems. The discovery of C. Methanothrix paradoxum and large oxic methane source from a freshwater wetland illustrate fundamental structural uncertainty in current global terrestrial methane models, motivating ongoing work in the E3SM land model.

Summary

Current ESM land model representations of wetland methane production, oxidation, transport, and emission to the atmosphere remain uncertain, as demonstrated in several recent publications. Our analysis here demonstrates a large model structural uncertainty that appears to be important globally, i.e., up to 80% of total methane emissions from the wetland we studied could be attributed to methanogenesis in oxygenated soils. Metagenomic and metatranscriptomic sequencing recovered the first near complete genomes for a novel methanogen species (Candidatus Methanothrix paradoxum), and showed acetoclastic production from this organism was the dominant methanogenesis pathway in oxygenated soils. Together our findings challenge widely-held assumptions about methanogenesis, with significant ramifications for global methane estimates and Earth system modeling.

Contact
William J. Riley
Lawrence Berkeley National Laboratory
Publications
Smith, G. "Methanogenesis in Oxygenated Soils is A Substantial Fraction of Wetland Methane Emissions." Nature Communications 8, (2017). [10.1038/s41467-017-01753-4].