Linking transcriptional dynamics of CH 4 -cycling grassland soil microbiomes to seasonal gas fluxes.

Soil CH 4 fluxes are driven by CH 4 -producing and -consuming microorganisms that determine whether soils are sources or sinks of this potent greenhouse gas. To date, a comprehensive understanding of underlying microbiome dynamics has rarely been obtained in situ. Using quantitative metatranscriptomics, we aimed to link CH 4 -cycling microbiomes to net surface CH 4 fluxes throughout a year in two grassland soils. CH 4 fluxes were highly dynamic: both soils were net CH 4 sources in autumn and winter and sinks in spring and summer, respectively. Correspondingly, methanogen mRNA abundances per gram soil correlated well with CH 4 fluxes. Methanotroph to methanogen mRNA ratios were higher in spring and summer, when the soils acted as net CH 4 sinks. CH 4 uptake was associated with an increased proportion of USCα and γ pmoA and pmoA2 transcripts. We assume that methanogen transcript abundance may be useful to approximate changes in net surface CH 4 emissions from grassland soils. High methanotroph to methanogen ratios would indicate CH 4 sink properties. Our study links for the first time the seasonal transcriptional dynamics of CH 4 -cycling soil microbiomes to gas fluxes in situ. It suggests mRNA transcript abundances as promising indicators of dynamic ecosystem-level processes.