Abstract
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.