Linking trace gas fluxes and microbial community characteristics along an alpine soil moisture gradient

Background/Question/Methods

Atmospheric carbon dioxide (CO₂) and methane (CH₄) have been increasing since the pre-industrial era, resulting in climate warming that has increased flooding, permafrost melt, hurricanes, and the loss of sea-ice and glaciers. In order to accurately characterize and predict fluxes of these trace gases in the future, it is necessary to understand the mechanisms of CO₂ and CH₄ production and consumption, principally including decomposition processes by soil microorganisms. Toward this aim, the principal objective of this work was to integrate the study of trace gas fluxes and microbial community characteristics along an alpine soil moisture gradient characteristic of dry meadow vegetation at Niwot Ridge, Colorado. To achieve this, specific goals of this project were to (1) characterize the magnitude, variability, and seasonality of alpine CO₂ and CH₄ cycling across a soil moisture gradient, (2) differentiate the relative autotrophic and heterotrophic components of bulk soil respiration, and (3) quantify the variability of total microbial biomass and methanogenic archaea over space and time.

Results/Conclusions

Soil moisture generally increased from winter to summer, as a result of increased infiltration associated with the seasonal transition from dry snow (scoured by wind from the study site) to wet snow and rain. Soil CO₂ fluxes increased with soil moisture until the point of saturation, and the seasonality of peak fluxes was strongly correlated with landscape position. Conversely, CH₄ production was observed uniquely in saturated soils, which were associated with ice lenses beneath the tundra soil surface that prevented water drainage in some locations. Precipitation was insufficient to saturate soils, and unsaturated soils were characterized by CH₄ oxidation. Overall, the ratio of heterotrophic to autotrophic respiration was proportional to the magnitude of the flux, as were microbial biomass and the presence of methanogenic archaea. We thus conclude that changes in soil moisture may produce divergent reactions from CO₂ and CH₄ producing (and consuming) soil microorganisms within this vegetation community, capable of affecting the both the magnitude and sign of the flux.