Elevated Atmospheric CO₂ Modifies Mostly the Metabolic Active Rhizosphere Soil Microbiome in the Giessen FACE Experiment

Elevated levels of atmospheric CO₂ lead to the increase of plant photosynthetic rates, carbon inputs into soil and root exudation. In this work, the effects of rising atmospheric CO₂ levels on the metabolic active soil microbiome have been investigated at the Giessen free-air CO₂ enrichment (Gi-FACE) experiment on a permanent grassland site near Giessen, Germany. The aim was to assess the effects of increased C supply into the soil, due to elevated CO₂, on the active soil microbiome composition. RNA extraction and 16S rRNA (cDNA) metabarcoding sequencing were performed from bulk and rhizosphere soils, and the obtained data were processed for a compositional data analysis calculating diversity indices and differential abundance analyses. The structure of the metabolic active microbiome in the rhizospheric soil showed a clear separation between elevated and ambient CO₂ (p = 0.002); increased atmospheric CO₂ concentration exerted a significant influence on the microbiomes differentiation (p = 0.01). In contrast, elevated CO₂ had no major influence on the structure of the bulk soil microbiome (p = 0.097). Differential abundance results demonstrated that 42 bacterial genera were stimulated under elevated CO₂. The RNA-based metabarcoding approach used in this research showed that the ongoing atmospheric CO₂ increase of climate change will significantly shift the microbiome structure in the rhizosphere.