Impact of elevated atmospheric CO₂ on the wheat rhizomicrobiome under the additional influence of warming, drought, and nitrogen fertilization

The rhizomicrobiome is an important component of agricultural soil that may face vulnerability due to global change. Here, we focused on its response in a 2-year cropland free air carbon dioxide enrichment (FACE) experiment on winter wheat in combination with warming (+3 °C) and drought. To characterize bacterial abundance and prokaryotic diversity, 16S rRNA gene PCR amplicons obtained from soil DNA were analysed. Elevated CO₂ (e[CO₂]) increased under conventional water supply bacterial abundance, along with higher above-ground plant biomass, but had no tangible effect on the overall diversity and community composition. In contrast, drought reduced bacterial abundance under e[CO₂] despite higher plant biomass, while it did not affect bacterial abundance at ambient CO₂. Warming and N-fertilizer supplies increased bacterial abundance and community composition, but these responses were not modified by CO₂. Compared to the wheat growth stage and variation in N-supply, the effect of CO₂ on the overall community composition was minor, explaining 2.2% of its variation. While 7 taxa, representing only 0.3% of relative abundance and most of them affiliated with Acidobacteria, changed with the CO₂ level, many more responded to growth stages and nitrogen supply. Fewer growth stage and N supply-responsive taxa were detected with e[CO₂]. Our results provide strong evidence that wheat plant responses to e[CO₂] transfer to the rhizomicrobiome affecting the abundance composition of their constituting microbes. Additional drought conditions can strongly change the direction of these responses and should thus be considered for predicting implications of global change for the cropland soil microbiomes.