Effects of elevated CO₂ concentration on Se accumulation and associated rhizobacterial community in Cardamine hupingshanensis

Background and aims: Selenium (Se) deficiency in soil and human diets may worsen with elevated atmospheric carbon dioxide (eCO₂). However, current research focused on essential nutrient elements, such as nitrogen, phosphorus and potassium, the effects of eCO₂ on Se accumulation in staple crops are understudied. Here, the Se-hyperaccumulator Cardamine hupingshanensis was selected to investigate the impacts on Se accumulation, and associated rhizobacterial communities under eCO₂ (800 ppm). Methods: Simulated CO₂-elevated greenhouse experiments were conducted, and Se concentrations were measured using inductively coupled plasma mass spectrometry (ICP-MS). Bacterial communities in bulk, rhizosphere and rhizoplane soils were analyzed via high-throughput 16 S rRNA amplicon sequencing. Results: The eCO₂ increased Se levels by three- to four-fold in C. hupingshanensis cultivated in natural soils. Rhizobacterial communities exhibited notable shifts under eCO₂ with increased relative abundances of Bacillaceae, Rhizobiaceae, Flavobacteriaceae and Xanthomonadaceae, but decreased Sphingomonadaceae, Gemmatimonadaceae and Micrococcaceae. Genera such as Nannocystis, Steroidobacter, Dactylosporangium and Brevundimonas, demonstrated significant positive correlation (P < 0.05) with total Se, bioavailable Se and pH in soils. The abundances of the bacteria involved in Se metabolism exhibited significant positive correlations (P < 0.05) with total inorganic carbon (TIC), total organic carbon (TOC) and carbon metabolism. Structural Equation Modelling analysis indicated that eCO₂ significantly increased soil bioavailable Se and C. hupingshanensis Se levels. Conclusion: The eCO₂ promoted Se accumulation in C. hupingshanensis roots by regulating soil pH, TIC and bioavailable Se levels, while reshaping rhizobacterial communities. This study contributes to understanding associated mechanisms of Se accumulation under eCO₂, particularly in plant-rhizobacterial interactions.