The influence of elevated CO₂ on bacterial community structure and its co-occurrence network in soils polluted with...

Elevated CO₂ (eCO₂) and nanoparticles release are considered among the most noteworthy global concerns as they may impose negative effects on human health and ecosystem functioning. A mechanistic understanding of their combined impacts on soil microbiota is essential due to the profound eCO₂ effect on soil biogeochemical processes. In this study, the impacts of Cr₂O₃ nanoparticles (nano-Cr₂O₃) on the activity, structure and co-occurrence networks of bacterial communities under ambient and eCO₂ were compared between a clay loam and a sandy loam soil. We showed that eCO₂ substantially mitigated nano-Cr₂O₃ toxicity, with microbial biomass, enzyme activity and bacterial alpha-diversity in clay loam soil were much higher than those in sandy loam soil. Nano-Cr₂O₃ addition caused an increase in alpha-diversity except for clay loam soil samples under eCO₂. 16S rRNA gene profiling data found eCO₂ remarkably reduced community divergences induced by nano-Cr₂O₃ more efficiently in clay loam soil (P < 0.05). Network analyses revealed more complex co-occurrence network architectures in clay loam soil than in sandy loam soil, however, nano-Cr₂O₃ decreased but eCO₂ increased modularity and network complexity. Rising CO₂ favoured the growth of oligotrophic (Acidobacteriaceae, Bryobacteraceae) rather than the copiotrophic bacteria (Sphingomonadaceae, Caulobacteraceae, Bacteroidaceae), which may contribute to community recovery and increase available carbon utilization efficiency. Our results suggested that the degree to which eCO₂ mitigates nano-Cr₂O₃ toxicity is soil dependent, which could be related to the variation in clay and organic matter content, resilience of the resistant bacterial taxa, and microbial network complexity in distinct soils.