Divergent Responses of the Diazotrophic Microbiome to Elevated CO₂ in Two Rice Cultivars

The species-specific responses of plant growth to elevated atmospheric CO₂ concentration (eCO₂) could lead to N limitation and potentially influence the sustainability of ecosystem. Questions remain unanswered with regards to the response of soil N₂-fixing community to eCO₂ when developing high-yielding agroecosystem to dampen the future rate of increase in CO₂ levels and associated climate warming. This study demonstrates the divergent eCO₂ influences on the paddy diazotrophic community between weak- and strong-responsive rice cultivars. In response to eCO₂, the diazotrophic abundance increased more for the strong-responsive cultivar treatments than for the weak-responsive ones. Only the strong-responsive cultivars decreased the alpha diversity and separated the composition of diazotrophic communities in response to eCO₂. The topological indices of the ecological networks further highlighted the different co-occurrence patterns of the diazotrophic microbiome in rice cultivars under eCO₂. Strong-responsive cultivars destabilized the diazotrophic community by complicating and centralizing the co-occurrence network as well as by shifting the hub species from Bradyrhizobium to Dechloromonas in response to eCO₂. On the contrary, the network pattern of the weak-responsive cultivars was simplified and decentralized in response to eCO₂, with the hub species shifting from Halorhodospira under aCO₂ to Sideroxydans under eCO₂. Collectively, the above information indicates that the strong-responsive cultivars could potentially undermine the belowground ecosystem from the diazotrophs perspective in response to eCO₂. This information highlights that more attention should be paid to the stability of the belowground ecosystem when developing agricultural strategies to adapt prospective climatic scenarios by growing high-yielding crop cultivars under eCO₂.