Impacts of Elevated CO₂ and a Nitrogen Supply on the Growth of Faba Beans (Vicia faba L.) and the Nitrogen-Related Soil Bacterial Community

Ecosystems that experience elevated CO₂ (eCO₂) are crucial interfaces where intricate interactions between plants and microbes occur. This study addressed the impact of eCO₂ and a N supply on faba bean (Vicia faba L.) growth and the soil microbial community in auto-controlled growth chambers. In doing so, two ambient CO₂ concentrations (aCO₂, daytime/nighttime = 410/460 ppm; eCO₂, 550/610 ppm) and two N supplement levels (without a N supply—N0—and 100 mg N as urea per kg of soil—N100) were applied. The results indicated that eCO₂ mitigated the inhibitory effects of a N deficiency on legume photosynthesis and affected the CO₂ assimilation efficiency, in addition to causing reduced nodulation. While the N addition counteracted the reductions in the N concentrations across the faba beans’ aboveground and belowground plant tissues under eCO₂, the CO₂ concentrations did not significantly alter the soil NH₄⁺-N or NO₃⁻-N responses to a N supply. Notably, under both aCO₂ and eCO₂, a N supply significantly increased the relative abundance of Nitrososphaeraceae and Nitrosomonadaceae, while eCO₂ specifically reduced the Rhizobiaceae abundance with no significant changes under aCO₂. A redundancy analysis (RDA) highlighted that the soil pH (p < 0.01) had the most important influence on the soil microbial community. Co-occurrence networks indicated that the eCO₂ conditions mitigated the impact of a N supply on the reduced structural complexity of the soil microbial communities. These findings suggest that a combination of eCO₂ and a N supply to crops can provide potential benefits for managing future climate change impacts on crop production.