Impacts of Elevated CO[sub.2] and a Nitrogen Supply on the Growth of Faba Beans and the Nitrogen-Related Soil Bacterial Community

Ecosystems that experience elevated CO[sub.2] (eCO[sub.2]) are crucial interfaces where intricate interactions between plants and microbes occur. This study addressed the impact of eCO[sub.2] 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 CO[sub.2] concentrations (aCO[sub.2], daytime/nighttime = 410/460 ppm; eCO[sub.2], 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 eCO[sub.2] mitigated the inhibitory effects of a N deficiency on legume photosynthesis and affected the CO[sub.2] 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 eCO[sub.2], the CO[sub.2] concentrations did not significantly alter the soil NH[sub.4] [sup.+]-N or NO[sub.3] [sup.−]-N responses to a N supply. Notably, under both aCO[sub.2] and eCO[sub.2], a N supply significantly increased the relative abundance of Nitrososphaeraceae and Nitrosomonadaceae, while eCO[sub.2] specifically reduced the Rhizobiaceae abundance with no significant changes under aCO[sub.2]. 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 eCO[sub.2] 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 eCO[sub.2] and a N supply to crops can provide potential benefits for managing future climate change impacts on crop production.