Elevated CO₂ alters the abundance but not the structure of diazotrophic community in the rhizosphere of soybean grown in a Mollisol

The effect of elevated CO2 (eCO2) on rhizospheric diazotrophic community in cropland has little been studied, although eCO2 facilitates nodulation and N2 fixation in legumes. In this study, four soybean cultivars (Xiaohuangjin, Suinong 8, Suinong 14, and Heinong 45) were grown in Mollisols for 65 days under ambient CO2 (aCO2) (390 ppm) or eCO2 (550 ppm). Quantitative PCR and Illumina MiSeq sequencing targeting the nifH gene that reflects the composition of diazotrophic community were determined. Elevated CO2 significantly increased the abundance of nifH gene copies in the rhizospheres of the Suinong 8 and Heinong 45 cultivars, but not in the Suinong 14 and Xiaohuangjin cultivars. The nifH abundance correlated negatively with nodule density (p ≤ 0.01) but positively with nodule size (p ≤ 0.01). Elevated CO2 did not significantly alter the composition of diazotrophic community, nor shift dominant bacterial operational taxonomic units (OTUs). These results indicated that eCO2 stimulated the growth but did not alter the community composition of diazotrophs in the rhizosphere of soybean, which depended on cultivar and might contribute to nodulation responses to eCO2.