Effect of elevated atmospheric CO₂ concentration on the metabolic function of microbe in rhizosphere of different soybean cultivars

Climate change, characterized by increased concentrations of CO₂, can substantially stimulate plant photosynthesis and growth, and subsequently change the quantity and quality of below-ground rhizodeposition. From the perspective of soil microbiology, these alterations may indirectly induce changes in soil microbial biomass and function, and further influence the processes of soil organism evolution, such as nutrient transformation and the balance of soil carbon storage. In this study, four soybean cultivars, ‘Xiaohuangjin’ (XH), ‘Mufeng 5’ (MF), ‘Suinong 14’ (SN), and ‘Dongsheng 1’ (DS), which are released in 1951, 1972, 1996 and 2003, respectively, widely planted in Northeast China then, were selected. Soil samples were collected from the rhizosphere of soybeans grown in either ambient CO₂ (410 mol∙L⁻¹, CK) or elevated CO₂ (550 mol∙L⁻¹, EC) at the beginning of the seed stage. Each CO₂ treatment was performed in triplicate. Thus, there were 12 OTCs (open top chamber) in total. The effects of elevated CO₂ on microbial metabolic function were studied using the BIOLOG technique. The results showed that different soybean cultivars had different metabolic patterns and the order of average well color development (AWCD) ranked as: MF > SN > DS > XH. Biodiversity indices and PCA analysis revealed that the metabolic patterns of cultivars XH, MF, and DS were resistant to elevated CO₂, while elevated CO₂ significantly changed the metabolic patterns of SN. The contribution of each carbon source to the PCA indicated strong correlations between the variations of carbon sources in the EC treatment and CK of SN with the positive and negative carbon sources in PC1, respectively. In addition, some specific carbon sources in CK of SN, such as L-arginine and 2-hydroxy benzoic acid, are harmful for soybean growth, and whether elevated CO₂ may increase soybean diseases resistance merits further analysis. Meanwhile, there were interaction effects between soybean cultivars and elevated CO₂ on specific carbon sources. In summary, this study illustrates that different soybean cultivars have different metabolic functions and respond differently to increased CO₂.