Impact of Elevated O₃ on Soil Microbial Community Function Under Wheat Crop

This study was initiated to explore the effects of ozone (O₃) exposure on potted wheat roots and soil microbial community function. Three treatments were performed: (1) Air with daily averaged O₃ concentration of 4-10 ppb (control situation, CK), (2) Air plus 8 h averaged O₃ concentration of 76.1 ppb (O₃-1), and (3) Air plus 8 h averaged O₃ concentration of 118.8 ppb (O₃-2). In treatments with elevated O₃ concentration (O₃-1 and O₃-2), the root and shoot biomass were reduced by 25% and 18%, respectively, compared to the control treatment (CK). On the other hand, root activity was significantly reduced by 58% and 90.8% in the O₃-1 and O₃-2 treatments, respectively, compared to CK. The soil microbial biomass was significantly reduced only in the highest O₃ concentration (O₃-2 treatment) in the rhizosphere soil. Soil microbial community composition was assessed under O₃ stress based on the changes in the sole carbon source utilization profiles of soil microbial communities using the Biolog[trade mark sign] system. Principal component analysis showed that there was significant discrimination in the sole-carbon source utilization pattern of soil microbial communities among the O₃ treatments in rhizosphere soil; however, there was none in the bulk soil. In rhizosphere soil, the functional richness of the soil microbial community was reduced by 27% and 38% in O₃-1 and O₃-2 treatments, respectively, compared to CK. O₃-2 treatment remarkably decreased the Shannon diversity index of soil microbial community function in rhizosphere soil, but the O₃-1 treatment did not. In the dominant microorganisms using carbon sources of carbohydrates and amino acids groups were significantly reduced by an elevated O₃ concentration in the rhizosphere soil. Our study shows that the elevated ozone levels may alter microbial community function in rhizosphere soil but not in the bulk soil. Hence, this suggests that O₃ effects on soil microbes are caused by O₃ detriments on the plant, but not by the O₃ direct effects on the soil microbes.