Interactive effects of ozone exposure and nitrogen addition on the rhizosphere bacterial community of poplar saplings

It is widely documented that elevated ground-level ozone (O₃) has negative effects on tree physiological characteristics, and in return, affects forest ecosystem function. However, the effect may be modified by soil nitrogen (N) availability. Numerous studies have focused on the aboveground part of trees under elevated O₃ alone or in combination with soil N; however, little is known about the response of soil bacterial communities. Here, we investigated the effects of O₃ (charcoal-filtered air, CF, versus ambient air +40 ppb of O₃, E-O₃), N addition (0 kg ha^-1 yr^-1, N0, versus 200 kg ha^-1 yr^-1, N200), and their combination on rhizosphere soil bacterial communities of hybrid poplar, using an MiSeq targeted amplicon sequencing of the bacterial 16S rRNA gene. E-O₃ significantly decreased bacterial abundance, and N200 significantly decreased the α-diversity. The negative impacts of N200 on α-diversity were alleviated by E-O₃. Nitrogen and E-O₃-N200 combination altered bacterial community composition, with a significant increase in the relative abundance of Proteobacteria and Bacteroidetes and a decrease in the abundance of Firmicutes. From an ecological network analysis, E-O₃, alone and in combination with N200, complicated the co-occurrence network of bacterial communities by inducing a microbial survival strategy, shifting the hub species from RB41 to Bacillus and Blastococcus. Conversely, N200 led to simplification and decentralization of the co-occurrence network. These findings demonstrate that the rhizosphere bacterial communities exhibit divergent responses to E-O₃ and N200, suggesting the need to consider the stability of the belowground ecosystem to optimize plantation management in response to environmental changes.