The Interactive Effects of Nitrogen Addition and Ozone Pollution on Cathay Poplar-Associated Phyllosphere Bacterial Communities

Ground-level ozone (O-3) can adversely impact tree productivity and the service functions of forest ecosystems. The deposition of atmospheric nitrogen (N) can enhance nutrient availability and mitigate the O-3-mediated impairment of plant-soil-microbe systems. Interactions between plants and associated microbial communities are integral to the ability of these plants to resist environmental stressors, yet studies examining the impact of increased O-3 and N levels, alone or in combination, on these phyllosphere bacterial communities have been lacking to date. Accordingly, this study was conducted to examine the impact of O-3 (charcoal-filtered air vs. non-filtered ambient air + 40 ppb of O-3), N addition (0, 50, and 100 kg N ha(-1) year(-1)), and a combination of these treatments on the phyllosphere bacterial communities associated with Cathay poplars. Higher O-3 levels were found to significantly reduce the relative abundance of Gammaproteobacteria phyla while increasing the relative abundance of the dominant Alphaproteobacteria and Betaproteobacteria, with these effects being independent of N levels. Consistently, while marked differences in the composition of phyllosphere bacterial communities were observed as a function of O-3 treatment conditions, they were largely similar across N treatments. Higher O-3 levels contributed to significant reductions in alpha diversity, including both observed OTUs and phylogenetic diversity, when no N or low levels of N were added. alpha diversity was not affected by the N addition irrespective of O-3 levels. A significant correlation was observed between photosynthesis rates and both alpha diversity and phyllosphere bacterial community composition, indicating a close relationship between photosynthetic activity and this microbial community. Together, these data offer new ecological insights regarding O-3-induced changes in the makeup of bacterial communities present on plant surfaces, providing a foundation for efforts to formulate novel management strategies aimed at adapting environmental stressors under conditions of O-3 pollution and in N-enriched environments.