Soil pH drives poplar rhizosphere soil microbial community responses to ozone pollution and nitrogen addition

Abstract

AbstractGround‐level ozone (O3) pollution frequently coincides with the deposition of anthropogenic nitrogen (N), and both factors can influence the structure and functionality of both above‐ and belowground ecosystems. Elevated O3 levels have been shown to adversely impact plants in many prior reports, but the interacting effects of high O3 levels and N addition on exposed plants remain to be clearly defined, and the changes in rhizosphere microbial community composition in this context have yet to be studied. The direct and indirect mechanisms and interactions among plants, microbes, and the soil that shape these O3 and N responses are also poorly understood. Herein, we explored the interactive effects of O3 exposure (five levels) and soil N (four levels) on the composition of rhizosphere soil microbial communities associated with poplar trees (Populus euramericana cv. ‘74/76’). In these analyses, exposure to higher levels of O3 was linked to significant decreases in bacteria, fungi, arbuscular mycorrhizal fungi, and to a reduction in the ratio of fungi‐to‐bacteria, whereas soil N addition had no impact on these parameters. No interactive effects between O3 and N were observed in the context of alterations in soil microbial community composition, and equivalent performance was observed for concentration‐based (AOT40, cumulative exposure to hourly O3 concentrations >40 ppb) and flux‐based [POD1 and POD7, cumulative stomatal uptake of O3 > 1 or 7 nmol O3 m−1 PLA (projected leaf area) s−1] dose–response analyses. Structural equation modelling revealed that changes in the composition of the microbial community were attributable to changes in soil pH but unrelated to plant characteristics. Overall, these findings indicated that increased O3 levels can induce soil alkalinisation and thereby influence soil microbial communities such that soil pH is a reliable predictor of O3 pollution‐related changes in these communities.Highlights Elevated O3 exhibited an overall negative influence on microorganisms. AOT40‐, POD1‐ and POD7‐based dose–responses performed equally well. Soil pH is a reliable predictor of O3 pollution‐related changes in microbial communities. N addition failed to affect O3 effects on soil microbial community.