Ozone will remain a threat for plants independently of nitrogen load

Elevated concentrations of ground‐level ozone (O₃) and atmospheric nitrogen (N) deposition occur concurrently. The negative effects of elevated O₃ on plants have been widely studied and are well understood nowadays. However, how the effects of elevated O₃ on plants may be driven by N deposition remains an unsolved puzzle. We conducted a meta‐analysis and showed that the negative effects of elevated O₃ on photosynthesis, stomatal conductance, growth and biomass production of semi‐natural and natural vegetation may remain unchanged by N deposition in the coming future under realistic increases in O₃ concentrations (+20 to 40 ppb) and N deposition (up to 60 kg ha⁻¹ year⁻¹). The negative effect of elevated O₃ on chlorophyll content is offset by soil N addition; however, the negative effect on biomasses is not offset by soil N addition. Across functional groups and O₃ levels, N addition exacerbated O₃ effects on root when N increased from 0–10 kg N ha⁻¹ year⁻¹ to 11–30 kg N ha⁻¹ year⁻¹. However, an analysis as per the plant functional group revealed that such a N‐dependent O₃ effect was significant only in perennial non‐woody plants, and was non‐significant when only realistic increases in O₃ concentrations were considered. Likewise, N addition appeared to exacerbate O₃‐negative effects on photosynthesis of trees when N increased from 0–30 kg N ha⁻¹ year⁻¹ to >60 kg N ha⁻¹ year⁻¹; however, this effect was significant only when realistic increases in O₃ concentrations were considered. The results suggest potential error in the current estimates of the overall O₃ impacts on plants due to no consideration of soil N availability, and encourage further studies on the interaction of O₃ and N availability that will permit more robust analyses in the future. Elevated O₃ will likely remain a persistent agricultural and ecological issue independently of N deposition. A free Plain Language Summary can be found within the Supporting Information of this article.