Ozone will remain a threat for plants independently of nitrogen load

Elevated concentrations of ground-level ozone (O-3) and atmospheric nitrogen (N) deposition occur concurrently. The negative effects of elevated O-3 on plants have been widely studied and are well understood nowadays. However, how the effects of elevated O-3 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 O-3 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 O-3 concentrations (+20 to 40 ppb) and N deposition (up to 60 kg ha(-1) year(-1)). The negative effect of elevated O-3 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 O-3 levels, N addition exacerbated O-3 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 O-3 effect was significant only in perennial non-woody plants, and was non-significant when only realistic increases in O-3 concentrations were considered. Likewise, N addition appeared to exacerbate O-3-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 O-3 concentrations were considered. The results suggest potential error in the current estimates of the overall O-3 impacts on plants due to no consideration of soil N availability, and encourage further studies on the interaction of O-3 and N availability that will permit more robust analyses in the future. Elevated O-3 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.