Anthropogenic activities have caused multiple simultaneous changes to the chemical composition of the atmosphere. While rising carbon dioxide (CO2) has been shown to accelerate plant growth and impact phenology, it is unclear how these outcomes may be modified by the simultaneous effects of co-occurring pollutants such as ozone (O3, a damaging oxidant), nitrogen dioxide (NO2, an oxidant or plant signal), or atmospheric nitrogen deposition. To test interactive effects of multiple pollutants on growth and phenology, Arabidopsis thaliana was grown in open-top chambers with a fully factorial combination of ambient or elevated pollutants: CO2, O3, and NO2 and soil nitrate. Previous observations of elevated CO2 effects on A. thaliana flowering time produced conflicting results, ranging from acceleration to delay or no change. This study suggests that some observed variability may arise from interactions with other pollutants; we find evidence for all three outcomes depending on treatment combination. When CO2 alone was elevated, accelerated phenology (1–3 days) was accompanied by increased early-season biomass. Interactions between elevated O3, NO2, and soil nitrate caused the strongest phenological delays but were damped when CO2 was also elevated. These findings caution against simple extrapolations that do not consider multiple coincident atmospheric changes with implications for design and interpretation of global change experiments.

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