Comparison of crop yield sensitivity to ozone between open‐top chamber and free‐air experiments

Assessments of the impacts of ozone (O3) on regional and global food production are currently based on results from experiments using open‐top chambers (OTCs). However, there are concerns that these impact estimates might be biased due to the environmental artifacts imposed by this enclosure system. In this study, we collated O3 exposure and yield data for three major crop species—wheat, rice, and soybean—for which O3 experiments have been conducted with OTCs as well as the ecologically more realistic free‐air O3 elevation (O3‐FACE) exposure system; both within the same cultivation region and country. For all three crops, we found that the sensitivity of crop yield to the O3 metric AOT40 (accumulated hourly O3 exposure above a cut‐off threshold concentration of 40 ppb) significantly differed between OTC and O3‐FACE experiments. In wheat and rice, O3 sensitivity was higher in O3‐FACE than OTC experiments, while the opposite was the case for soybean. In all three crops, these differences could be linked to factors influencing stomatal conductance (manipulation of water inputs, passive chamber warming, and cultivar differences in gas exchange). Our study thus highlights the importance of accounting for factors that control stomatal O3 flux when applying experimental data to assess O3 impacts on crops at large spatial scales. Present global and regional assessments of O3‐induced crop yield losses are based on results from Open‐Top Chambers (OTC) experiments, which suffer from less ecologically realistic environmental conditions than Free‐Air Concentration experiments (O3‐FACE). In wheat and rice, O3 sensitivity was higher in O3‐FACE compared to OTC experiments, while the opposite was found for soybean. These differences could be linked to factors influencing stomatal conductance (irrigation, passive chamber warming, cultivar differences), highlighting the importance of accounting for stomatal O3 flux in large‐scale impact assessments.