What was done
The authors grew three soybean varieties in pots placed within open-top chambers that received atmospheric CO2 concentrations of ambient, 1.3, 1.6, and 1.9 times ambient in combination with O3 (ozone) concentrations of 0.4, 0.9 and 1.5 times ambient to determine the interactive effects of elevated CO2 and O3 on yield.

What was learned
Overall, O3 reduced plant growth and seed yield, while atmospheric CO2 enrichment increased these parameters.  The percentage increase in yield resulting from elevated CO2 was typically greater for O3-stressed plants than it was for plants grown at sub-ambient O3 concentrations.  Across three cultivars and two growing seasons, plants exposed to 716 ppm CO2 and the lowest O3 level of 20 ppb displayed an average CO2-induced seed weight increase of 20%, while plants grown at the same CO2 concentration and the highest and most stressful O3 concentration of 79 ppb exhibited an average yield stimulation of 74%.

Atmospheric CO2 enrichment clearly ameliorated the growth-reducing effects associated with O3 stress; and at double the ambient CO2 concentration, it actually overcame them.  At ambient CO2 concentration, for example, seed yield was reduced by 16 and 37% at O3 concentrations of 50 and 79 ppb, respectively, compared to plants grown at 20 ppb O3.  At 716 ppm CO2, however, seed yield at 50 and 79 ppb O3 was actually 1 and 6% greater, respectively, than it was at 20 ppb O3.

What it means
As the atmospheric CO2 content rises, it is likely that the O3 content of the air will also rise.  Although O3-induced crop damage results in billions of lost dollars per year, the rising CO2 content will likely reduce, and perhaps completely overcome, the deleterious effects of O3 on crop productivity.  In the present study, for example, a doubled atmospheric CO2 concentration actually increased seed yield, even at an O3 concentration that was 1.5 times greater than current O3 levels.  Thus, soybean growers should be able to look forward to progressively greater yield increases as the air's CO2 content climbs higher and higher.