Stimulation of C3 crop yield by rising [CO2] is widely expected to counteract crop losses to greater drought that is predicted this century. But these expectations come from sparse field trials that have been biased towards mesic growth conditions. This eight-year study used precipitation manipulation and year-to-year variation in weather conditions at a unique open-air field facility to show that the stimulation of soybean yield by elevated [CO2] diminished to zero as drought intensified. Contrary to the prevalent expectation in the literature, rising [CO2] did not counteract the effect of strong drought on yield because elevated [CO2] interacted with drought to modify stomatal function and canopy energy balance. These results highlight the potential for unexpected, negative impacts of interacting global change factors on a key global commodity crop in its primary region of production.
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