Nitrogen application is required to realize wheat yield stimulation by elevated CO but will not remove the CO -induced reduction in grain protein concentration.

Elevated CO (eCO ) generally promotes increased grain yield (GY) and decreased grain protein concentration (GPC), but the extent to which these effects depend on the magnitude of fertilization remains unclear. We collected data on the eCO responses of GY, GPC and grain protein yield and their relationships with nitrogen (N) application rates across experimental data covering 11 field grown wheat (Triticum aestivum) cultivars studied in eight countries on four continents. The eCO -induced stimulation of GY increased with N application rates up to ~200 kg/ha. At higher N application, stimulation of GY by eCO stagnated or even declined. This was valid both when the yield stimulation was expressed as the total effect and using per ppm CO scaling. GPC was decreased by on average 7% under eCO and the magnitude of this effect did not depend on N application rate. The net effect of responses on GY and protein concentration was that eCO typically increased and decreased grain protein yield at N application rates below and above ~100 kg/ha respectively. We conclude that a negative effect on wheat GPC seems inevitable under eCO and that substantial N application rates may be required to sustain wheat protein yields in a world with rising CO .