Abstract Rising atmospheric CO2 has stimulated plant productivity, with terrestrial ecosystems currently absorbing nearly one-third of anthropogenic CO2 emissions. Increases in photosynthesis can subsequently lead to increased carbon (C) storage in plants and soil. However, there is growing evidence that nitrogen (N) availability constrains elevated CO2 (eCO2) responses, yet we know much less about the role of phosphorus (P) limitation on productivity under eCO2.