Rising levels of atmospheric CO2 have been implicated in changes in the nitrogen (N) and phosphorus (P) content of terrestrial vegetation; however, questions remain over the role of C, N and P interactions in driving plant nutrient stoichiometry, particularly whether N and P additions alter vegetation responses to CO2 enrichment singly. Here we use meta-analysis of 46 published studies to investigate the response of plant N and P to elevated CO2 alone and in combination with nutrient (N and P) additions across temperate vs. tropical biomes. Elevated CO2 reduces plant N concentrations more than plant P concentrations in total biomass pools, resulting in a significant decline in vegetation N/P. However, elevated CO2 treatments in combination with N additions increase plant P concentrations, whereas P additions have no statistical effect on plant N concentrations under CO2 enrichment. These results point to compensatory but asymmetrical interactions between N, P and CO2; that changes in N rapidly alter the availability of P, but not the converse, in response to increased CO2. Our finding implies widespread N limitation with increasing atmospheric CO2 concentrations alone. We also suggest that increased anthropogenic N deposition inputs could enhance plant N and P in a progressively CO2-enriched biosphere.