Increased plant growth under elevated CO2 slows the pace of climate warming and underlies projections of terrestrial carbon (C) and climate dynamics. However, this important ecosystem service may be diminished by concurrent changes to vegetation carbon to nitrogen (C:N) ratios. Despite clear observational evidence of increasing foliar C:N under elevated CO2 , our understanding of potential ecological consequences of foliar stoichiometric flexibility is incomplete. Here, we illustrate that when we incorporated CO2-driven increases in foliar stoichiometry into the Community Land Model the projected land C sink decreased two-fold by the end of the century compared to simulations with fixed foliar chemistry. Further, CO2-driven increases in foliar C:N profoundly altered Earth’s hydrologic cycle, reducing evapotranspiration and increasing runoff; and reduced belowground N cycling rates. These findings underscore the urgency of further research to examine both the direct and indirect effects of changing foliar stoichiometry on soil N cycling and plant productivity.