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 show that incorporating CO2-driven foliar stoichiometry into the Community Land Model reduced the projected land C sink two-fold by the end of the century compared to simulations with fixed foliar chemistry. Further, flexible foliar C:N profoundly altered Earth’s hydrologic cycle, reducing evapotranspiration and increasing runoff. Belowground N cycling rates were reduced in the flexible scenario, highlighting the urgency of further research examining both the direct and indirect effects of changing foliar stoichiometry on soil N cycling and plant productivity.