Carbon (C) enters terrestrial ecosystems via photosynthesis and cycles through the system together with other essential nutrients (i.e., nitrogen (N) and phosphorus (P)). This coupling of C, N, and P leads to the theoretical prediction that limited nutrient availability will limit photosynthesis and plant growth, leading to a strong constraint on future terrestrial C dynamics. However, the lack of reliable information about plant tissue stoichiometric constraints remains a challenge to quantifying nutrient limitations on projected global C cycling. We harmonized observed plant tissue C:N:P stoichiometry from more than 6,000 plant species using the Plant Functional Type (PFT) framework common in global land models. Using observed C:N:P stoichiometry and the flexibility of these ratios as emergent plant traits, we show that observationally‐constrained fixed plant stoichiometry does not improve model estimates of present‐day C dynamics compared with unconstrained stoichiometry. However, adopting stoichiometric flexibility significantly improves model predictions of C fluxes and stocks. 21st century simulations with RCP8.5 CO2 concentrations show that stoichiometric flexibility, rather than baseline stoichiometric ratios, is the dominant controller of plant productivity and ecosystem C accumulation in modeled responses to CO2 fertilization. The enhanced nutrient limitations and plant P‐use efficiency mainly explain this result. This study is consistent with the previous consensus that nutrient availability will limit future land carbon sequestration but challenges the idea that imbalances between C and nutrient supplies and fixed stoichiometry limit future land C sinks. We show here that it is necessary to represent nutrient stoichiometric flexibility in models to accurately project future terrestrial ecosystem carbon sequestration.