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 carbon dynamics. However, the lack of reliable information about plant tissue stoichiometric constraints remains a challenge to quantifying nutrient limitations on projected global carbon 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 necessarily improve model estimates of present day carbon dynamics compared with unconstrained stoichiometry. However, adopting stoichiometric flexibility significantly improves model prediction of gross primary productivity, carbon use efficiency, vegetation biomass, and soil carbon stocks. 21^st 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 carbon accumulation in modeled responses to CO₂ fertilization. We show that, at a global level, enhanced plant phosphorus-use efficiency and nutrient limitations 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 carbon and nutrient supplies and fixed stoichiometry limit future land carbon sinks. We show here that it is necessary to capture the flexibility in models to accurately project future terrestrial ecosystem carbon sequestration.