Accurately representing the relationships between nitrogen supply and photosynthesis is crucial for reliably predicting carbon-nitrogen cycle coupling in Earth System Models (ESMs). Most ESMs assume positive correlations among soil nitrogen supply, leaf nitrogen content, and photosynthetic capacity. However, leaf photosynthetic nitrogen demand may influence the leaf nitrogen response to soil nitrogen supply, thus responses to nitrogen supply are expected to be largest in environments where demand is greatest. Using a nutrient addition experiment replicated at 26 sites spanning four continents, we demonstrated that climate variables were stronger predictors of leaf nitrogen content than soil nutrient supply. Leaf nitrogen increased more strongly with soil nitrogen supply in regions with highest theoretical leaf nitrogen demand, increasing more in colder and drier environments than warmer and wetter ones. Thus, leaf nitrogen responses to nitrogen supply are primarily influenced by climatic gradients in photosynthetic nitrogen demand, an insight that will improve ESM predictions.