Abstract Photosynthesis links terrestrial carbon, water and nutrient cycles. Photosynthetic least-cost theory suggests that plants optimize photosynthesis at the lowest summed investments in nutrient and water use. The theory predicts that increasing nutrient availability should increase nutrient allocation toward photosynthetic enzymes and reduce stomatal conductance, allowing similar photosynthetic rates achieved at a lower ratio of leaf intercellular to atmospheric CO2 concentration (χ) and reduced water loss. The theory suggests similar responses to increasing soil pH in acidic soils due to common correlations between soil pH and nutrient availability. However, empirical tests of the theory outside of environmental gradients are rare. To test this theory experimentally, we measured photosynthetic traits in mature Acer saccharum Marshall trees growing in a 9-year, nitrogen-by-pH manipulation in the northeastern USA. Increasing soil nitrogen availability did not affect net photosynthesis (Anet) or stomatal conductance (gs) rates, but was associated with increased area-based leaf nitrogen content (Narea), increased photosynthetic capacity (Vcmax, Jmax) and decreased χ (i.e, increased water-use efficiency). These patterns strengthened the tradeoff between nitrogen and water use, indicated by steeper slopes of Narea–χ and Vcmax–χ with increasing soil nitrogen availability. When examined across all plots, soil pH had no effect on any traits. However, in plots without nitrogen additions, increasing soil pH increased the slopes of Narea–χ and Vcmax–χ, though did not modify χ. Supporting the theory, A. saccharum maintained Anet across the soil nitrogen availability gradient by trading less efficient nitrogen use for more efficient water use. Additionally, the effects of soil pH on nitrogen–water use tradeoffs appear to occur through indirect pH effects on soil nitrogen availability. These results indicate that elevated nitrogen deposition could stimulate photosynthesis less than commonly expected and instead reduce water losses, and conversely, that reductions in photosynthesis expected from increasing nitrogen limitation in some regions could be lessened if accompanied by increased transpiration.