Human societies depend on an Earth System that operates within a constrained range of nutrient availability, yet the recent trajectory of terrestrial nitrogen (N) availability is uncertain. Examining patterns of foliar N concentrations ([N]) and isotope ratios (15N) from more than 42,000 samples acquired over 37 years, here we show that foliar [N] declined by 8% and foliar 15N declined by 0.8 – 1.9 ‰. Examining patterns across different climate spaces, foliar 15N declined across the entire range of MAT and MAP tested. These results suggest declines in N supply relative to plant demand at the global scale. In all, there are now multiple lines of evidence of declining N availability in many unfertilized terrestrial ecosystems, including declines in 15N of tree rings and leaves from herbarium samples over the past 75-150 years. These patterns are consistent with the proposed consequences of elevated atmospheric CO2 and longer growing seasons. These declines will limit future terrestrial C uptake and increase nutritional stress for herbivores.