These results suggest declines in N supply relative to plant demand at the global scale. Examining patterns across different climate spaces, foliar δ 15N declined across the entire range of mean annual temperature and mean annual precipitation tested. Examining patterns of foliar N concentrations and isotope ratios (δ 15N) from more than 43,000 samples acquired over 37 years, here we show that foliar N concentration declined by 9% and foliar δ 15N declined by 0.6–1.6‰. 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. Nature Ecology & Evolution volume 2, pages 1735–1744 ( 2018) Cite this article Isotopic evidence for oligotrophication of terrestrial ecosystems