The use of stable nitrogen (N) isotopes as an integrated signal of ecosystem dynamics has grown markedly, but an understanding of large-scale isotope patterns and processes is has not been determined. We compiled new and published data on the natural abundance N isotope composition (d¹⁵N values) of soil and plant organic matter from around the world. Across a broad range of climate and ecosystem types, we found that soil and plant d¹⁵N values systematically decreased with increasing mean annual precipitation (MAP) and decreasing mean annual temperature (MAT). Because most undisturbed soils are near N steady state (inputs=outputs), the observations suggest that an increasing fraction of ecosystem N losses are ¹⁵N-depleted forms (NO₃, N₂O, etc.) with decreasing MAP and increasing MAT. Wetter and colder ecosystems appear to be more efficient in conserving and recycling mineral N. Globally, plant d¹⁵N values are more negative than soils, but the difference (d¹⁵Nplant- d¹⁵Nsoil) increases with decreasing MAT (and secondarily increasing MAP), suggesting a systematic change in the source of plant-available N (organic/NH₄⁺ vs. NO₃^-) with climate. Understanding controls on N storage at steady-state is critical for predictions of how these ecosystems will respond to human-mediated disturbances of the global N cycle.