GLOBAL PATTERNS OF THE ISOTOPIC COMPOSITION OF SOIL AND PLANT NITROGEN

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.