Global patterns of the isotopic composition of soil and plant nitrogen

We compiled new and published data on the natural abundance N isotope composition (delta(15)N 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 delta(15)N values systematically decreased with increasing mean annual precipitation (MAP) and decreasing mean annual temperature (MAT). Because most undisturbed soils are near N steady state, the observations suggest that an increasing fraction of ecosystem N losses are N-15-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 delta(15)N values are more negative than soils, but the difference (delta(15)N(plant)-delta(15)N(soil)) increases with decreasing MAT (and secondarily increasing MAP), suggesting a systematic change in the source of plant-available N (organic/NH4+ versus NO3-) with climate. Nitrogen isotopes reflect time integrated measures of the controls on N storage that are critical for predictions of how these ecosystems will respond to human-mediated disturbances of the global N cycle.