BG Biogeosciences BG Biogeosciences 1726-4189 Copernicus Publications Göttingen, Germany 10.5194/bg-9-3287-2012 Isotopic identification of nitrogen hotspots across natural terrestrial ecosystems Bai E. 1 Houlton B. Z. 2 Wang Y. P. 3 State Key Laboratory of Forest and Soil Ecology, Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang 110164, China Department of Land, Air and Water Resources, University of California, Davis, California, USA CSIRO Marine and Atmospheric Research and Centre for Australian Weather and Climate Research, Aspendale VIC 3195, Victoria, Australia 23 08 2012 9 8 3287 3304 Copyright: © 2012 E. Bai et al. 2012 This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/ This article is available from https://bg.copernicus.org/articles/9/3287/2012/bg-9-3287-2012.html The full text article is available as a PDF file from https://bg.copernicus.org/articles/9/3287/2012/bg-9-3287-2012.pdf

Nitrogen (N) influences local biological processes, ecosystem productivity, the composition of the atmospheric-climate system, and the human endeavour as a whole. Here we use natural variations in N isotopes, coupled with two models, to trace global pathways of N loss from the land to the water and atmosphere. We show that denitrification accounts for approximately 35 % of total N losses from the natural soil, with NO, N<sub>2</sub>O, and N<sub>2</sub> fluxes equal to 15.7 &plusmn; 4.7 Tg N yr<sup>−1</sup>, 10.2 &plusmn; 3.0 Tg N yr<sup>−1</sup>, and 21.0 &plusmn; 6.1 Tg N yr<sup>−1</sup>, respectively. Our analysis points to tropical regions as the major "hotspot" of nitrogen export from the terrestrial biosphere, accounting for 71 % of global N losses from the natural land surface. The poorly studied Congo Basin is further identified as one of the major natural sources of atmospheric N<sub>2</sub>O. Extra-tropical areas, by contrast, lose a greater fraction of N via leaching pathways (~77 % of total N losses) than do tropical biomes, likely contributing to N limitations of CO<sub>2</sub> uptake at higher latitudes. Our results provide an independent constraint on global models of the N cycle among different regions of the unfertilized biosphere.

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