Isotopic identification of nitrogen hotspots across natural terrestrial ecosystems

Abstract. 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, N₂O, and N₂ fluxes equal to 15.7 ± 4.7 Tg N yr⁻¹, 10.2 ± 3.0 Tg N yr⁻¹, and 21.0 ± 6.1 Tg N yr⁻¹, 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 N₂O. 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 CO₂ uptake at higher latitudes. Our results provide an independent constraint on global models of the N cycle among different regions of the unfertilized biosphere.