Isotopic identification of nitrogen hotspots across natural terrestrial ecosystems

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 N-2 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 (similar to 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.