Global distribution of gaseous nitrogen losses from the unmanaged terrestrial biosphere: results from a stable nitrogen isotope model

Background/Question/Methods

Nitrogen (N) availability limits primary productivity in many regions. Gaseous losses of N via denitrification pathways have the potential to remove a significant fraction of fixed N from the biosphere, thus constraining global N supplies.  Global-scale estimations of gaseous N losses remain uncertain, owing to difficulties in scaling local soil measures to regions coupled with the intractability of N₂ gas measurements. We use a novel stable nitrogen isotope model to estimate the global distribution of gaseous N losses from the unmanaged (i.e., non-agrarian) terrestrial biosphere.  In the model, terrestrial N isotope ratios are determined by the isotopic composition of N inputs and losses.  The δ¹⁵N of inputs is based on the mean δ¹⁵N of deposition (-1.5 ‰) and fixation (0 ‰) and their relative contributions to total N inputs among regions.  The isotope effect of leaching losses is approximately 0 ‰ based on data synthesis.  The isotope effect of gaseous losses varies from 16 ‰ to 20 ‰, depending on local denitrification conditions.  The fraction of N lost via gaseous pathways is modeled by formulating these processes into a simple analytical model.  We then estimate the steady-state distribution of gaseous N fluxes from land using estimates of  N₂ fixation (1 degree × 1 degree) generated from the CASA-CNP model, and N deposition (5 degree × 3.75 degree) from a three dimensional chemistry-transport model.  

Results/Conclusions

Results indicate that gaseous N losses are highest in tropical areas where the combination of warm temperatures, moist soil conditions, and high N availability favors denitrification.  We estimate that the South American and African continents account for 67 % of global gaseous N emissions from soil.  Gaseous N emissions from Pan-tropical forests fall between 11.4 and 14.3 Tg N yr^-1 while denitrification from temperate forest soils is estimated to vary from 0.91 to 1.14 Tg N yr^-1.  In sum, our model indicates that soil denitrification accounts for 49.5 to 61.9 Tg of N losses globally from unmanaged ecosystems, or approximately 30 % to 40 % of annual N inputs. Our results suggest that N gaseous losses (mainly by denitrification globally) remove a significant amount of N and may play an important role in determining the available nitrogen for plant growth and soil microbial activity and therefore constraining global net primary productivity.