PS 45-39 - Global distribution of gaseous nitrogen losses from the unmanaged terrestrial biosphere: results from a stable nitrogen isotope model

Wednesday, August 5, 2009
Exhibit Hall NE & SE, Albuquerque Convention Center
Edith Bai, State Key Laboratory of Forest and Soil Ecology, Instituted of Applied Ecology, Chinese Academy of Sciences, Shenyang, China, Benjamin Z. Houlton, Land, Air and Water Resources, University of California, Davis, Davis, CA and Yingping Wang, CSIRO Marine and Atmospheric Research, Victoria 3195, Australia
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.

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