In this project, we use the stable isotopes of N, 15N and 14N, to model soil denitrification rate.  15N/14N ratios vary naturally among biogenic materials owing to isotope fractionations, particularly kinetic ones, which are most commonly associated with organisms’ enzymatic preferences for light N (14N).  In terms of the terrestrial biosphere, coherent variations in 15N/14N composition of soils and ecosystems are widely observed across gradients in temperature, precipitation, and latitude.  Such 15N/14N patterns in bulk soil N are thought to reflect the dominant pathways and isotopic ratios by which N enters and leaves different environments and across varying environmental conditions.  Thus, in a steady state framework, the partitioning of denitrification gaseous (fgas_loss) can be analyzed (see figure legend for details).  Using estimates of symbiotic and asymbiotic N2 fixation and global N deposition generated from a three dimensional chemistry-transport model, we convert these gas loss fractions to steady-sate fluxes.  This simulation points to a substantial pattern in denitrification fluxes in terrestrial soil, with losses varying from 0 to 6.45 g N m-2 yr-1 across global ecosystems.  Regionally, the highest fluxes are found in central Africa, South America, and Southeast Asia, where the combination of warm temperatures, moist soil conditions, and high N availability favors soil microbial activities.  Incorporating spatially and temporally explicit phenomena (hotspots and hot moments) has become the biggest challenge in denitrification models.  Our N isotope model integrates denitrification processes or nitrogen gas emissions resulting from hot spots and moments and provides a framework for large-scale long-term studies. 

Fig. 1 Model of soil nitrogen isotope ratios:  I, N input flux; U, plant uptake; R, plant return; L,leaching losses; G, gaseous losses; εL and εG, effective enrichment factor for leaching and gaseous losses [ε (‰) = (14k/15k – 1) ∙ 1000, where k is the rate constant]; εden, enrichment factor for denitrification process; fgas loss, fraction of loss by gases; fleaching, fraction of loss by leaching (fgas loss + fleaching = 1); p, proportion of NO3- consumed by denitrification.  At steady state, plant uptake is equal to plant return, resulting in no isotope effects on soil δ15N by plant uptake process.  Soil δ15N can be modeled independent of plants: δ15Nsoil = δ15Ninputs + εG ∙ fgas loss + εL ∙ fleaching.