Mechanistic modeling of nitrite accumulation and nitrogen oxide gas emissions during nitrification

Nitrite (NO2-) accumulation in soil following nitrogen (N) fertilizer application has been observed under a variety of conditions. The presence of NO2- together with soil acidity results in the formation of nitrous acid (HNO2), which decomposes abiotically to produce nitric oxide (NO) and nitrous oxide (N2O). These N oxide trace gases have potential effects on several atmospheric processes. Presented here is a model that describes some of the interactions between microbial, chemical, and physical processes that influence NO2- accumulation and N oxide gas emissions following applications of NH4+-based fertilizers. The model is applied to hypothetical, and actual field scenarios. A two-step, two-population nitrification submodel is linked to gas production and transformation submodels. Transport of all chemical species occurs by diffusion. The model results suggest that some degree of transient nitrite accumulation following NH4+ application is a consequence of the nature of nitrification itself. Model simulations and sensitivity analysis indicate that (i) soils receiving similar fertilizer treatments but differing in their ability to buffer nitrification-induced acidity may produce dramatically different N oxide gas emissions, (ii) subsurface fertilizer placement can significantly reduce net NO emissions, and (iii) the differential responses of Nitrosomonas and Nitrobacter populations to chemical toxicities associated with the form and/or rate of fertilizer application may significantly affect the extent of NO2- accumulation and corresponding gas emissions. Overall, the results contribute to our basic understanding of how multiple microbial, chemical, and physical factors can interact to control the net soil-to-atmosphere emission of nitrification-derived NO and N2O.