Identifying Pathways of Nitrous Oxide Flux from Agricultural Soils Using Nitrogen Isotopomer Analysis

Applied N fertilizer, not assimilated by crops, is subject to N loss pathways mediated by microbial processes. Nitrous oxide (N₂O) is released during oxidation and reduction of soil N species and often augmented by applied N. Loss of N₂O results in economic loss and environmental degradation as N₂O is a potent greenhouse gas and the leading cause of stratospheric ozone depletion. The main microbial processes responsible for loss are nitrification and denitrification, but the contribution from each is not fully understood. We hypothesized that fertilizer and water addition would lead to an increase in N₂O emissions and deviation from the microbial pathway responsible for baseline emissions; we predicted that denitrification would be the primary pathway after water and fertilizer addition, while nitrification would dominate in control plots. An automated soil flux chamber system was used to test this hypothesis. Potassium nitrate fertilizer (KNO3), was broadcast applied to treatments planted with winter barley, at a rate of 112kg KNO₃-N ha^-1 with water to reach water-filled-pore-space of 80% within the surface 5 cm. The N₂O flux and isotope composition of the terminal and central N in N₂O were measured using an enhanced cavity laser absorption spectroscopy system at three-hour intervals. The difference in N isotope composition between the terminal and central N is defined as site preference (SP) and can reflect the microbial pathway responsible for N₂O efflux. Microbial N₂O emissions were greater with water and N addition, and results from SP suggest a shift in the dominant pathway for N₂O flux. In contrast, no changes in flux or pathway were detected for the control treatment. Fertilizer application is necessary for optimizing crop yield, but its application has shown to create soil conditions that favor denitrification resulting in N loss.