Deciphering Soil Nitrogen Biogeochemical Processes Using Nitrogen and Oxygen Stable Isotopes

Variations in stable isotope abundances of nitrogen (δ¹⁵N) and oxygen (δ¹⁸O) of nitrate are a useful tool for determining sources of nitrate as well as understanding the transformations of nitrogen within soil (Chapter 2). Various sources of nitrate are known to display distinctive isotopic compositions, while nitrogen transformation processes fractionate both N and O isotopes and can reveal the reaction pathways of nitrogen compounds. However, to fully understand the δ¹⁵N and δ¹⁸O values of nitrate sources, we must understand the chemistry and the isotopic fractionations that occur during inorganic and biochemical reactions. Among all N cycle processes, nitrification and denitrification displayed some of the largest and most variable isotope enrichment factors, ranging from -35 to 0‰ for nitrification, and -40 to -5‰ for denitrification. In this dissertation, I will first characterize the isotopic enrichment factors of ¹⁵N during nitrification and denitrification in a Midwestern agricultural soil, two important microbial processes in the soil nitrogen cycle. Nitrification incubations found that a large enrichment factor of -25.5‰ occurs during nitrification NH₄⁺ è NO₃^-, which agrees well with previous studies (Chapter 3). Additionally, oxygen isotopic exchange that occurs between nitrite and water during nitrification was also quantified and found that 82% of oxygen in NO₃^- are derived from H₂O, much greater than the 66% predicted by the biochemical steps of nitrification. The isotopic enrichment that occurs during denitrification was assessed by measuring the change in δ¹⁵N as the reactant NO₃^- was reduced to N₂ gas (Chapter 4). The incubations and kinetic models showed that denitrification can causes large isotopic enrichment in the δ¹⁵N of remaining NO₃^-. The enrichment factor for NO₂^- è gaseous N was -9.1‰, while the enrichment factors for NO₃^- è NO₂^- were between -17 to -10‰, both of which were within the range of values report in literature. The results demonstrated that nitrification and denitrification caused large isotope fractionation and can alter the presumed δ¹⁵N and δ¹⁸O values of nitrate sources, potentially leading to incorrect apportionment of nitrate sources.

The results of the denitrification incubation experiments were applied to a field study, where the measured enrichment factor was utilized to quantify loss of N by field-scale denitrification (Chapter 5). Field-based estimates of total denitrification have long been a challenge and only limited success has been found using N mass balance, N₂O gas flux, or isotope labeling techniques. Here, the flux of nitrate and chloride from tile drain discharge from a small field was determined by measuring both dissolved ions (ion chromatography) and monitoring water discharge. The δ¹⁵N and δ¹⁸O of tile nitrate was also measured at a high temporal resolution. Fluxes of all N inputs, which included N wet and dry deposition, fertilizer application, and soil mineralization were determined. The d¹⁵N and d¹⁸O values of these nitrate sources was also determined. Using this data, I first detected shifts in δ¹⁵N and δ¹⁸O values in the tile drain nitrate, which indicated variable amounts of denitrification. Next, a Rayleigh distillation model was used to determine the fraction of NO₃^- loss by field scale denitrification. This natural abundance isotope method was able to account for the spatial and temporal variability of denitrification by integrating it across the field scale. Overall, I found only 3.3% of applied N was denitrified. Furthermore, this study emphasized the importance of complementary information (e.g. soil moisture, soil temperature, precipitation, isotopic composition of H₂O, etc.), and the evidence it can provide to nitrogen inputs and processes within the soil.