Date of Award

Summer 2025

Document Type

Open Access Thesis

Department

Chemistry and Biochemistry

First Advisor

Wendell Walters

Second Advisor

Susan Richardson

Abstract

Nitrogen oxides (NOx = NO + NO₂) are trace gases that play a critical role in the atmosphere by influencing air quality, oxidation chemistry, and the deposition of fixed nitrogen. Historically, NOx emissions have been primarily linked to anthropogenic sources such as fossil fuel combustion, industrial activities, and agriculture. However, as successful legislation has significantly reduced these emissions, the relative importance of natural NOx sources has become an increasing concern. Soil NOx emissions are of particular interest due to their strong temperature dependence and their connection to global climate change. Rising global temperatures are expected to accelerate soil NOx emissions as well as increase the likelihood and intensity of biomass burning events, which can further alter the nitrogen cycle. Further, biomass burns can trigger significant perturbations in the cycling of oxidized and reduced nitrogen species in soils, ultimately affecting NOx emission rates. While some field studies have documented variable impacts of biomass burning on soil NOx emissions across different landscapes, controlled laboratory investigations into the specific drivers of these changes remain limited. In this study, we present findings from controlled incubation experiments designed to investigate soil NOx emissions before and after biomass burning events of varying intensities. Soil core samples were collected from a rural forest site at the Savannah River Site both pre- and post-burn, and the impact of fire on soil properties—including pH, nutrient levels, and water content—was measured. Despite variations in burn intensity, no consistent significant changes were observed in soil pH, ammonium concentrations, or nitrate concentrations (p > 0.05). A custom-built soil flux chamber was used to measure NOx and ammonia (NH3) fluxes from the collected soil samples, both before and after the burn. There was no consistent result observed between the soil samples before and after the biomass burning event. The average NOx flux before the biomass burning was determined to be 680 ± 540 ng-N/m2 (n=11) hour and after the biomass burning was 1,100 ± 860 ng-N/m2 hour (n=14). No statistical differences were detected between the NOx flux for the different plot conditions (p<0.05). There were also no statistical differences detected between the NOx flux before and after the biomass burning event, which can be attributed to by the high variability in the flux (p<0.05). One post-burn soil had unusually high flux compared to the other soils. The NH3 flux from the soil samples was determined to be nearly negligible across all experiments, likely due to the low soil acidity conditions (pH = 4.03 ± 0.22; n = 30), limiting NH3 volatilization. Additional flux experiments were performed to examine the response to an ammonium nitrate (NH4NO3) spike. This spike indicated a strong NOx flux response (p<0.05) which indicates that the absence of change in the NOx flux after the biomass burning is attributed to no increase in fixed nitrogen content in the soil. The results of this study suggest that a lack of increased nitrogen deposition or altered soil nitrogen pools can make biomass burning events alone insufficient to drive measurable changes in soil NOx flux or NH3 emissions in forested ecosystems in the southeastern United States.

Rights

© 2025, Olivia Rae Steinbeck

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