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Transformation of soil organic matter in forest fire impacted watersheds elucidated by FT-ICR mass spectrometry




Bahureksa, William, author
Borch, Thomas, advisor
Farmer, Delphine, committee member
Ackerson, Chris, committee member
Heuberger, Adam, committee member

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Soils provide numerous ecosystem services that are essential to life on Earth, including food security, water filtration and purification, and infrastructure for biodiversity. Soil properties (e.g., soil productivity, moisture retention, structure and aggregation, and nutrient supply) that facilitate these services depend on the soil organic matter (SOM), which can be dramatically impacted from ecosystem disturbances such as wildfires. Wildfires can provide benefits to an ecosystem through the cleaning of the forest floor, soil nourishment, and the removal of competitive underbrush. However, wildfires have grown in frequency and severity around the world, and there is great interest in resolving changes to SOM composition under wildfire conditions to secure water resources and recover fire-affected areas. In the following work, Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) was critically evaluated for the analysis of SOM. Data processing methods for FT-ICR MS were investigated to improve compositional analysis. Laboratory-simulated and field-based burn samples were collected and used to investigate changes to water-soluble fractions over progressive series of fire intensity, burn severity, and burn extent gradients. FT-ICR MS currently achieves the highest mass resolving power in the world, which makes it suitable for the study of complex mixtures with tens of thousands of compounds that are separated by mass on the order of a few electrons. Recent strategies for SOM characterization by FT-ICR MS are critically reviewed, with emphasis on SOM sample collection, preparation, analysis, and data interpretation. Importantly, the range of structures, functionalities, and mass means no technique achieves "complete" characterization, and methods used for processing and visualizing FT-ICR MS spectra can influence representation and interpretation of data. The complexity of DOM and influence of post-data processing was demonstrated by studying the effect of peak-picking threshold (3σ, 4σ, 5σ, and 6σ) on a Suwannee River Fulvic Acid standard measured by a custom 21 tesla FT-ICR mass spectrometer. Applying a 3σ peak-picking threshold revealed an additional 13,000 peaks that could be assigned compared to a 6σ peak-picking threshold with a difference of only 12 ppb root-mean-square mass error. Furthermore, isobaric overlaps differing by as little as the mass of an electron are identified up to m/z 1000, and 18O and 17O isotopologues were assigned for the first time in DOM at 3σ. Ecosystem recovery after wildfires in forested watersheds depends on revegetation and soil microbial communities and is therefore limited by the availability of nutrients. The remaining nutrients and substrate available for microbes depends on specific wildfire intensities and are poorly understood. To investigate SOM byproducts during heating and mechanisms that contribute to pyrogenic organic matter (pyOM) formation and mobilization, water-extractable organic matter was extracted from soils heated at discrete temperatures using laboratory microcosms. Relative to the unburnt control, dissolved organic carbon and nitrogen increased at ≥150°C and decreased when ≥450°C. Nitrogen-containing species predominated mass spectra at temperatures >150°C, and mass difference-based analysis suggested that products formed during heating could be used to model transformations along the Maillard reaction pathway. To investigate the short-term impacts of burn extent on water chemistry and dissolved organic matter (DOM) in fire-affected watersheds, streams originating from catchments of low, moderate, and high burn extent within the area of the Cameron Peak Fire of 2020 were sampled before, during, and after the first large rainstorm following the fire. Water chemistry parameters (DOC, TDN, turbidity) for moderate and high burn extents streams tended to increase during the storm and decrease following the storm in high burn extent streams. Fluorescence indices indicated that low/moderate burn extent streams exhibited an increase in microbially-derived residues compared to high burn extent. While a substantial portion of DOM species between every stream were common between each event and included labile and aromatic residues during the storm, the low burn extent exhibited the most unique aromatic features after the storm. When chlorinating stream samples to simulate drinking water treatment, the total DBPs were greater in streams of moderate/high burn extents compared to low burn extent. When DBP concentrations were normalized to DOC, the DOM introduced during the storm resulted in fewer DBPs, suggesting the increase in DBP formation is due to increased DOM loading overall rather than increased reactivity of the DOM. In total, the work presented here contributes to the mechanistic understanding of the residues produced during SOM heating that can be mobilized and impact water chemistry in fire-affected watersheds.


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dissolved organic matter
nitrogen cycling
carbon cycling
pyrogenic organic matter


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