Hydrologic and spatiotemporal controls on pyrogenic carbon export from fire-affected, coastal california streams

Abstract

May2025School of ScienceWildfires play a crucial role in shaping ecosystems but are increasingly driven by anthropogenic factors, leading to shifts in fire regimes and their associated environmental impacts. One significant consequence of fire is the production and mobilization of pyrogenic carbon (PyC), a complex mixture of thermally altered organic compounds that influence carbon cycling in terrestrial and aquatic systems. Recent increases in wildfire frequency and severity, driven by climate change and land-use alterations, have amplified the need to understand the environmental fate of fire-derived materials in order to mitigate and prepare for continued fire perturbations. The chapters of this dissertation aim to improve our understanding of PyC in fire-affected aquatic systems by assessing the efficacy of current PyC quantification methods, determining the variability in PyC concentration and character, and/or identifying any drivers of PyC export from riverine systems. In Chapter 1, I introduce the two specific fractions of PyC targeted in following chapters: levoglucosan (a rapidly degraded anhydrosugar) and black carbon (a refractory condensed aromatic compound class). In Chapter 2, I assess the degree of impact free benzenepolycarboxylic acids (BPCAs) have on the current dissolved black carbon quantification method. Black carbon is most commonly quantified via oxidation of its characteristic condensed aromatic structures into BPCA molecular markers for analysis. Results show that dissolved black carbon concentrations are not artificially overestimated by the presence of free BPCAs as long as only certain conversion factors are utilized. In Chapter 3, I investigate two paired temporary streams that were equivalently impacted by the 2020 SCU Lightning Complex Fires. Through high-temporal resolution measurements, watershed-specific drivers of dissolved organic and black carbon export were identified. These streams also showcased that while there is a significant first-flush of dissolved black carbon exported during the first stream wet-up post-fire, the greatest concentrations were observed during wet-up of the second year post-fire. In Chapters 4 and 5, I investigate the post-fire PyC export behavior from five coastal mountain watersheds variably impacted by the 2020 CZU Lightning Complex Fires. In Chapter 4, results from black carbon concentrations suggest that hydrology rather than the percentage of watershed area burned is the primary driver of post-fire export behavior. In Chapter 5, levoglucosan concentrations were compared to that of black carbon from these same watersheds. Distinct molecular class-specific characteristics were revealed. Levoglucosan, being highly soluble, was rapidly mobilized during the first events post-fire. In contrast, black carbon concentrations were more closely linked with discharge and the ratio of dissolved black carbon to organic carbon increased with time since fire highlighting the role of black carbon aging increasing solubility and therefore mobilization. In Chapter 6, I contextualize the findings from the previous chapters, commenting on spatiotemporal controls on PyC export and make suggestions for future avenues of research. Overall, these findings contribute to a more comprehensive understanding of PyC fluxes and their implications for carbon budgets, water quality, and role in the Earth system.Ph