Environmental Dynamics of Dissolved Organic Matter and Dissolved Black Carbon in Fluvial Systems: Effects of Biogeochemistry and Land Use

Black carbon (BC) is an organic residue formed primarily from biomass burning (e.g., wildfires) and fossil fuel combustion. Until recently, it was understood that BC was highly recalcitrant and stabilized in soils over millennial scales. However, a fraction of the material can be solubilized and transported in fluvial systems as dissolved BC (DBC), which represents on average 10% of the global export of dissolved organic carbon (DOC) from rivers to coastal systems. The composition of DBC controls its reactivity, and it has been linked with a variety of in-stream processes that induce both carbon sequestration and evasion of CO₂ from aquatic systems, which suggest DBC may have a significant contribution within the global carbon cycle. The primary objectives for the thesis were to elucidate environmental factors that control the fate and transport of DBC in fluvial systems. Ultra-high resolution mass spectrometry was used to characterize DBC on a molecular scale whereas benzenepolycarboxylic acids were used to quantify and characterize BC in both dissolved and particulate phases (PBC). Sinks for polycondensed DBC were linked to a series of in-stream biogeochemical processes (e.g., photodegradation, metal interactions); whereas photooxidation of particulate charcoal led to production of DBC, suggesting photodissolution as a previously unrecognized source of DBC to fluvial systems. Coupling of DBC with PBC, however, was hydrologically constrained with sources varying over temporal scales and land use regimes. For DBC in particular, an enrichment of heteroatomic functionality was observed as a function of anthropogenic land use. Furthermore, land use coupled with stream order (a proxy for in-stream processing as defined by the River Continuum Concept) could explain significant spatial variability in organic matter (e.g., DOC) composition within an anthropogenically impacted system. With an increase in wildfire frequency projected with on-going climate change trends, parallel projections for increases in BC production are also expected. Furthermore, conversion of natural landscapes for urban and agricultural practices is also expected to continue in the coming decades. Thus, it is imperative to reach a comprehensive understanding of processes regulating the transport of DBC in fluvial systems with efforts to constrain future BC budgets and climate change models

Photodissolution of charcoal and fire-impacted soil as a potential source of dissolved black carbon in aquatic environments

This study investigates the effect of photodissolution on the production of dissolved black carbon (DBC) from particulate charcoal and a fire-impacted soil. A soil sample and a char sample were collected within the burn vicinity of the 2012 Cache La Poudre River wildfire and irradiated in deionized water with artificial sunlight. Photoexposure of the suspended char and soil significantly enhanced production of DBC after 7 days continuous exposure to the simulated sunlight. The increase was coupled with an increase in the DBC polycondensed character. In agreement with this, characterization using Fourier transform-ion cyclotron resonance-mass spectrometry (FT-ICR-MS) showed an increase in the number of BC molecular formulae detected and in their average molecular weight, suggesting that increasing photoexposure is required for dissolution of larger, more polycondenced DBC compounds. An increase in molecular signatures with lower H/C ratio and higher O/C ratio after 7 days photoexposure suggested increasing functionality of newly produced DBC with irradiation time, and therefore photooxidation as a potential mechanism for the photodissolution of BC. The photoproduced DBC was also strongly coupled with the photoproduced bulk dissolved organic carbon (DOC). The results suggest that photodissolution may be a significant and previously unrecognized mechanism of DBC translocation to aquatic systems