Evaluating the effects of fire on carbon and nitrogen biogeochemistry in forested ecosystems

Roth, Holly, author
Borch, Thomas, advisor
Henry, Chuck, committee member
Reynolds, Melissa, committee member
Prenni, Jessica, committee member
Wilkins, Mike, committee member
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Forests provide ecosystem services (e.g., carbon storage, nutrient processing, and water filtration) valued at ~$5 trillion per year which are vulnerable to disturbances such as wildfire. Although fires are a natural component of healthy forests, climate change has begun to increase the size, frequency, and severity of wildfires outside of their historic range. Expected increases in burn severity have implications for carbon (C) and nitrogen (N) cycling, with the potential to shift forests from C sinks to C sources due to long delays in tree re-establishment. There is great interest in resolving changes to soil organic matter (SOM) composition, especially organic nitrogen, to predict how forests respond to wildfires. Therefore, the purpose of the work included in this dissertation was to improve nitrogen analysis in fire-impacted forest systems and apply these methods to soil and water samples. In the following work, a suite of advanced analytical approaches were used to determine the molecular composition of SOM, which was evaluated for the impacts of severe wildfires on microbially-mediated SOM processing and water quality in fire-impacted watersheds. Field-based soil and water samples were collected from subalpine forests in the Colorado Rocky Mountains and investigated for shifts in the water-soluble and solid fractions of SOM in lodgepole pine-dominated forests and their influence on microbial processing and water quality was determined. The objectives of this study were to leverage ultrahigh mass spectrometry to improve N analysis in fire-impacted systems (Objective 1), determine the post-fire changes to surface water C and N chemistry in reducing conditions (Objective 2) and to characterize how fire severity influences SOM composition along soil burn severity gradients (Objective 3). Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) currently achieves the highest mass resolving power in the world, which allows for the study of complex mixtures with tens of thousands of compounds that are separated by the mass of an electron across a wide molecular weight range. The most widely used FT-ICR MS analytical approach uses negative-ion mode electrospray ionization (-ESI) to selectively ionize highly abundant carboxylic acids in SOM. The application of this approach has allowed for rigorous analysis of C composition; however, -ESI FT-ICR MS vastly underestimates N-dense species which are formed during combustion. The biases associated with ionization are propagated in chemical property calculations that are determined by elemental compositions and which must be fully understood for proper use in C and N cycling models. We compared traditional -ESI with positive-ion mode electrospray ionization (+ESI) of burned soil extracts and found that +ESI increased compositional coverage by 87%, including nearly 10,000 additional N species (Objective 1). We applied our +ESI FT-ICR MS findings on a burn severity gradient (low, moderate, and high severity) to evaluate the compositional changes to SOM with increasing severity, with a specific focus on organic nitrogen. We collected soils from burned lodgepole pine forests along the Colorado-Wyoming border from two depths to characterize changes to organic N chemistry. Since organic N is the most abundant form of soil N in conifer forests, a better understanding of post-fire organic N will help address a critical gap in our understanding of fire severity-induced changes in the molecular composition of soil organic nitrogen. Nuclear magnetic resonance spectroscopy and FT-ICR MS analysis showed that N content and aromaticity of water-extractable SOM (0-5 cm depth) increased with burn severity, while minimal changes to the 5-10 cm depth were observed. Heterocyclic N species are generally higher in toxicity compared to their non-nitrogenated counterparts, which prompted soil toxicity measurements. We used Microtox ® to determine that soil toxicity increased with increasing burn severity, which may be partly attributed to newly formed N-species (Objective 2). In conjunction with increased fire activity, North American beaver (C. canadensis) populations have steadily increased since the early 1900s. The ponds that beavers create slow or impound hydrologic and elemental fluxes, increase soil saturation, and have a high potential to transform redox active elements (e.g., oxygen, nitrogen, sulfur, and metals). While surface water runoff composition has been studied in many environments, the effects of reducing conditions (i.e., beaver ponds) on these products are not well known. We collected surface water and sediment samples to investigate the impact of beaver ponds on the chemical properties and molecular composition of dissolved forms of C and N, and the microbial functional potential encoded within these environments from a combination of FT-ICR MS and metagenomics. We found that N-containing compounds and aromaticity increased in the surface water of burned beaver ponds, and that C/N and O/C ratios decreased. Microbial communities within the ponds did not have the capacity to process aromatic species, but they did have the potential for anaerobic metabolism and the potential to respire on microbial necromass (Objective 3). Fires burn heterogeneously across the landscape, and overstory vegetation likely plays a large role both in the way fires burn and how soils recover post-fire. Site factors such as soil type affect the interactions of SOM with abiotic soil components and can have cascading effects on soil C storage, including SOM partitioning between particulate organic matter (POM) and mineral associated organic matter (MAOM). POM is generally considered to have a faster turnover time than MAOM, which is physically protected from microbial degradation. Soil under two common trees in Colorado (lodgepole pine and aspen) are known to differ in SOM quantity and composition, including their relative proportions of POM and MAOM but post-fire products in these soils are relatively uncharacterized. To determine the differences in post-fire SOM between aspen and pine soils, we collected soils from under aspen and pine stands and burned them in open-air pyrocosms to minimize environmental variables which confound field-based studies. We concluded that fire influenced the dissolved fraction of the soils, with higher concentrations of dissolved organic carbon, dissolved total nitrogen, ammonium-N, and nitrate-N in burned aspen soil extracts. To determine the implications for less bioavailable carbon fractions, we will determine %C and %N in soils that have only been dried and sieved, as well as separated into POM and MAOM. We will also characterize the dissolved fractions using FT-ICR MS and NMR to evaluate differences in soil functional groups. Complementary microbiome analyses will be performed to determine the implications of shifts in soil functionality for microbial processing and C and N sequestration. The novel application of +ESI in this dissertation allowed for the identification of increasingly N-dense species at high burn severities which were not previously observed in field samples. N-dense species are enriched under reducing conditions as they are unable to be processed by local microbial communities. In total, these findings contribute to our understanding of newly formed organic C and N species in soils, with implications for microbial activity in fire-affected watersheds.
Includes bibliographical references.
2023 Spring.
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mass spectrometry
soil organic matter
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