Molecular Characterization of Dissolved Organic Matter in Northern Peatlands

Northern peatlands are the most effective terrestrial ecosystems for storing carbon. While covering only 3% (>350 million ha) of the Earth's surface, they account for about 1/3 of the carbon stored in global soils. Although the formation of this large carbon sink would tend to dampen climatic warming, peatlands are an important source for the greenhouse gas methane. The chemical and biochemical processes that link solid phase peat and dissolved organic matter (DOM) within peatland porewaters are not well understood, and thus the response of these large carbon reservoirs to climate change remains uncertain. It is therefore important to investigate the composition of bog and fen peatland DOM to better understand its role and effect on the carbon cycle with global warming. In this dissertation, I will be describing the results of experiments designed to identify the differences in the reactive and refractory DOM pools from two distinctly different peatland formations within the Glacial Lake Agassiz Peatlands (GLAP) of northern Minnesota: bogs and fens. UV-Vis absorption spectroscopy, Excitation/Emission Matrix (EEM) fluorescence spectroscopy and ultrahigh resolution Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR MS) were used for DOM characterization. Although DOM, an extremely heterogeneous mixture of organic compounds, plays a versatile role in many important natural processes, its real chemical structure has remained hitherto unknown. With continuing advances in readily available highly developed mass spectrometers, the amount of information generated for analysis is steadily rising. Currently, FT-ICR MS at high magnetic fields (> 9 Tesla), is the only advanced analytical technique capable of ultrahigh resolution and mass accuracy that can distinguish upwards of 10,000 spectral peaks. This technique facilitates the identification of thousands of unambiguous molecular formulas for complex DOM. The most commonly used ionization method, Electrospray Ionization (ESI), has proven to be an excellent source for DOM ionization prior to MS analysis. ESI coupled to FT-ICR MS provides an ideal combination necessary for investigating and characterizing DOM from unique natural environments. Chapter 1 is divided into two parts. The first part provides an introduction to DOM sources, composition, sinks and the different methods used for DOM characterization so far. The second part provides an introduction to peatlands, their distribution and characteristics and the impacts of future climate change on them. In Chapter 2, a brief introduction to UV/Vis absorption and fluorescence spectroscopy, FT-ICR principles and the figures of merit which make them indispensable for complex DOM mixtures. Chapter 3 investigates acidification as a preservation technique for DOM samples that cannot be analyzed quickly or for archiving DOM samples for later analysis. Here, we show that acidification of dissolved organic matter samples prior to analytical characterization can lead to changes in the composition and optical properties of the samples. The two DOM isolation methods, dialysis and solid-phase extraction (SPE), are compared in Chapter 4. The isolates from both methods are characterized using ESI FT-ICR MS. In this study, we show that the SPE method is not very effective in extracting several major classes of DOM compounds that have high ESI efficiencies, including carboxylic acids and organo-sulfur compounds, and that out compete other less functionalized compounds (e.g. carbohydrates) for charge in the ESI source. The large abundance of carboxylic acids, likely the result of in-situ microbial production, in the dialysis-extracted DOM makes it difficult to see other (mainly hydrophilic) compounds with high O/C ratios. Our results indicate that, while dialysis is generally preferable for the isolation of DOM, for samples with high microbial inputs, the use of both isolation methods is recommended for a more accurate molecular representation. Chapter 5 shows the importance of investigating the molecular characterization of terrestrial (DOM) from the GLAP of northern Minnesota, to reveal the potentially pivotal role it plays in global carbon cycling. EEMS fluorescence spectroscopy and UV/Vis absorption spectroscopy are used to identify changes in the optical properties associated with the chromophoric fractions of DOM (CDOM) in fen and bog porewaters of the Red Lake II system in the GLAP. EEMS and absorption spectra results were also found to be in agreement with molecular characterization determined previously by others. Here, we suggest that optical spectroscopy techniques represent an effective surrogate approach to characterizing DOM provided some detailed molecular information is available for calibrating the observed correlations. In chapter 6, the three analytical techniques ESI FT-ICR MS, EEMS fluorescence spectroscopy and UV/Vis absorption spectroscopy are used to characterize the composition, reactivity, and dynamics of DOM porewaters from a fen and bog in the Red Lake IV complex of the GLAP of northern Minnesota. Compared to fen DOM, a large fraction of bog DOM appeared to be unreactive. SUVA values indicated less aromaticity in deep fen samples relative to deep bog samples. FT-ICR MS data showed high abundance of compounds with low O/C and high H/C elemental ratios in deep fen samples. Those compounds were absent in both surface fen and surface and deep bog samples, providing further evidence of qualitative differences between fen and bog DOM. These differences were attributed to either differences in vegetation and organic production or environmental factors that render DOM less reactive in the bog sites and/or more reactive in the fen sites. In chapter 7 the optical and molecular composition data of porewater DOM collected from different bog and fen sites in the GLAP are compared using the three analytical techniques ESI FT-ICR MS, PARAFAC-EEMS fluorescence spectroscopy and UV/Vis absorption spectroscopy. Our objective was to (1) compare the reactivity of fens and bogs porewater DOM from different bog and fen sites in the GLAP and see if the low reactivity of bog DOM compared to the more reactive fen DOM is valid across different fens and bogs and (2) compare between the composition of bog and fen DOM from different sites. We used PARAFAC modeling of fluorescence EEMS, SUVA254 measurements, and FT-ICR MS to investigate the chemical quality and molecular composition of DOM from 4 different bog and 4 different fen sites in the GLAP. We believe this will provide an improved understanding of the role that peatland DOM plays in the global carbon cycle and its response to climate change. To the best of our knowledge, this is the first study that correlates fluorescence components identified from PARAFAC analysis with compositional information developed by ultrahigh resolution FT-ICR mass spectrometry.