Northern peatlands store significant amounts of carbon as organic matter in water-saturated, anoxic peat. Redox transformations of electron-donating phenols and electron-accepting quinones in peat organic matter are increasingly recognized as key processes that govern carbon cycling in peat soil, including the release of the greenhouse gases carbon dioxide and methane from these systems. First, phenolic dissolved organic matter (DOM) in peat is considered to inhibit the activity of extracellular hydrolytic enzymes, thus decreasing peat decomposition rates. Increasing oxygenation events as a result of changing climatic conditions may lead to enzymatic oxidation of these phenols and thus loss of their inhibitory pressure, leading to accelerated decomposition of peat organic matter. Second, quinone moieties in peat organic matter in water-saturated, anoxic peat are considered to serve as terminal electron acceptors for anaerobic respiration. Respiration to quinones under formation of the corresponding hydroquinones is considered to suppress methane emissions from peatlands, provided that the electron-accepting quinones are regularly regenerated through oxidation with O2. Despite the importance of phenols and quinones to peatland carbon cycling, little information was available on the concentration ranges of these moieties in peatlands as well as on their redox transformations in oxidative and reductive environments. This lack of information reflected analytical challenges associated with the quantification of these moieties in organic matter. The goal of my dissertation research, summarized herein, was to overcome existing knowledge gaps by (i) developing analytical approaches to quantify the redox properties of peat organic matter, (ii) providing a systematic characterization of the redox properties of peat organic matter and (iii) elucidating changes in the redox properties of peat organic matter during key reduction and oxidation processes that occur in peatlands. The research involved both field and laboratory studies.
In the first part my PhD research, I developed and validated a flow-injection analysis (FIA) system coupled to electrochemical detection to accurately determine the redox properties of dilute DOM under defined EH and pH conditions. This novel system has significantly higher sensitivity to oxidizable and reducible moieties than previously employed analytical techniques. Furthermore, the system allows for automated sample analysis which was not previously possible. The development of this method was instrumental to the characterization of the redox properties of peat DOM in the next part of my PhD research.
In the second part of my PhD work, I used the FIA system to quantify the number of electron-donating and electron-accepting moieties in peat DOM collected from pristine ombrotrophic bogs in Värmland, Sweden. DOM sampled from anoxic peat pore water in these bogs had high phenol contents as compared to a diverse set of model DOM isolates from different aquatic and terrestrial environments. Analysis of DOM collected from oxic pools located in the same bogs rather than anoxic pore water revealed that the pool DOM had lower phenol contents, suggesting oxidative processes in the pools that resulted in a loss of phenolic moieties. Consistently, incubating peat DOM with phenol oxidase under oxic conditions resulted in irreversible oxidation of phenols. The finding of oxidative removal of phenols in peat DOM supports that oxygenation events remove phenols in peat organic matter and thus possibly also their inhibitory role in peat organic matter decomposition. The FIA analysis further demonstrated, for the first time, the presence of electron-accepting quinones in peat DOM from both, peat pore water and pool water. Electron transfer to peat DOM over a cycle of electrochemical reduction and subsequent O2 reoxidation was fully reversible, supporting the hypothesized role of quinone moieties in DOM as regenerable terminal electron acceptors.
In the third part of my PhD research, I switched focus from DOM to particulate organic matter (POM) in bogs. More specifically, we studied the redox state and reactivity of POM in anoxic peat with dissolved O2 directly in the field. Using push-pull tests with dissolved O2 as reactant, we demonstrated rapid and complete consumption of dissolved oxygen injected in anoxic peat as a result of abiotic electron transfer from reduced moieties in POM to O2. We thereby demonstrated that POM was in a highly reduced state and that the reduced moieties in POM rapidly transfer electrons to oxygen, consistent with the hypothesized role of peat POM as a regenerable terminal electron acceptor in temporarily anoxic parts of peats. Finally, we highlighted the potential of using consecutive oxygen push-pull tests to quantify the total reducing capacity of POM directly in the field.
In the last part of my PhD research, we demonstrated the applicability of the developed FIA system to monitor changes in the redox state of DOM also in the context of chemical water treatment. To this end, treated DOM samples and DOM isolates with increasing specific doses of chlorine and ozone and subsequently quantified the resulting decrease in electron-donating phenols in the DOM samples. The results from this work highlight the potential of the FIA system to be employed in routine analysis during chemical water treatment to monitor changes in the redox state of DOM.
Taken together, my PhD research summarized in this thesis advances analytical methods to quantify phenols and quinones in organic matter in environmental and engineered systems and contributes to a more holistic understanding of the role of these moieties in the carbon cycling of peatlands
