Effects of Organic Matter Redox Reactions on Carbon Cycling in Boreal Peatland Ecosystems.

다국어 초록 (Multilingual Abstract)

Boreal peatland ecosystems are key players in the global carbon (C) cycle, storing large amounts of C in the form of organic matter while also acting as one of the largest natural sources of methane (CH4) to the atmosphere. These peatlands currently act as net C sinks, but, under global climate change, may shift from C sink to source. To predict future peatland C sink-source dynamics it is critical to better constrain the processes controlling CH4 production in peatland ecosystems and how these processes may respond to global change. The goal of my research is to investigate how redox-active organic matter (RAOM) in peat can serve as an electron acceptor for microbial respiration, thereby suppressing CH4 production. To do so, I investigated both the response of RAOM to different effects of global change and what current measurement techniques tell us about RAOM reduction in different peat types. First, I investigated how long-term changes to water-table levels in an Alaskan fen directly and indirectly affected RAOM reduction. I found that sustained changes to water-table levels over multiple growing seasons did not directly affect RAOM pools but indirectly affects RAOM reduction, likely via changes in microbial processing of the RAOM pool. To further investigate the possible resiliency of the RAOM pool, I investigated how ~10 years of warming and elevated atmospheric carbon dioxide (CO2) concentrations impacted RAOM reduction. Once again, I found that RAOM pools were resilient to these global change drivers. Taken together, both studies emphasize RAOM pools may be resistant due to their complex structure, making these compounds hard to degrade and resistant to the effects of global change on decadal time scales. Next, I investigated what insights two different techniques could give us into the relationship between RAOM reduction and CH4 production, absorbance and fluorescence spectroscopy and electron shuttling capacity (ESC). First, I investigated how optical properties of dissolved organic matter (DOM) might predict C greenhouse gas production and RAOM reduction in three different peatland types. I found that a greater percentage of protein-like components led to more CO2 and CH4 production, and these protein-like components are likely serving as the main, bioavailable electron donor in the optically reactive DOM pool. Finally, I investigated the relationship between the ESC of RAOM and CH4 production in both laboratory and field experiments. I combined the above datasets with other studies that had measured ESC across a variety of peatland types, and found that the redox state of RAOM, as measured by ESC, is a strong predictor of CH4 production in both laboratory and field incubations. The findings from my dissertation work highlight the importance of RAOM as a key control on CH4 production in boreal peatland ecosystems. Altogether, this body of work emphasizes the need to continue investigating RAOM’s response to global change to better predict current and future peatland C greenhouse gas emissions.