| Summary: | 2018 Spring. Includes bibliographical references. Organic matter turnover and mobilization links the productivity of terrestrial and fluvial ecosystems and regulates global climate. The first part of this dissertation reviews how our conceptual framework of soil organic matter (SOM) and dissolved organic matter (DOM) cycling has evolved, and emphasizes the role of microbial communities in controlling SOM stability. Chapter two investigates how fresh carbon (C) influences SOM cycling in soils underlying two dominant Arctic plant species. We amended soils colonized by Eriophorum vaginatum—a tussock-forming sedge—and Betula nana—a competitive dwarf shrub—with glucose, and employed stable isotope tracing to quantify substrate conversion to CO2, incorporation in microbial biomass, and retention in bulk soil. We measured responses during peak biomass, fall senescence, and spring thaw to assess interactive effects of glucose amendment and season. We also captured legacy responses to amendment by assessing the fate of glucose over short, intermediate, and longer-term periods. We found that glucose conversion to CO2 was twice as high in tussock soils, while stabilization in bulk soils was significantly higher in shrub soils. Our results highlight the extraordinary C storage capacity of these soils, and suggest shrub expansion could mitigate C losses even as Arctic soils warm. Chapter three evaluates the mobilization and transformation potential DOM of flowing through an Arctic hillslope. Widespread permafrost thaw is expected to increase CO2 release from soils to the atmosphere and transform the hydrological routing of water and DOM across Arctic landscapes. We traced the mobilization potential of DOM at two landscape positions (hillslope and riparian) and from two soil horizons (organic and mineral) using bromide, and characterized the chemical composition of DOM using solution state 1H-NMR and fluorescence spectroscopy. We found that compounds mobilized through the porous organic horizon were associated with ...
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