Lynch, Laurel M., authorWallenstein, Matthew D., advisorBoot, Claudia M., committee memberCovino, Timothy P., committee memberCotrufo, M. Francesca, committee member2018-06-122020-06-072018https://hdl.handle.net/10217/189278Organic 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 plant-derived molecules, while those flowing through mineral soils had a microbial fingerprint. Landscape position also influenced the chemical diversity of DOM, which increased during downslope transport from hillslope to riparian soils. While the chemical composition of DOM varied across the landscape, the potential for rapid lateral flow across Arctic hillslopes and along the mineral-permafrost interface was uniformly high, suggesting DOM mobilization is an important mechanism of C loss from Arctic soils. Chapter four explores how geomorphic complexity and seasonal hydrology influence the cycling and transformation of DOM in alpine headwater streams. We collected surface and hyporheic water samples from two watersheds varying in channel complexity (single-thread and multi-thread) at eight time points spanning the seasonal hydrograph. We found that connectivity across the terrestrial-aquatic interface was maximized during peak discharge and decreased through the season. The chemical composition of DOM, evaluated using electron impact gas chromatography mass spectrometry and fluorescence spectroscopy, varied with watershed connectivity, with increasingly divergent DOM profiles observed with a loss of hydrologic connectivity. We suggest that widespread channel simplification, resulting from land-use and management changes, will reduce DOM processing and compromise ecosystem function.born digitaldoctoral dissertationsengCopyright and other restrictions may apply. User is responsible for compliance with all applicable laws. For information about copyright law, please see https://libguides.colostate.edu/copyright.dissolved organic matter chemistrymicrobial metabolismstable isotope tracingglobal change biologyArctic carbon cyclingmicrobial substrate use efficiencyText