Capturing and Characterizing Soluble Organic Matter Dynamics in Soil Formation Processes
Duston, Stephanie Ann
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Dissolved organic matter (DOM) is a highly complex, heterogeneous mix of compounds with diverse functional groups that contribute to several environmental processes such as organo-mineral complexation, nutrient bioavailability, and mineral dissolution. Because of these contributions of DOM to important ecosystem processes, it is often of interest to quantify the flux of DOM moving through different parts of ecosystems. Unfortunately, the complexity and variability of DOM makes quantification and chemical analysis of fluxes challenging. This thesis has two components, the first examines the potential of using four different resins for the purpose of quantifying time-integrated DOM fluxes across two source (e.g. Douglas fir and Yellow poplar) and concentration (30 and 5 mg C/L) leaf-extracts. The second explores how water soluble organic matter (WSOM) changes along spatial gradients of podzolization in a northeast glaciated headwater catchment. Findings from the resin study suggest that quaternary amine Cl- resins with a cross-linked polyacrylamide matrix and gel structure have the best suitability for in-situ sampling of DOM over time. While these resins only captured and allowed for the analysis of ~ 30% of dissolved organic carbon (C) in a series of laboratory studies, it is recognized that only ~50% of natural DOM may be ionized and sorbed electrostatically. Thus, for mass balance approaches, the use of resins would require an adjustment factor to better estimate soluble loads. Though, the observed robustness across source and concentration suggests that resins may be appropriate for indexing DOM fluxes to compare across space, time, or treatments. The second portion of this study examined chemical characteristics of water-soluble organic matter (WSOM) extracted from soils and of DOM sampled from shallow groundwater wells. Quantification of WSOM carbon content and spectroscopic analyses were used to compare samples based on genetic horizon and to compare differences along gradients of lateral and vertical podzolization. Findings show that there were significant trends in WSOM characteristics along podzolization horizon sequences which are indicative of microbial processing along the hillslope. Comparing spatial development of podzols (e.g. lateral versus vertical) found that WSOM in laterally developed E horizons are more microbial in nature when compared to vertically developed E horizons. There were also significant trends between WSOM extractions and groundwater collected from zones of soil development along a hillslope transect, which suggests some homogenization of WSOM as it is processed and transported downslope. This is evidenced by corresponding trends in fluorescence index, freshness index, and protein percent that were indicative of biogeochemical changes due to microbial processing and complexation. Characterizing WSOM can help predict trends in podzolization, and can help identify hotspots of biogeochemical processing.
General Audience Abstract
Dissolved organic matter (DOM) is made up of many different compounds that collectively contribute to several important environmental processes. The quantity and chemistry of DOM are known to vary by location on the landscape. Often, these differences are important indicators of ecosystem properties or processes. Despite the importance of DOM to ecosystem processes, sampling and analysis remain a key challenge because of DOM variability over time. This research tested a way to passively sample how much DOM moves through a location over a period of time using resins, which attract and bind a large proportion of DOM compounds so that they can be quantified. Overall, I found that one type of resin, with specific chemical and physical structure, was better at attracting and quantifying DOM and may be useful in comparing the quantity of DOM that moves through a system over time. Separately, I also looked at the chemical characteristics of DOM in different types of soils at different points along a forested hillslope in order to better understand spatial patterns of important soil processes. The results of this work identified trends in fluorescence characteristics of DOM, and indicate changes in DOM between different types of soil along the hillslope as they undergo microbial processing. There is also evidence from comparing water and soil samples that DOM from different sources become more similar as the DOM is processed and transported downslope. Characterizing DOM can help predict trends with regards to important environmental processes and allows us to identify hotspots of nutrient sources and sinks in forests.
- Masters Theses