Graduate Thesis Or Dissertation
 

Sources of Soil Carbon Loss from Soil Density Fractionation: Soluble Pools or Methodology?

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/w0892j08t

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  • Soil is the largest terrestrial store of carbon (C) making it a critical component of the global C cycle. To model global carbon budgets more accurately we need to understand dynamics and turnover of the many different functional soil C pools that exist in soil. Many different methods are used to fractionate soil organic C including both chemical and physical methods. One of the most popular methods is soil density fractionation (SDF). This method fractionates soil based on density which effectively separates the mineral-associated components from the free organic components of soil. However; several studies noted differences in the total amount of soil organic C (SOC) in samples before SDF compared the sum of the organic C in the density fractions. The amount of SOC lost with fractionation was variable, leading to the central question of this thesis: are the losses of SOC after SDF an artifact of laboratory techniques, or do these losses represent a real pool of potentially soluble C that can vary by ecosystem and/ or season? I hypothesized that in the Oregon summer-dry climate soluble organic products of decomposition accumulate in soil over the summer when temperatures and microbial activities are high and precipitation is low, and during the fall and winter these soluble products are leached with heavy precipitation. However, lab handling techniques such as air-drying soil could also have an impact on SOC losses with fractionation: microbial activity can persist in soils if slowly dried over time in the lab, yielding increased soluble products. To address these divergent hypotheses, soil samples were collected seasonally and subsamples were subjected to 4 different laboratory handling procedures prior to fractionation: air-drying and oven-drying to simulate common drying techniques; leaching to evaluate if potentially soluble C represented a pool that could be removed with simulated precipitation; and immediate fractionation of fresh soil to determine if drying of soil caused artifacts in pools of potentially soluble soil C. Results showed that the light fraction (LF) made up a mean of 58.30  1.81% of total C and the heavy fraction (HF) made up a mean of 32.78  1.07% of total C. These proportions remained consistent with season and treatment. Total soil mass loss and total C loss was on average 8.8  1.3 % and 8.36  1.36 %, respectively, indicating that loss of high C-LF did not dominate over low C-HF. No seasonal or treatment trend was found in either soil mass or soil C loss. C loss as DOC was greatest in the oven dried treatment compared to other treatments across all seasons. DOC loss on average made up 23% of total C loss while particulate loss accounted for 73% of the total C loss. This study suggests that oven drying soil can lead to DOC loss while air drying soil does not impact DOC. When using the SDF method careful steps should be taken to minimize particulate loss as this study shows it contributes to the bulk of total C loss. In addition, oven drying of soil should be avoided, although air drying appears not to contribute to either DOC loss or changes in LF or HF recovery.
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  • Intellectual Property (patent, etc.)
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  • 2020-03-25 to 2021-04-25

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