Homogenization of Detrital Leachate in an Old-Growth Coniferous Forest, OR : DOC Fluorescence Signatures in Soils Undergoing Long-Term Litter Manipulations Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/p5547v217

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  • Soil dissolved organic carbon (DOC) is a small but crucial part of the forest carbon cycle. Characterizing the relationship between detrital inputs and soil DOC chemistry is crucial to understanding the ultimate fate of root carbon, fallen wood and needles. Chemical differences in the DOC pool may help to explain whether fractions are sorbed to mineral surfaces and contribute to accumulation of soil organic carbon, respired as CO₂, or exported to nearby catchments. Soil solution DOC was sampled from the detrital input and removal treatment (DIRT) plots located in the H.J. Andrews Experimental Forest, OR to determine whether detrital inputs impart a detectable signal on DOC in mineral soil. Multiple types of fresh litter extracts, along with lysimeter and soil extracts from DIRT treatment plots were characterized using UV-Vis and fluorescence spectroscopy coupled with the Cory and McKnight (2005) parallel factor analysis (PARAFAC) model. Principal component analysis of 13 unique fluorophores distinguished using PARAFAC show that litter and soil extracts (Douglas-fir needles, wood of decomposition Class 1, Class 3 and Class 5, O-horizon, and 0-5 cm A-horizon) each have distinct fluorescence signatures. However, while litter-leached DOC chemistry varies by litter type, neither lysimeter-collected DOC or soil extracts show statistically significant differences in fluorescence signatures among treatments, even after 17 years of litter manipulations. The lack of observed differences among DIRT treatments suggests an exchange hypothesis whereby both abiotic and biotic mechanisms effectively homogenize organic carbon constituents within the dissolved pool. Change in PARAFAC components and optical indices with soil depth and during a 1-month biodegradation incubation of litter and soil extracts provided insight into the characteristics of PARAFAC components and the application of the Redox Index in soils. Patterns in the Freshness Index and Fluorescence Index as a result of incubation varied widely among replicates, indicating the indices may not be suitable in these soils. The results of this work emphasize the ability of sorption and biodegradation to homogenize soil DOC and demonstrate that fluorescence can be an effective fingerprinting technique for soil DOC composition.
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