Characteristics of dissolved organic matter (DOM) and its stabilization in forest soil Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/73666713f

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  • Dissolved organic matter (DOM) plays an important role in transport of C and essential nutrients such as N, P and S. DOM is also critical for the formation of soil organic matter (SOM), which is the largest terrestrial C pool. Nonetheless, we lack a basic understanding of what controls immobilization and mobilization of DOM. I conducted a parallel laboratory and field study to examine: 1) the effect of litter quality on DOM chemistry, and 2) the effect of DOM chemistry on immobilization in the mineral soil. For field study, an ongoing long-term manipulation of litter inputs (Double Litter=doubled annual leaf litter inputs; Double Wood=doubled woody litter inputs; and Control= normal litter inputs) in a Douglas-fir old-growth forest in the Pacific Northwest was chosen. In the lab study, degree of litter decomposition strongly influenced the chemical composition of the water extracts. For both needle and wood extracts, the hydrophobic acid fraction increased and the hydrophilic neutral fraction decreased from newly-fallen to well-decomposed DOC sources. Contrary to the laboratory results, no difference was found in composition of the 0-horizon leachate among Double Litter, Double Wood and Control treatments after 4 years of litter manipulation, in spite of significant differences in total C, total N and C:N ratios of the 0-horizon material. Possibly, microbial degradation decreased differences in DOM chemistry. Alternately, DOM production from native 0-horizon material may be much greater than from newly added litter. In field and lab studies, the removal of two acid fractions (hydrophobic and hydrophilic acids) accounted for most of the total DOC decrease. Because the hydrophobic neutral fraction was little sorbed, ligand exchange rather than hydrophobic interaction was suggested to be the major mechanism of DOM sorption. Concentrations of DOM in incoming water and DOM removal were positively correlated with a slope of ~1.0 and negative intercept regardless of season and %hydrophilic neutral, the most biodegradable fraction, of incoming DOM, suggesting that the removal was mainly abiotic (sorption) and that there was constant net release of DOM from the soil layers independent of sorption.
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