The chemical fingerprint of solubilized organic matter from eroded soils and sediments

Organic matter (OM) transfers between solid and water phases are critical components of OM cycling in surface waters as they contribute to compositional differences between dissolved OM (DOM) and particulate OM (POM). However, fractionation effects during such phase changes are not well-characterized, especially during the release of soluble OM from sediments and eroded soils. Agricultural practices alter the magnitude and timing of OM export and constitute a major source of sediment through soil erosion. This study assessed the elemental, biomolecular, and optical characteristics of DOM solubilized from sediments and soils in an agricultural watershed of northern California to trace compositional changes during OM flow from mobilized soils in streams. Relative to mineral-bound OM, solubilized DOM was nitrogen-poor (more than doubled C: N ratios) and depleted in amino acids and lignin (three- to six-fold decrease in carbon-normalized yields). Distinct fractionation patterns of individual amino acids and lignin phenols led to a more acidic pool of solubilized DOM that appeared substantially more degraded than its source POM, with decreased degradation index (DI) values and more than doubled molar contributions of non-protein amino acids and processing ratio (PR) values. Lignin composition also greatly differed in solubilized DOM compared to mineral-bound OM, with cinnamyl:vanillyl phenol ratios up to six-fold lower in solubilized lignin than in particulate lignin. Solubilized DOM generally resembled stream DOM more than its source POM and was chemically distinguishable from DOM leached from plants. Absorption coefficients and fluorescence peak intensities were strongly correlated with solubilized DOM concentrations and composition, while optical parameters established to characterize DOM origin and reactivity such as spectral slope, fluorescence index, and carbon-specific fluorescence intensities suggested unique compositional traits for solubilized DOM compared to stream DOM. This study therefore documented strong fractionation patterns during solubilization, linking eroded soil OM and stream DOM and highlighting a pathway that can account for compositional differences between DOM and POM in surface waters. A new amino acid parameter, the Solubilization Index (SI), was defined to capture the effects of solubilization processes on OM composition. SI values in solubilized DOM were up to an order of magnitude higher than in its source POM and were similar to SI values in stream DOM. Because the SI is based on empirical behaviors of individual amino acids, each tied to mechanistic interpretations, the SI is demonstrating a strong potential for mechanistic-driven research on the extent of phase changes in streamwater DOM. (C) 2019 Elsevier Ltd. All rights reserved.