Partitioning of sediment-associated organic matter in agricultural watersheds: controlling parameters and water quality implications

Sediment-associated organic matter (OM) may constitute a significant source of dissolved organic matter (DOM) in agricultural watersheds, but the partitioning of sediment-bound OM is still poorly characterized in such systems. The Willow Slough agricultural watershed in the Central Valley of California, USA, is the focus of a study of DOM dynamics in agricultural watersheds. Weekly surface water samples collected at the watershed outlet since January 2006 show that dissolved organic carbon (DOC) concentrations increase during summer irrigation up to 7 mg/L, and steadily return to winter baseline concentrations (2 mg/L). A similar trend is observed for total suspended sediment concentrations (TSS), which peak about five weeks earlier than DOC (late June) around 200 mg/L, suggesting that sediments may contribute significantly to the delivery of DOM. We investigated the potential impact of sediment-bound OM partitioning on DOM concentration and composition in agricultural surface waters and will present laboratory data related to the equilibrium abiotic release of DOM from suspended, bed, and bank sediments, collected in the Willow Slough watershed over a range of land uses and hydrologic conditions. Desorption isotherms show distinct DOC contributions between sediment types (from 2 to 15% OC desorbed), with suspended sediment from summer irrigation yielding up to 3.4 times more DOC than winter storm suspended sediment, and 4.8 times more than summer bed sediment. In addition, environmental parameters such as water temperature, conductivity, and pH were found to affect OM partitioning differently, pH displaying the most control on DOC release (up to 60% increase from pH 6.5 to pH 9.5). Finally, a detailed biogeochemical characterization of the desorbed DOM quality (including amino acids composition) will serve as a basis for comparing riverine and sediment-derived DOM. By assessing the contribution of DOM from sediments in agricultural watersheds, this research provides a better understanding of DOM dynamics and composition at the interface between soil and water ecosystems