Storm event dissolved organic carbon chemodiversity reveal (Dys)connectivity of hillslope sources and downstream propagation in a Midwest drained agricultural watershed

Authors: HOU, TINGYU - Purdue University; BLAIR, NEAL - Northwestern University; Papanicolaou, Athanasios; SPARKS, JANI - University Of Oklahoma; FILLEY, TIM - University Of Oklahoma

Submitted to: Hydrological Processes
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/1/2025
Publication Date: N/A
Citation: N/A

Interpretive Summary: The dissolved organic matter (DOM) plays a significant and contemporary role in exchanges between terrestrial and aquatic pools within the global carbon cycle, owing to its high mobility and reactivity. Anthropogenic activities, such as intensive agricultural practices and drainage/channel modifications, have altered the transfer transport dynamics and delivery of water flows, thereby influencing the connectivity of DOM from various source areas within the landscape. Agricultural practices, including row crop planting and tillage, significantly impact both the source and flow dynamics of DOM by replacing species of plant inputs, creating oriented surface roughness through plowing, and exposing bare soil surfaces after harvest. Storm events amplify hydrological connectivity between terrestrial and aquatic environments, facilitating the transfer of water and associated DOM from the landscape to streams and rivers. Understanding the mechanisms governing the variability in the quantity, composition, and reactivity of DOM in response to storm events is critical for better identifying and predicting carbon budget apportioning and processing within agricultural-dominated inland water systems. This research provides research ecosystem scientists, as well as water researchers, potential mechanisms for future studies aiming to differentiate DOM sources within landscapes and help inform management decisions on DOM provenance and transport.

Technical Abstract: Anthropogenic activities, such as agricultural management practices and drainage modifications, alter the modes modes of transport, i.e., surface vs. sub-surface, travel time, and flux magnitude of water, and associated dissolved organic matter (DOM) from the landscape to streams and rivers. However, the mechanisms governing the variability found in the quantity, composition, and reactivity of DOM in response to storm events, including their frequency of occurrence and magnitude, remain poorly characterized. In the Clear Creek Watershed (CCW), a tile-drained watershed representative of much of the agricultural land in the US Midwest, DOM source and composition variations were monitored across multiple storm events and scales to investigate the effects of hydro-climatological controls and landscape connectivity on DOM evolution from upland and upstream to lower streams. Stable carbon isotope composition and fluorescence spectroscopy coupled with parallel factor analysis (PARAFAC) identified the chemodiversity of DOM between potential surface and subsurface hillslope sources, with a vascular plant-derived PARAFAC component (C4) and an enriched d^13 C signal dominating in surface waters (surrogate of soil water extracts), whereas a protein-like component (C5) with depleted 13C dominated dominating in hillslope groundwater. In tile drain effluents, this DOM chemodiversity was clearly categorized, suggesting that the availability and timing of these sources to be hydrologically connected during storms were primarily controlled by hillslope flow generations and antecedent wetness conditions. Duration and sequence of storms and watershed topography (slope, cover patchiness, and riparian zone) for example in the headwaters vs. lowland areas may change the type of hillslope flow i.e., saturation excess (non Hortonian) or infiltration excess (Hortonian) thereby affecting delivery transport, reaction time and flux of water and dissolved phase of DOM. DOM dynamics in streams indicated a major contribution of surface soil and vascular plant-derived OM primarily exported through overland flow during storm-flow periods, which controls the hysteresis patterns between discharge and DOM composition. For example, during one storm, a fast delivery of a clockwise pattern observed in headwater shifted to counterclockwise patterns at lower reaches when connectivity was attenuated downstream by wider riparian areas with more perennial vegetation coverage. Moving longitudinally along a stream, lower reaches tend to diminish the DOM chemodiversity, suggesting an equal weight in the dominance of hydrological and biogeochemical processes through water diffusion and mixing together transporting and transforming DOM from upland/upstream inputs. The sampling scheme we deployed in the CCW is essential for investigating the degree of connectivity from terrestrial OM sources and subsequent processing within flow paths and downstream, which is critical for better identifying and predicting carbon processing within agricultural-dominated inland water systems.