Molecular-level exploration of spatiotemporal dynamics of fluvial particulate organic carbon sources during storm...

Identifying the origins of storm fluvial particulate organic carbon (POC) provides information about the hydrological connectivity within the river corridor and the roles of the land-stream interface in the carbon cycle. However, current understanding of storm-induced POC source dynamics is constrained by observations limited in space and time. This study presents a unique approach integrating higher spatial and temporal resolution sampling with a multi-biomarker analysis to better understand POC source dynamics across scales. Storm POC samples were collected at ∼2 h intervals at three locations along the flow trajectory of an agricultural stream during six storm events with varied storm characteristics and seasonality, and characterized for their concentrations, C and N contents, stable C isotopes, and biomarker contents. Our results showed a source transition from in-stream algal production during early storm stages to surface soils with vascular plant signatures during peak precipitation and discharge across events and stations. Biomarkers further resolved the terrestrial signature into one likely from bank vegetation and another from row crop soils. This additional separation appeared conditionally, with the magnitude and sequence influenced by environmental factors such as storm trajectory, antecedent conditions, and management/vegetation cover. Source transitions were less distinctive in the lower reaches due to the greater integration of inputs, although one storm with localized precipitation showed the opposite pattern. Both scenarios align with the expected lower hydrological connectivity downstream. With the employed approach, the evolution of the storm pulse POC as it responds to river corridor processes could be visualized both temporally and spatially.