Major Reservoir Regulation Reshapes the Composition and Fate of Particulate Organic Carbon in the Yellow River

Abstract Reservoir regulation significantly alters the export and flux of terrestrial particulate organic carbon (POC), while also modifying its composition and fate within fluvial systems. However, the effects of different regulatory regimes on riverine POC persistence and transformation remains uncertain. Here, we investigated POC dynamics during both impoundment and water‐sediment regulation periods of the Xiaolangdi Reservoir (XLDR) in the Yellow River. By analyzing carbon isotopic signatures and reactive iron‐bound OC (OC FeR ), we found that under the interception of XLDR, petrogenic OC (OC petro , 54.3 ± 5.0%) replaced pre‐aged soil as the dominant component of riverine POC. The strong affinity of OC petro for fine‐grained fractions suggested it was primarily bound to clay minerals and originated from midstream 14 C‐free loess and paleosol. Due to reservoir‐induced sediment fining, OC petro enrichment outweighed the contribution of primary production to the sedimentary OC in XLDR. In contrast, FeR selectively preserved younger, modern C 3 plant‐derived OC (OC bio ) in the catchment, while preferentially absorbing phytoplankton‐derived OC in the reservoir environment. Under the sediment‐regulation mode, XLDR‐released POC underwent significant mineralization during rapid transport. Intensified hydrodynamic disturbances and oxic environment led to substantial losses or oxidation of OC petro (43 ± 10%) and most autochthonous OC from XLDR. Affected by downstream redox transitions, FeR reduction induced the desorption and release of OC FeR from suspended sediments, further accelerating OC bio mineralization. Our finding suggests that, rather than a long‐term carbon sink, reservoir may function as a transient carbon storage with its regulation regime profoundly altering the heterogeneity and preservation of riverine POC in aquatic systems.