Regulation of Particulate Organic Carbon by Cascade Mega‐Reservoirs in the Changjiang Basin: Enhanced Sequestration and Altered Downstream Composition

Abstract Artificial reservoirs significantly alter the natural transport of suspended particulate matter (SPM) from rivers to oceans, thereby reshaping the global carbon cycle through changes in particulate organic carbon (POC) dynamics over decadal to millennial timescales. Here, we investigate dam‐induced perturbation of POC composition, transport, and fate within the Changjiang (CJ) River basin in response to the operation of cascade mega‐reservoirs (CMRs) along the Jinshajiang (JSJ) in the upper CJ. The CMRs have introduced new perturbations to SPM and POC delivery, compounding the effects of the Three Gorges Dam (TGD). We analyzed elemental, stable, and radiogenic isotopic compositions of POC, as well as the inorganic chemistry of SPM collected from both the upper and lower CJ. Since the construction of CMRs, POC sequestration in artificial reservoirs reaches approximately 6.6 megatons carbon per year (MtC yr −1 ), 3.8 MtC yr −1 of which being POC of biospheric origin (POC bio ). Notably, the flux of POC trapped in the TGD declined from 1.6 to 0.4 MtC yr −1 , while CMRs sequestered 0.7 MtC yr −1 . This shift highlights the relocation of POC burial sites from the TGD and estuary to upstream reservoirs. The rapid burial of terrestrial POC in large mountainous river reservoirs is expected to enhance POC preservation by minimizing mineralization caused by prolonged transport to estuaries. The significant reduction in sediment load and the increased proportion of POC bio due to reservoir retention have substantially altered the composition and flux of exported POC, impacting downstream and estuarine carbon cycles.