The ballast effect controls the settling of autochthonous organic carbon in three subtropical karst reservoirs.

The change in hydrodynamics by damming facilitates the terrestrial biological‑carbon-pump (BCP) effect and promotes the generation and burial of autochthonous organic carbon (OC). To constrain the burial fluxes of OC is crucial when assessing the role of inland waters in the global carbon cycle as OC originating mainly from weathering-derived dissolved inorganic carbon is overlooked in current global carbon budgeting. Here we examined the elemental and carbon isotopic compositions of the settling organic matter collected by sediment traps in three subtropical karst reservoirs (Hongfenghu, Pingzhai and Puding), SW China. The results show that 30- 60% of the settling OC in the studied reservoirs is autochthonous. The proportion of OC correlates inversely with OC's settling flux. Interestingly, Hongfenghu Reservoir, featured by the highest trophic state and POC concentration, has the highest fraction of OC but the lowest settling flux of OC among three reservoirs. The ballast effect of biogenic silica and lithogenic materials, rather than the aquatic primary productivity, is supposed to be the primary factor that governs the settling flux of OC in the studied reservoirs. Finally, it is estimated that the settling flux of OC in the three reservoirs is 47- 119 g C/m/yr while the burial flux of OC is 10- 26 g C/m/yr if assuming about 80% of OC is remineralized after sedimentation. This study demonstrates for the first time the role of biogenic silica and lithogenic materials' input in reservoir OC's settling which may be the further important due to the strengthening agricultural activity and the increasing fast-flow hydroelectric reservoirs.