Carbon transfer from land to fluvial networks in a typical karst river-reservoir system

Although terrestrial ecosystems have been widely recognized as an important atmospheric carbon (C) sink, the net C sink capacity may have been overestimated due to C loss through aquatic ecosystems, particularly in catchments with fragile landscapes and intense human disturbances. Here, we integrated the three primary pathways of aquatic C export, including C burial, gaseous C emissions, and downstream C export, into the terrestrial-aquatic C assessment within the Wujiang River basin (WRB) in Southwest China, a typical karst river-reservoir system with cascade reservoirs. The assessment reports a net landscape C sink of 12.0, 13.8, 14.0, and 16.1 Tg C/yr in the WRB in the years 2000, 2006, 2013, and 2017, respectively, with the aquatic C export counteracting 10.6%, 11.9%, 14.6%, and 14.1% of the terrestrial C sink in these years. The aquatic C export exhibited a discernible increasing trend, indicating that dam construction and ecological restoration have profoundly altered the C biogeochemical processes and terrestrial-aquatic C transfer dynamics. Particularly, downstream C export contributed 61.8%-82.1% to the aquatic C export with approximately 72% occurring during the wet season, due largely to enhanced rock weathering and allochthonous C supply under severe soil erosion in this karst region. Organic C burial in reservoirs accounted for 0.7%-2.0% of the terrestrial C sink, which was primarily regulated by autochthonous C biogeochemical processes and terrestrial C input. Simultaneously, CO₂ and CH₄ emissions counteracted 1.2%-3.7% of the terrestrial C sink, and this counteracting effect was intensified if the gaseous emissions from depth-profile waters that are characterized by elevated microbial degradation and anoxic conditions were considered. This study emphasizes the substantial role of terrestrial-aquatic C transfer in offsetting the terrestrial C sink, which underscores the need of integrating aquatic C export for a holistic understanding of the net C sink capacity at the landscape scale.