Unexpected shifts of dissolved carbon biogeochemistry caused by anthropogenic disturbances in karst rivers.

• Anthropogenic disturbances cause large shifts in dissolved carbon turnover. • River damming and agriculture modulate aquatic metabolism and CO 2 outgassing. • Dissolved carbon turnover follows the associated trajectories of biogeochemistry. • A part of dissolved carbon is cycled rapidly in karst aquatic ecosystems. • Biological carbon pump triggers overall dissolved organic carbon metabolism. Dissolved carbon (C) provides critical feedbacks to regional biogeochemical processes and global C cycling. Yet to date, the specific pathways of fluvial dissolved C turnover, particularly with human-induced shifts involved, are still poorly understood. Here, we examined dissolved inorganic (DIC) and organic C (DOC), as well as human disturbances i.e., river damming and land use in karst rivers. We show that anthropogenic activities caused unexpected shifts to dissolved C biogeochemistry. Specifically, we found that human disturbances accelerated aquatic metabolism, ultimately causing more river CO 2 generation than fixation. The extended hydrological retention by damming greatly stimulated biological utilization of dissolved C. River DOC was sourced largely from farmland and forest, while land-use fragmentation increased DOC diversity. Artificial dams and land uses intensified the transformations between DIC and DOC within karst environments. Based on these findings, we provided a process-based conceptual model regarding the rapid cycle of active C in karst waters, revealing the associated trajectories of DIC and DOC biogeochemistry. This study suggests that reducing anthropogenic disturbances essentially decelerates organic C metabolism, and therefore promotes riverine CO 2 sequestration in the context of global C neutrality. [Display omitted] [ABSTRACT FROM AUTHOR]