Abstract:

Karst terrain covers approximately 20 million km² globally, with karst groundwater accounting for about 26.4% of the world’s groundwater resources. In karst regions, the CO₂-H₂O-CaCO₃ system drives carbonate rock dissolution, a key surface process that facilitates substantial atmospheric CO₂ uptake. A significant portion of this absorbed CO₂ enters karst aquifers, making these systems a critical component of carbon sequestration within the Earth’s surface system. The karst carbon cycle is fundamentally driven by hydrological processes, where the spatial distribution and regimes of water bodies significantly influence its pathways, efficiency, and ultimate sink effect. This paper synthesizes the fundamental characteristics of karst groundwater systems and reviews recent advances in understanding their carbon cycle, focusing on three key aspects: carbon sources, carbon dynamics, and carbon sequestration. Karst groundwater systems are characterized by multi-spatial-scale heterogeneity, structural instability of aquifers, high hydrological variability, frequent groundwater-surface water exchange, low self-purification capacity, and high vulnerability to contamination. These features collectively give rise to unique hydrological processes that control the pathways and efficiency of the system’s carbon cycle. Carbon within these systems exists primarily in four forms: Dissolved Inorganic Carbon (DIC), Particulate Inorganic Carbon (PIC), Dissolved Organic Carbon (DOC), and Particulate Organic Carbon (POC). The concentrations of these carbon species result from a dynamic equilibrium between endogenous and exogenous sources. Their relative contributions are co-determined by regional climatic, geological, and hydrological conditions, as well as ecological and biological factors. The dynamics of DIC are modulated by the CO₂ effect, dilution effect, and piston effect, exhibiting significant spatial variability due to differences in hydrogeological conditions and the intensity of water-rock-gas interactions. In contrast, variations in Organic Carbon (OC) are primarily governed by source effects and hydrological effects. Key carbon sequestration mechanisms within karst groundwater systems include carbon assimilation by autotrophic microorganisms, the Biological Carbon Pump (BCP) effect, and the Microbial Carbon Pump (MCP) effect, all of which show considerable potential for long-term carbon storage. Future research should prioritize developing distributed coupled water-carbon cycle models, establishing comprehensive monitoring and analytical frameworks for the entire carbon pathway (including uptake, transformation, and sequestration) within karst subterranean river systems, and formulating artificial regulation strategies to enhance the carbon sequestration capacity of karst groundwater systems. Addressing these challenges will provide essential scientific insights for deepening our understanding of karst carbon cycling, accurately assessing regional carbon budgets, safeguarding karst ecosystem security, and supporting global “carbon neutrality” goals.

Key words: karst groundwater system, carbon source, carbon dynamics, carbon fixation