Coupled water-carbon cycling in karst regions: a review of processes and modeling

The role of inland water bodies in the global carbon cycle is receiving increasing attention, particularly in karst regions where high hydrological connectivity and abundant carbonate rocks have resulted in active carbon cycle processes that respond rapidly to global climate change. Consequently, karst regions have emerged as a focus of global carbon cycle research. While water movement drives the carbon cycle, the complex interplay among biogeochemical processes, hydrodynamics, and anthropogenic interventions has hindered a comprehensive understanding of the coupled water-carbon cycling processes in karst environments. Especially, the carbon storage mechanisms in karst regions are not well understood. In addition, there is an urgent need to improve the applicability and accuracy of models in coupled karst water-carbon studies. This study provides a systematic review of the unique water-carbon cycling processes in karst regions, including rapid epikarst-conduit-surface exchanges, hydrological regulation of dissolved inorganic versus organic carbon partitioning, and microbial-geochemical feedback mechanisms. We emphasize the regulatory controls and carbon sequestration mechanisms, especially the importance of microbial dark fixation in groundwater for the regional and global carbon cycle. Additionally, we summarize recent advances in water-carbon coupling modeling within karst regions, with the aim of identifying more effective methodologies for quantifying these coupled cycling processes. Furthermore, we compare existing models by comparing their strengths, limitations, and applicability to karst systems and discuss how emerging technologies (e.g., machine learning, isotopic tracers) can improve the accuracy of models. Looking ahead, continued long-term and large-scale monitoring and simulation efforts are recommended to better understand the coupled water-carbon cycling dynamics in karst regions. An in-depth exploration of the multifactorial mechanisms driving these processes can contribute to an improved understanding of the interactions between water and carbon cycles in karst surface water ecosystems. Additionally, we propose in-depth studies of microbial carbon sequestration mechanisms in karst groundwater and their integration with global carbon modeling. This research will also serve as a foundation for evaluating the carbon sink potential and guiding the ecological management of karst regions.