Land-use management and climate change can enhance the autotrophic capacity and reduce the CO2 emissions of karst aquatic ecosystems.

The carbon flux involved in aquatic metabolism in karst surface waters is an important component of both regional and global carbon cycling. Yet, the mechanisms of how aquatic metabolism and the related carbon flux respond to human land use and climate change in a high-pH aquatic environment remain unclear. To address this, we conducted continuous high-frequency (15-min interval) monitoring of hydrochemical parameters, combined with a bookkeeping model and gas transport velocity model, to estimate the aquatic net ecosystem primary (NEP) and water-air CO2 exchange flux (FCO2) under different land-use types at a simulation test site. We then used a structural equation model (SEM) and Random Forest model (RF) to determine the relationship between NEP, land-use type, and climatic factors, and to determine how NEP variations alter the FCO2. The results showed that the annual NEP of karst surface water systems under bare rock (0.01 g C m-2 day-1) was significantly lower than under vegetated land (shrubs, grass and cropland, 0.38-0.75 g C m-2 day-1). This high NEP demonstrates a strong autotrophic capacity and the potential to reduce CO2 emissions in these aquatic systems. Our results also suggest that differences in groundwater HCO3- inputs between bare rock/soil and vegetated land can explain their NEP differences. We applied the RF model to predict the variation of the NEP of different land-use systems by the end of this century, under different CMIP6 scenarios. The results suggested that land-use regulation (the conversion from bare rock or soil to grass or shrubs) can increase the autotrophic capacity of karst surface systems by 42.3 % (SSP126) and 51.5 % (SSP585). The results of this study indicate that human land-use change can potentially enhance the autotrophic capacity and lower the CO2 emissions of high-pH karst aquatic ecosystems.