Soil CO2 cycle models in karst critical zone: A case study of the soil-cave system

Soil CO2 release is the second-largest carbon flux between the atmosphere and terrestrial ecosystems, playing a critical role in regulating atmospheric CO2 concentrations. However, the carbon flux estimation, especially regarding carbon exchange between soil and underground cracks or pipes in karst critical zone, remain poorly understood. From 2019 to 2021, we conducted a three-years field measurement in Shawan soil-cave system in Southwest China, to reveal the dynamics of CO2 concentrations, temperature and moisture in the cave and the overlying soil profiles. We found that CO2 was transmitted mutually between the cave and the overlying soil through diffusion and/or convection. Biotic processes constrained seasonal patterns of soil CO2 concentrations, while precipitation and cave ventilation modulated vertical variations of soil CO2. We highlighted that soil moisture governed short-term shifts of soil CO2 in karst critical zone. The intensity of water-rock interaction at the bottom of soil profile controls the consumption or accumulation of soil CO2. Cave ventilation and air diffusion can enhance CO2 exchange at the soil-cave interface, altering CO2 levels in the overlying soil. Based on these findings, we proposed four critical CO2 cycle models for soil-cave systems in the karst critical zone, which are crucial for accurately estimating carbon fluxes in karst ecosystems.