CO 2 is an important driving force in karstification. Studying the variation characteristics of CO 2 in each zone of a karst cave system during different seasons might contribute to revealing its migration path and to further exploring CO 2 sources in caves. To this end, we monitored the soil, cave drip water, and cave air CO 2 in Mahuang Cave, SW China. We analyzed the vertical migration characteristics of CO 2 based on the carbonic acid equilibrium system theory and the stable isotopes of carbon through statistical analysis. The results show the following: (1) There are obvious seasonal changes in CO 2 in each zone of Mahuang Cave, generally indicating high rainy season and low dry season characteristics. (2) The CO 2 in Mahuang Cave is mainly supplied by the soil, bedrock, and the external atmosphere. The rainy season has plentiful rainfall, and the CO 2 in the cave is mainly derived from soil CO 2 . In the dry season, with lower temperature and little rain, cave CO 2 is mainly attributed to the external atmosphere, brought by the ventilation effect, and carbon in the bedrock. (3) Carbonate precipitation in Mahuang Cave is dominant throughout the year. The seepage migration process and carbonate precipitation are controlled by the soil "CO 2 effect" and drip water type.

Ventilation modes in karst caves are of great significance for exploring issues regarding "carbon sources and sinks" in karst areas. Therefore, this study conducted continuous monitoring of air temperature, humidity, and CO2 concentrations inside and outside the Dafeng cave in Suiyang, Guizhou from August 2015 to July 2020 in order to comprehensively analyze each element using a systematic analysis method. The results revealed that: (1) the Dafeng cave is mainly divided into three ventilation modes: ① during summer and autumn, the inside of the cave is mainly in a restricted ventilation mode, wherein air exchange inside and outside the cave is suppressed, and the CO2 concentration in the cave easily forms a cumulative effect; ②during winter, the inside of the cave is mainly in an active ventilation mode, wherein there is strong air exchange, and the CO2 concentration inside the cave is close to that outside the cave; ③ during spring, due to the variable climate and rising temperature, the inside of the cave gradually transitions from an active to restricted ventilation mode and the air exchange intensity gradually changes. The isotope data change characteristics outside the cave at the Yemingzhu monitoring site verifies the ventilation mode of the tunnel during each season. (2) Due to the influence of tourists, air changes inside and outside the cave, and the structure of the tunnel, there may be multiple ventilation modes within a single season. Tourists and tunnel structure primarily affect the structure of the tunnel. A change in the air environment outside the cave mainly affects the virtual temperature outside the cave, increasing or reducing the virtual temperature difference between the inside and outside of the cave, thereby affecting the ventilation mode. Thus, our study suggests that more consideration should be given to changes in external climate or weather conditions when studying the conversion mechanism of karst cave ventilation modes.