Characteristics and influencing factors of vertical carbon migration in the cave system of Liangfeng cave in Guilin

In order to study the characteristics of vertical carbon migration in open karst cave system and its influencing factors. From November 2017 to November 2018， taking Liangfeng cave in the underground river basin of Maocun village， Guilin as the research object，we conducted real-time monitoring of the atmospheric environment and cave environment in the field. At the same time, a test piece dissolution experiment was carried out at 30 cm and 60 cm of the cave overlying soil ，and the test piece dissolution and drip degassing monitoring were carried out simultaneously in the cave so as to measure the CO2 concentration and δ13C—CO2 value in the main process of vertical carbon migration.The results show，（1）The "atmosphere-soil-cave" vertical carbon migration system can affect the CO2 distribution pattern inside the cave system together with cave ventilation. The change of the ventilation direction of the cave depends on the temperature difference between the inside and outside of the cave.The ventilation direction in the cold season with little rain was from outside the cave to the inside of the cave. At this time， the vertical carbon migration ability was weak， and the distribution of CO2 inside cave was dominated by the ventilation of cave entrance. From early March to mid-September， the temperature difference between inside and outside of the cave gradually transitioned and reversed. The ventilation direction of the cave entrance was from the inside to the outside， with high rainfall intensity. The vertical carbon migration was active and dominated the CO2 distribution in the cave； （2） The intensity of soil respiration in the karst critical zone determines the amount of carbon that can be transferred by the vertical carbon migration system. The seasonal variation of CO2 distribution in the cave was essentially the response of the external environment to the vertical carbon migration system and cave ventilation； （3） The dissolution experiment of carbonate rock on the overlying roof of the cave shows that the dissolution under the soil can weaken the role of soil carbon source. The dissolution rate of carbonate rock at 30 cm and 60 cm of soil are 0.48 mol·m-2·a-1 and 0.96 mol·m-2·a-1， respectively； while the carbonate rock precipitation-degassing rate at the monitoring point of the first cave hall was 49.35 mol·m-2·a-1 and 9.07 mol·m-2·a-1 in the second cave hall. The drip degrassing of dissolved soil CO2 transported by the vertical carbon migration system to the inside of the cave was significantly.