Characteristics Of Vertical CO₂ Migration And Response Of Carbon Source And Sink In Karst Key Zone

The global carbon cycle is one of the core issues in climate change research today.Carbonate rock as the world's largest carbon pool,karst carbon cycle research is of great significance to reveal the natural carbon cycle law and global carbon "sink".The unique "binary structure" in the karst area makes it possible to study the Karst Critical Zone.At present,the study of the Karst Critical Zone is one of the innovative issues in the field of carbon cycle and karst resources and environment in the karst area.The study of the carbon cycle driven by karstification in the Karst Critical Zone has become a hot spot.Since the operation of the karst dynamic system is accompanied by changes in carbon migration,the carbon migration between phases is accompanied by changes in carbon concentration and isotope fractionation.On the Karst Critical Zone,on the one hand,CO₂ is driven by concentration to the karst underground space;on the other hand,CO₂ is transported by water as a link and participates in underground karstification,which changes the hydrochemical characteristics of the infiltrated karst water and carries karst information.Infiltrate into the cave space,contribute to the cave air environment through degassing,and record the upper karst information in the cave secondary chemical deposits through sedimentation.Therefore,the vertical upward CO₂ migration study in the Karst Critical Zone is of great significance for clarifying the main karst processes occurring in the karst key zone and the source of CO₂ in the underground karst space,and provides a scientific reference for further research on the carbon cycle of the Karst Critical Zone.Based on this,the study takes the Mahuang branch cave of the Shuanghe Cave system in Suiyang,Guizhou as an example.From October 2018 to October 2019,through field monitoring sampling and indoor experiments,combined with the characteristics of CO₂ spatio-temporal changes in key belts,stable and fixed carbon isotopes,Hydrogeochemistry and soil physical and chemical properties and other indicators,comprehensive use of GIS means and related mathematical statistical analysis methods for analysis and research.Our research indicates:?1?The average value of soil CO₂ concentration at the soil monitoring point above Mahuang Cave is 15653 ppm,and the monthly CO₂ change of the soil profile is consistent,showing the characteristics of rainy>dry season and increasing with the increase of soil depth.The soil CO₂ concentration of different monitoring points are:Forested land?Soil1#?>Dry land?Soil2#?>Idle land?Soil3#?.The content of major elements and organic carbon isotope(?¹³C)on the soil profile increase with the depth of the soil layer,while the change of soil organic matter?organic carbon,nitrogen?content is opposite.?2?The order of CO₂ concentration in the Karst Critical Zone of Mahuang Cave is: soil CO₂ concentration> deep saturation?assumed?CO₂ concentration> cave water CO₂ concentration> cave air CO₂ concentration> atmospheric CO₂ concentration,except for atmospheric CO₂,there is no obvious seasonal change In addition to the characteristics,the CO₂ concentration in the remaining belts shows the characteristics of rainy>dry season,and the seasonal characteristics of soil CO₂ are the most obvious,indicating that the changes of cave water and air CO₂ on the seasonal scale may be greatly affected by the CO₂ overlying the cave.?3?At times,?¹³C values in soil,cave water,and cave air show the characteristics of heavy dry season opposite to CO₂ and li ghter in the rainy season.The atmospheric CO₂ concentration and ?¹³CCO₂ value have no obvious seasonal changes,and the relative per formance stable,indicating that atmospheric CO₂ may have a stable source.In space,the values of ?¹³C in each phase state are: Cave drip>Atmosphere>Cave>Soil.?4?The hydrogeochemical characteristics of cave water have a positive response to changes in CO₂ migration in Critical Zone.It shows that there is a significant correlation between the ?¹³C_DIC value of Mahuang cave water and most hydrogeochemical indicators.Among them,the ?¹³C_DIC value of cave water is negatively correlated with water temperature,EC,Ca2+,HCO3-,and SIc in hydrogeochemical indicators,and positively correlated with p H,Mg/Ca,Sr/Ca,etc.?5?The carbon in the cave water mainly comes from the bedrock of the Karst Critical Zone,but its source is relatively stable.There is a significant change in the ?¹³C_DIC value of the cave water in the dry season and the rainy season,which is consistent with the change of the soil ?¹³CCO₂ value,indicating that the cave water changes are mainly controlled by the cave cover the soil with CO₂.The karst processes on the Karst Critical Zone belts in the dry and rainy seasons are significantly different.The PCP and WR effects in the dry season are significantly stronger than in the rainy season,and the carbon sink effect is also stronger than in the rainy season.