Variation Of CO₂ Content In The Overlying Soil Of Xueyu Cave And Its Influences On Chemical Characteristics Of Cave Drip Water

Karst dynamics system is a solid-gas-liquid phase system,featured with its unbalanced characteristics,responding quickly to the external environment.Research on transport and conversion mechanism of CO₂ in karst dynamic system and the quantification of carbon input and output in the system have very important theoretical significance to solve the problem of“missing carbon sink”in the carbon cycle.As the input section of the karst dynamic system,soil CO₂ is one of the driving forces and the important material source of the entire system.On the one hand,the changes in content of soil CO₂ affect the intensity of karstification and carbon sink efficiency will affect the chemical characteristics of the water in karst aquifers.On the other hand,it also determines the CO₂ partial pressure difference at the soil-air interface,which directly affects the CO₂ efflux at the soil-air interface.Studying the characteristics of soil CO₂concentration in the karst area and the chemical characteristics of the dripping water in the underground cave is of great scientific significance for understanding the driving mechanism and process of the karst dynamic system,as well as improving the carbon cycle model in the karst area.Xueyu Cave in Chongqing was selected as the study area.From February 2015 to January 2018,the high-precision monitoring of precipitation,temperature,soil CO₂content,soil temperature,and soil moisture above the cave was performed using the automated instruments.We use gradient method based on Fick's First Law to calculate the soil CO₂ efflux.Inside the cave,we selected four drip sites and monitored their water chemistry indicators monthly(including conventional anions and cations such as Ca²⁺,Mg²⁺,and HCO₃^-,and geochemical index such as EC,pH,pCO₂,and SIc).By considering the interaction between the lithosphere,biosphere,and atmosphere,we discussed the variation and influencing factors of CO₂ concentration and its efflux on the daily scale,rainy scale,and seasonal scale.Furthermore,we analyzed the response processes of the dripwater chemistry in the cave to the variation of soil CO₂ content.The study found:?1?On the daily scale,the differences in soil moisture and soil temperature conditions and their daily fluctuations in different seasons caused different daily variations and fluctuations of soil CO₂ concentration and CO₂ efflux in different seasons.In the spring and autumn,the main influencing factor of soil CO₂ concentration and efflux was soil temperature,both soil CO₂ concentration and efflux was correlated with soil temperature well.In summer,the increase of root respiration intensity makes soil CO₂ concentration and efflux subject to the transfer process of"new carbon"which fixed by recent photosynthesis,so there was a lag between soil temperature and soil CO₂ concentration.In winter,soil CO₂ concentration and efflux had no obvious daily fluctuations due to the limitation of soil temperature.?2?During the rainfall event,the variation and fluctuation of CO₂ concentration and efflux varied depending on the rainfall intensity.Soil CO₂ concentration in the heavy rain event was significantly reduced,while it increased with different amplitudes during the light and moderate rain events.The rate of increase was proportional to the intensity of the previous rainfall.The CO₂ efflux dropped sharply during the rain,and the decrease is proportional to the rainfall intensity.Based on the principal component regression model,it is found that soil moisture is the main factor that causes changes in soil CO₂ concentration and efflux during rainfall events.With increasing rainfall intensity,this dominant role of soil moisture was also increasing.Because rainwater carried CO₂ infiltration and was involved in karstification,the recovery of soil CO₂efflux after rain was not as non-karst area.The soil CO₂ efflux did not exceed the pre-rain level after three rainfall events.?3?On the seasonal scale,the variation of soil CO₂ concentration and efflux were consistent with the seasonal variation of soil temperature and moisture,but the main control factor varied in different seasons.During the dry season,soil temperature was the main influencing factor of CO₂ concentration and efflux,both of which changed with soil temperature.During the rainy season,soil CO₂ concentration and efflux were affected by the soil temperature and humidity.The influence of soil temperature on CO₂concentration and CO₂ efflux has a threshold.When it was less than this value,the soil temperature showed a promoting effect,and when it was greater than the value,the soil temperature showed a inhibition effect.Being different from the soil temperature,the increase of soil moisture always promoted soil CO₂ concentration,but it also had a“bidirectional”control effect on soil CO₂ efflux,and only when the soil moisture was<15%or>26%,the soil moisture was the main factor controlling the change of CO₂efflux.?4?The Ca²⁺,Mg²⁺,and HCO₃^-concentrations and geochemical indexes like EC,pH,pCO₂,SIc of drip water at different sites showed a good response to changes in soil CO₂ concentration.The increase of soil CO₂ concentration caused the rise of pCO₂,Ca²⁺,Mg²⁺,HCO₃^-,EC,and the drop of pH and SIc in the drip water.However,as a key indicator of the extent of drought outside the cave,the Mg/Ca ratio showed different variation characteristics among different sites,the Mg/Ca ratio in"steady"drip sites had a distinct seasonal variation while it was very stable in"sensitive"drip sites throughout the year.On the contrary,ion concentration at the“sensitive”drip sites showed a“dilution effect”in the rainy season when high-intensity rainfall happened,while the ion concentration in the“stable”drip sites was relatively stable.This indicates that there were differences in the intensity of PCP and dilution effect from sites to sites due to the different migration paths,so that some hydrochemical indicators of different dripping sites have a differential response to the changes of CO₂ content in the overlying soil.