The Change Process Of Soil CO₂ Migration In Karst Area And Its Environmental Response Analysis

Carbon cycle is a hot topic in today's climate change research.It has an important impact on global climate change.Carbonate as the largest carbon reservoir in the world,it's karst process plays an important role in the global carbon cycle.In the global carbon cycle,soil is the largest carbon storage of the planet surface and the most active part of Karst's environment.It's CO₂ concentration is significantly affected by soil physical and chemical properties and the condition of geological and geomorphological.As the main source of air CO₂ in Karst caves,the influence of soil CO₂ on the water-gas and depositions environment in cave is far more than that of vegetation coverage,cave roof characteristics and human activities.Soil CO₂ flux is an important step to study the migration process of CO₂between soil environment and air environment and cave environment.Meanwhile,the distribution of CO₂ flux of soil vertical profile is an important way to understand the carbon cycle of ecosystems.Taking DaFeng cave of Shuanghe cave system in Guizhou province as an example,the indexes of soil CO₂ concentration and soil temperature were monitored monthly and soil samples were collected from January 2016 to December 2016.The environmental indexes such as CO₂ concentration and temperature in the cave were monitored and the samples of the drip water were collected in the cave.Combined with soil CO₂ concentration,soil physical and chemical properties,cave environment,drip water physical and chemical properties and other index data,based on Fick's law,migration coefficient model,Multivariate regression modeling to analyze the seasonal variation characteristics of soil and soil environmental indexes,the diffusion flux of soil CO₂,and the migration of soil principal elements quantitatively.Thus,the mechanism of interaction between atmosphere-soil-cave environment system could be revealed.The result showed that:?1?The CO₂ concentration and temperature of soil showed significant seasonal characteristics which in summer were higher than winter.The seasonal difference between the two indexes could reach 35758 mg/kg and 13.4?.The porosity of shallow soil was significantly higher than deep soil.The aeration porosity and total porosity of shallow soil were 17.85%and 17.75 higher than deep soil.The fluctuation of soil bulk density and soil volume water content were highest in the surface layer,and as a whole it decreased with the increase of depth.The migration ability of main elements of soil was:Na>Ca>mg>K,and the main factor which affected the migration of main elements were precipitation,soil temperature and soil CO₂ concentration that the cumulative contribution rate was 32.31%.?2?The CO₂ concentration of cave atmosphere in June to September was higher than other months,during this period,the CO₂ concentration of cave air was higher than 900 mg/kg in generally.The air humidity was maintained more than75%and the variation of pressure was less than 25 hPa.The drip water P_CO2 and drip water SIc?Calcite saturation index?had significant seasonal characteristics,but they were opposite to soil CO₂ concentration.Atmospheric CO₂ concentration and air temperature of summer and autumn were higher than winter and spring which the difference value of CO₂ concentration and temperature were 102 mg/kg and 26.2?respectively.With the influence of Siberia High,the largest air pressure difference of different season was 17.4 hPa and the precipitation difference was more than 120 mm.?3?Soil CO₂ was the main source of cave air CO₂.The lag phenomenon was existed in soil CO₂ concentration and the corresponding cave CO₂ concentration and between soil water P_CO2 and cave drip water P_CO2 showed a one-month lag characteristic too.Lag phenomenon was important to the response between soil environment and cave environment.Under the condition of hysteresis,the correlation between soil environment and cave environment was significantly higher than that under non-lag condition.The soil CO₂ diffusion flux was one of the main responses between soil environment and atmospheric environment.Under the influence of atmospheric pressure and precipitation,the diffusion flux of soil CO₂had significant seasonal variation characteristics.The response between soil environment and atmospheric environment was also reflected in the effect of precipitation on the migration of soil geochemical elements and the contribution rate was up to 12.77%.In addition,the atmospheric precipitation had a significant effect on the P_CO2 changes of cave water-gas environment.