Changes Of Soil CO₂ Concentration And Karstification In Rocky Desertification Area Of Longtan Karst Trough Valley,Youyang County,Chongqing

Soil CO₂ concentration is closely related to soil respiration,and land surface CO₂emission is strongly dependent on the accumulation and distribution of soil CO₂concentration.The karst soil is considered to be the main source of CO₂ in karst air and karst caves.In karst area,the soil layer is a key and special link.The soil CO₂concentration in surface karst zone directly affects karstification.Soil CO₂ is soluble in water and easily forms carbonic acid,it reacts with bedrock such as carbonate rock to consume CO₂ in soil,which is an important carbon sink of atmospheric CO₂.The ecological environment in karst trough rocky desertification area is fragile,which is a good combination of geological and biological effects.Understand the pattern of changes in carbon cycle in karst trough rocky desertification area can be used for land use,agricultural development and economic construction in karst trough area of China.And provides a theoretical basis for the realization of the harmonious cycle of CO₂ in karst regions.In this study,the rocky desertification area of Longtan karst trough valley in Youyang County,Chongqing,was used as the research area.The dynamic high-resolution monitoring were used to measure the temperature,precipitation,soil temperature,soil moisture,soil CO₂ concentration,and monitoring soil water and karst spring water chemical index by month(include HCO₃^-,Ca²⁺,EC,pH)on both sides of Longtan karst trough valley from June 2017 to December 2018.Exploring the pattern of changes and influencing factors of soil CO₂ concentration on daily scale,seasonal scale and different daily rainfall.And discussing the response of karst spring water chemistry and soil water chemistry to soil CO₂ concentration and karstification.The study found:?1?On daily scale,the diurnal variation of soil CO₂ concentration is consistent with soil temperature and air temperature.Soil CO₂ concentration rises with the increase of air temperature and soil temperature,and decrease with air temperature and soil temperature.The soil CO₂ concentration in daytime is greater than night.The daily peak and valley times of soil temperature behind the peaks and valleys times of soil CO₂concentration,have a hysteresis effect.In daily scale,soil CO₂ concentration in sloping slope reached valley value from 7:00 to 9:00 am,and the soil CO₂ concentration in reversed slope reached valley value from 9:00 to 12:00 am.However,the soil CO₂concentration in sloping slope and reversed slope both reached peak value at 16:00 pm.Soil CO₂ concentration reaches peak value 1 to 4 hours earlier than soil temperature,and reaches the valley value 1 to 2.5 hours earlier than soil temperature.This may be related to the rising sun's rays.Soil CO₂ concentration mainly comes from plant root respiration and soil microbial respiration,and soil root respiration and microbial respiration in karst rocky desertification area are highly sensitive to light and air temperature changes,so soil CO₂ concentration has a rapid response to the change of light and air temperature.On seasonal scale,soil CO₂ concentration have a higher content in summer and autumn,a lower content in winter and spring.Soil temperature have the greatest impact on soil CO₂ concentration in winter and spring.Soil moisture has the greatest impact on soil CO₂ concentration in summer and autumn.The correlation between soil CO₂ concentration,soil temperature and soil moisture is closely related to the climate characteristics of this area.?2?Under the rainfall event,soil temperature,soil moisture and soil CO₂concentration have different responses to different intensity rainfall events.Soil temperature,soil moisture and soil CO₂ concentration have no significant change under light rain events.Soil CO₂ concentration was mainly affected by soil temperature.Under moderate rain events,soil temperature,soil moisture and soil CO₂ concentration responded significantly to rainfall events.Soil CO₂ concentration was mainly affected by soil moisture.Under the rainstorm event,in addition to soil CO₂ concentration increase with soil moisture increase on early stage of rainstorm,the rainstorm event promoted the increase of soil CO₂ concentration.?3?Under different geomorphological area,Soil CO₂ concentration and its variation range in reversed slope are larger than that of the sloping slope.In short-term cooling and warming period,the correlation between the soil CO₂ concentration and the soil temperature?R²=0.76?in sloping slope is stronger than the correlation between the soil CO₂ concentration and the soil temperature?R²=0.46?in reversed slope.The response speed of sloping slope to the external changes?air temperature,precipitation?is faster than that of the reversed slope.Especially under the influence of rainstorm,the soil temperature and soil moisture in sloping slope are prone to extreme changes,which are mainly related to the sloping slope soil is easy to be lost,and the high rate of rock outcrops.This indicates that the rocky desertification is more serious,the soil temperature,soil moisture and soil CO₂ concentration are more sensitive to the external environment.?4?Ca²⁺,HCO₃^-,and conductivity in karst spring water and soil water peaked in hot and rainy season,and reached the valley value in low temperature and low rain season,which is consistent with the seasonal variation of soil CO₂ concentration.Soil CO₂ concentration and soil water chemical concentration in reversed slope are larger than sloping slopes.This indicates that the rocky desetification is more lighter,the soil CO₂ concentration is greater and the karstification is stronger.