Study On δ¹³C_DIC Ecohydrological Characteristics And Carbon Flux In Longtan Trough Valley Rocky Desertification Control Area Of Chongqing

Karst rocky desertification is great environmental problem in China.At the same time,it also has a great impact on the social life and economic activities of local residents.Alleviating the contradiction between human land relationship,production and living and environmental restoration will be an important problem faced by rocky desertification areas.The government has focused on the continuous consolidation and improvement of carbon sink capacity in karst areas.The statistical monitoring capacity still needs to be strengthened in karst areas and implement the monitoring and evaluation of the effectiveness of ecological protection and restoration of carbon sinks.Due to the unique geographical background of Chongqing karst rocky desertification trough valley area,it restricts the differential distribution of gas-water-soil-rock-biology on the dip and anti-dip slopes of the trough valley area,forming different spatial laws and variation characteristics.Taking Longtan trough area of Youyang County,Chongqing as an example,this paper analyzes the water body in the trough area Ecological and hydrological characteristics ofδ¹³C_DIC.In this paper,the soil water,groundwater and river water at the bottom of Longtan trough in Youyang,Chongqing are taken as the research objects.Conduct rainstorm monitoring and sampling in the rainy seasons of 2018 and 2021 and sampling were carried out to collect slope overland flow and groundwater samples of dip and anti-dip slopes.Day and night monitoring and stratified sampling shall be carried out during the rainless period of the pond from September 25,2021 to September 26,2021.The environmental indexes,anions,cations andδ¹³C_DIC of samples were measured.The dynamic characteristics of the dip and anti-dip slopes in Karst trough area were analyzedδ¹³C_DIC ecohydrological characteristics,and make a preliminary estimation of spring point carbon flux in the study area.research shows:（1）From 2017 to 2021,the HCO₃^-of groundwater was continuously enriched and theδ¹³C_DICof groundwater showed a negative trend,which may indicate that the carbon sink capacity of the study area will be enhanced.The HCO₃^-andδ¹³C_DICof groundwater have obvious seasonal variation characteristics.The concentration of HCO₃^-in groundwater is mainly controlled by soil CO₂.In summer,vegetation root respiration and microbial activities are strong,resulting in a large amount of soil CO₂ dissolved in water and replenished to groundwater,resulting in high HCO₃^-concentration and lowδ¹³C_DIC.In winter,groundwater and soil water with low HCO₃^-concentration and highδ¹³C_DIC.The soil layer of the anti-dip slope is thicker and the vegetation recovery is better,resulting the higher concentration HCO₃^-and more negativeδ¹³C_DIC of groundwater than the groundwater of dip slope.The dissolved inorganic carbon and carbon stable isotope in soil water-groundwater-river have obvious seasonal characteristics,and show obvious inheritance characteristics,and in the processδ¹³C is gradually positive.The migration process of inorganic carbon sink flux is not stable,so it is difficult to realize long-term carbon sink sequestration.（2）The results of rainstorm monitoring show that soil nutrients are generally lost through overland flow and groundwater at the beginning of rainfall.The dynamic change characteristics of groundwater are diversity in the difference of rainfall intensity:Theδ¹³C_DIC of groundwater are gradually negative at the initial stage of rainfall in two monitoring.At the monitoring with high rainfall intensity,theδ¹³C_DIC of groundwater are gradually positive in the middle and late stage of rainfall,and the monitoring with low rainfall intensity has the opposite characteristics.After rainstorm monitoring,indicating characteristics ofδ¹³C_DIC of groundwater in karst trough valley should take into account the influencing factors of rainfall intensity,soil and vegetation cover.The hydrochemistry andδ¹³C_DIC ecohydrological characteristics of the dip and anti-dip slopes indicate that the current situation of soil layer and vegetation restoration on the dip and anti-dip slopes is different,and different treatment schemes should be adopted.We should give priority to natural restoration or planting pioneer species in the dip slope.And we should improve slope prevention and control works,precision fertilization and avoid over development of planting industry in the anti-dip slope.（3）The monitoring indexes of groundwater-fed pool have the characteristics of daily variation due to the activities of aquatic plants.The diurnal monitoring of the groundwater-fed pool showed that the photosynthesis of aquatic plants led to the decrease of HCO₃^-and Ca²⁺concentrations and the positiveδ¹³C_DIC in the upper water during the daytime,having the opposite characteristics during the nighttime.Due to the water depth and hydrodynamic conditions,there may be two differentδ¹³C_DIC variation modes in the groundwater-fed pool.It is estimated that the absorption of DIC by aquatic photosynthesis in September is about 167.18 kg/day,accounting for 64%of the total reduction of DIC in that day,indicating that aquatic plants play an important role in natural carbon sink in karst area.（4）From 2018 to 2021,the highest carbon flux of Laoquan in the study area was mainly distributed in the wet season,which were 2652.91,5127.15,6281.55 and3686.97/（km²·mon）respectively.The lowest value of carbon sink flux is mainly concentrated in the summer drought period every year.From 2018 to 2021,the annual cumulative carbon flux of Laoquan increased year by year,reaching 11891.40,12315.94,20912.29 and 16422.14 kg/（km²·a）respectively.In 2020,the carbon flux of Laoquan is the highest,which may indicate that the vegetation restoration is better in that year.Flow is one of the main factors affecting the carbon sink flux of Laoquan.The climate factors（precipitation and temperature）of the study area ultimately affect the dynamic change of water-soil CO₂ by regulating the biological activities of surface cover,and indirectly regulate the inorganic carbon sink flux of Laoquan.