Characteristics Of Karst Hydrochemistry And Carbon Sink At Yaji Test Site,Guilin

Carbonate rocks are the world’s largest carbon reservoir,and the carbon sinks generated by their weathering are an important part of the’missed sink’.In this study,the physicochemical parameters,water chemistry,dissolved inorganic carbon characteristics and sources of karst springs（spring S55 and spring S31）and boreholes（borehole CF1 and borehole CF2）from March to December 2022 were analysed using a combination of field tests and indoor laboratory analyses,and the water chemistry-runoff method was used to analyse The karst carbon sink fluxes on long time scales before（2009）and after（2022）vegetation restoration were compared,and the factors influencing the karst carbon sink were explored,with the following results:（1）The p H of the water bodies in the study area ranged from 6.92 to 7.91 with a mean value of 7.22,which is a weak alkaline characteristic of the karst area;the water temperature ranged from 15.57 to 28.58℃with a mean value of 20.12℃;the dissolved oxygen ranged from 0.39 to 13.33 mg/L with a mean value of 7.48 mg/L;the conductivity ranged from 364.40 to 567.80μs/cm with a mean value of 466.78μs/cm.The average mass concentrations of each anion and cation were,in descending order,HCO₃^->SO₄^2->Cl^->NO₃^-and Ca²⁺>Mg²⁺>K⁺>Na⁺,with Ca²⁺and HCO₃^-being the major ions in the study area.The type of water chemistry in the study area is HCO₃-Ca type obtained by Piper diagram and Gibbs diagram,which is mainly influenced by the weathering of rocks,whose weathering characteristics are mainly carbonate weathering carbonate rocks.（2）The analysis of hydrogen and oxygen isotopes revealed that atmospheric precipitation is the main source of recharge for karst groundwater in the study area.The dissolved inorganic carbon concentration in the water bodies of the study area ranged from 204.81 to 373.56 mg/L,with a mean value of 300.79 mg/L.The dissolved inorganic carbon at each point varied less during the dry season and more during the rainy season.Theδ¹³C_DICof karst waters ranged from-16.70 to-11.58‰,with a mean value of-14.63‰.The analysis revealed that there was a negative correlation between HCO₃^-,conductivity and dissolved inorganic carbon andδ¹³C_DICvalues.Based on the isotope mass balance principle,the contribution of carbonate rock and soil CO₂to theδ¹³C values of karst groundwater was calculated,and the dissolved inorganic carbon was obtained mainly from soil CO₂.（3）Calculating the karst carbon sink flux before（2009）and after（2022）revegetation,the average monthly carbon sink flux before revegetation was 4.89 t/km²and the annual carbon sink flux was 59.48 t/km²·a.After revegetation,the average monthly carbon sink flux was 10.25 t/km²and the annual carbon sink flux was 102.50t/km²·a.102.50 t/km²·a.The comparison shows that with the restoration of vegetation,the karst carbon sink flux also increases,and the carbon sink flux in the rainy season is significantly higher than that in the dry season.By analysing the relationship between carbon sink flux and flow and HCO₃^-mass concentration,it is found that flow is the main influencing factor of carbon sink flux.