Temporal And Spatial Characteristics And Influencing Factors Of Karst Carbon Sink In The Maxi River Basin

As the main transport carrier connecting the two major carbon pools of land and ocean,rivers are easily affected by biophysical and chemical processes in the process of inputting dissolved inorganic carbon（DIC）into the ocean.For example,CO₂is released into the atmosphere by degassing at the water-air interface or converted into stable organic carbon（OC）by photosynthesis of aquatic plants in rivers,thus affecting the carbon sink effect formed by karstification.In this thesis,the Maxi River in the typical karst area of Fuling District of Chongqing was selected as the research object,and the surface water samples of the selected 25 monitoring points were collected in April and July 2021.The temporal and spatial variation and influencing factors of hydrogeochemical parameters of river water were analyzed from the watershed scale,and the carbon sink generated by karstification was quantitatively estimated by using the water chemical mass balance method.The dynamic change process of CO₂at the water-air interface was revealed by the variation characteristics of SIc and p CO₂in river water and their influencing factors.The effects of CO₂degassing and aquatic photosynthesis on karst carbon sink were quantitatively distinguished by using the changes of water chemistry along the process between the upstream and downstream tributaries.According to the analysis of the monitoring data,the conclusion shows that:（1）The basic physical and chemical parameters of river water show obvious seasonal variation characteristics.Water temperature（T）is sensitive to external climate change,which is greater in the rainy season than in the dry season.However,due to the complex spatial heterogeneity of the basin,there is no obvious change in water temperature in space.Dissolved oxygen（DO）in river water is controlled by aquatic photosynthesis and temperature.The good hydrological environment in dry season makes the aquatic photosynthesis stronger,resulting in higher DO concentration in river water than that in rainy season.During the monitoring period,the p H value of the river water was greater than 7.5,and the water body was weakly alkaline,showing the seasonal variation characteristics of dry season greater than rainy season.The concentration of electrical conductivity（EC）and total dissolved solids（TDS）in water is high and varies widely,reflecting the severe karstification of the basin.Due to the obvious dilution effect of precipitation in the rainy season,it is also shown that the dry season is greater than the rainy season.（2）Combined with piper diagram,correlation,Gibbs diagram,element ratio and other analysis methods,the average concentration of river water ions and the coefficient of variation were used to determine the chemical composition,source and temporal and spatial variation characteristics of river water.The results showed that the average mass concentration of anions in river water was in the order of HCO₃^->SO₄^2->Cl^->NO₃^-,and the average mass concentration of cations was in the order of Ca²⁺>Na⁺>Mg²⁺>K⁺.HCO₃^-and Ca²⁺were the dominant anions and cations in river water,and the chemical type of river water was mainly HCO₃-Ca type.In terms of time,due to the dilution effect of precipitation,the river water chemistry showed obvious seasonal variation characteristics that the dry season was greater than the rainy season.In space,controlled by the weathering of carbonate rocks,the characteristics of HCO₃^-,Ca²⁺and Mg²⁺ion concentrations are similar and the variation is not large.SO₄^2-ions are concentrated in monitoring points 1 and 21,mainly from the weathering of mineral components in rock and soil.The concentration of NO₃^-ion fluctuates greatly and is greatly affected by agricultural farming activities.The three ions of Cl^-,K⁺and Na⁺were significantly positively correlated with each other,and the maximum concentration appeared at 3 monitoring points in the downstream of Jiaoshi Town,mainly from the input of human activities.（3）According to the variation characteristics of dissolved inorganic carbon stable carbon isotope(δ¹³C_DIC)in river water,it is very uncertain to useδ¹³C_DICto trace the source of DIC.Therefore,the hydrochemical mass balance method[（HCO₃^-）_mol-(Ca²⁺+Mg²⁺)_mol]was used to calculate the karst carbon sink in the Maxi River Basin.The results showed that the range of variation in the dry season was 0.61mmol/L～2.94mmol/L,and the range of variation in the rainy season was 0.09mmol/L～3.20mmol/L,with an average of 2.1mmol/L and 2.05mmol/L,respectively.（4）During the monitoring period,the calcite saturation index（SIc）in the river water was greater than 0,which was saturated or supersaturated,and showed the seasonal variation characteristics that the dry season was greater than the rainy season.The average p CO₂of river water was 42.66±25.73Pa and 178.54±153.79Pa,respectively.The p CO₂of river water at most monitoring points was greater than the average atmospheric CO₂partial pressure（38.4Pa）.In terms of time,it shows the seasonal variation characteristics that the rainy season is greater than the dry season,which is related to the increase of rainfall in the rainy season carrying a large amount of soil CO₂into the river.In the dry season,the p CO₂of 60%of the river water monitoring points is less than the average atmospheric CO₂partial pressure,which may be related to the strong aquatic photosynthesis in the dry season.Spatially,due to CO₂degassing,p CO₂decreased from upstream to downstream,causing HCO₃^-,Ca²⁺concentration to decrease,p H and SIc to increase,δ¹³C_DICto be positive,and the production of aquatic photosynthesis also led to the increase of DO concentration.（5）In terms of time,the karst carbon sink in the Maxi River Basin showed two seasonal variation characteristics:the dry season was greater than the rainy season,and the dry season was less than the rainy season.The seasonal variation characteristics of the dry season greater than the rainy season may be mainly related to the strong precipitation dilution in the rainy season and the lower p CO₂in the dry season.The seasonal variation characteristics of the dry season are less than those of the rainy season.This may be because the SIc in the river water in the dry season is greater than that in the rainy season,resulting in calcium carbonate precipitation.（6）In this thesis,the amount of CO₂removal from the river water-air interface and the amount of carbon fixation by aquatic photosynthesis were quantitatively estimated by the change of water chemical quality between the upstream and downstream of the tributary.The results showed that the CO₂degassing amount was 0.81mmol/L～20.98mmol/L,accounting for 19.45%～98%of the total carbon transfer.Due to the limitation of hydrological conditions,the carbon fixation effect of aquatic photosynthesis is not strong,only 0.05mmol/L～1.12mmol/L,accounting for 0.33%～13.28%of the total carbon transfer.It can be seen that the effect of CO₂degassing at the river water-air interface on karst carbon sink is greater than that of aquatic photosynthesis,which reflects the strong CO₂degassing phenomenon of the source stream and reduces the carbon sink effect caused by karstification.