Carbon Cycle And Carbon Sink Effects In Typical Karst Springs In Ji'nan City

The karst carbon sink is a new carbon sink found at home and abroad in recent years and plays an important role in stabilizing carbon loss.Jinan is located in the typical northern karst area in China.It has a wide karst area,strong surface karst development,and widespread karst springs.The reputation of“Quancheng”is famous both at home and abroad.The karst carbon sink has great potential,but as of now,it has not been studied systematically.Studies have shown that the non-karst area produces exogenous water with low pH and calcite saturation.After entering the karst area,it will accelerate the chemical weathering of carbonate rocks and improve the karst carbon sink of the basin.The mountains in the south of Jinan City are metamorphic rocks of the Taishan Group,which is conducive to the increase of carbon sinks in the basin.At the same time,the mountains in the south of Jinan,as an important food and vegetable production area in Shandong Province,with the development of agriculture and urbanization,the use of large amounts of chemical fertilizers,sewage discharge,and the participation of exogenous acids?HNO₃,H₂SO₄?in the dissolution of carbonate rocks in karst areas.Does not consume CO₂ in the atmosphere or soil,but increases the amount of carbonate rock erosion and HCO₃^-content in the water.Therefore,in the process of calculating the karst carbon cycle in the watershed,its effect on the karst carbon sink effect should be considered,and the calculation accuracy of the karst carbon sink should be improved.This article take the main recharge areas of Jinan Springs,Yufu River Basin and Jinan Springs as research objects,uses water chemistry,Galy model,soil CO₂ analysis combined with stable isotopes?¹³C_DIC,?¹⁵N_NO3 analysis.Taking Jinan as the research object,through field investigation and indoor testing analysis,and access to drainage area,area of karst watershed,area of non-karst area,area of cultivated land,annual average precipitation,leakage coefficient of karst area,and leakage coefficient of non-karst area.A comprehensive study of the carbon cycle process in Jinan karst area was conducted.The effects of exogenous water and external acid in the river basin on the karst carbon cycle process were discussed.The rate and intensity of carbon sinks in the basin were estimated.From 2016 to July 2017,19 sampling points in the Jinan spring area were sampled,and three months of surface water and groundwater dynamic observations were made in the Jinan watershed.Analyze conventional water chemistry,?¹³C and?¹⁵N_NO3,and soil CO₂ is sampled,the?¹³C value is measured,and soil CO₂ concentration data is collected,etc.The contribution of exgenous water to carbon sinks and the impact of agricultural activities in the basin on the karst carbon sink process were discussed.The contribution rate of exogenous water to karst carbon sinks was accurately calculated,and the introduction of external acid?H₂CO₃,H₂SO₄?was deducted from agricultural activities.Most of the carbon sinks were estimated and a more accurate estimate of the karst carbon sink in the Jinan River Basin was made.The results show that exogenous water can increase the erosive capacity of karst water and promote the dissolution of carbonate rocks.Exogenous water is more aggressive than karst water due to its lower pH,HCO₃^-concentration,SIc?calcite saturation index?,and SId?dolomite saturation index?.After entering the karst area,the exogenous water meets the karst water,the karst water SIc and SId values are reduced,and the dissolution of karst water is enhanced.The weathering rates of silicate rocks and carbonate rocks in the Yufu River Basin were 8.06 mm·ka^-11 and 18.51 mm·ka^-1,respectively.The rate of weathering after the exogenous water flowed into the karst water was significantly increased to 20.13 mm·ka^-1.The influence of agricultural activities on groundwater chemistry has obvious time characteristics.The nitric and sulphuric acid produced by agricultural activities erode carbonate rocks and increase the concentration of Ca²⁺and Mg²⁺in the groundwater system.According to the comparison of the chemical parameters of the dry and rainy seasons,the increase of nitric acid and sulphuric acid in the groundwater system during the rainy season.Nitric acid and sulfuric acid increased the amount of dissolved carbonate rock,resulting in an increase in the proportion of NO₃^-and SO₄^2-in total anion content,resulting in a lower proportion of HCO₃^-in anions than in the dry season.Under different cropping systems,the average NO₃^-concentration in groundwater within the concentrated orchard planting area?sporadic farmland?is the highest,and the cultivated land and woodland?including orchards?in the basin area account for more than 15%of the total drainage area.Therefore,the introduction of external acid into agricultural activities has important research significance on the impact of karst carbon sinks.Through the?¹³C_DIC test of water samples,it was found that the karst springs in the pooled drainage area are characterized by the chemical composition of carbonic acid and sulfuric acid that jointly dissolve the carbonate rocks,and there is dissolution of silicate rocks by carbonic acid.According to the calculation of the water chemistry method,the HCO₃^-produced by carbonic acid,nitric acid,sulfuric acid-dissolved carbonate rocks,and carbonate-dissolved silicate rocks in the Jinan spring area was obtained,and the ratios to total HCO₃^-were 20.7%,11.7%,33.5%and 34.1%,respectively.The model estimation results show that the carbon sink rates of exogenous and pure carbonate rocks are 6.82 tCO₂·?km²·a?^-11 and14.34tCO₂·?km²·a?^-1,respectively.After the exogenous water enters the karst area,the carbon sink rate of the mixed water is15.87tCO₂·?km²·a?^-1,and the karst carbon sink rate is increased by nearly 2.3 time.The contribution rate of exogenous water is 60.08%.Accelerated dissolution of carbonate rocks by exogenous water is the main reason for increasing carbon sinks.As the exogenous acid is involved in the rock dissolution process in the karst area,resulting in the increase of HCO₃^-concentration in the water,it will interfere with the calculation of the karst carbon sink flux.The[HCO₃^-]_H2SO4+HNO3 generated from the exogenous acid needs to be carried out in the total HCO₃^-.After deduction,the calculated rate of carbon sink in Jinan Springs is12.27tCO₂·?km²·a?^-1,and the total annual average carbon sink in spring area is18407tCO₂·a^-1,which is 68.01%before deduction.Therefore,the introduction of exogenous acid in agricultural activities lead to an increase in the concentration of HCO₃^-in the water,and the calculated result was larger than the exact value.This study provides basic data and technical methods for the study of karst carbon cycle in northern China.It is of great significance for enriching the research results of carbon cycle and deepening the research theory of karst dynamics.