| Carbon cycle not only supports life on the earth and being the feedback of ecosystem,but also directly affects atmospheric CO2 concentration and global climate change.The Chinese Government has set the goal of reaching CO2 emission peak by 2030 and realizing carbon neutralization by 2060.Karst carbon sink can reach 16.1%of the annual reduction of industrial carbon emission in China,but it has not been taken as a specific action to deal with climate warming.Relative to conventional organic carbon sink,carbon sink from carbonate weathering is inorganic and extra.It is often studied separately in a watershed.So,its contribution to area's total carbon sink is not very clear,and the systematic evaluation has been relatively scarce.As the latest achievement of earth system science,the study of karst critical zone requires that the carbon cycle process and carbon storage in multiple spheres of the earth should be unified and evaluated systematically.The karst critical zone of Miaoguan underground river(KCZ-M)in Guohua,Guangxi was studied in this paper.The horizontal and vertical boundaries of KCZ-M were identified by a variety of technical methods.Then the carbon sink from vegetation,soil and carbonate weathering were calculated and compared by biomass survey,field experiments of standard limestone tablets and hydrochemical method.The controlling factors of vegetation ecological process and the ecological and hydrological mechanism of carbonate dissolution were analyzed.1.Through the field hydrogeological survey,the hydrological analysis based on digital elevation model,tracer experiment,and on the basis of previous studies,the hydraulic connection and vertical and horizontal boundaries of KCZ-M were further studied.It shows that the flow direction of Miaoguang River is NE,which is controlled by structure and occurrence of strata.It is an unclosed watershed with inconsistent surface and underground watersheds.The areas of underground and surface watersheds are 1.6km2 and 1.8 km2,respectively.The bottom of the conduit of Miaoguang underground river was taken as the bottom of KCZ-M.The depth of KCZ-M is about 80m.2.Based on the radiometric and geometric correction of high-resolution remote sensing images,the inversion model between vegetation index and biomass was studied.Low biomass pixels and high biomass pixels were separated to improve the fitting effect.And the average biomass was 0.45 kg/m2 and 4.0 kg/m2,respectively.The biomass pattern of KCZ-M was mainly affected by land uses.The biomass of artificial forest land,which has been growing for more than ten years,was 4?15 kg/m2 higher than that of sparse shrub grassland which has been naturally restored for 5-6 years.And it was 4?19 kg/m2higher than that of slope farmland.From 2000 to 2018,the biomass increment was 344.6t/a,which was converted into 234.8?313.1 t CO2/(km2·a)according to the carbon content and area.The carbon sink effect of vegetation was significant.Referring to the previous research results,it was estimated that the soil carbon sink was 82.2?109.6 t CO2/(km2·a).The sum carbon sink of vegetation and soil was 317.0?422.7 t CO2/(km2·a).3.Considering the influence of season,landform,hydrogeological condition and land use,a dissolution experiment was carried out in a hydrological year.Principal component analysis shows that the dissolution system of carbonate rocks has seasonal differences.It was showed that the high temperature and rainy summer is an open system and the autumn and winter is a closed system,which makes the dissolution rate and its controlling factors different.On the whole,with the increase of soil depth,the increase of soil CO2 concentration,water content and temperature promoted the increase of dissolution rate,and the soil profile was dominated by open dissolution system.Influenced by the vertical infiltration of rainwater,the time of water rock interaction and the soil water retention,even under the same land use,the difference of slope or geomorphic position will lead to the great difference of dissolution rates.Hydrogeological condition is an important controlling factor.According to the dissolution rates of tablets,the calculated karst carbon sink of KCZ-M is 17.8 t CO2/(km2·a).4.Through the monitoring of water discharge and water hydrochemistry in a year,the water balance of KCZ-M was analyzed.It was showed that the water amount of rainfall,evapotranspiration and runoff of underground river can reach balance.And the water exchange though the non-closed boundaries had little effect on the balance result.Besides,the reduced water in KCZ-M was basically the same as the water discharged into the below conduit during this observed period.The exogenous acids from rainfall and soil fertilization can only dissolve the carbonate minerals in the surface soil and bare bedrock,and the chemical type of underground river was not affected.Ca2+and HCO3-were the main ions in the underground river.Karstification in KCZ-M was affected by both soil environment and hydrological process,and runoff played a decisive role in the karst carbon sink and data of?13CDIC.According to the monthly runoff and hydrochemistry of underground river,the calculated karst carbon sink was 51.5 t CO2/(km2·a).Moreover,the calculated method of tablet dissolution rate was modified,and the new calculated karst carbon sink of KCZ-M is 43.0 t CO2/(km2·a),which was close to the hydrochemical calculation result.To sum up,in karst areas,significant organic carbon sink has been achieved by ecological measures such as returning the grain plots to forestry and artificial afforestation.And the karst carbon sink can reach 12.2?16.2%of organic carbon sink.The total carbon sink of KCZ-M was 368.5?474.2 t CO2/(km2·a),which has significant effect on atmospheric CO2 sink. |
PDF Full Text Request