Contribution And Mechanism Of Microbes And Carbonic Anhydrase To Carbon Storage In Karst Soil Ecosystems

There is an missing sink of the atmospheric carbon dioxide（CO₂）.The dissolution of carbonate and silicate rocks in the crust may be the cause of this missing sink.However,due to the rate and stability of carbon sink caused by karstification,the argument of carbon sink caused by karst is questioned.Recent studies of indoor simulations have shown that microorganisms and carbonic anhydrase（CA）can accelerate karstification.Moreover,results of field observations indicate that microorganisms can increase soil carbon storage by changing soil carbon conversions.However,it remains unclear whether the microorganisms can increase karst soil carbon storage through the carbon conversion process.Understanding the role and the mechanisms of microorganisms and CA in carbon storage in karst soil ecosystems,can help clarify the contribution of karst carbon sinks to missing sink.In order to study the above problems,the following work were carried out:Firstly,the role and mechanism of purified microorganism CA in accelerating CO₂ capture of CO₂-H₂O-carbonate system were studied through laboratory simulation of CO₂-H₂O-carbonate system.And the contribution of microbial CA to the carbon storage in the karst system is estimated.Then,through a field soil column experiments,the contribution of microorganisms and CA to the carbon storage in karst soil ecosystem under natural conditions was studied.Finally,through the indoor simulation experiment of soil-limestone system,the results of field soil column experiments under natural karst conditions are verified.At the same time,the carbon conversion and carbon storage in the soil-limestone system were calculated.The mechanism of microbial and CA promoting carbon storage in karst soil ecosystem was analyzed.The main results obtained are as follows:（1）The contribution of microbial CA to CO₂ capture in the CO₂-H₂O-carbonate system was quantitatively analyzed The contribution of microbial CA to the carbon storage of the system reaches over 1/5.Flow dissolution experiment was designed to simulate the CO₂ absorption process in the CO₂-H₂O-carbonate system during rainwater infiltration or surface water scouring.The effect of CA to this system on capture CO₂ was studied,which was purified from bacterial culture.The contribution of CA to the carbon storage of the CO₂-H₂O-carbonate system is estimated.The results showed that,bacterial CA not only accelerate the reaction speed but also increase the amount of CO₂ absorbing process in the CO₂-H₂O-carbonate system.The bacteria CA contribute at least 18.9%of the CO₂ captured by the CO₂-H₂O-limestone system and 22.1%by the CO₂-H₂O-dolomite system.In summary,microbial CA can increase carbon storage in the form of dissolved inorganic carbon（DIC）in the CO₂-H₂O-carbonate system.Thus,bacteria CA contribute at least 1/5 of the carbon storage in the DIC form of the system.If estimated according to the amount of C fixed by global carbonate dissolution,the contribution of CA in the soil to CO₂ capture may reach 0.567¹.938×10¹⁴gC/a.（2）Microorganism and CA could increase the SOC form of carbon storage in karst soil ecosystem by 19.22%.A field soil column experiment system was designed.Field experiments were carried out in the Guilin Karst Experimental Site and the Maocun Silica clastic control zone to study the effects of microorganisms and CA on the soil carbon storage processes in the karst carbon cycle under natural conditions.The results of 3 years of research showed that when adding representative microorganisms,the dissolution flux of soil samples increased,soil organic carbon（SOC）content increased,soil inorganic carbon（SIC）content decreased to a certain extent,while the total amount of solid carbon in the soil（SOC+SIC）increases.When the microorganisms and/or CA activity in the soil column were inhibited,the dissolution flux of the soil column decreases and the SOC decreases.Although the SIC does not necessarily increase,but the total amount of solid carbon in the soil column decreases.In addition,δ¹³C-org values in the soil column supplemented with representative CA microorganisms were negatively biased compared with those in the control group,indicating that the increase of soil carbon storage was caused by microorganisms other than animals and plants.The representative CA-producing microorganisms and CA increased the SOC in soil by 19.22%（p<0.05）,which increased the carbon storage in soil of SOC form.（3）The main mechanisms by which microbes and CA promote carbon storage in karst soil,is to change the direction of carbon element transformation.A new type of indoor soil column experiment system simulating karst soil ecosystem was designed to accurately monitor the carbonaceous processes of all kinds of carbonaceous elements in the soil-limestone simulation system and to study the effects of microorganisms.Then the contribution of microorganisms and their CA to carbon conversion and carbon storage capacity of soil-limestone systems can be studied.The results showed that under the combined action of soil microbes and CA,the carbon stored in solid state（SOC+SIC）decreased while the carbon emitted from gaseous and liquid forms（CO₂+DOC+DIC）increased.Different types of microbes can change the carbon element transformation direction in the soil,while reducing the amount of SIC and DIC release and increasing the SOC content.Addition of microbes and their CA could lead the soilδ¹³C-org bias negative,compare to the unadulterated control,indicating a microbial-driven carbon input.In addition,some CA-producing microorganisms in karst soil can release DOC that is more stable than DIC.And then carbon storage in karst soil was formed.It is estimated that the southwestern karst area,which accounts for one third of China’s land area,can increase soil carbon storage by 2.04 Pg annually due to the effect of soil microorganisms,and has a significant ecological effects.The main mechanisms by which microbes and CA promote carbon storage in karst soil may be as follows:（1）There may be a way to fix carbon:First,CA accelerates the formation of DIC,and then soil microorganisms were converted to SOC using DIC.（2）Microorganisms directly form carbon storage in the form of SOC,or the added microorganisms weaken the predominant microbial decomposition of SOC in the soil,resulting in an increase of soil carbon storage.（3）Microorganisms use DOC or DIC to generate DOC.DOC forms stable carbon storage by burying,which is more stable than DIC.In summary,this study uncovered carbon storage in a neglected karst soil ecosystem.This study found that microbes and CA in karst soil are an important contributor to a carbon stocks which was neglected.Through this new discovery,the consistent view that microorganisms cause the release of soil carbon will be changed,in karst area.This finding helps to answer the questions about the rate and stability of karst carbon sequestration.The results of this paper provide new evidence for further enriching karst dynamics,and provide new way to increase carbon storage in karst ecosystems by regulating soil microbes.