Study On Dissolution Process And Carbon Sink Effect Of Carbonate Minerals In Loess

The Loess Plateau in Northern China is a huge carbon pool with the inorganic carbon storage of8.5×10² PgC,and the scale of intercepting atmospheric carbon is also about 1.5 million tons of carbon per year.Carbonate minerals are very rich in Chinese loess,and their content can be up to 20%?of which about90%are secondary carbonate minerals?.Like the dissolution process of carbonate rocks,carbonate minerals in loess consume CO₂ in the atmosphere/soil under water dissolution to form carbon sinks and participate in the global carbon cycle process.Due to the dispersed structure,the fine particles,and the large water-soil contact surface of the carbonate minerals in the loess,the dissolution process of the carbonate minerals and increases the carbon sink flux is accelerated.Therefore,the study of carbon sinks in the Loess region is of great significance in the estimation of the global carbon budget.The Qingliangsigou watershed in the Loess District of Luliang County,Shanxi Province was selected as the research area to analyze the content,distribution characteristics and influencing factors of inorganic carbon in the loess in different land use in the area;study the ion sources in the loess water and explore the differences through ion ratio and isotope analysis end element contribution ratio;evaluate the dissolution and carbon sink effect of carbonate minerals and silicate minerals in the watershed using hydrochemical runoff method and method of limestone tablets to.Finally,considering the results of the calculation of carbon sinks in the loess area by the method of limestone tablets are relatively small,the dissolution test data of karst watersheds in different climate zones in China are compared and analyzed to explore the influencing factors of this method and its suitability in the calculation of loess carbon sinks.The main conclusions are as follows:?1?Compared with the loesses in the world,the loess in the study area shows lack of SiO₂,Fe₂O₃,rich in CaO and MgO,and its calcium carbonate content is between 1.80%and 18.31%,with an average value of 11.64%.The order of soil organic carbon content in different land use is field cropland>wasteland>fruit forest land;and soil inorganic carbon content order is field cropland>fruit forest land>wasteland.The soil CO₂ content of the different land use in the study area varies greatly:the range of CO₂ content in fruit forest land,field cropland and wasteland soil is 0.210%⁰.643%,0.246%⁰.603%,0.190%⁰.677%,respectively.Among them,the apricot field has the highest CO₂ content in the fruit forest land,and the soybean field has the highest CO₂ content in the field crop field.The crop types,root development,soil microbial activity,and farming model is the factors that influence soil CO₂ concentration.From the perspective of carbon sequestration and increasing sinks,it is more favorable to plant soybeans in field crops and plant apricot trees in fruit trees.In addition to land use,soil respiration is also affected by hydrothermal factors and temperature.Its response to temperature is highly dependent on soil water content,and the rate of soil CO₂generation is in ture more affected by the coupling of hydrothermal factors.?2?Water chemistry types of the Qingsisigou Loess Valley are bicarbonate-sodium-type(HCO₃^--?SO₄^2-Na⁺)?for surface water?and bicarbonate-sodium-type?HCO₃^--Na⁺??for groundwater?.The end-member element analysis of loess water chemistry using stoichiometry,Gibbs graphical method and weathering parameters combined with the Galy forward model shows that the loess water chemistry in the study area mainly comes from atmospheric subsidence,human contribution,evaporative salt minerals,silicate minerals and carbonate minerals.Their contribution ratios for the total dissolved matter in the watershed are:1.66%,6.32%,10.38%,62.23%,and 19.31%,respectively.Long-term water-mineral interaction and fine silicate mineral particle morphology in the loess area are the main reasons for the contribution of silicate mineral dissolution.The cation displacement reaction,soil-salt leaching and evaporation,and human input also contribute to the dissolution of silicate minerals.?3?The average chemical weathering rate of loess minerals is 9.31 t/?km²?a?,and the consumption rate of CO₂ in the atmosphere is 6.34 t CO₂/?km²?a?,which is significantly higher than that in the Sanchuan River karst basin at the same latitude(5.28 t CO₂/?km²?a?.Based on the hydrochemical runoff method calculation,the atmospheric CO₂ consumption of mineral chemical weathering in the basin is 0.18?10⁴ t/a of which the carbonate mineral weathering consumption accounts for 44.5%,i.e.0.81?10³ t/a;weathering consumption of silicate minerals accounts for 55.5%,which is 0.10?10⁴ t/a.the analysis of DIC end-member contribution and the contribution ratios of different carbonate mineral show that the contribution rate of atmospheric/soil CO₂ to DIC in groundwater in the watershed varies from 37.22 to57.61%,with an average value of 48.45%.The contribution of primary carbonate minerals to DIC of groundwater is between 16.86 and 40.05%,with an average value of 30.03%.The contribution of secondary carbonate minerals to the DIC of groundwater is between 17.70 and 26.21%,with an average value of 21.52%.?4?The carbon sink strength calculated by the method of limestone tablets is only 1/15 of the carbon sink strength calculated by the hydrochemical runoff method.In order to clarify whether the dissolution test method is suitable for application in the loess area,we collected data on the dissolution of 30 dissolution test points from north to south in China,and at the same time selected the Nanchuan River Basin and the middle and upper reaches of the Wujiang River Basin for comparative analysis,including the impact factors analysis such as climatic conditions,soil physical and chemical properties,and land use of the dissolution rate of the tablets.The analysis shows that the effect of precipitation on the dissolution rate is greater than that of temperature,especially on the dissolution rate of the tablets under the soil.At the same time,the results show that:Inorganic carbon in the soil is the main reason for the carbon deposition on the surface of the test piece and the calculation result of the carbon sink is obviously small.Therefore,when using the method of limestone tablets to calculate carbon sinks in the loess area,the effect of high content of soil inorganic carbon on the calculation results should be fully considered to avoid the dissolution test method directly to calculate the dissolution rate and carbon sink strength in the soil environment with high content of inorganic carbon.