The Carbon Sequestration Effect In Semi-arid Karst Area

The continuous rise of atmospheric CO₂ concentration has led to the intensification of global climate warming,resulting in higher temperature,increased evaporation and intensified water cycle,which has led to more and more frequent extreme weather events such as global rainstorm and drought,and has brought huge losses to the lives and property of the country and the people.Taking effective methods to increase carbon sinks and sequester carbon to inhibit the rise of atmospheric CO₂ concentration and slow down global warming has become the top priority for scientists in various countries,whose study global change.Karstification is a special geological process occurring in the shallow surface environment of the earth,which is closely related to atmosphere,hydrosphere and biosphere.As a low-temperature geochemical open system,karst dynamic system is characterized by open sensitivity and biological participation.It responds quickly to environmental changes and actively participates in the global carbon cycle.There are mainly four types of karst in China,namely,southwest tropical subtropical karst,north arid and semi-arid karst,northeast wet temperate karst and Qinghai-Tibet alpine karst.The arid and semi-arid karst in the north lies to the north of 35 N.Qinling is the boundary zone of karst types in the south and north.Carbonate rocks in arid and semi-arid karst areas in northern China have an area distribution of68.5×10⁴km²,of which the exposed area is 7.78×10⁴km²,the coverage area is 8.74×10⁴km²,and the buried area is 51.95×10⁴km²,which is rich in karst groundwater resources.However,compared with the karst areas in the south of China,the research work on karst carbon sink in the north is less.On the one hand,due to the high content of secondary carbonate in the soil of the northern karst region and its easy deposition,there is a big error in calculating the intensity of karst carbon sink by using the test piece method.On the other hand,karst water in the north mainly occurs in the Cambrian-Ordovician strata with an older horizon,while coal mines occur in the Carboniferous-Permian strata with a newer horizon.The special hydrogeological pattern of"coal is upstairs and water is downstairs"leads to sulfuric acid generated by sulfide oxidation in coal measures strata or acidic water generated by leaching of a large amount of piled slag by rainwater,which continuously infiltrates into karst aquifer and participates in carbonate dissolution,thus reducing carbon sink effect.In addition,the Ordovician limestone stratum contains several gypsum interlayers,which dissolve and input a large amount of SO₄^2-.Under such complicated conditions,which method should be adopted to accurately calculate the intensity of karst carbon sink and how to deduct sulfuric acid to calculate karst carbon sink flux are two important aspects of current karst carbon sink research.In this study,the representative Liulinquan karst basin in northern semi-arid karst region is taken as the research object.Through dissolution test piece method and monitoring of soil chemical indexes,and isotope technology is used to distinguish primary and secondary carbonates,the proportion of secondary carbonates in soil carbonates is quantitatively calculated,and the characteristics and sources of carbonates in soil layers in arid and semi-arid karst regions in China are revealed.The mechanism of soil system affecting dissolution and sedimentation in semi-arid karst region is clarified,and the problems existing in calculating karst geological carbon sink effect in semi-arid region by karst test piece method are solved.At the same time,systematic sampling and research were carried out on karst groundwater in the basin,revealing the evolution law and influencing factors of dissolved inorganic carbon cycle in karst dynamic system.Galy model is used to quantitatively evaluate the karst carbon sink effect of the basin where sulfuric acid participates in dissolution.It reveals that sulfuric acid participates in the process of karst carbon sink under dissolution.The following conclusions can be drawn:1.The dissolution rate of test pieces is affected by different vegetation conditions and controlled by soil chemical indexes.?1?Vegetation condition is the main factor that affects the dissolution rate of carbonate rocks.The dissolution rate of test pieces in the same horizon shows the rule that forest land is larger than shrub and shrub is larger than grassland.With the positive succession of vegetation,the dissolution rate of carbonate rocks has an increasing trend.?2?The dissolution rate of test pieces under soil is mainly controlled by soil organic carbon,soil inorganic carbon and soil moisture content,but is less affected by soil CO₂ concentration.Soil organic carbon content and soil water content have a promoting effect on the dissolution rate of the test piece,while soil inorganic carbon content has an inhibiting effect on the dissolution rate of the test piece.?3?The mass fraction of inorganic carbon in soil is 2.85⁵.06 times of that of organic carbon,which is easy to supersaturate and precipitate under the climate conditions of drought,little rain and large evaporation.As a result,the test pieces buried in the soil layer are affected not only by dissolution,but also by soil carbonate deposition,resulting in the mass increase of some test pieces after dissolution during one hydrological year.Therefore,the test piece buried in the soil is corroded in the rainy season and becomes lighter in weight,while in the dry season,it mainly receives carbonate deposition and gains weight.?4?According to the standard dissolution test piece method,the average karst carbon sink intensity in the exposed karst area is calculated to be 1.51 t/?km²·a?,while according to the Galy model method,the calculated result is 3.93 t/?km²·a?,and the test piece method is only 1/3 of the Galy model method.Due to the high content of inorganic carbon in soil and easy deposition in semi-arid areas,the dissolution rate of test pieces embedded in soil system cannot truly represent the dissolution rate of bedrock surface,and the strength of karst carbon sink will be underestimated by the dissolution test piece method.The use of dissolution test pieces to study the effect of karst carbon sink is limited in the soil of semi-arid areas in northern China.2.The proportion of secondary carbonate in soil is high,and the deposition of secondary carbonate is the main reason for the weight increase of test pieces in soil.?1?The content of soil carbonate and its?¹³C value in the study area are mainly controlled by the proportion of secondary carbonate,which has light carbon isotope.The carbonate content in the upper layer?0⁵0 cm?of the soil profile in woodland,cropland and shrub land increases with the increase of depth,while the?¹³C value of carbonate gradually decreases with the increase of depth,and the change trend in the lower layer?50⁷0 cm?of the profile is opposite.This is due to the fact that soil secondary carbonate dissolved in soil water moves downward and deposits,but the lower layer?50⁷0 cm?is limited in depth by the downward movement of soil water and is closer to the parent layer,so the content of secondary carbonate decreases while the proportion of primary carbonate increases.?2?CO₂ concentration in 3 soil profiles increased with the increase of depth in 0⁵0 cm,and decreased with the increase of depth in 50⁷0 cm.Due to the exchange of air and atmosphere in the upper soil and the consumption of karstification at the rock-soil interface in the lower layer.The?¹³C value of soil CO₂ in the upper layer?0-50cm?becomes more and more negative with the increase of soil depth,while in the lower layer?50-70cm?becomes more and more positive with the increase of soil depth.The two-way gradient changes of soil CO₂ concentration and?¹³C value all reflect the combined effects of air exchange,decomposition of soil organic matter and karstification at the soil-rock interface on the upper soil CO₂.?3?Secondary carbonate in soil comes from recrystallization after dissolution of primary carbonate.In the presence of organic carbon,secondary carbonate crystals are formed by reprecipitation through reaction SOC?CO₂?g??CO₂?aq??HCO₃^-?aq??CaCO₃?s?.The proportion of secondary carbonate in the three soil profiles is relatively large,with the proportion of farmland,woodland and shrub land being 0.52,0.42 and 0.32 respectively.The high proportion of secondary carbonate deposition is the main reason for the weight gain of test pieces buried in semi-arid soil.Therefore,when calculating the intensity of karst carbon sink in the semi-arid karst region in the north,the carbon isotope value should be used to quantitatively calculate the secondary carbonate deposited on the test piece,and the dissolution rate of the test piece should be corrected according to the deposition amount of the secondary carbonate rock to eliminate the influence of the secondary carbonate,and then the intensity of karst carbon sink in the basin should be calculated according to the dissolution rate of the test piece.3.The hydrochemical characteristics of karst groundwater are controlled by runoff conditions and have regional evolution laws?1?The chemical characteristics of karst groundwater in the basin are controlled by hydrogeological conditions.The water temperature,conductivity?EC?,TDS,total ions,Na⁺,Ca²⁺,Mg²⁺,Cl^-,HCO₃^-,SO₄^2-contents increase continuously with the increase of runoff path from recharge area,runoff area,drainage area,and deep buried area.The content of Na⁺,Cl^-,SO₄^2-varies widely,and the maximum values are 50 times,80 times and 32 times of the minimum value respectively.However,the variation range of Ca²⁺and HCO₃^-is relatively small,and the maximum is 2-3 times of the minimum.In recharge area and runoff area,Na⁺content is less,Ca²⁺is the main cation in groundwater,but in discharge area and deep buried area,Na⁺content obviously exceeds Ca²⁺and Mg²⁺,becoming the most main cation in groundwater.The hydrochemical types changed from HCO₃-Ca.Mg type to HCO₃.SO₄-Ca.Mg type and HCO₃.SO₄-Ca.Na.Mg type,and finally evolved into Cl.HCO₃-Na.Ca,Cl.HCO₃-Na type and Cl-Na.Ca type.?2?Controlled by the high concentration of CO₂ in the soil,the PCO₂ of karst groundwater in the basin is significantly higher than the partial pressure of atmospheric CO₂?300-350 ppm?,reflecting the process that atmospheric rainfall absorbs the high concentration of CO₂ in the soil layer to form carbonic acid to dissolve the lower carbonate rock.The saturation indexof calcite?SIC?and dolomite?SID?are both greater than 0,and both minerals are in a strong supersaturated state.However,due to the complicated sources of Ca²⁺and HCO₃^-,there is no obvious trend change rule of SIC and SID from recharge area,runoff area,discharge area,and deep buried area.?3?The saturation index?SIG?of gypsum in karst groundwater in the basin is less than 0,and gypsum has been dissolved in karst groundwater.From recharge area,runoff area,drainage area and deep buried area,the saturation index of gypsum increases continuously,indicating that with the continuous dissolution of groundwater runoff gypsum layer.?4??¹³C_DIC of karst groundwater in the basin has a large variation range,with a difference of5.569‰between the minimum value and the maximum value.The?¹³C_DIC values in recharge and runoff areas are relatively light and have a wide distribution range,while runoff and deep buried areas are relatively heavy and concentrated.The?¹³C_DIC value in the recharge area,the runoff area,the drainage area and the deep buried area shows a constant weight gain change rule.With the increase of runoff path,the proportion of CO₂ from soil in DIC decreases and the proportion of carbon from carbonate rocks increases.?5?The?³⁴S of karst groundwater in the basin has a large variation range,with the maximum value of 24.11‰and the minimum value of 2.56‰.The?³⁴S in the recharge area is relatively light and concentrated,mainly from atmospheric precipitation.The?³⁴S in runoff area increased in weight and was relatively scattered,resulting from atmospheric precipitation and gypsum dissolution.The sulfur isotope in the drainage area is heavy,but the karst water on the south bank of the Sanchuan River is 4-5‰lighter than that on the north bank,reflecting that the karst water on the north and south banks have different sulfur isotope sources.The deep buried area has high content of SO₄^2-and the heaviest?³⁴S,and its water has long runoff path,deep circulation depth,longest water-rock interaction time and the most sufficient gypsum dissolution.On the whole,with the increase of runoff path,the proportion of gypsum dissolution sources increases continuously.4.Galy model is used to evaluate the influence of silicate rock weathering and sulfuric acid dissolution on CO2 flux consumed by rock weathering in the basin?1?K⁺,Na⁺,Cl^-in karst groundwater mainly comes from atmospheric precipitation and dissolution of salt rock.Silicate weathering also has important contribution to SiO₂ and K⁺,Na⁺.Ca²⁺,Mg²⁺,HCO₃^-,SO₄^2-are mainly contributed by dissolution of calcite and dolomite in recharge area,runoff area and discharge area.However,the deep buried area mainly comes from gypsum dissolution and dolomitization.?2?Sulfuric acid participates in dissolution of carbonate rock and has important contribution to hydrochemical composition of karst groundwater in recharge area,runoff area and drainage area,but its contribution to deep buried area is not obvious.Groundwater in the recharge area mainly comes from carbonate rocks dissolved in carbonic acid.Groundwater in runoff area is mainly controlled by carbonic acid dissolution,but sulfuric acid weathered carbonate rock also has certain influence.Groundwater in the drainage area is more affected by sulfuric acid weathering.?3?The average proportion of HCO₃^-produced by sulfuric acid participating in carbonate dissolution is less than 10%in recharge area and runoff area,and more than 10%in drainage area and deep buried area.That is,the proportion of carbonic acid participating in carbonate dissolution gradually decreases with the increase of runoff path,while the proportion of sulfuric acid participating in carbonate dissolution continuously increases.?4?The Galy model is used to calculate the CO₂ fluxes consumed by weathering of rocks in the upper Qinglongquan L02 and Yangjiagang Spring L29 in the drainage area,which are30.80×10³mmol/km².a^-1 and 31.63×10³ mmol/km².a^-1 respectively;L01 of Chemingyu-Guankou Spring in the recharge area is 89.25×10³mmol/km².a^-1,and the CO₂ flux consumed by rock weathering in upper Qinglong Spring and Yangjiagang Spring in the discharge area is 1/3 of that of Chemingyu-Guankou Spring in the upstream recharge area.?5?The CO₂ flux consumed by silicate weathering accounts for a large proportion in the basin.In CO₂ flux consumed by rock weathering,silicate rock weathering accounts for the largest proportion of L29 in Yangjiagang Spring,accounting for 51.92%;The second is L02 in Shangqing Longquan,with 44.98%.The minimum is L01 in Chemingyu-Guankou spring area,which is 11.09%;From the recharge area to the discharge area,the proportion of silicate weathering and the consumed CO₂ flux increase continuously.?6?When sulfuric acid is considered to participate in carbonate dissolution,the carbon sink fluxes of the three karst springs are deducted 10.35%,5.77%and 8.63%respectively.From the recharge zone to the discharge zone,the proportion of carbonate rocks dissolved by sulfuric acid increases.