Effects Of Allogenic Acids(sulfuric Acid And Nitric Acid)on Karst Carbon Cycle

The imbalance of atmospheric carbon flow results in 1.8³.4 Pg C/y “Missing sink” in the research of global climate change and carbon cycle.The carbon stored in carbonate rocks is the largest carbon pool in the earth.The processes related to carbonate rocks?karst processes?respond quickly to the global changes and take part in global carbon cycle at different time scales,forming karst carbon sink that is equal to 1/3 of the “Missing sink”.Based on the traditional view,the dissolution of carbonate rocks due to atmospheric and soil CO₂ that dissolves in water to form H2CO₃,which contributes half of HCO₃-in karst water,one form of carbon sink.However,the increasing anthropogenic activities?urbanization,agricultural activities?have been brought allogenic acids?sulfuric and nitric acids?that can also dissolve carbonate rocks,but without consuming atmospheric and soil CO₂.This process will emit CO₂,to influence karst carbon cycle and the global carbon cycle.Thus,to study the effects of allogenic acids on karst carbon cycle will benefit for the accurate evaluation of karst carbon sink and improve the global carbon cycle models in academic field.In this thesis,we selected one typical karst subterranean river catchment called Laolongdong which is suffering increasing interference from urban activities,and another catchment called Xueyudong,a comparison,which is free of discernible human influence in Fengdu.We monitored water in the field by dynamical monitoring and sampled every month as well as high-frequency sampling during rainstorm by using the high-accuracy water level and multi-parameter monitor.The data of water chemistry and isotopes at different time scales from minutes to months were obtained in the lab and field.Two catchments have similar geological backgrounds,climate and area.While their indicators in water chemistry and ?¹³CDIC were combined to qualify the effects of allogenic acids?sulfuric and nitric acids?on the processes of karst carbon cycle.Also,combined with isotopes,such as ?34SSO₄,?¹⁸OSO₄,?¹⁵NNO₃,?¹⁸ONO₃ and ?¹⁸OH2O,the model SIAR?Stable Isotope Analysis in R?was used to quantify the contributions of SO₄^2-and NO₃^-from different sources in the underground river.On a basis of conceptual model,water chemistry and runoff were used to form a method for calculating carbon sink under the influence of allogenic acids?sulfuric and nitric acids?,to quantify their effects on the calculation on net carbon sink in the two catchments and to reveal the rules of karst carbon sink flux and dominant factors under the influence of allogenic acids.According to the research planning,the main results are shown as following:1.In Laolondong,allogenicacids?sulfuric and nitric acids?take part in dissolving carbonate rocks.Acid rain,pyrites,sewage,fertilizers are important sources of these acids.Xueyudong is dominated by karst process.Laolongdong shows HCO₃-SO₄-Ca type in hydrochemistry,while HCO₃-Ca type in Xueyudong.The ratio of [Ca²⁺+Mg²⁺] and [HCO3-] is between 0.5 and 1.0 in Laolongdong,not significantly correlated.However,[Ca²⁺+Mg²⁺] and [HCO3-+ SO₄^2-+ NO₃^-] are markedly correlated with the ratio of 1:1.On the contrary,[Ca²⁺+Mg²⁺] and [HCO₃-] are highly correlated in Xueyudong,the ratio is close to 1:1.Moreover,the ?¹³CDIC in Laolongdong is positively correlated to HCO₃-,ranging between-13.78^-8.25‰.?¹³CDIC in Xueyudong is from-15.15^-11.98‰,not correlated to HCO₃-content.According to the relationships of [SO₄^2-+ NO₃^-]/ [HCO₃-] and [Ca²⁺+Mg²⁺]/ [HCO₃] in equivalent concentration and the relationships between ?¹³CDIC [Ca²⁺+Mg²⁺]/[HCO3-],[SO₄^2-+NO₃^-]/[Ca²⁺+Mg²⁺],[SO₄^2-+NO₃^-]/[HCO₃-],Laolongdong is located between the ends of hydrocarbon dissolving carbonate rocks and sulfuric/nitric acids dissolving carbonate rocks while Xueyudong is close to the end of hydrocarbon dissolving carbonate rocks.The concentration of TDS?the Total Dissloved Solids?,SO₄^2-,DIN?the Dissolved Inorganic Nitrogen?,Na+ and Cl-ions are much higher in Laolongdong than in Xueyudong.A tracing test proves that Laolongdong is connected to sewages accompanying with nitrification from urban living and agricultural usage.Laolongdong always?73.68%?suffers the acid rain that contains high contents of SO₄^2-?NH⁴⁺?NO₃^-in the study year,two times than in Xueyudong.2.In Laolongdong,about 98.2% of total SO₄^2-was interpreted: 42.7% was from sulfuric acid,of which 21.6% was from human-related activity.Under the conditions of rainstorm,the ratios changed into 45.5% and 15.0%,respectively.In Xueyudong,SO₄^2-is mainly from rainfall.There are five sources of SO₄^2-in Laolongdong based on field investigation and samplings: rainfall,dissolution of evaporites,oxidation of hydrogen sulfides,sewage and chemical fertilizer.According to different values of ?34SSO₄??¹⁸OSO₄ in different sources,we used the model SIAR to calculate the contributions from different sources every month.The results show that contributions from acid rain and oxides of pyrites were32.4%?March?⁵0%?April?with a weighted mean of 42.7%.The contribution from human-related acid rain was 2.85?March?²4.5%?August?,which was positively correlated to rainfall.The output of SO₄^2-responds quickly to rainstorm rain.Different sources of SO₄^2-play different roles to the contributions with/without rainstorm.Thus,the rainstorm should be independently considered when we study the effects of sulfuric acid on karst carbon cycle in Laolongdong.3.The processes of nitrification and denitrification co-exist in Laolongdong,86.4% of the NO₃^-sources can be explained.Among it,98.7% is from nitric acid and 97.1% of which is human-related.Under the condition of rainstorm,82.9% NO₃^-sources can be explained and the ratios changed into 99.3% and 94.7%.In Laolongdong,three forms of nitrogen have different immigrating and transmitting methods in dry season,rainy season and rainstorm condition.In Xueyudong,NO₃^-is mainly from soil organic N.Similarly,there are five sources of NO₃^-in Laolongdong: rainfall,chemical fertilizer,sewage/feces,soil organic N and nitrification of rainfall and fertilizer.NO₃^-can be from fertilizer and any other sources that produce H+ to dissolve carbonate rocks.We found that in normal seasons the fractionation factors of ?¹⁵N??¹⁸O are-4.43‰ and-4.00‰,indicating that there is significant process of denitrification in Laolongdong.Under a rainstorm,NO₃^-was increasing with the nitrification process accompanying with variable ?¹⁵NNO₃ and ?¹⁸ONO₃.In Laolongdong,three forms of nitrogen have different immigrating and transmitting methods.In dry season,they go into sink holes and shafts as channel flow to the underground river,and exist mainly as NH⁴⁺;In rainy season,added to the channel flow,nitrified NO₃^-in epikarst zones going downward along deep fractures with diffusion flow into the underground river,NO₃^-is the main ions.Under the rainstorm,there are four stages of the migration and transformation processes of three kinds of nitrogen: I,NH⁴⁺ mainly immigrates in the channel flow;II,NO₃^-goes along with “old water” in diffusion flow;III,NO₂-goes along with “new water” in diffusion flow and finally,NO₃^-migrates with “new water” in diffusion flow.In Xueyudong,NH⁴⁺?NO₂^-is under the detected limit,NO₃^-is low throughout the year,mainly from soil-N.However,?¹⁸ONO₃ values are different from rain,which confirm that soil-N is the dominant source of NO₃^-in the underground water.4.On the basis of conceptual model,We established an equation to calculate karst carbon sink under the effects of allogenic acids?sulfuric and nitric acids?based on water chemistry and runoff.Where NCSF is the net carbon sink flux?t CO₂/km^·a?,Q is runoff?L/s?,A is the area of watershed?km^?,[HCO3-],[SO₄^2-]??and[NO₃^-]??stand for mol concentration of related production in the dissolving processes and 44 is molecular weight of CO₂.5.We calculated that in the Laolongdong,the carbon sink flux is 35.50 t CO₂/km^·a,specifically,potential carbon emission flux from sulfuric and nitric acid dissolving carbonate rocks are 17.17 t CO₂/km^·a and 3.58 t CO₂/km^·a,respectively.Anthropogenic sulfuric and nitric acids dissolved carbonate rocks and potentially emitted carbon flux 7.44 t CO₂/km^·a and 3.48 t CO₂/km^·a,respectively.In Xueyudong,the karst carbon sink flux is 30.34 t CO₂/km^·a,specifically,potential carbon emission flux from sulfuric and nitric acids dissolving carbonate rocks are 2.16 t CO₂/km^·a and 1.17 t CO₂/km^·a,respectively.Karst carbon emission/sink flux is higher in rainy season than in dry season and discharge is the main control factor in both catchments.Without considering the effects from allogenic acids?sulfuric and nitric acids?,the estimated karst carbon sink fluxes are 56.34 t CO₂/km^·a in Laolongdong and 33.67 t CO₂/km^·a in Xueyudong,increasing 58.42% and 10.69%,respectively.Therefore,effects of allogenic acids on karst carbon cycle are much larger in the catchment with increasing human-related activity than that with less human interference,which should be considered when calculating carbon sink.In Laolongdong,effects of allogenic acids on karst carbon cycle are higher in rainy season,especially with a rainstorm than in dry season.The ratios of potential carbon emission from allogenic acids and net carbon sink is annually 0.58,0.35 in dry season,0.73 in rainy season and 0.77 under rainstorm.In Xueyudong,these ratios are annually steady,indicating that the effects of allogenic acids on karst carbon cycle are relatively stable.