Characteristics Of Dissolved Inorganic Carbon In Karst Watersheds And The Mechanisms Influencing Carbon Fluxes At The Water-Air Interface

Increasing atmospheric CO₂ concentration leads to global climate change,resulting in frequent extreme weather and ecological environment deterioration.An important scientific problem brought about by climate change is the imbalance of global CO₂ budget.Understanding the process of carbon cycling in various circles is an important way to change the global ecological environment.As an important part of the carbon cycle in terrestrial ecosystems,rivers act as the main transport carrier to transport terrestrial carbon sources to the ocean.The carbon input from rivers to the ocean is susceptible to biophysical and chemical processes that affect the carbon output.Due to its special environmental conditions,the carbon source and sink effect exists in karst areas,and the CO₂ exchange flux at the water-air interface affects the carbon budget.This study takes Chwuxi Watershed in Suiyang County,Guizhou Province as the research object,respectively in 2019 in November and December,in June 2020 in July and collecting water samples,test water ion of Yin and Yang and dissolved inorganic carbon isotope(?¹³C_DIC),through the model method to calculate water-gas interface CO₂ flux,deeply understand the riverine carbon transport and the hydrologic and biogeochemical process,It provides theoretical support and reference for exploring carbon source and sink.Based on the analysis of sampling results,the following conclusions are obtained:(1)Under the influence of rain and heat in subtropical monsoon climate,the p H of The Basin varies from 7.96 to 8.63,showing weak alkalinity on the whole.TDS varied from 233.74 mg/L to 275.90mg/L,and EC varied from231.00?s·cm^-1 to 356.00?s·cm^-1.Both TDS and EC showed higher in rainy season than in dry season.Dissolved oxygen DO varies from 7.45 mg/L to11.32mg/L,and it is also more in dry season than in rainy season.(2)The main cation and anion in the river are Ca²⁺,Mg²⁺and HCO₃^-,followed by SO₄^2-,NO₃^-,and Cl^-,respectively.The dilution effect leads to the concentration of each ion in the rainy season is less than that in the dry season.The hydrochemical type of chwuxi water is HCO₃-Ca type or HCO₃-Ca·Mg type,and the ions in the river mainly come from rock weathering.Ca²⁺,Mg2+and HCO₃^-are weathering products of carbonate rocks.NO₃^-comes from agricultural activities and atmospheric deposition.SO₄^2-may come from gypsum dissolution in addition to atmospheric deposition and agricultural activities.(3)The range of DIC variation in Chiwuxi river 2.56?3.10mg/L,and the DIC concentration in rainy season was lower than that in dry season.?¹³C_DIC ranges from-12.00‰to-6.95‰,and is heavier in dry season than in rainy season.Soil CO₂ and carbonate weathering are the main sources of DIC in rainy season,and atmospheric CO₂ is also one of the sources of DIC in dry season due to photosynthesis of aquatic plants.The DIC proportion of soil CO₂ in dry season was 37.00 to 51.50%,and that of soil CO₂ in rainy season was 50.46 to 58.69%.(4)Carbonate weathering carbonate rocks are still the dominant role in controlling HCO₃^-output from the watershed,and sulfuric acid and nitric acid produced are involved in the weathering process of carbonate rocks,and the HCO₃^-produced will become a source of atmospheric CO₂ and a potential factor leading to the bias of?¹³C_DIC in rivers.Calculations using chemical equilibrium quantification indicate that HCO₃^-produced by sulfuric acid and nitric acid occupy 11.63 to 32.86%of the total HCO₃^-,12.90 to 32.86%in the dry season and 11.63 to 27.57%in the rainy season.(5)The average CO₂ uptake per unit area during the dry season sampling period was 120.87g·s^-1 and the carbon sink intensity was about 5.92t CO₂·km^-2·a^-1,while the average CO₂ uptake per unit area during the rainy season was 300.09g·s^-1 and the carbon sink intensity was 14.69 t CO₂·km^-2·a^-1,according to the calculation results of the hydrochemical-runoff method.(6)The variation of pCO₂ ranged from 323.59 to 870.96?atm,and the variation of CO₂ flux at the water-gas interface ranged from-24.31 to158.18mmol·(m~2·d)^-1,both of which exhibited the characteristics that the rainy season was larger than the dry season.Spatially,Ca²⁺,HCO₃^-,and TDS decreased from the river source along the river flow,while SIc and p H rised gradually from the upstream to the downstream of the river.Carbonate rock mineral dissolution influenced pCO₂ changes,while soil CO₂ influx and photosynthesis were also factors affecting pCO₂ changes.According to the calculated CO₂ fluxes at the water-air interface,the rivers in Chiwuxi all exhibit carbon"sources".The dry season is favorable for the growth of aquatic plants to convert carbon"sources"into carbon"sinks"and absorb atmospheric CO₂.