Variations Of CO₂ Exchange Fluxes Across Water-air Interface And Environmental Meaning In A Surface Steam In Subtropical Karst Area,SW China

Karst groundwater with high water CO₂ partial pressure?pCO₂?often shows highly positive CO₂ concentration gradient to atmosphere after it was discharged to surface,so the evaluation of CO₂ exchange fluxes across karst water-air interface for karst carbon cycle research is important.CO₂ cycle process or sources/sinks are not only the basis of understanding and responding to global climate change,but also the core of the current global climate change research.Gas exchange across water-air interface in terrestrial surface water is an important way of nutrient elements?carbon,nitrogen?exchange between aquatic ecosystems and ambient air.Escaping CO₂ gas from surface water is also actively involved in the modern carbon cycle.In the material cycle in karst regions,CO₂ plays a key role in karst processes,driving the formation of karst features.Due to the impact of the ecological environment,soil CO₂ content and carbonate dissolution of karst groundwater with high HCO3-content,when it is discharged from the aquifer,with the formation of high positive CO₂ concentration gradient with atmosphere and strong degassing,and drive the elements such as calcium and magnesium in the karst water to deposite.This process is affected by rainfall,air temperature,photosynthetic metabolism,land use change and other factors.CO₂ exchange fluxes across karst water-air interface is the key to the evaluation of karst carbon sink flux and solve the problem of the stability of karst carbon sink,the current study CO₂ exchange fluxes across karst water-air interface mainly driven calcium carbonate deposition in CO₂ degassing?travertine cave sediments?and change of water chemical properties of water.CO₂ degassing are key factors driving calcium carbonate deposition.CO₂ exchange fluxes across karst water-air interface is the key to accurately evaluate groundwater release contribution to global terrestrial water CO₂,however the research results of the present study published specifically for typical karst water CO₂ exchange fluxes across karst water-air interface and CO₂ fluxes of degassing and water chemical interaction mechanism is less.This paper researched CO₂ exchange fluxes across water-air interface and its influence in the karst surface stream in detail in a hydrological year multi temporal and spatial scaleswhich it was fed by Guancun subterranean stream in Liuzhou city,Guangxi province,the result of the study:The stream due to its special geological background,water chemistry type is Ca-HCO3-type,reflect the lithology control on water chemical characteristics.After the stream surface,due to the concentration difference of CO₂ degassing occurs,pCO₂ continues to decrease along the process,while pH,SIc continue to increase along the process.The stream exposed to the surface,because of the CO₂ concentration difference occurred degassing,on time scale,the four monitoring points showed the rainy season CO₂ degassing strength is lower than the dry season,showed the same change trend and stream pCO₂,no obvious regularity in the spatial scale,but in general is a process from upstream to downstream decline.In the monitoring period from September 2015 to August2016,carbon isotope from CO₂ gas(?¹³C-CO₂)and the concentration of CO₂ at four monitoring points is higher with the passage of time,the concentration of CO₂ in the floating chamber increases with the CO₂ degassing of water continuously,because CO₂ degassing of water makes 12 C rich CO₂ gas preferentially escape from water,so that the floating box ?¹³C-CO₂ values gradually smaller.The?¹³C-CO₂ value of the air CO₂ in a certain distance above the water surface can be caused significantly smaller by the CO₂ degassing of the streams,so in considering the anthropogenic CO₂ release for effects of atmospheric CO₂ content and isotope effect,need to consider or exclude water escaping CO₂ gas area for gas CO₂,in order to be more accurate the evaluation of CO₂ resulting in increased environmental changes.By principal component analysis of the 15 factors affecting the CO₂ fluxes,by KMO and Bartlett test,the factors that affect the CO₂ degassing flux can be roughly divided into 3 categories:?1?effects of carbonate balance system and Photosynthesis of aquatic plants on CO₂ degassing flux;?2?atmospheric environmental impact factors on CO₂ degassing flux;?3?effect of organic carbon on water CO₂ degassing flux,from the variance contribution rate,the carbonate balance system has the greatest influence on CO₂ degassing flux.From December 13,2016 to December 14 th in G1,the downstream Lei Ya monitoring station of G4 for 24 consecutive hours of day and night monitoring stream upstream of export,export area stream upstream of the G1 temperature,p H value,dissolved oxygen,conductivity remained unchanged,no obvious change and the indexes,the water ion changed little,and tends to be stable.The downstream G4 water chemical characteristics showed a relatively obvious diurnal variation.The influencing factors include physical factors?solar radiation,water temperature,etc.?and biological effects?photosynthesis and respiration?.18:00,0:00 and the next day 6:00 and 10:00 were used to monitor the diurnal variation of CO₂ degassing flux in the stream in December 13 th.The flux diurnal variation of G1 in the outlet area ofthe stream was significant,which showed that the night was an upward trend and reached the maximum value at about 6:00 in the morning,and showed a downward trend during the day.The temperature will affect the emission of CO₂ directly or indirectly,G1 temperature at about 6:00 is the minimum value,the water temperature drop will usually increase the solubility of O₂ and CO₂ in the water,otherwise it will reduce the solubility,illustrate the main reason for the G1 night degassing flux temperature were to change the solubility of CO₂ in water.The diurnal variation of the degassing flux of the G4 point stream in the lower reaches of the stream is also shown in the morning > night>noon>afternoon,the night to the morning,the stream degassing flux rising,and in the morning around 6:00,the degassing flux reaches the maximum value,The diurnal variation of water chemistry at night was mainly affected by the respiration of aquatic plants,streams of aquatic plants respiration is greater than photosynthesis,to sum up,the main factor of the diurnal variation of G1 point CO₂ degassing flux is the influence of water temperature on the solubility of CO₂ in water,and the biological respiration is the dominant factor of the diurnal variation of G4 flux in the lower reaches of the stream.