[Variations of CO2 Exchange Fluxes Across Water-air Interface and Environmental Meaning in a Surface Stream in Subtropical Karst Area, SW China].
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. Karst groundwater with high water CO2 partial pressure (pCO2) often shows highly positive CO2 concentration gradient to atmosphere after it is discharged to surface, so the evaluation of CO2 exchange fluxes across karst water-air interface is important for karst carbon cycle research. This paper researched CO2 exchange fluxes across water-air interface in the karst surface stream in detail which was fed by Guancun subterranean stream in Liuzhou city, Guangxi province. Closed static chamber method and portable hand-holding CO2 sensor (GM70) were both employed in CO2 exchange fluxes monitoring. The results showed that CO2 degassing was the mainly form of CO2 exchange across the steam water-air interface. CO2 degassing flux in subterranean stream outlet (G1 site) ranged from 139.48 to 890.84 mg·(m2·h)-1 with an average of 445.72 mg·(m2·h)-1. CO2 degassing flux in stream downstream site (G2 site) ranged from 16.54 to 844.18 mg·(m2·h)-1 with an average of 159.81 mg·(m2·h)-1. The CO2 degassing flux in G1site was higher than that in G2 site. CO2 degassing fluxes in rainy season in both G1 and G2 site were higher than those in dry season. Stable carbon analysis of CO2 gas (δ13C-CO2) found that CO2 degassing from karst stream might influence air CO2 carbon isotope near water surface, which resulted in the more negative δ13C-CO2 value with the increase of CO2 degassing flux. Significant spatio-temporal variations of δ13C-CO2 were found, and the δ13C-CO2 in the rainy season was more negative than that in dry season and δ13C-CO2 in G1 site was more negative than that in G2 site. As a result of stream CO2 degassing, the hydrochemical characteristics of steam varied along stream running, which resulted in decrease of HCO3-, EC and pCO2 and increase of pH, SIc and δ13C-DIC in the stream.