[Variations of CO Exchange Fluxes Across Water-air Interface and Environmental Meaning in a Surface Stream in Subtropical Karst Area, SW China].

CO 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 CO gas from surface water is also actively involved in the modern carbon cycle. In the material cycle in karst regions, CO plays a key role in karst processes, driving the formation of karst features. Karst groundwater with high water CO partial pressure (CO) often shows highly positive CO concentration gradient to atmosphere after it is discharged to surface, so the evaluation of CO exchange fluxes across karst water-air interface is important for karst carbon cycle research. This paper researched CO 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 CO sensor (GM70) were both employed in CO exchange fluxes monitoring. The results showed that CO degassing was the mainly form of CO exchange across the steam water-air interface. CO degassing flux in subterranean stream outlet (G1 site) ranged from 139.48 to 890.84 mg·(m·h) with an average of 445.72 mg·(m·h). CO degassing flux in stream downstream site (G2 site) ranged from 16.54 to 844.18 mg·(m·h) with an average of 159.81 mg·(m·h). The CO degassing flux in G1site was higher than that in G2 site. CO degassing fluxes in rainy season in both G1 and G2 site were higher than those in dry season. Stable carbon analysis of CO gas (δC-CO) found that CO degassing from karst stream might influence air CO carbon isotope near water surface, which resulted in the more negative δC-CO value with the increase of CO degassing flux. Significant spatio-temporal variations of δC-CO were found, and the δC-CO in the rainy season was more negative than that in dry season and δC-CO in G1 site was more negative than that in G2 site. As a result of stream CO degassing, the hydrochemical characteristics of steam varied along stream running, which resulted in decrease of HCO, EC and CO and increase of pH, SIc and δC-DIC in the stream.