Carbon dioxide exchanges and their controlling factors in Guijiang River, SW China

The important flux estimate of carbon exchange across the water-air interface in river systems of the global carbon cycle are being increasingly recognized. However, these estimates have large uncertainty due to the incomplete temporal and spatial coverage of CO2 flux. Here, we evaluated the seasonal and spatial dynamics and controlling factors of CO2 exchange across the water-air interface and their potential role in regional and global carbon budgets in 25 river transects along Guijiang River (GJR), southwestern China, in July and December 2016. These river transects were divided into upstream (US), middle (MS), and downstream (DS) reaches in non-karst, karst and mixed areas, respectively. The riverine CO2 fluxes showed obvious seasonal variations, ranging from 5.62 +/- 5.93 g/(m(2).d) in summer to 1.25 +/- 4.02 g/(m(2).d) in winter, with an average value of 3.44 +/- 5.48 g/(m(2).d). The riverine CO2 fluxes exhibited significant spatial patterns, where the highest and lowest averaged CO2 fluxes occurred at MS in summer and winter, respectively. Generally, the river was a carbon source with respect to the atmosphere throughout the year. However, the CO2 fluxes were obviously restrained by aquatic metabolism and the carbonate buffer system in winter. Additionally, the absorbed CO2 at MS was due to strong photosynthetic and carbonate buffer activities. Carbon loss via atmospheric exchange in the GJR river basin was 0.032 +/- 0.047 Tg C/yr, which accounted for 20.1% of the riverine dissolved carbon fluxes (0.160 +/- 0.075 Tg C/yr). Thus, 0.128Tg C yr(-1) was sequestered in the GJR basin. Our results demonstrate the need to emphasize the importance of aquatic photosynthesis and the carbonate buffer system in restraining CO2 degassing in karst rivers. It is important to underscore the detailed spatiotemporal changes in CO2 flux to reduce uncertainty in current estimates of global riverine CO2 emissions.