River sequesters atmospheric carbon and limits the CO₂ degassing in karst area, southwest China

CO₂ fluxes across water-air interfaces of river systems play important roles in regulating the regional and global carbon cycle. However, great uncertainty remains as to the contribution of these inland water bodies to the global carbon budget. Part of the uncertainty stems from limited understanding of the CO₂ fluxes at diurnal and seasonal frequencies caused by aquatic metabolism. Here, we measured surface water characteristics (temperature, pH, and DO, DIC, Ca²⁺ concentrations) and CO₂ fluxes across the air-water interface at two transects of Guijiang River, southwest China to assess the seasonal and diurnal dynamics of fluvial carbon cycling and its potential role in regional and global carbon budgets. The two transects had differing bedrock; DM transect is underlain by carbonate and detrital rock and PY is underlain by pure carbonate. Our results show that the river water both degasses CO₂ to and absorbs CO₂ from the atmosphere in both summer and winter, but the degassing and absorption varied between the two transects. Further, CO₂ fluxes evolve through diurnal cycles. At DM, the river evaded CO₂ from early morning through noon and absorbed CO₂ from afternoon through early morning. At PY in summer, the CO₂ evasion decreased during the daytime and increased at night while in winter at night, CO₂ uptake increased in the morning and decreased in the afternoon but remained relatively stable at night. Although the river is a net source of carbon to the atmosphere (~15mMm^-2day^-1), the evasion rate is the smallest of all reported world's inland water bodies reflecting sequestration of atmospheric carbon through the carbonate dissolution and high primary productivity. These results emphasize the need of seasonal and diurnal monitoring of CO₂ fluxes across water-air interface, particularly in highly productive rivers, to reduce uncertainty in current estimates of global riverine CO₂ emission.