Earth Science Frontiers ›› 2022, Vol. 29 ›› Issue (3): 179-188.

### Carbon flux in a typical dolomite-dominated drainage basin in humid subtropical climate

ZENG Cheng1(), HE Chun1,2, XIAO Shizhen3, LIU Zaihua1,*(), CHEN Wangguang1,2, HE Jianghu3

1. 1. State Key Laboratory of Environmental Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550081, China
2. University of Chinese Academy of Sciences, Beijing 100049, China
3. State Engineering Technology Institute for Karst Desertification Control, School of Karst Science, Guizhou Normal University, Guiyang 550001, China
Karstification is directly involved in the global carbon cycle. Understanding the mechanism of karst carbon sink is of great significance for an accurate evaluation of the carbon cycle. Carbonate rocks are widely distributed in China, especially in southwestern China. Although quite amount of research on inorganic carbon sequestration in limestone-dominated watersheds of this area has been carried out, only few involved the dolomite counterparts, which has adverse impacts on the accurate assessment of China’s carbon sink potential in karst systems and the national effort toward carbon neutrality. To fill this research gap, we investigated the Huangzhou River Basin in Shibing, Guizhou Province, a representative domolmite-dominated drainage basin in humid subtropical climate in southwestern China. We conducted a year-long (May 2018 to May 2019) automated hydrological, hydrochemical monitoring study of water-rock-gas interaction and inorganic carbon sequestration in the drainage basin, and obtained high temporal resolution continuous monitoring data on regional rainfall and water flow, water temperature, water conductivity and pH at the outlet area of the Huangzhou River Basin. The saturation indexes of calcite and dolomite and the partial pressure of CO2 in river water were then calculated based on a chemical thermodynamic model. Our study show that (1) the Huangzhou River Basin is a typical drainage basin in mountainous area that is significantly impacted by rainfall. Although flash flooding happens frequently, the chemical regime of the dolomite-dominated drainage basin is still controlled by its chemostatic behavior. (2) The river is sourced from dolomite aquifer upstream on a residual plateau where good quality water-bearing media made of karstified dolomite are developed on either side of a shallow waterbed; whereas its middle and downstream sections lack well developed karst system, with runoff overland into streams. Due to CO2 release from river surface, the calcite saturation index of river water is high at the outlet, where travertine deposition could be found at the riverbed. (3) According to the chemical thermodynamic model of open system for dolomite, the $HCO 3 -$ concentration in karst groundwater discharged to the river was calculated to be 5.1 mmol/L using the soil CO2 data for different land use types. This value is 16% higher than the average concentration of $HCO 3 -$ in river water at the outlet of the Huangzhou River Basin, indicating the dissolved inorganic carbon in the river water is reduced due to turbulence-promoted degassing and aquatic photosynthesis. (4) The carbon flux in the dolomite-dominated watershed, calculated by the load estimation method, was 36.43 t CO2/(km2·a). This value is not significantly higher than the carbon flux in other carbonate-dominated drainage basins in southern China, suggesting the CO2 concentration in land cover is still inversely proportional to watershed runoff in the dolomite-dominated drainage basin, so that its carbon flux remains relatively constant even in humid subtropical climate.