THE CARBON CYCLE INDUCING BY CARBONATE ROCK WEATHERING IN A KARST CRITICAL ZONE--AN CASE STUDY IN ZHUJIANG RIVER BASIN, SOUTH CHINA (Invited Presentation)

In a watershed, the carbon cycle induced by carbonate rock weathering mainly consists of three parts: carbonate rocks dissolution removing the atmospheric/ soil CO₂ to water and to produce inorganic carbon; inorganic carbon transfer and conversion along with water flow; inter-conversion between inorganic carbon and organic carbon with the aquatic plants photosynthesis, part of the organic carbon deposit on the river/lake/reservoir beds mixing with sediments. Zhujiang river is a typical karst watershed with the 36.32% karst cover. The investigation results show: (1) The results of soil carbon cycle in karst area in Maocun, Guilin, revealed that the weathering and dissolution of carbonate rocks under the soil resulted in a lower soil respiration of 25% than that in non-karst area(sandstone and shale), it indicates the karst carbon cycle happening. (2) The comparison on carbon exchange and isotopes at the water-gas interface between the granite and carbonate rock basins in the Lijiang River, found that the CO₂ emission of the karst water is 10.92 times that of the allogenic water from non-karst area, while the carbon isotope of HCO₃^- in karst water is lighter by 8.62‰, it means inorganic carbon contained in karst water body dominantly from soil source and a greater karst carbon sink effect. (3) the CO₂ concentration of cave air of Panlong cave in Guilin, the minimum is 300-500ppmv with heavy δ¹³C from Oct. to April next year, and maximum is 1000-1500ppmv with light δ¹³C from May to Sept. (4) When the karst subterranean stream in Zhaidi, Guilin, is exposed to surface, karst water with carbon-rich stimulated the growth of aquatic plants, and the calculation indicated that 12.52% of inorganic carbon is converted into organic carbon along the 512-meter water flow. (5) With the investigation and monitoring, the estimation of carbon source and carbon sink inducing by carbonate rock weathering in Zhujiang river has made, the total carbon flux by the carbonate rocks weathering is 1167×10⁴ tCO₂/a(100%), of them, 177.83×10⁴ tCO₂/a(15%) emitted from water to air, 46.6×10⁴ tCO₂/a(5%) deposited on the river bed, 623×10⁴ tCO₂/a(53%) inorganic carbon and 320.8×10⁴ tCO₂/a( 27%)organic carbon entered sea.