Biogeochemistry process is one of three key processes in Earth's critical zones. In the research field of karst carbon cycle and global change,the study of biogeochemical processes,mechanism and their influence factors can help to understand better of the short-time scale property of karst processes and the carbon sink stability in karst systems. Taking Chaotian River (a tributary of Lijiang River),about 20km east of Guilin, China as an example, the diurnal fluctuation of dissolved inorganic carbon (DIC),isotope,specific conductivity and other chemical parameters were examined by conducting high resolution field monitoring and high frequency sampling. The monitoring and sampling work was deployed from 9 to 13,August, 2013 at the two sites of the reach near hydrological station (25°11'41.8"N, 110°28'59.5"E) of Chaotian River, i.e. Niaolingqiao site (NLQ) (25°11'37.3N,110°29'22.7"E) and Lianghegou site (LHG) (25°11'44.7"N,110°28'27.4"E),representing gravel and muddy bed respectively. Auto data logger (YSI 6920) and auto water sampler (Teledyne ISCO 6700) were installed at two sites. pH, dissolved oxygen, specific conductivity (SpC),HCO_3~- and Ca~(2+) all showed diurnal variations, reflecting influence of photosynthesis and calcite precipitation. The SpC, Concentrations of Ca~(2+) and HCO_3~- at NQL and LHG monitoring sites showed a diel cycle of daytime decrease and nighttime increase, with an amplitude of 10%, 11.5%~12.5% and 10%~12% respectively at Niaolingqiao site. Conversely, the delta~(13)C_(DIC) values presented a diel cycle of daytime increase and nighttime decrease. Using the diurnal amplitude of DIC and calcium concentrations,the mean conversion rate of DIC by aquatic vegetation photosynthesis at Niaolingqiao site is calculated to be 0.64 * 10~(-5) mmol/L · s during a 48-h period, daily DIC transform magnitude is about 1484kg/d (be equivalent to 292kg C/d),accounting for 4% of total DIC input; the average Ca precipitation rate is estimated to be 0.38* 10~(-5) mmol/L · s. The small rate for Ca precipitation could be related to the presence of inhibiting solute of dissolved organic carbon in water of Chaotian River. The nighttime increase in DIC and calcium could be explained by groundwater input from the upstream karst aquifer. Hydrochemical data of water samples collected in different seasons from October 2010 to April 2012 showed that the concentrations of Ca~(2+) and HCO_3~- of karst water are much higher than that of reservoir water and surface water draining from silicate-rich rock areas upstream. The values of delta~(13)C_(DIC) of different water bodies exhibit remarkable differences, for example, the delta~(13)C_(DIC) values of Si'anjiang reservoir and Zhaidi underground stream range -8.39%o ~-13.52 and -11.57~-15.67 respectively, with an average of -10.69 and -13.78 respectively. The values of delta~(13)C_(DIC) at Chaotian hydrological station are between the surface water and the groundwater, ranging from -10.47%o to -15.57%o with an average of - 13.15. It suggested that DIC could be sourced by the mixture process of surface water in silicate-rich system upstream and carbonate system groundwater. DIC-supply from groundwater is estimated using isotope constraint end-member method, roughly 75% ~80% of DIC at Chaotian station is contributed by karst underground water on a yearly basis.