Abstract:

 Continuous samplings of surface water, including stream flow and surface flow, in a small karst catchment (Huanjiang Karst Agricultural Ecological Station, Chinese Academy of Sciences) were conducted during two rainfall events on Aug. 23 and Aug. 24, 2007, with the main purpose of understanding the responses of dissolved carbon (DOC and DIC) and stable carbon isotopic composition of DIC (δ13CDIC) to the rainfall. Both the dissolved carbon concentrations and δ13CDIC values varied quickly in response to the rainfalls. About 107.1 kg of DOC and 812.5 kg of DIC were transported out of the catchment during two rainfall events, about 64 and 8 times respectively those of normal days during the same period. According to δ13CDIC values of the water, about 56%61%, 56%63%，and 50%58% of DIC in surface flow, upstream flow and downstream flow, respectively, were estimated to be originated from soil CO2. About 406.2 kg soil CO2 was dissolved in downstream water during rainfall periods, while the amount during the same period of normal days was only about 48.3 kg. It is concluded that rainfall can significantly affect the carbon cycling of the catchment，and the studies of cycling of carbon and other nutrients on different time scales and rainfall events are important for understanding local and global carbon cycling.

Key words:  , karst ecosystem； dissolved inorganic carbon； dissolved organic carbon； stable carbon isotopic composition；soil CO₂