Carbon plays a vital role in karst ecosystem.A proportion of carbon dioxide produced by microbial activities and respiration of roots can be dissolved and carried by the infiltration water and then dissolve carbonate materials in the karst soil system.This process can produce a lot of dissolved inorganic carbon(DIC) into the soil water and groundwater.The stable carbon composition of DIC(δ13CDIC) can be used as a tracer for studying biogeochemical processes in soil water because the potential biogenic,atmospheric and geologic sources of C have distinct δ13C signatures.In order to reveal the characteristics of temporal and spatial variations of DIC fluxes,which can help us to get an insight into the carbonate dissolution and its related carbon cycling,and evaluate the carbon isotopic evolution of soil water in karst areas,soil water derived from different vegetations in Qingmuguan karst areas,Chongqing,SW China,were collected monthly from July,2010 to July,2011,and the DIC concentrations and δ13CDIC values of soil water were analyzed.The results show that:(1) DIC concentrations and δ13CDIC of soil water showed distinct temporal and spatial variations in the study area.DIC concentrations and δ13CDIC values of soil water varied from 21.56 mg/L to 265.35 mg/L,and from-7.73‰ to-20.68‰,respectively.(2) Soil water derived from grass and coniferous forest land had lower DIC concentrations and δ13CDIC values,with a mean value of 59.12 mg/L and-17.22‰,31.47 mg/L and-16.37‰,respectively.The δ13CDIC indicated that these DIC were mainly derived from the dissolution of soil CO2.(3) Soil water derived from the dry land,shrub land and afforestation land had higher DIC concentrations and δ13CDIC values,with the averages of 153.88 mg/L and-12.2‰,221.82 mg/L and-11.9‰,97.30 mg/L and-11.23‰,respectively.Meanwhile,these δ13CDIC values of soil water were generally 4‰—5‰ higher in the rainy season than those in the dry season.Moreover,these δ13CDIC values showed positive correlations with corresponding DIC concentrations.These indicated that DIC in these soil water were primarily derived from dissolution of carbonate materials by soil CO2,suggesting that great carbonate dissolution occurs in the karst soil system which could contributes to the reduction of soil CO2 released to the atmosphere to some extent.Nevertheless,our research also indicated the carbonate was dissolved partly by sulphuric acid which derived from the atmospheric acid precipitation in the rainy season,supported by the stoichiometric analysis,elevated δ13CDIC and δ34S of sulfate in the soil water.Besides,organic acids in the soil system could also facilitate the dissolution of carbonate materials,and result in the elevated δ13CDIC of soil water.Hydrion produced by oxygenation of inorganic nitrogenous fertilizers from agricultural activities also probably participated in the dissolution of carbonate and elevated δ13CDIC of soil water.This study revealed the variations of DIC concentrations and δ13CDIC values in the karst soil water and their influencing factors,which will aid in understanding of the migration and transformation of soil CO2 in karst soil system,especially the reduction effects of soil CO2 released to the atmosphere.However,the accurate estimation of soil CO2 reduction requires further research.