Abstract: In this study, Wanan Reservoir was investigated with four seasonal samplings from September 2014 to June 2015, to understand the characters and sources of dissolved inorganic carbon (DIC) and its carbon isotope (δ¹³C_DIC). Nine sampling sites covering the reservoir were included in the field survey. Results showed that the concentration of DIC in water was lower in summer and autumn than that in other seasons. The value of δ¹³C_DIC was more positive in winter and spring. From upstream to downstream, DIC concentration showed a decrease trend, whileδ¹³C_DIC become more positive gradually. Photosynthesis and organic matter decomposition playedimportant roles in controlling the variation of δ¹³C_DIC in the surface water. Along the water column in the central reservoir, in situ parameters such as water temperature had no significant difference, indicating a weak thermal stratification in reservoir. However, DIC concentrations had an increase trend downwardly, while δ¹³C_DIC gradually become more negative from surface water to deep water. Based on massbalance calculation, about 57% of DIC derived from soil CO₂ at the reservoir entrance, while this value was 49% at the site of reservoir discharge. This result means that river damming could change the weathering information contained in the chemical composition of river water.

Key words: low hilly area, slope position, soil organic carbon, forest type, total nitrogen