| Summary: | As an important part of the global carbon cycle, dissolved inorganic carbon (DIC) concentration and its stable carbon isotopic composition (δ<sup>13</sup>C<sub>DIC</sub>) have been used to constrain the sources of DIC in rivers. In this study, we systematically investigated the water chemistry, DIC contents, and δ<sup>13</sup>C<sub>DIC</sub> values in a tropical agricultural river in northeast Thailand. The water temperature ranged from 20.3 to 31.3 °C, and water pH values ranged from 6.4 to 8.4, with seasonal variations. Based on the major ion compositions, the hydro-chemical type of the Mun River water was a unique Na–Ca–Cl–HCO<sub>3</sub> type, controlled by evaporite and silicate weathering. Seasonal variation of DIC concentrations and its carbon isotopic composition was obvious; DIC and δ<sup>13</sup>C<sub>DIC</sub> were significantly lower in the wet season (135 to 3146 μmol/L and −31.0‰ to −7.0‰) compared to the dry season (185 to 5897 μmol/L and −19.6‰ to −2.7‰). A high level of <sup>12</sup>C-enriched DIC/CO<sub>2</sub> from soil respiration and organic matter oxidation may cause the low pH values, δ<sup>13</sup>C<sub>DIC</sub> values, and high partial pressure of CO<sub>2</sub> (<i>p</i>CO<sub>2</sub>) in the middle and lower reaches during the wet/rainy season compared to the dry season. This may be responsible for the seasonal and spatial variations of DIC concentrations and δ<sup>13</sup>C<sub>DIC</sub> values in the Mun River. According to the relationship between <i>p</i>CO<sub>2</sub> and δ<sup>13</sup>C<sub>DIC</sub> values, CO<sub>2</sub> outgassing may be more significant in the dry season, due to the greater influx of groundwater with higher <i>p</i>CO<sub>2</sub> levels; and the rapid CO<sub>2</sub> diffusion into the atmosphere will continuously increase the δ<sup>13</sup>C<sub>DIC</sub> values and decrease <i>p</i>CO<sub>2</sub> levels. These results show that riverine biologic effects and CO<sub>2</sub> outgassing play important roles in the DIC and δ<sup>13</sup>C<sub>DIC</sub> evolution of this typical agriculturally-dominated watershed.
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