Abstract: Dissolved inorganic carbon (DIC) export and carbon dioxide (CO₂) emissions are important components of watershed carbon cycle and play an important role in the biogeochemical cycle of watersheds. To understand the impact of vegetation differences on DIC and CO₂ emissions, we selected three small watersheds with different forest vegetations in the northern Da Xing’an Mountains as quasi-paired watersheds and measured DIC and CO₂ in the rivers from April to October 2023. The results showed that: (1) The DIC concentration in the river showed a decreasing trend from the upper reaches to the lower reaches. The concentration in the upper reaches was significantly higher than that in the middle and lower reaches (P<0.05). There were no significant differences in the pressure of CO₂ (pCO₂) and CO₂ emission flux (F_CO2) in the upper, middle, and lower reaches (P>0.05). (2) The average DIC concentration of rivers in Laoyeling (the highest comprehensive index of forest vegetation), Sandaohe, and Shengdancun (the lowest comprehensive index of forest vegetation) had the same trend with the comprehensive index of forest vegetation, which were 16.24, 12.85, and 10.77 mg·L^-1, respectively, and were significantly different among the watersheds (P<0.05). The average DIC daily flux (F_DIC) arranged as Shengdancun (9.38 kg·km^-2) > Laoyeling (8.51 kg·km^-2) > Sandaohe (6.40 kg·km^-2), and the difference between Shengdancun and Sandaohe was significant (P<0.05). (3) There was no significant difference in the mean value of pCO₂ among the three watersheds (P>0.05). The average F_CO2 of Shengdancun (9.72 g·m^-2·d^-1) was significantly higher than that of Laoyeling (3.80 g·m^-2·d^-1) and Sandaohe (3.27 g·m^-2·d^-1) (P<0.05). (4) DIC concentration, F_DIC, and F_CO2 were significantly correlated with water quality indices, runoff and velocity, while p_CO2 was significantly correlated with dissolved oxygen and water temperature. In conclusion, the DIC concentration in the river decreased along the flow direction in the Laoyeling watershed. The differences of forest vegetation could cause significant changes in river DIC, which was positively correlated with the comprehensive index of forest vegetation. The higher the comprehensive index of forest vegetation in the watershed is, the higher the output concentration of DIC in the river. pCO₂ and F_CO2 did not show obvious patterns with the comprehensive index of forest vegetation.

Key words: Da Xing’an Mountains, small watershed, dissolved inorganic carbon, carbon dioxide, forest difference