Abstract: The Three Rivers (Jinshajiang River, Lancangjiang River, and Nujiang River) region (TRR) in Southwest China was selected to study the characteristics of chemical weathering during river erosion. With the forward model, hydrochemical characters and dissolved inorganic carbon isotopes of river waters were used to interpret the sources and their corresponding contributions to dissolved components, and to confirm the participation of sulfuric acid in carbonate weathering and the net release of CO₂. The results showed that: (1) Hydrochemical compositions of river water were controlled by complex lithology. The ions of NaCa-Cl-HCO₃ were dominated in Jinshajiang River due to outcrop of evaporates in its upstream region, while Ca-HCO₃ was dominated in Lancangjiang River and Nujiang River due to the existence of abundant carbonate and silicate minerals; (2) Sources and ratios of dissolved components in river waters in TRR were variable. The dissolution of evaporates might control the sources of ions in the main stream of Jinshajiang River with an average percentage of 52%, but was minor in Lancangjiang River and Nujiang River with an average percentage of 11% and 2%, respectively. On the contrary, the dissolution of carbonates controlled the sources of ions in the main stream of Lancangjiang River and Nujiang River with an average of 70% and 78%, respectively, and was minor in the main stream but considerable in major tributary drainage of Jinshajiang River with a corresponding average of 38% and 74%. In addition, the contribution ratios of silicates weathering to the main streams of Jinshajiang River, Lancangjiang River and Nujiang River were 8%, 16%, and 15% respectively; (3) The weathering rates of silicates in the catchments of Jinshajiang River, Lancangjiang River and Nujiang River were 1.14, 3.32 and 4.32 t·km^-2·a^-1, respectively, and the corresponding CO₂ consumption rates were 0.35, 1.19 and 1.41×10⁵ mol·km^-2·a^-1. By contrast, the weathering rates of carbonates in the three studied rivers were 17.41, 34.04 and
34.84 t·km^-2·a^-1 with the CO₂ consumption rates of 1.46, 2.69 and 2.53×10⁵ mol·km^-2·a^-1 respectively; (4) Sulfate in the TRR was mainly derived from sulfide oxidation which led to net release rates of CO₂ as 0.87, 3.01 and 4.36×10⁵ mol·km^-2·a^-1 in Jinshajiang River, Lancangjiang River and Nujiang River, respectively. These values were higher than those in Guizhou, China, so attention should be paid to the important role of sulfuric acid in rock weathering during the study of regional carbon cycle and global climate change.

Key words: soil quality, heavy metal, Xinjiang, cotton field., physicochemical property