<p>Agricultural HNO₃ can affect mineral weathering, regional carbon cycling, and river NO₃^– pollution, which are linked to complex biological and hydrogeochemical processes. However, the involvement of HNO₃ in carbonate weathering and associated CO₂ fluxes is still not fully understood, especially in piedmont areas. Herein, 20 river water samples were collected from a typical piedmont region (Qingshui River basin) in September 2023 and analyzed for hydrochemical ions and multiple isotopes. The hydrogeochemical analyses, coupled with the Bayesian and forward models, and stoichiometric relationships, were employed to elucidate the major mechanisms underlying the release of HNO₃ from agricultural NH₄⁺ inputs and its quantitative effects on carbonate weathering and CO₂ sink flux. The hydrochemical evolution was driven by the piedmont strong runoff condition, resulting in the dominance of the HCO₃-Ca facies with less-soluble minerals. On the other hand, the riverine NO₃^– concentrations in the piedmont agricultural region were primarily derived from sewage and manure, with a mean contribution rate of 92.0 ± 2.07%. A large amount of NH₄⁺ from manure and sewage was rapidly oxidized through microbial nitrification, producing 35.9 ± 12.7&#xa0;mg/L of HNO₃. According to the forward model results, carbonate weathering was the major contributor to water–rock interactions (49.8 ± 3.96%). Furthermore, carbonate rocks (mainly calcite) were preferentially and rapidly weathered by the produced HNO₃, increasing and decreasing the Ca²⁺ + Mg²⁺ concentrations and CO₂ sink flux by the end of the sampling period by 18.5 ± 6.21% and 13.0 ± 5.07%, respectively. Our study provides further insights into mineral weathering and carbon cycling associated with anthropogenic acids in piedmont areas.</p>