River Water Hydrochemical Characteristics and Weathering Fluxes of a Subtropical Monsoon Basin, South‐Central China

ABSTRACT Quantifying weathering‐driven carbon sinks in human‐impacted subtropical basins remains a critical gap. In the carbonate‐dominated, densely populated Xiangjiang River Basin, this study tracked hydrochemistry through two hydrological years to partition solute sources and constrain CO 2 consumption at high resolution. Findings revealed precipitation total dissolved solids (TDSs) spanning 1.7–201.4 mg/L, where Ca 2+ , K + , SO 4 2− and NO 3 − constituted the principal ionic constituents and the values for Xiangjiang River water were 25.8–183.1 mg/L, with Ca 2+ and HCO 3 − as the dominant ions. The chemical weathering contributions from carbonate rock (62.2%) and silicate rock (14.5%) were estimated using a forward model, indicating that carbonate rock weathering is the primary source of the Xiangjiang River solutes. The average annual weathering fluxes of carbonate and silicate rocks in the Xiangjiang River were 21.6 and 3.4 t/km 2 /year, respectively, with corresponding CO 2 consumption rates of 293.1 × 10 3 and 81.0 × 10 3 mol/km 2 /year. This study provides the first high‐resolution quantification confirming that the subtropical monsoon climate of the Xiangjiang River Basin generates intermediate weathering‐driven CO 2 consumption rates, positioning its carbon sink capacity between that of arid/plateau regions and tropical zones. The findings establish hydrochemical baselines for the Xiangjiang and offer quantitative benchmarks for regional water‐resource management and carbon‐cycle modelling under intensifying anthropogenic pressures.