Extreme precipitation amplified the cumulative effects of DOM availability on organic-sourced DIC in the Yangtze River.

Frequent extreme climate events are restructuring riverine carbon cycles dominated by dissolved inorganic carbon (DIC). However, the variability of dissolved organic matter (DOM) induced by rainstorm and its linkage to riverine DIC dynamics remain unclear, limiting an in-depth understanding of carbon transport and fate across the river-ocean continuum. This study employed Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) coupled with stable carbon and water isotope tracing techniques to investigate DOM-DIC interactions in the Yangtze River. Results demonstrated that organic matter constituted a major DIC source, contributing 13.52 ± 1.66% and 23.15 ± 3.27% of total DIC during normal (May) and rainstorm (September) periods in 2021, respectively. Extreme precipitation events (>150 mm·day-1) elevated dissolved organic carbon (DOC) concentration and the biological transformation index (Ibio) of DOM, while reducing molecular mass and double-bond equivalents (DBE) compared to the normal condition. During the rainstorm period, DOC concentration and Ibio values progressively declined downstream with increasing distance from the precipitation core, while molecular mass and DBE increased, contrasting with the spatially homogeneous DOM distribution characteristic of the normal period. Rainstorm enhanced terrestrial organic matter inputs, increasing DOC concentration and enriching low-molecular-weight, highly saturated CHO and CHON compounds. These synergistic effects accelerated DOM biodegradation to organic-sourced DIC (DICoc). Structural equation modeling further confirmed that extreme precipitation primarily promoted DICoc production through stimulated DOM biodegradation rather than photochemical oxidation. Storm events mobilized protein-like compounds from residential wastewater, while elevated water temperatures and nutrient levels collectively enhanced DOM biodegradability. Conversely, rainstorm-induced turbidity plumes suppressed photodegradation of terrestrial aromatic humic substances. Our findings highlight that precipitation-driven DOM loading and molecular transformations significantly accelerate biogeochemical carbon cycling.