Molecular Transformation of Dissolved Organic Matter during Dark Oxygenation in Paddy Soil: Insights into Abiotic Processes from High-Resolution Mass Spectrometry Combined with Two-Dimensional Correlation Spectroscopy

Dissolved organic matter (DOM) engages in complex interactions with reactive oxygen species generated in redox oscillations. However, the molecular-level transformations of DOM in paddy soils during reoxygenation, particularly their dynamic evolution, remain poorly understood. This study revealed that protein-like components in DOM underwent pronounced desorption within 0–0.25 h and then were converted, whereas fulvic- and humic-like components exhibited greater stability. Molecular transformations were characterized by polymerization, aromatization, and the degradation of high-molecular-weight and highly unsaturated compounds into simpler aliphatic structures. Lignin-like and condensed aromatic compounds showed high reactivity and were transformed. Furthermore, molecules with 0.6 ≤ H/C ≤ 1.0 and 0.75 ≤ O/C ≤ 0.8 were preferentially transformed and removed, while molecules with 1.0 ≤ H/C ≤ 1.4 accumulated at the later stage. Overall, DOM experienced redistribution, oxidative condensation, and structural simplification during oxygenation. These findings advance the molecular-level understanding of DOM transformation and its environmental behavior in paddy systems.