Abiotic Pathways Bridge the Molecular Transformation from Terrestrial to Lacustrine Dissolved Organic Matter in...

Terrestrial dissolved organic matter (DOM) is a dominant allochthonous input to shallow lakes but exhibits low bioavailability due to prior microbial degradation and complex aromatic structures. In contrast, lacustrine DOM is more saturated and structurally homogeneous, implying in-lake transformation. Herein, we investigated the abiotic transformation of terrestrial and lacustrine DOM using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), linkage reaction classification, and XGBoost-SHAP machine learning. Under coupled simulated sunlight and MnO2 exposure, terrestrial DOM is more prone to dearomatization and partial oxidation, with limited carbon backbone cleavage due to stable condensed aromatic frameworks. Conversely, while partial oxidation remains a primary pathway, aliphatic-rich lacustrine DOM readily undergoes carbon backbone fragmentation. Notably, MnO2 acts as an electron acceptor and reactive oxygen species regulator, mitigating direct photosensitized carbon backbone cleavage while enhancing oxidation and structural complexity. Machine learning confirms that, despite distinct origins, both DOM types exhibit consistent shifts toward higher oxygenation and aliphatic character. Furthermore, this analysis provides new insights, identifying the nitrogen content and DBE/O as universal indicators of DOM reactivity. Ultimately, abiotic processes impose common molecular constraints, guiding DOM transformation along partially convergent pathways and driving structurally distinct DOM pools toward a shared chemical fate.