Stability of sedimentary organic matter: Insights from molecular and redox analyses.

Sedimentary organic matter (SOM) affects the stability of the aquatic carbon pool. The degradation process of SOM is complex for its multifaceted composition. The concentration and properties of SOM affect its steady state, yet the transformation processes of SOM in lakes remain unclear. Here we show the molecular and redox perspectives of SOM stability in polluted sediments with high organic matter content and diverse vegetation. We find significant differences in carbon fractions across various sites. The origin of the organic matter, determined using excitation-emission matrix spectra, influences the consistency of organic matter composition and biochemical degradation in lacustrine sediment. We also observe that sulfur-containing substances decrease carbon chain length and reduce organic matter stability. Fourier-transform ion cyclotron resonance mass spectrometry shows that sulfur-containing substances decrease the degree of saturation and cause reduction. In contrast, nitrogen-containing compounds increase the modified aromaticity index and humin content, enhancing organic carbon complexity and stability (p < 0.05). These results complement the characteristics and transformations of SOM. In a broader perspective, this study contributes to laying the foundation for understanding SOM stability in the carbon cycle and its future effects.