The molecular weight-dependent redox capacity of soil dissolved organic matter: Roles of aromaticity and organic sulfur

The redox capacities of soil DOM regulate the oxidation–reduction reactions by accepting or donating electrons, which influences a variety of critical redox processes such as greenhouse gas emissions and the (bio)geochemical cycle of contaminants. The composition and reactivity of DOM components vary with their molecular weight (MW), and, therefore, the redox capacities of soil DOM are expected to depend on the MW. However, the limited data that are available on how specific molecular compositions affect the MW-dependent soil DOM redox capacities are inconclusive. In this study, we investigated how soil DOM fractions, separated according to their MW, differed in redox capacity as quantified with electron accepting and donating capacity, and evaluated the relationships between redox capacity of DOM fractions and their optical indices and molecular compositions. As expected, with increasing MW, aromaticity of DOM fractions increased and aliphaticity and protein-like composition decreased, which were measured with Fourier transform-ion cyclotron resonance mass spectrometry (FT-ICR-MS) and the optical methods. For the high MW fraction, electron-accepting capacity (EAC) as measured by mediated chronoamperometry was positively correlated with condensed aromatics and polyphenol content and negatively correlated with aliphatic content. More interestingly, our data show that electron-donating capacity (EDC) correlated with sulfur-containing components of the low MW fraction but did not show any correlations with other molecular compounds/properties of soil DOM. Our results highlight the importance of considering both aromaticity and organic sulfur and integrating the molecular information of soil DOM (e.g., MW) for predicting soil DOM redox capacities.