Linking Molecular Diversity to Electron Transfer Capacity of Soil Dissolved Organic Matter: Role of Molecular Composition and Structure

Soil dissolved organic matter (DOM) plays a pivotal role in various environmental redox processes due to its abundant redox active functional groups, but how the molecular diversity of soil DOM regulates its redox capacity remains poorly understood. In this study, we assessed how the molecular diversity affected the redox capacity of 22 soil DOM samples. Despite the diverse DOM compositions shown in Fourier transform ion cyclotron resonance mass spectrometry data, we found that all DOM can be represented by three molecular groups. We subsequently constructed molecular models for soil DOM and used these results to explain variations in the soil DOM redox capacity measured by mediated chronoamperometry. The redox capacity of soil DOM was significantly correlated with the content of major redox-active functional groups within the supramolecular 3D structure of the DOM. The distances between the key redox-active functional groups, quinoid C═O and Ar-OH pairs, mostly were between 17 and 43 Å, and the pairs close to the surfaces of the molecular structure may be more redox active. The findings from this study provide a basis for predicting the electron exchange processes in soil DOM, thereby enhancing our ability to explain and predict DOM-mediated redox reactions in soil environments.