Molecular Alteration of Dissolved Organic Matter in Intertidal Sediments: The Role of Fe(II) Reoxidation

Iron redox cycling is pivotal in coastal sediment organic matter (OM) processing. However, molecular-level effects of iron reoxidation on dissolved organic matter (DOM) during redox oscillations remain underexplored. This study investigated DOM transformations driven by iron reoxidation in Changjiang Estuary porewaters using controlled oxidation experiments and Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). Iron reoxidation significantly altered DOM composition and regulated nutrient (P, S, N) availability. Dissolved organic carbon (DOC) decreased by up to 37 ± 17% (initial oxidation induced 22 ± 12% loss), driven by ROS-mediated degradation and association with the precipitating iron (oxy)hydroxides. Concurrently, substantial abiotic sulfurization and nitrogen incorporation into DOM were evidenced, primarily transforming lignin- and protein-like components into new CHOS and CHONS molecular formulas. Furthermore, iron (oxy)hydroxides were associated with the formation of new, more complex polymeric structures, shifting the DOM pool toward a more aromatic and refractory composition. These findings demonstrate iron reoxidation as a key abiotic process controlling DOM fate, potentially promoting carbon sequestration and regulating nutrient availability in dynamic coastal environments, and underscoring its importance for coastal biogeochemical cycling.