Mechanisms of hydrophilic and hydrophobic dissolved organic matter influencing nitrogen dynamics in sediments

Dissolved organic matter (DOM) composition critically influences sediment nitrogen (N) cycling, yet the distinct roles of its hydrophilic (HIM) and hydrophobic (HOM) fractions remain incompletely characterized. Through controlled simulation experiments, we characterized the structural differences between HIM and HOM using techniques such as elemental analysis and electron paramagnetic resonance (EPR). By correlating these findings with quantitative monitoring of N fluxes in the overlying water column and key physicochemical parameters (pH and redox potential), we systematically evaluated the effects of HIM and HOM on N dynamics under natural, hypoxic, and alkaline hypoxic (pH ∼ 10) conditions. Our results demonstrate that HIM markedly enhanced ammonium-N (NH₄N) release, increasing fluxes by 60 % over controls (p < 0.001), with a further two-fold amplification under hypoxia. In contrast, HOM consistently yielded significantly lower NH₄N fluxes compared to HIM across all conditions (p < 0.001). Under alkaline hypoxia, a universal suppression of NH₄N release (50 % reduction) was observed, while HIM uniquely promoted nitrate-N (NO₃N) flux, elevating it to a level significantly above that of all other treatments (p < 0.05). The differences in N dynamics at the sediment-water interface caused by HIM and HOM may be attributed to their distinct compositions. This study elucidated the mechanisms of DOM-mediated N release and transformation, providing a basis for mitigating lake eutrophication and advancing insights into the potential role of DOM in global carbon and N cycles.