Hydrophobic and hydrophilic dissolved organic matter: Mechanistic insights into its influence on sediment phosphorus release revealed through multifaceted characterization

The role of dissolved organic matter (DOM) in phosphorus (P) cycling in aquatic ecosystems is essential. To the best of our knowledge, no study has yet examined the impacts of the hydrophilic (HIM) and hydrophobic (HOM) fractions of DOM on P release at the molecular level. In this work, Fourier transform infrared spectroscopy (FTIR), electron paramagnetic resonance (EPR), carbon-13 nuclear magnetic resonance (¹³C NMR), elemental analysis (EA), and Fourier transform ion cyclotron resonance mass spectrometry coupled with electrospray ionization (ESI-FT-ICR-MS) were employed to gain deeper insights into the characterization of two sets of DOM fractions. Incubation experiments were then designed to analyse their impact on soluble phosphorus (SRP) release. Herein, we demonstrated the distinctly different roles of HIM and HOM in P release from sediments. Adding HIM inhibited SRP release from the sediment, whereas HOM promoted it. Mechanism diagrams illustrated that the carboxyl and hydroxyl groups on HIM, along with the superoxide radical ions (•O₂⁻) produced by HIM, play a critical role in suppressing the release of Fe-bound P. Furthermore, the results of the incubation experiments indicated that the maximum release and average flux in the HOM treatment were 2.35 mg/kg and 0.66 mg/(m²·d), respectively, representing a 73.17 % increase in SRP release compared to the control. In contrast, in the HIM treatment, the values were 0.28 mg/kg and 0.10 mg/(m²·d), respectively, indicating a 79.08 % decrease. Therefore, future efforts should focus on leveraging the P removal capacity of HIM while prioritizing the elimination of HOM. Our study uncovers the effects of HIM and HOM on P release at the molecular level, offering a scientific foundation for effective management of P cycling in aquatic ecosystems.