Environment and Sources Affect the Molecular Composition and Characteristics of Sediment Dissolved Organic Matter in Coastal Environments in China

   Sediment dissolved organic matter (DOM) on the coast of China plays a critical role in the terrestrial and marine carbon cycle process of the global environment. In this study, surface sediment samples were collected from coastal areas including the Bohai Sea (BS), Yellow Sea (YS), East China Sea (ECS), and South China Sea (SCS) in China (Figure 1). We considered environmental factors, sediment sources, and the molecular compositions of sedimental DOM on coast of China in combination with the stable isotopic analysis and FT-ICR-MS. We analyzed the δ¹³C and δ¹⁵N stable isotopic compositions and DOM molecular characteristics of sediments collected from coastal areas spanning the BS, YS, ECS, and SCS by using the isotope ratio mass spectrometry and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). Stable isotope (δ¹³C and δ¹⁵N) analysis and the MixSIAR model showed that the sediments on the coast of China are affected by three main end-members. 

    Isotopic analysis indicated that most of the C and N in coastal sediments of the BS, YS, and ECS came from terrigenous sources (45.0%~73.3%), whereas anthropogenic sources were the main end-members in the SCS (49.8%). There were no significant regional differences in the concentrations of DOM（Figure 2）. The major molecular compounds detected in the DOM on the coast of China were lignins (22.17%~25.83%) and proteins/amino sugars (47.95%~54.35%). The sources of the sediments affect the composition of DOM molecules, and mainly terrigenous inputs to the DOM were affected. The terrigenous DOM present in the sediments is mainly affected by the dissolution of sediments, and the DOM molecules in this fraction are mainly condensed aromatics and unsaturated hydrocarbons. Environmental factors play a vital role in the molecular composition of DOM. Lignin molecules in DOM increase with increasing salinity and depth. Autochthonous sediment sources likely contribute to the O-poor and unsaturated compounds present in the coastal DOM（Figure 2）. Baased on a combination of Spearman’s correlation analysis and stable isotopic analysis, it was found that DOM molecular chemodiversity was influenced by spatial environmental factors (mainly salinity, depth, and temperature.) and by the origin of coastal sediments. DOM molecules containing nitrogen were mainly influenced by marine sources in the sediments (p<0.05). Our findings are of significance for understanding the carbon cycle on the coast of China. These findings can facilitate a better assessment of the contribution of DOM sources and highlight the importance of maintaining a balance between development and coastal ecosystem sustainability under increasing human disturbances.