The study of chemical composition of dissolved organic matter (DOM) can provide key chemical information for hydrospheric carbon cycle, which has important environmental and biogeochemical significance. In recent years, significant progress has been made in the study of DOM molecular composition and structure, including average structure analysis based on nuclear magnetic resonance and fluorescence spectroscopy, and molecular composition analysis based on high-resolution mass spectrometry. Molecular composition information opens a new chapter for DOM research, but the limited molecular structure information still cannot answer many questions about the source and transformation mechanism of DOM. There are a large number of oxygen atoms existed in the form of different functional groups in DOM molecules, leading to the diversity of DOM molecular properties and bringing insuperable difficulties to the study of molecular structure.

In order to obtain the molecular structure information of DOM, this research used a hydrodeoxygenation method to convert high oxygen-containing DOM into hydrocarbons or low oxygen-containing compounds with low molecular polarity. The carbon skeleton of DOM molecules was preserved as far as possible without damage by controlling the reaction conditions. These compounds can be characterized by well-established analytical methods in the field of petroleum analysis to obtain molecular composition and structural information. In this study, the structure and distribution of various types of compounds in reaction products of typical DOM samples such as Suwannee River Fulvic Acid (SRFA) and Jiufeng Forest Soils Fulvic Acid (JFSFA) were obtained using this method. The long-chain n-alkanes (C₂₃~C₃₅), alkaloids and diterpenoids in JFFA revealed the occurrence of prominent input of terrigenous higher plants. The n-alkanes in SRFA had a bimodal distribution which was dominated from C₁₁ to C₁₅ and C₁₉ to C₂₃, and the diversity of steroid compounds together indicated that SRFA had a mixed source of aquatic algae, bacteria and terrestrial higher plants.