Deciphering the Molecular Signatures and Optical Properties of Dissolved Organic Matter in the Aksu River (Xinjiang, China) via FT-ICR MS and 3D-EEM Spectroscopy

Desert oases and river systems are complex and dynamic ecosystems featuring unique hydrological patterns. The system significantly influences the production, degradation, and transformation of dissolved organic matter (DOM), thereby further regulating DOM in the desert oasis. However, the molecular composition and significance of DOM in rivers within desert oases are rarely studied. In this paper, the optical properties and spatial variation in molecular characteristics of surface water DOM in the Aksu River were investigated using three-dimensional fluorescence spectroscopy (3D-EEM) and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). The results indicated that the rivers possess distinct molecular compositional characteristics of DOM, with high spatial heterogeneity (variations in optical parameters and molecular compounds). Diversity in DOM is revealed at the molecular level primarily through S- and N-containing functional groups. Unlike large rivers (e.g., the Yangtze) dominated by terrestrial inputs or algal blooms, our study reveals that the DOM in the Aksu River (a desert-oasis river) is characterized by highly unsaturated and phenolic compounds and is primarily driven by intense photodegradation and evaporation rather than by microbial or terrestrial allochthonous inputs. This highlights a distinct photochemical signature unique to arid river systems. The findings will deepen the understanding of the DOM in desert-oasis river systems. Based on this research, seasonal variation in DOM in the Aksu River under different hydrological conditions can be further studied, thereby enriching the understanding of the carbon cycle in desert-oasis river systems.