Impact of organic carbon concentration on the molecular properties and assembly of riverine dissolved organic matter.

Dissolved organic matter (DOM) plays a central role in ecosystem health and carbon cycling. However, uncertainties remain regarding DOM molecular assembly mechanisms under varying organic carbon concentrations triggered by natural and anthropogenic factors. This study integrates geochemical analyses with molecular characterization of surface water and pore water DOM from seven rivers spanning diverse biomes and non-purgeable organic carbon (NPOC) gradients. Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS) revealed that the low NPOC groups were characterized by the increase of polycyclic condensed aromatics and sugar-like substances. Intermediate NPOC conditions favored the accumulation of DOM molecules with higher thermodynamic efficiency and activity compared to low- and high-NPOC groups. Alpha diversity analysis showed that decreasing NPOC concentrations led to narrower molecular coverage, reduced dispersion, and lower divergence. Stochastic processes primarily governed the DOM assembly process in high-NPOC environments, indicating constrained production and transformation pathways of DOM molecules. Conversely, deterministic processes dominated the assembly process for low-NPOC conditions, suggesting selective preservation or transformation of specific groups of DOM molecules. These findings highlight organic carbon concentration as a key driver of DOM molecular architecture and assembly processes, with implications for carbon sequestration and nutrient cycling in river ecosystems.