Spectroscopic properties and driving factors of dissolved organic matter in the Yellow River Delta

River deltas are hot spots of biogeochemical cycling. Understanding sources and driving factors of dissolved organic matter (DOM) in river deltas is important for evaluating the role of river deltas in regulating global carbon flux. In this study, spectroscopic properties of soil DOM were analyzed in both freshwater and tidal areas of the Yellow River Delta. Five fluorescent components of soil DOM (two humic-like DOM, two protein-like DOM, and one possible contaminant) were identified by parallel factor analysis and further confirmed by comparison with an online database. Concentration, spectroscopic properties, and sources of soil DOM and its components differed between freshwater and tidal areas. DOM concentration was much higher in freshwater areas than in tidal areas. In freshwater areas, soil DOM was mainly derived from phytoplankton and microorganisms, while in tidal areas, it was mainly derived from microorganisms and human activities. These differences in DOM between both areas were strongly driven by environmental factors, especially soil carbon (C), nitrogen (N), and its stoichiometric ratio C/N. These explained 80.7% and 69.6% of variations in DOM and CDOM, respectively. In addition, phytoplankton also contributed to soil DOM, CDOM, and fluorescent components C1–C4 as identified by significant positive correlations between them. The results imply that in the Yellow River Delta, both the concentration and composition of soil DOM are strongly driven by soil properties and phytoplankton density.