Spectral Properties and Their Seasonal Variations of Soil Dissolved Organic Matter in Typical Vegetation of the Yellow River Estuary Wetland

To investigate the properties of soil dissolved organic matter (DOM) in typical vegetation of the Yellow River Estuary wetland, soil samples from the root surfaces of three dominant plants (Tamarix chinensis, Phragmites australis, and Suaeda salsa) were collected in July and September 2022 and February 2023. The physicochemical properties of the soil and spectral characteristics of DOM were analyzed to explore the dynamics of soil DOM during the growing and stagnant periods, as well as their relationships with environmental factors. The results revealed that the concentrations of dissolved organic carbon (DOC) and total dissolved nitrogen (TDN) in the soils followed an overall trend of Tamarix chinensis＞Phragmites australis＞Suaeda salsa. However, variations were observed across growth stages: during the rapid growth period, Tamarix chinensis had the highest content, followed by Phragmites australis and Suaeda salsa; at the end of growth period, Tamarix chinensis remained the highest, with Suaeda salsa surpassing Phragmites australis; whereas in the senescence phase, Suaeda salsa exhibited the highest content, followed by Tamarix chinensis and Phragmites australis. Utilizing excitation-emission matrix spectroscopy coupled with parallel factor analysis (EEMs-PARAFAC), three humic-like components (C1, C2, C3) and one protein-like component (C4) were identified in the soil CDOM, with humic-like components accounting for over 86% of the total. Comprehensive analysis using optical indices such as SUVA₂₅₄, S_R, FI, BIX, and HIX revealed that soil DOM in the study area was predominantly hydrophobic, highly humified and aromatic, with terrestrial sources as the primary origin and minimal contribution from autochthonous sources.