Characterizing the seasonal variation and key influencing factors of dissolved organic matter in the Luohe River, a main tributary of the Yellow River, China
摘要
Dissolved organic matter (DOM) is the largest reactive organic C pool and plays a significant role in different biogeochemical processes in river ecosystems. In recent decades, climate change and anthropogenic activities (e.g., reservoir construction and changes in land-use types) have obviously influenced the quality and quantity of DOM. However, the responses of DOM chemistry in rivers to both seasonal alteration and land-use types remain poorly understood. Here we investigated the influence of seasonal alteration and land-use types on the concentration, compositions, and sources of DOM in the Luohe River, one of the largest tributaries of the Yellow River Basin in China, using ultraviolet (UV)–visible and three-dimensional fluorescence excitation–emission matrix (3D-EEM)-parallel factor analysis (PARAFAC). The results showed that the concentration of dissolved organic carbon (DOC) in the Luohe River during the summer (8.40 ± 2.85 mg L−1) was significantly higher than that during both the spring (5.36 ± 1.00 mg L−1) and autumn (4.24 ± 1.33 mg L−1). The absorption properties of DOM exhibited the presence of DOM composition with high hydrophilic substances and low molecular weight. The fluorescence index showed the source of DOM with both autochthonous and allochthonous sources. Three types of humic-like components (C1 + C2 + C4, Em. > 380 nm) and one type of protein-like component (C3, Em. < 380 nm) were identified using the PARAFAC model. The optical properties of DOM exhibited significant seasonal variation. The partial least squares structural equation modeling (PLS-SEM) analysis indicated that the direct effects of land use on protein-like component and humic-like component in spring were 0.2286 and −0.2593, respectively, and those in summer were 0.2241 and −0.4824, respectively. In addition, land use indirectly influenced the protein-like component by changing DOM origin (0.3455) and nutrients (0.1205) in spring, and affected the humic-like component by changing DOM origin (−0.3417) and nutrients (−0.1184) in spring. These findings suggested that both seasonality and land-use types were key drivers for the DOM sources and compositions, which are useful to understand the vital role of DOM in the biogeochemical cycle, and can provide a theoretical foundation for water quality protection.