As an important sources of dissolved organic matter (DOM) in aquatic system, DOM releases from flooding or submerged soils is a key process in water-level fluctuation zones. In this study, four typical sites in water-level fluctuation zones of Three Gorges Reservoir areas were selected to conduct simulated soil flooding experiments, under ambient (open air) and anoxic conditions. By using fluorescence spectrum technique, the dynamic and geochemical characteristics of DOM releases from flooding soils were investigated. Results showed that the trend or model of DOM releases observed by fluorescence spectrum in all soils from four sampling sites was similar to the observation by UV-Vis spectrum. Fluorescence property of DOM releases showed an important contribution to DOM fluorescence in overlying waters. The rapid releases at initial stage and removal mechanism for later dynamic equilibrium were crucial to explain the dynamic "source-sink" equilibrium in whole flooding period. Effect of inorganic mineral adsorption-desorption on humic-like components (A and C peaks) was significant. Also, impact of microbial utilization on protein-like components (B and T peaks) was confirmative. Additionally, all DOM samples had "autochthonousness plus allochthonousness" property. Except Shibaozhai (SB) site in Zhongxian county, during the whole flooding period, all three other sites didn't show any significant difference between ambient and anoxic conditions. They could be explained by the high heterogeneity of soil property including minerals and organic components. Meanwhile, both the UV-Vis and fluorescence spectra were complementary for each other, and they evidentially showed that the "source-sink" (release and removal) mechanism in DOM releases from submerged soils was the core to decide the dynamics of DOM in overlying waters. Importantly, DOM showed higher aromaticity and humification at the initial release stage when flooding occurred, as well as the greater terrestrial source characteristics, which further helped to explain the environmental fates of pollutants in these environmental sensitive areas. It would also be useful for unveiling the role of DOM in environmental system in future.
Keywords: Three Gorges Reservoir Areas; dissolved organic carbon (DOC); dissolved organic matter (DOM); flooding; soil; three-dimensional fluorescence spectrum; water-level fluctuation zone.