Temporal and Spatial Distribution Characteristics and Source Analysis of Dissolved Organic Nitrogen in the Surface Sediment of the Caohai Lake, Southwest China

Abstract The Caohai is a typical natural lake-type karst plateau wetland, which is sensitive to environmental changes, and the release of dissolved organic nitrogen (DON) in the sediment is a potential factor of nitrogen pollution in the lake. Therefore, this study aimed to reveal the temporal and spatial distribution characteristics and source analysis of DON in karst lake sediments. Surface sediment samples were collected from the Caohai Lake in southwest China, and the sample locations were distributed across almost the entire wetland area during both the wet and dry seasons. The DON content and fluorescence spectroscopy were determined and analysed using a three-dimensional fluorescence-parallel factor analysis (3DEEM-PARAFAC) model. The mean DON content in the sediment during the dry season (67.79 mg/kg) was higher than in the wet season (28.57 mg/kg). There were four fluorescent components of DON in the sediment: C1 (tyrosine-like protein), C2 (terrestrial humus), C3 (tryptophan-like protein), and C4 (terrestrial humus). The maximum fluorescence intensity of each component in both seasons showed the trend of C2 > C1 > C3 > C4. The DON in the surface sediment of the Caohai Lake was influenced by terrestrial sources and biological activities. These findings help reveal the circulation mechanism of endogenous nitrogen in the lake and provide theoretical support for the prevention and control of nitrogen pollution in the Caohai Lake.