Characteristics of soluble organic nitrogen composition and sources in sediments from Erhai Lake in China and the effect on the water quality

Soluble organic nitrogen (SON) has been hypothesized to play a central role in biogeochemical nitrogen cycle in freshwater lake resulting in deterioration in water quality. In this study, we investigated the composition and structure of DON in sediments from a typical eutrophic lake, Erhai Lake in China, to understand organic nitrogen molecular transformation involved in nitrogen release in the lake, and to assess DON potential effect on the lake water quality. Our results indicated that total soluble nitrogen (TSN) and SON in sediments had similar spatial distribution trend. The dominant component of SON fractions in the lake region had a significant difference that the north region was HON, the south region was hydrophobic acidic (HOA), but the middle region was hydrophilic matter (HIM). Three-dimensional fluorescence spectroscopy analysis of SON fractions indicated that HOA was dominated by humic-like substances, whereas HIM, HON, and hydrophobic basic were dominated by protein-like substances, which suggested that SON of the south region in the lake mostly originates from human activities. The higher content (31.7 %) and fluorescence characteristics of HIM in SON indicated that the SON of the lake sediments had a higher bioavailability and might remarkably impact the nitrogen cycle in water. Thus, the control of SON and HIM fraction in lake sediments was important to protect the lake water and to prevent the lake eutrophication.