Fractions and mineralization potential of the sediment organic nitrogen in Daya Bay, South China Sea: Anthropogenic influence and ecological implications

Nitrogen mineralization is a critical biogeochemical process that transfers organic nitrogen into inorganic forms using heterotrophic microorganisms. However, few studies have focused on this potential nutrient supplier. In this study, the composition of sediment organic nitrogen (SON) was studied, and nitrogen mineralization flux entering the water column was quantified. The results indicate that acid-hydrolyzable nitrogen (AHN) accounts for more than 40% of the SON, especially in the riverine input and marine aquaculture areas, which had significantly higher concentrations than the bay mouth area. Similar results were found for the ammonium nitrogen (AN), amino-sugar nitrogen (ASN), the total hydrolyzable amino acid (THAA), and unidentified hydrolyzable nitrogen (HUN). The mineralization rate in the marine aquaculture area was as high as 9.03 ± 1.33 mg·kg-1·d-1, while those of the riverine input (4.77 ± 1.55 mg·kg-1·d-1) and bay mouth (5.12 ± 1.42 mg·kg-1·d-1) areas were lower. The SON fractions, including the AHN, AN, ASN, and AAN, could obviously affect the mineralization of the SON. However, the extracellular enzymes, including proteinase and urease, are the predominant factors controlling the SON mineralization process. Anthropogenic activities, including riverine input and marine aquaculture, exert significant influences on the fractions and mineralization of the SON, and thus, they may increase the amount of dissolved inorganic nitrogen in the bottom of the water column in Daya Bay.