Fate of Organic Nitrogen in Estuaries: A Hotspot for the Interaction, Mixing, and Biological Activity of Terrestrial and Marine Sources

Estuaries are key areas for terrestrial material transport and marine biogeochemical processes, particularly those of dissolved inorganic nitrogen (DIN) and dissolved organic nitrogen (DON). However, the fate of DON in estuaries with a high runoff remains poorly understood. We explored the translocation and transformation of DON in the Pearl River Estuary (PRE) and adjacent coastal areas of southern China based on DON concentrations, optical and fluorescence characteristics, and molecualr composition. The distribution of DIN was primarily influenced by freshwater‐seawater mixing. In contrast, biological processes and freshwater‐seawater mixing shaped the distribution of DON. High levels of DON in the terrestrial‐dominated zone were predominantly anthropogenic sources through terrestrial inputs, whereas DON in the freshwater‐seawater mixing zone and seawater‐based zone were mainly influenced by biological activities. DON exhibits fast dilution in low‐salinity areas and retention in moderate‐salinity areas (freshwater‐seawater mixing area) in summer and winter, while undergoes a rapid decrease in open water areas during winter. Consequently, DON in the PRE and adjacent coastal areas exhibited nonconservative mixing despite seasonal variations. At the molecular level, tDON exhibited reduced unsaturation and aromaticity, coupled with an elevated abundance of DON compounds containing one‑nitrogen atom (1 N-DON, 53.17 %) and compounds containing carbon, hydrogen, oxygen, nitrogen, and sulfur (CHONS) (27.46 %). It was evident that lignin was depleted while more oxygenated tannin compounds were generated in the freshwater-seawater mixing zone. This transformation is attributed to heightened biological activities, likely influenced by the priming effect of terrestrial nutrient inputs. In summer, the prevailing plume combined with biological activities in the strong mixing area and outer estuary increased the abundance of 3 N-DON molecules and a concurrent rise in the abundance of DON compounds containing only carbon, hydrogen, oxygen, and nitrogen (CHON), DON compounds containing carbon, hydrogen, oxygen, nitrogen, sulfur, and phosphorus (CHONSP), and CHONS. This trend also underscores the expanding role of marine plankton and microbes in the utilization of DON compounds containing carbon, hydrogen, oxygen, nitrogen, and phosphorus (CHONP). These findings provide novel insights into the role of DON in nitrogen biogeochemical processes in river‐dominated estuaries and adjacent coastal areas and revealed the details of tDON transformation processes at the molecular level in a river-dominated estuary and underline the estuarine hydrodynamics involved in transporting and altering DON within the estuary.