The excess reactive nitrogen (Nr) delivery driven by intensive anthropogenic activities has led to unprecedented pressures on estuaries and coastal oceans. How to maintain a healthy and sustainable ecosystem has become an ecological and socio-economic problem of global concern, which calls for a deeply understanding of Nr biogeochemical cycle, particularly, the fate of Nr in systems. Here, we quantitatively assess the two sedimentary Nr removal processes (denitrification and anammox) and associated nitrous oxide (N₂O) production in the Pearl River Estuary (PRE). The coupling between the biogeochemical potentials and chlorophyll in spatial indicates that the quantity and quality of organic matter are important drivers for Nr loss and N₂O production. Direct denitrification and coupled nitrification-denitrification are two main mechanisms of Nr removal in the PRE, which are largely depended on nitrate concentration and labile organic matter, respectively. The Nr self-purification capacity is 1.1×10⁷ mol N d^-1, with 9.1×10⁶ mol N d^-1 from denitrification and 2.2×10⁶ mol N d^-1 from anammox, accounting for 25% of the total riverine Nr input. Such a result indicates that most of the Nr will be delivered to shelf seas under current high Nr input stress. In addition, sedimentary denitrification-induced N₂O production (2.4×10⁵ mol N d^-1) occupies 66 ± 27% of the daily sea-air N₂O emission flux, playing a significant role in climatic feedback. We argue that riverine inorganic Nr controls the self-purification capacity of estuary via affecting the primary productivity and the subsequent labile organic matter sedimentation. Effective control measures in the future reduce the upstream Nr load maintaining the resilience of estuarine ecosystems and mitigating the climatic feedback. These findings contribute to policy makers to develop knowledge-based management actions for achieving a sustainable estuary.
 

Nitrogen purification,Denitrification,Anammox,Climatic feedback,Knowledge-based management