Biological nitrogen fixation is a fundamental process in the marine nitrogen cycle, playing a critical role in the ocean's capacity for atmospheric CO₂ uptake and sequestration. Historically, research on nitrogen fixation has predominantly concentrated on oligotrophic marine regions, largely due to traditional paradigms and the limitations of analytical methods. Consequently, the understanding of nitrogen fixation in coastal waters, which are strongly influenced by terrestrial inputs, remains comparatively underdeveloped. This study focuses on the river-estuary-coastal ocean continuum in China's coastal region, specifically the Jiulong River-Xiamen Bay system, employing the isotope ¹⁵N tracer technique (revised ¹⁵N₂ gas bubble method), to quantify nitrogen fixation rates (NFRs) across varying salinity gradients. Preliminary findings reveal that significant NFRs were observed across a range from the eutrophic freshwater zones of the Jiulong River to the high-salinity waters of Xiamen Bay. The nitrogen fixation activity was predominantly concentrated in the estuarine zones where freshwater and seawater mix, as well as in the high-salinity bay areas, indicating pronounced spatial heterogeneity. Moreover, the distribution of the nifH gene in total environmental DNA across different salinity regimes suggests a widespread potential for nitrogen fixation from the estuary to adjacent coastal regions.Overall，the observations from this study of nitrogen fixation along the river-estuary-coastal ocean continuum enhance our understanding of global nitrogen fixation dynamics and underscore the critical role of such continuums in refining estimates of the global nitrogen budget.

 

nitrogen fixation, river-estuary-coastal ocean continuum, Jiulong River, Xiamen Bay