Typhoon landfall track significantly changes coastal nitrogen cycling pathways by altering hydrodynamic regimes

Coastal nitrogen (N) cycling is vulnerable to typhoon disturbances, yet the pathway-specific mechanisms, particularly how typhoon tracks regulate N sources and transformations, remain unclear due to methodological limitations in distinguishing biotic and abiotic processes. Here, we employed nitrate (NO₃^-) dual isotopes (δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^-) to study the distinct N cycling pathways in Zhanjiang Bay following two typhoons with contrasting landfall tracks. The results demonstrate that the typhoon track not only determines hydrodynamic patterns but also fundamentally alters the nature of N cycling. Specifically, different landfall tracks of typhoons lead to distinct N cycling patterns. For the left-side landfall typhoon (Lionrock), onshore winds established a salinity front that intensified sediment resuspension. The isotopic evidences revealed that this physical regime triggered adsorptive NO₃^- loss onto suspended particles, a quantitatively important but previously overlooked abiotic NO₃^- removal pathway during such events. Concurrently, source apportionment indicated that this physical retention mechanism trapped > 70 % of municipal sewage-derived NO₃^- within the bay. In contrast, the right-side landfall typhoon (Chaba) generated offshore winds, flushing the bay with terrestrial nutrients and stimulating intense phytoplankton blooms. The isotopic evidences suggested that phytoplankton assimilation is the dominant NO₃^- consumption process. Therefore, the typhoon track acts as a switch that flips between two distinct N fates, including physicochemical processes (dominated by the left-side landfall typhoon) and biological process (dominated by the right-side landfall typhoon). This N-centric framework provides actionable insights for coastal management, showing that left-side typhoons may exacerbate localized eutrophication risks from point sources, while right-side typhoons drive basin-wide biological responses.