Abstract: The hydrological circulation driven by tidal pumping has potentially important impacts on geochemical cycling within shallow mangrove aquifers. However, the nitrogen cycling mechanisms modulated by tidal pumping, as well as the nitrogen utilization strategies of mangrove plants, remain poorly understood. In this study, the nitrogen content and stable nitrogen isotopic compositions of plants, shallow aquifer sediments, porewater, and sediment extracts were collected and determined. Additionally, compound-specific isotope analysis of amino acids (CSIA-AA) was performed on the plant and sediment samples. The results indicate that the total nitrogen content in the aquifer sediments ranges from 0.04% to 0.22%. Terrestrial nitrogen dominates the recharge zone, whereas marine-derived nitrogen is predominant in the discharge zone. Organic nitrogen accounts for 73.2% of the dissolved nitrogen in the sediments, while the remaining inorganic nitrogen is predominantly in the form of ammonium. Ammonium derived from the degradation of organic nitrogen in the recharge zone is rapidly assimilated by plants or microorganisms, preventing its accumulation in the porewater. Conversely, the porewater ammonium concentration in the discharge zone averages 0.68 mg/L, yielding an estimated seaward export flux of 2.23 ± 1.41 mmol m⁻² d⁻¹. Furthermore, CSIA-AA reveals that the nitrogen source utilized by mangrove plants is substantially enriched, with δ¹⁵N values up to 10.7‰, more positive than those of the sediment nitrogen sources. This pronounced enrichment is likely the result of plant litter or sedimentary organic nitrogen degrading into ammonium, which subsequently undergoes isotopic fractionation via ammonia volatilization and anaerobic ammonium oxidation (anammox) before being ultimately recycled and assimilated by the plants. This study elucidates the primary nitrogen cycling processes and plant nitrogen utilization strategies in mangrove ecosystems, providing a scientific basis for evaluating mangrove nitrogen fluxes and regulating coastal nitrogen cycles.