The concentration of NH₄⁺ in pore water shows significant seasonal variations, with the highest levels in summer. The δ¹⁵N-NH₄⁺ indicator presents three distinct patterns: increasing with depth, decreasing with depth, and remaining relatively stable, reflecting the complex interactions among mineralization, nitrification, denitrification, and anaerobic ammonium oxidation (anammox). Stations with higher δ¹⁵N-NH₄⁺ values are mainly concentrated in the western part of the Bohai Sea and the strait area, while the downward trend is mainly observed in winter. The concentration of NO₃^- is generally low in all seasons, but reaches a peak within the top 3 cm and drops sharply with depth. The ratio of δ¹⁵N-NO₃^- to δ¹⁸O-NO₃^- shows a good 1:1 relationship, indicating the role of denitrification. The total nitrogen (TN) content in sediment is significantly higher in clayey sediments and nearshore/horizonal areas. There is a strong positive correlation between TN and δ¹⁵N-TN values, and the δ¹⁵N-NH₄⁺ value in the deep interstitial water is close to the δ¹⁵N value of marine organic matter after decomposition, indicating that TN is preferentially decomposed by heavy ¹⁵N-containing marine organic matter during the mineralization process.
Furthermore, we employed a non-steady-state diagenesis model to simulate the vertical distribution of nutrients and isotopes, taking into account molecular diffusion, sediment transport, and biogeochemical reactions. The model results indicated that the coupling process of nitrification and denitrification, as well as the mineralization and ammonia oxidation processes, are the main driving forces influencing the nitrogen isotope fractionation in the sediments of the Bohai Sea. This study provides new insights into the nitrogen transformation mechanism in shallow marginal sea sediments and emphasizes the importance of seasonal dynamics and sediment resuspension events in regulating the nitrogen cycle.