**5. Conclusions**

It was confirmed from the study that the groundwater level in the silty-clay riparian zone presented a slow response to the tide-driven fluctuation of the river stage. The delayed response time reached a few hours within 1–4 m away from the bank, which could help to extend the hydraulic and solute retention time. The continuous infiltration of the river water under tide action contributed to the aerobic and neutral riparian hyporheic zone conductive to nitrification. Within 4 m away from the bank, the dominant inorganic nitrogen form changed from NO− <sup>3</sup> -N to NH<sup>+</sup> <sup>4</sup> -N, upon increasing the distance from the bank. In addition, the removal of nitrogen could occur in the riparian hyporheic zone with an aerobic and neutral environment under the joint control of nitrification, microbial assimilation, and aerobic denitrification. It is important to consider that nitrogen occurrence in riparian hyporheic zones of tidal rivers is governed by many factors including tidal strength, permeability and chemical compositions of sediments, DO concentration, vegetation coverage, and even nitrogen concentration in river water.

**Author Contributions:** Conceptualization, Y.C. and J.X.; methodology, Y.C. and J.X.; software, J.X.; validation, J.X.; formal analysis, Y.C. and J.X.; investigation, Y.C., J.X., N.Z., R.H., X.R. and D.Y.; resources, Y.C. and J.X.; data curation, J.X., R.H. and X.R.; writing—original draft preparation, Y.C. and J.X.; writing—review and editing, Y.C. and N.Z.; funding acquisition, Y.C. and N.Z. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was financially supported by the National Natural Science Foundation of China (Grant No. 42077176) and the Natural Science Foundation of Shanghai (Grant No. 20ZR1459700).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Data are contained within the article.

**Acknowledgments:** The authors specially thank the Shanghai Institute of Geological Survey for providing assistance with sample testing.

**Conflicts of Interest:** The authors declare no conflict of interest.
