## 5 Conclusions  
This study was carried out in the Shamei Inland Sea, Boao town, Qionghai City, Hainan Province. Based on samples of the soil and sediments around the Shamei Inland Sea, analyses of the samples and relevant data obtained from the study area, the following conclusions were drawn:  
1. Nitrogen in soils and sediments around the Shamei Inland Sea showed obvious variations among inland areas, lagoon areas, sand bar areas and offshore areas. There is an overall deficiency of total nitrogen in the inland sea area of Shamei; the nitrogen form is mainly organic nitrogen, the organic nitrogen level in the soil is greater than that in the sediment and the distribution of total nitrogen is characterized by inland diffusion to offshore waters. Consequently, the productivity of terrestrial ecosystems in this area is greater than the productivity of aquatic ecosystems. The average contents of nitrate, ammonium and nitrite in the soil around the lagoon were higher than those in the sediment, and they were significantly enriched in the lagoon mouth area, which may be related to the dynamic sedimentation and colloidal condensation in the lake-sea boundary area. The content of nitrate is highest in the estuary area to the northwest of the lagoon, while the contents of ammonium and nitrite are highest in the estuary area to the south of the lagoon, which may be related to the difference in upstream parent rocks and the different types of industrial and agricultural activities. 2. There is a positive correlation between soil-sediment total nitrogen and the organic matter content, cation exchange capacity and clay content in the Shamei Inland Sea, indicating that changes in basic physical and chemical soil-sediment properties have important effects on nitrogen enrichment, preservation and mineralization. In the lagoon soil-sediment, the organic nitrogen content in the soil is greater than that in the sediment, indicating that the productivity of the terrestrial ecosystem is greater than that of the aquatic system. Additionally, the high proportion of ammonium in the sediment and the high proportion of nitrate in the soil may be related to heavy sedimentary material, poor soil ventilation and weak nitrification. 3. Changes in the basic physical and chemical properties of soils and sediments in the Shamei Inland Sea have important coupled effects on nitrogen enrichment, preservation and mineralization. Through RDA, it was found that the organic matter content is closely related to the nitrogen content and is the main factor influencing (or controlling) the nitrogen content, with an explanatory degree of 17.2% and a contribution degree of 80.9% in the nitrogen content. Data curation, K.Y. ; formal analysis, K.Y. and Y.S. ; funding acquisition, Y.S., K.Y. and G.F.; investigation, K.F. and Z.W. ; methodology, Y.S. ; project administration, K.Y. and Y.S. ; writing--original draft, K.Y. ; writing--review and editing, K.Y., B.L., G.F. and Y.S. ; visualization, K.Y. ; supervision, G.F. All authors have read and agreed to the published version of the manuscript. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Environmental Variable** & **Explanation \%** & **Contribution \%** & **Pseudo-F** & \(\mathbf{p}\) \\ \hline OM & 17.2 & 80.9 & 13.9 & 0.002 \\ pH & 2.1 & 9.9 & 1.7 & 0.206 \\ CEC & 1.8 & 8.5 & 1.5 & 0.184 \\ Caly & 0.2 & 0.8 & 0.1 & 0.88 \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Redundancy analysis results between the soil–sediment nitrogen content and environmental variables. **Funding:** Comprehensive Survey of Natural Resources in HaiChengWen Coastal Zone, grant number DD20230414; Comprehensive Survey of Natural Resources in Huizhou-Shanwei Coastal Zone, grant number DD20230415. **Institutional Review Board Statement:** Not applicable. **Informed Consent Statement:** Not applicable. **Data Availability Statement:** Not applicable. **Acknowledgments:** HaiChengWen Coastal Zone, grant number DD20230414; Huizhou-Shanwei Coastal Zone, grant number DD20230415.