## 3 Results
### Spatial Distribution of Soil-Sediment Inorganic Nitrogen  
The soil-sediment inorganic nitrogen content was 0-37.71 mg/kg (Table 1, Figure 2), with an average content of 4.25 mg/kg and a coefficient of variation of 1.55. The overall fluctuation was small. The average content of inorganic nitrogen in the soil around the lagoon was higher than that in the surface layer. The sediments exhibit the terrigenous enrichment characteristics of nitrogen. The lagoon sediments have the highest ammonium content, the ammonium content of the terrestrial soils is lower than that of the aquatic sediments and the nitrate content of the terrestrial soils is higher than that of the aquatic sediments. The content of nitrite was slightly higher in the aquatic sediments.  
in this area. In addition, the saturation levels of the soil near the lagoon and the offshore water body fluctuate greatly due to the fluctuation in the tidal process, and a dry soil effect may occur, which enhances the mineralization rate, increases the release of ammonium and leads to inorganic nitrogen supplementation.  
From the distribution point of view, the content of ammonium was higher in the estuary area of the lagoon because the river flowing into the lagoon flows through a rural area, and the concentrations of nitrogen pollutants carried in the runoff were high, potentially due to nitrogen from agricultural sources. Overall, ammonium, nitrate and nitrite were enriched in the lagoon mouth area, which may be related to a stable hydrodynamic environment and colloidal condensation and sedimentation in the area between the lake mouth and the sea (Figure 2d).  
The content of nitrate in the estuary area of the northwestern part of the study area and in the lagoon was higher than that in other areas (Figure 2e). The upper reaches of the Wanquan River, which flows into the northwestern lagoon, flows through urban areas that are densely populated. Rapid urbanization has led to the increased discharge of nitrogen-containing wastewater, thereby providing more nitrogen from industrial sources.  
The content of nitrous nitrogen in the estuary of the lagoon in the west and south of the study area was relatively high, the distribution in different geological layers was heterogeneous and the distribution was high in the relatively young Yandun Formation (Figure 2f). The Jiuqu River and Longgun River, which flow into the southwestern lagoon, flow through rural areas, and the runoff from agricultural production areas and livestock areas has high concentrations of nitrogen pollutants.  
Based on the ratio of inorganic nitrogen/organic nitrogen, the northern, western and southern estuaries of the study area were favorable for the accumulation of inorganic nitrogen, while the land and other water areas were favorable for the accumulation of organic nitrogen (Figure 2g), which may be associated with higher mineralization rates. Figure 2h shows that from the ratio of ammonium/nitrate, the proportion of ammonium in river estuaries and lagoons was high, but the proportion of ammonium in land and sea areas was low. This may be related to the adsorption of ammonium ions during the flocculation and sedimentation of clay minerals in the estuary area, which is related to the rapid nitrification of nitrogen caused by the oxidative environment of terrestrial and offshore currents.