Spatial Distribution and Main Controlling Factors of Nitrogen in the Soils and Sediments of a Coastal Lagoon Area (Shameineihai, Hainan)

Abstract

As the relationship between the spatial distribution characteristics and physicochemical properties of nitrogen in lagoon soil and sediment is still unclear, we obtained and systematically analyzed soil and sediment samples from the surroundings of a lagoon in the Shamei Inland Sea, Qionghai City, Hainan Province. The spatial distribution of nitrogen forms was investigated, and the soil physicochemical properties that predominantly influenced the nitrogen distribution were characterized. The results were as follows: (1) There are differences in nitrogen content in the soil and sediment around the Shamei Inland Sea. The total nitrogen levels in the Shamei Inland Sea were low, and organic nitrogen was dominant. The soil samples showed higher organic nitrogen concentrations than the sediment samples. Thus, the characteristics of dispersion in offshore waters differ from those in lagoons. The average contents of nitrate, ammonium and nitrite in the soil around the lagoon were higher than those in the sediment, and they were especially high in the lagoon mouth area. The nitrate content was greater in the estuary area in the northwest of the lagoon, and the contents of ammonium and nitrite in the estuary area in the south of the lagoon were the highest. (2) The changes in the basic physical and chemical properties of the soils and sediments in the Shamei Inland Sea area have an important coupled effect on the enrichment, preservation and mineralization of nitrogen. Through redundancy analysis (RDA), the explanatory degree of the organic matter content in relation to the nitrogen content was determined to be approximately 17.2%, contributing 80.9% of all nitrogen. The organic matter content, cation exchange capacity and clay content were positively correlated with the nitrogen content, indicating that changes in the basic physical and chemical properties of the soil and sediment had an important impact on nitrogen enrichment, preservation and mineralization processes. The total nitrogen and the organic matter content, cation exchange capacity and clay content of the soil and sediment were positively correlated. The high proportion of nitrate in the soil, the high proportion of ammonium in the sediment, the heavy texture of the sediment, poor soil ventilation and weak nitrification were related.

1. Introduction

Nitrogen is an important element for many organisms, and nitrogen cycling is often coupled with the carbon cycle and affects the operation of ecosystems. The relatively low nitrogen content in the marine environment is often an important limiting factor for the primary productivity of marine ecosystems [1]. A lagoon is a dynamic and complex ecosystem located at the intersection of land, sea, brackish water and freshwater. Lagoons are sensitive to weather and climate change, making them important areas for studying climate change [2]. Lagoons also face ecosystem degradation issues such as eutrophication, and nitrogen is the main driver of eutrophication in coastal ecosystems. Thus, it is important to clarify the spatial distribution and main controlling factors in lagoon soil and sediment to maintain the health and productivity of the associated ecosystems.

Research on lagoons started early, and the research mainly focused on the development and formation conditions of lagoons, salinity evolution, migration mechanisms, eutrophication and other factors. Kroon FJ et al. [3] reported that the average total nitrogen load has increased by a factor of 5.7, severely impacting coastal ecosystems. Hayn M et al. [4] observed a nitrogen and phosphorus exchange in the West Falmouth Harbor lagoon and coastal waters off the coast of the United States. Shallow water systems dominated by benthic producers have the potential to retain substantial terrestrial nitrogen loads when adequate phosphorus supplies are available through exchange with coastal waters. Nazneen et al. [5] studied the largest lagoon in Asia and found that primary production within the lagoon, terrestrial inputs from river discharge and anthropogenic activities near the lagoon controlled the distribution of total nitrogen in surface and core sediments.

Other scholars have also conducted research on the transformation mechanism of nitrogen in lagoons, and the research has typically involved saturated sediments from inland lakes and marine continental shelves. Liu Xiaotong et al. [6] found that the nitrogen content in the vertical direction changed greatly according to the nitrogen geochemical characteristics of the sedimentary column in the lagoon of the East Island of the Xisha Islands. Based on this, they speculated that the nitrogen content was greatly influenced by the ocean and made a useful attempt to infer the sedimentary environment. Ge Chendong et al. [7] conducted an isotopic analysis of total nitrogen and other indicators based on sediment column samples from the Shamei Inland Sea and concluded that the lagoon in the Shamei Inland Sea is gradually closing. Since the 19th century, the lagoon has been continuously filling with sediment and shallowing, with stable sedimentation at a rate of approximately 0.15 cm/a. Zhou Meiling et al. [8] collected surface sediment samples from the Qilihai Lagoon and analyzed the factors influencing different nitrogen forms and the external environment; they found that the nitrogen content was related to the median particle size and the organic matter content. Zhang et al. [9] analyzed the relationship between the particle size and total nitrogen in sediments from Shuang Lagoon, Lingshui County, Hainan Province, and found that total nitrogen was correlated with the average particle size of the sediments. Jiangjun Jia et al. [10] studied the sources and burial patterns of organic matter in Shamei Lake, Hainan. Ruihuan Li et al. [11] conducted a study on the nutrient dynamics of the Wanquan River estuary. However, these studies were limited to a single sediment or soil location, and there is a lack of integrated soil–sediment research.

Therefore, this study focuses on nitrogen in Hainan coastal lagoon sediments and surrounding soils, which have been rarely studied in China. The high-resolution spatial distribution characteristics of nitrogen in the inland sea and surrounding areas are considered, the relationships between the basic physical and chemical properties of the lagoon soil and sediment and the pH, organic matter content, cation exchange capacity, clay content and nitrogen forms are analyzed and the main factors affecting the spatial distribution of nitrogen forms are identified. Furthermore, this study provides theoretical support for eutrophication prevention, protection and restoration in lagoon ecosystems and comprehensive coastal zone management in the Shamei Inland Sea area. Moreover, the results of this study can guide ecological restoration, ecological protection and coastal agricultural management and can provide theoretical support for assessing the transformation mechanisms of typical lagoons in southern China from a new perspective.

2. Research Area and Research Method

2.1. Study Area

The soil–sediment samples used in this study came from the Shamei Inland Sea (Figure 1), which is a naturally formed lagoon to the southeast of Qionghai City, Hainan Province. It is located in the tropical monsoon climate zone. From north to south, the Wanquan River, Jiuqu Jiang River and Longgun River flow into the lagoon. The area spans latitudes from 19°05′~19°09′ N and longitudes from 110°32′~110°34′ E. The lagoon is surrounded by a north–south spit known as the Jade Belt Beach on the coast and has a slender shape, shallow water depth and gentle slope. The southern part is approximately 6 km long and 500–700 m wide, the northern part is approximately 2.5 km long and the narrowest part is only approximately 10 m, with a total area of approximately 25.87 km². Due to the barrier of the Jade Belt Beach, there is no direct water exchange between the inner sea and the outer sea, and the salinity of the inner sea is less than 1% [7,10]. No lagoon dredging work has been carried out in the past three years.

2.2. Research Materials

The sampling was performed in the summer of 2021, the working scale was 1:50,000 and the sampling resolution was 2 samples/km². A total of 69 samples were collected, including 41 soil (25 inland soil samples and 16 sand bar soil samples) and 28 sediment samples (19 lagoon deposits and 9 offshore deposits). The distribution of the sample points and the elevation of the study area are shown in Figure 2 and Figure 3. The soil samples were collected at a depth of 20–40 cm and passed through a 2 mm sieve on site, and the sample weight was not less than 0.5 kg. A grab sampler was used to collect sediment samples, visible foreign objects were removed and the sediment was passed through a 2 mm nylon sieve on location to ensure that the sieved sediment was no less than 1.5 kg. The wet soil that had been sieved was wrapped with 60-mesh nylon mesh, and the water was drained to the extent possible.

2.3. Test Method

• Total nitrogen and forms of nitrogen

Total nitrogen: To determine the total nitrogen content, soil samples passed through a 0.25 mm sieve were weighed, placed into deionized water and boiled in concentrated sulfuric acid to convert all nitrogen-containing compounds into ammonium nitrogen. Then, a standard hydrochloric acid titration solution, distillation and titration were used in conjunction with an automatic nitrogen analyzer.

Ammonia and nitrite were extracted by potassium chloride solution in alkaline and acidic environments, respectively. After the addition of the chromogenic agent, calibration curves were drawn at 630 nm and 543 nm by a Perkin Elmer Lambda 900 ultraviolet/visible/near-infrared spectrophotometer, and the absorbance was measured. The nitrate was reduced to nitrite, the total amount of nitrite was measured and the amount of nitrate was obtained by subtracting the amount of original nitrite obtained in the previous step [12].

The detection limits for ammonia, nitrite nitrogen and nitrate nitrogen in the sample were 0.2 mg/kg, 0.15 mg/kg and 0.25 mg/kg, respectively, with an error range of 5%.

2.

Granularity

Air-dried soil samples were passed through a 2 mm sieve, and hydrogen peroxide and hydrochloric acid were added to remove organic matter and CaCO₃. Finally, the soil samples were rinsed with deionized water to remove excess HCl and other chlorides; then, the soil particle size was determined by a Mastersizer 2000 laser particle size analyzer [13].

3.

Organic matter

While being heated, excess potassium dichromate-sulfuric acid solution was used to oxidize soil organic carbon. The excess potassium dichromate was titrated with a ferrous sulfate standard solution, and the organic carbon content was calculated from the amount of consumed potassium dichromate according to the oxidation correction coefficient amount multiplied by the constant 1.724 to obtain the organic matter content [14].

4.

pH

Air-dried soil samples that had been passed through a 2 mm sieve were weighed, deionized water was added according to the soil–water ratio of 1:2.5 to prepare a solution and three standard buffer solutions of phthalate, phosphate and borate at room temperature were added. To measure the pH, a 3510 Portable Multiparameter Water Quality Analyzer for Determination of Solution pH was used [15].

5.

CEC (cation exchange capacity)

Air-dried soil samples that had been passed through a 2 mm sieve were weighed, and ammonium acetate solution was added repeatedly to treat the soil. The excess ammonium acetate was rinsed off with 95% ethanol; then, deionized water was used to wash all the soil sample residues into the NKB-3200 automatic nitrogen analyzer for digestion. The tube was distilled, the absorption solution was titrated with a 0.05 mol/L hydrochloric acid standard solution and a blank experiment was performed at the same time [16].

3. Results

3.1. Spatial Distribution of Soil–Sediment Total Nitrogen

Overall, the soil–sediment total nitrogen content was 0–2360 mg/kg, the average content was 90 mg/kg, the variation coefficient was 3.88 and the fluctuation range was large. In other regions, the total nitrogen content in the Qilihai Lagoon in Hebei Province ranges from 90 to 1180 mg/kg [8], the total nitrogen content in the sediments of Dianchi Lake in Yunnan Province ranges from 1600 to 5560 mg/kg [17] and the total nitrogen content in the sediments of Puppet Lake in Jiangsu Province ranges from 670–2570 mg/kg [18]. A comparison indicated that the soil–sediment total nitrogen content in the lagoon of the Samet Inland Sea was low. Using ArcGIS software, the spatial distribution of nitrogen content in the study area was obtained by kriging interpolation analysis. In Figure 2a, it can be seen that the total nitrogen content of terrestrial soil is higher than that of aquatic sediments, and the highest concentration of total nitrogen appeared in the southern part of the inland soil, followed by the Wanquan River estuary and the adjacent land areas in the north. Overall, the distribution of total nitrogen showed characteristics of diffusion from inland to offshore waters. This indicated that a large amount of nitrogen-containing nutrients carried by the rivers that enter the lagoon may be input into the lagoon from the inland sea of Samet, and at the same time, under the influence of hydrodynamics, nitrogen becomes enriched near the estuary of the lagoon. The overall nitrogen deficiency in the offshore area may be related to the exchange of water between the lagoon and the ocean, and frequent coastal currents and tidal action accelerate the process of nitrogen volatilization. This result confirmed that the denitrification effect in the offshore area is strong [19]. The concentrated distribution area of total nitrogen is basically consistent with the distribution of organic matter, suggesting the connection between the two in terms of genesis and enrichment, which is consistent with previous research results [20].

3.2. Spatial Distribution of Soil–Sediment Organic Nitrogen

The soil–sediment organic nitrogen content ranged from 0 to 2354.29 mg/kg (

Table 1. Statistical results of different forms of nitrogen in soil and sediment.

Type	Index	Total Nitrogen (mg/kg)	Organic Nitrogen (mg/kg)	Inorganic Nitrogen (mg/kg)	Ammonium (mg/kg)	Nitrate (mg/kg)	Nitrite (mg/kg)
Terrestrial soil	Max	2360	2354.29	37.71	2.28	35.87	1.49
	Min	0	0.00	0.23	0.00	0.12	0.00
	Average value	290	280.53	7.82	0.49	7.16	0.17
	Standard deviation	680	679.20	10.53	0.60	9.96	0.37
	Coefficient of variation	2.35	2.42	1.35	1.22	1.39	2.21
Lagoon sediment	Max	110	84.12	26.51	8.78	16.07	1.67
	Min	0	0.00	0.21	0.15	0.00	0.04
	Average value	30	20.63	4.46	2.47	1.64	0.34
	Standard deviation	20	19.81	5.88	2.44	3.65	0.36
	Coefficient of variation	0.97	0.96	1.32	0.99	2.22	1.04
Bar soil	Max	70	68.49	18.26	1.38	17.56	0.26
	Min	0	0.00	0.20	0.07	0.00	0.00
	Average value	30	26.09	2.39	0.23	2.12	0.04
	Standard deviation	20	19.28	3.51	0.25	3.38	0.06
	Coefficient of variation	0.63	0.74	1.47	1.10	1.59	1.70
Oceanic sediment	Max	140	136.23	8.82	4.07	4.66	0.23
	Min	0	0.00	0.76	0.34	0.00	0.00
	Average value	20	20.72	2.62	1.37	1.17	0.09
	Standard deviation	40	44.06	2.57	1.34	1.37	0.08
	Coefficient of variation	1.87	2.13	0.98	0.98	1.17	0.97
Region of interest	Max	2360	2354.29	37.71	8.78	35.87	1.67
	Min	0	0.00	0.20	0.00	0.00	0.00
	Average value	90	82.88	4.25	1.06	3.03	0.16
	Standard deviation	340	337.93	6.59	1.67	5.91	0.29
	Coefficient of variation	3.88	4.08	1.55	1.58	1.95	1.82

), with an average content of 82.88 mg/kg and a coefficient of variation of 4.08, reflecting a large fluctuation range. Based on Figure 2b, the organic nitrogen content of terrestrial soil is higher than that of aquatic sediments. As shown in Figure 2b, organic nitrogen is enriched in terrestrial areas, and the enrichment of organic nitrogen in terrestrial areas may be related to the surface return of terrestrial ecosystems and is also greatly affected by human activities. The distribution of organic nitrogen is roughly consistent with the distribution of organic matter, which may be due to the abundant precipitation in the region, abundant river flow, strong erosion and transport capacity on the surface and the organic matter and nitrogen-containing nutrients carried by the three rivers in the north, west and south. Nutrient salts have been shown to be injected into the lagoon of the inland sea of Samet, which is hydrodynamically affected, and are not transported offshore over long distances [12].

3.3. 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.

Ammonium and nitrate, as forms of soil inorganic nitrogen, can be absorbed and utilized by plants in large quantities, which is of great significance for crop growth. However, the action of running water may cause nitrogen to migrate into the water body, and an excessive amount may cause eutrophication in the water body, which is potentially harmful. From the distribution in Figure 2c, inorganic nitrogen is enriched in the terrestrial area, and the inorganic nitrogen in the young Yandun Formation is slightly higher, which may be related to the rapid nitrification process of organic nitrogen under oxidative conditions 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.

3.4. Spatial Distribution of the Soil–Sediment Particle Size

The average content of sand in the soil around the lagoon was higher than that in the sediment (

Table 2. Soil–sediment texture characteristics.

Type	Index	Clay	Silt	Sand	<0.031 mm	0.031~0.053 mm	0.053~0.25 mm	0.25~1 mm	1~2 mm
		(%)	(%)	(%)	(%)	(%)	(%)	(%)	(%)
Terrestrial soil	max	448.22	85.2	65.2	83.4	90.1	9.8	51.5	40.3
	min	5.27	16.6	12.8	14.8	19.7	4.4	5.2	0.0
	average value	56.26	48.5	39.0	51.5	54.1	7.4	23.3	14.1
	standard deviation	108.58	17.0	14.3	17.0	17.7	1.7	9.7	10.1
	coefficient of variation	1.93	0.4	0.4	0.3	0.3	0.2	0.4	0.7
Lagoon sediment	max	467.50	73.6	64.2	96.7	83.1	19.4	59.8	79.4
	min	8.49	3.3	3.1	26.4	4.3	0.3	2.7	0.0
	average value	62.75	50.8	46.2	49.2	60.7	9.9	17.3	11.6
	standard deviation	118.89	20.0	18.1	20.0	23.3	5.3	14.1	20.9
	coefficient of variation	1.89	0.4	0.4	0.4	0.4	0.5	0.8	1.8
Bar soil	max	448.22	56.6	51.8	81.4	66.5	9.8	30.3	40.3
	min	15.91	18.6	15.1	43.4	21.6	4.4	15.2	7.7
	average value	127.11	43.2	36.2	56.8	50.1	8.3	20.6	16.4
	standard deviation	214.12	17.1	15.4	17.1	19.8	2.6	6.9	15.9
	coefficient of variation	1.68	0.4	0.4	0.3	0.4	0.3	0.3	1.0
Oceanic sediment	max	541.03	68.0	62.3	100.0	80.6	11.4	94.5	67.5
	min	12.31	0.0	0.0	32.0	0.0	0.4	6.9	1.1
	average value	244.43	15.7	14.2	84.3	18.3	3.2	43.2	29.8
	standard deviation	204.87	23.4	21.3	23.4	27.5	3.7	39.3	30.0
	coefficient of variation	0.84	1.5	1.5	0.3	1.5	1.2	0.9	1.0
Region of interest	max	706.85	85.2	68.7	100.0	90.1	12.0	51.5	85.6
	min	5.27	0.0	0.0	14.8	0.0	0.5	3.2	0.0
	average value	237.63	28.8	23.4	72.6	32.4	4.9	19.8	41.2
	standard deviation	205.46	26.4	21.7	25.0	29.0	3.3	10.8	29.1
	coefficient of variation	0.86	0.91	0.93	0.34	0.90	0.68	0.55	0.71

), while the average contents of silt and clay were lower than those in the sediment, indicating that the grain size of the soil is coarser than that of sediment. This was related to the sorting effect that moving water has on soil particles. Regarding the distribution of single particle size components, as shown in Figure 3a, a high-value area was observed for clay in the terrestrial zone in the western part of the study area, and this area was found to extend to the lagoon water area. The content of sand in the sea area was the lowest; the content of sand in the study area showed a completely opposite distribution trend to that of clay and silt. Specifically, the sand content was highest in the lagoon bar and the sea area to the east and gradually decreased in the transition to the land area. This showed that the texture of the land area was the coarsest, the texture of the lagoon bar and the sea area was the finest and the sedimentary texture of the lagoon was in the middle. The distribution characteristics of median-size particles were basically consistent with the distribution of sand particles, reflecting the overall coarse characteristics of soil particles in the study area. In addition, in the western continental area, the soil–sedimentary material geography displayed obvious parent material differentiation; that is, the old continental strata were finer in texture, the younger continental strata were macroscopically granular and the lagoon sedimentary texture was in the middle. This may be due to the longer weathering and soil-forming process experienced by the old terrestrial bottom layer. By comparing additional particle size groups, it can be seen that the 0.25–1 mm component was consistent with sand; the content of this component was high in the soil, and the content of the component smaller than 0.25 mm was high in the sediment.

3.5. Spatial Distribution of Soil–Sediment Organic Matter

Soil–sediment organic matter is an important part of soil–sediment fertility and is an important indicator reflecting soil–sediment maturity and fertility level. The level of soil–sediment organic matter also affects other nutrients. In turn, nutrients affect crop growth and development, as well as yield and quality. The average content of soil organic matter around the lagoon was 13,080 mg/kg, and the content of organic matter in the surface sediments was 23,580 mg/kg. The former was significantly lower than the latter, which may be related to the favorable soil aeration conditions for the mineralization and decomposition of organic matter. Erosion of terrestrial soils and subsidence in lagoon water environments may also affect the organic matter content. Specifically, in terms of its distribution, as shown in Figure 3b, the most concentrated areas of organic matter were the land area and water area near the entrance to the river in the southwestern part of the study area, followed by the land area on the south bank of the entrance of the Wanquan River in the northwest. Additionally, a high-value area of organic matter appeared at the mouth of the lagoon. Enrichment of organic matter was observed in the land and waters near the mouth of the lagoon. This pattern reflects the erosion of terrigenous organic matter and the recharge process of the lagoon. The enrichment of organic matter at the mouth of the lagoon may be related to the condensation and deposition of organic matter in the context of high ion concentrations at the lake–sea interface.

3.6. Spatial Distribution of the Soil–Sediment pH

The average pH value of the soil in the study area was 5.1, which is acidic, reflecting the leaching of base ions, such as K, Na, Ca and Mg, and the enrichment of acid ions, such as H and Al. The average pH of the sediment was 7.3, which is neutral and is largely from the acceptance of terrestrial salt ions, reflecting the sediment’s enrichment process. From the distribution point of view, as shown in Figure 3c, the soil pH in the western land area was the lowest and slightly acidic, the pH value in the sea area east of the sand bar was the second highest and neutral and the pH value in the northeastern sea area was the highest. The values ranged from acidic to neutral and exhibited transitional characteristics. Overall, the pH difference in the study area reflected the enrichment and migration process of base ions.

3.7. Spatial Distribution of the Soil–Sediment Ion Exchange Capacity

The CEC is one of the most important features of sediments. The average CEC of the soil around the lagoon was 8.97 cmol/kg, and the average CEC of the surface sediment was 12.52 cmol/kg. This reflected the sorption phenomenon of base ions. As shown in Figure 3d, the distribution characteristics of the CEC and organic matter were basically consistent; that is, the highest values were found in the southwestern land area and its adjacent waters, as well as in the lagoon mouth area. This result indicated that the migration process of cations was related to their adsorption by organic matter. In addition, by comparing the two high-value areas, it was found that the diffusion of marine CEC values in the estuary was more obvious than that of the continental facies to the lagoon waters, which may also be related to the lower organic matter content of the open water that ensures CEC diffusion.

4. Discussion

4.1. Correlations among Different Forms of Nitrogen in the Soil–Sediment

• Total nitrogen and organic nitrogen

Total nitrogen in soil and sediment can generally be divided into inorganic nitrogen and organic nitrogen. In different regions, the content of each form of nitrogen and the proportions of the total nitrogen have great differences. Figure 4a shows that, overall, the soil–sediment organic nitrogen and total nitrogen contents in the lagoon of the Samet Inland Sea were significantly and positively correlated, and the correlation between the two was more similar in different soil–sediment samples.

Table 3. Correlation analysis of the basic physical and chemical properties of inland soil and nitrogen forms (25 samples).

	pH	Organic Matter	CEC	Clay	Total Nitrogen	Organic Nitrogen	Inorganic Nitrogen	Ammonium	Nitrate	Nitrite
pH	1.00
organic matter	−0.42	1.00
CEC	−0.37	0.693 **	1.00
clay	−0.28	0.06	0.28	1.00
total nitrogen	−0.37	0.34	0.36	0.12	1.00
organic nitrogen	−0.37	0.33	0.35	0.12	1.000 **	1.00
inorganic nitrogen	−0.24	0.42	0.35	0.33	−0.04	−0.05	1.00
ammonium	−0.32	0.832 **	0.517 *	0.01	−0.14	−0.15	0.47	1.00
nitrate	−0.23	0.39	0.33	0.33	−0.03	−0.05	0.999 **	0.43	1.00
nitrite	−0.03	0.05	0.05	0.31	−0.02	−0.04	0.795 **	0.14	0.794 **	1.00

* At the 0.05 level (two-tailed), the correlation is significant. ** At the 0.01 level (two-tailed), the correlation is significant.

,

Table 4. Correlation analysis between the basic physicochemical properties and nitrogen forms of lagoon sediments (19 samples).

	pH	Organic Matter	CEC	Clay	Total Nitrogen	Organic Nitrogen	Inorganic Nitrogen	Ammonium	Nitrate	Nitrite
pH	1.00
organic matter	−0.485 *	1.00
CEC	−0.31	0.878 **	1.00
clay	−0.10	0.38	0.40	1.00
total nitrogen	0.13	0.20	0.23	0.06	1.00
organic nitrogen	0.13	0.17	0.23	0.02	0.985 **	1.00
inorganic nitrogen	0.13	0.24	0.17	0.18	0.811 **	0.697 **	1.00
ammonium	−0.17	0.518 *	0.34	0.20	0.556 *	0.43	0.862 **	1.00
nitrate	0.30	0.03	0.04	0.13	0.857 **	0.768 **	0.950 **	0.662 **	1.00
nitrite	0.19	0.04	0.11	0.22	0.797 **	0.718 **	0.872 **	0.602 **	0.905 **	1.00

* At the 0.05 level (two-tailed), the correlation is significant. ** At the 0.01 level (two-tailed), the correlation is significant.

and

Table 5. Correlation analysis between the basic physical and chemical soil properties and nitrogen forms on the sand bar (16 samples).

	pH	Organic Matter	CEC	Clay	Total Nitrogen	Organic Nitrogen	Inorganic Nitrogen	Ammonium	Nitrate	Nitrite
pH	1.00
organic matter	−0.490 *	1.00
CEC	−0.35	0.609 **	1.00
clay	−0.508 **	0.854 **	0.861 **	1.00
total nitrogen	0.04	−0.15	−0.35	−0.24	1.00
organic nitrogen	0.06	−0.21	−0.484 *	−0.33	0.985 **	1.00
inorganic nitrogen	−0.15	0.34	0.883 **	0.590 **	−0.27	−0.438 *	1.00
ammonium	−0.37	0.944 **	0.580 **	0.801 **	−0.03	−0.10	0.37	1.00
nitrate	−0.13	0.28	0.859 **	0.541 **	−0.28	−0.439 *	0.997 **	0.30	1.00
nitrite	−0.08	0.495 *	0.827 **	0.635 **	−0.33	−0.454 *	0.777 **	0.524 **	0.750 **	1.00

* At the 0.05 level (two-tailed), the correlation is significant. ** At the 0.01 level (two-tailed), the correlation is significant.

support this result, and organic nitrogen was the main component of total nitrogen in the soil and sediment.

2.

Total nitrogen and inorganic nitrogen

The relationship between total nitrogen and inorganic nitrogen is complex among the different soil–sediment samples.

Table 6. Correlation analysis between the basic physicochemical properties and nitrogen forms of offshore sediments (9 samples).

	pH	Organic Matter	CEC	Clay	Total Nitrogen	Organic Nitrogen	Inorganic Nitrogen	Ammonium	Nitrate	Nitrite
pH	1.00
organic matter	−0.55	1.00
CEC	−0.52	0.996 **	1.00
clay	−0.15	0.34	0.36	1.00
total nitrogen	−0.53	−0.12	−0.12	−0.22	1.00
organic nitrogen	−0.52	−0.11	−0.10	−0.24	0.998 **	1.00
inorganic nitrogen	−0.15	−0.28	−0.30	0.28	−0.12	−0.18	1.00
ammonium	−0.20	−0.28	−0.28	0.49	0.02	−0.04	0.946 **	1.00
nitrate	−0.05	−0.31	−0.33	0.02	−0.23	−0.28	0.948 **	0.795 *	1.00
nitrite	−0.63	0.803 **	0.765 *	0.42	−0.26	−0.25	0.02	0.01	−0.03	1.00

* At the 0.05 level (two-tailed), the correlation is significant. ** At the 0.01 level (two-tailed), the correlation is significant.

shows that, overall, the contents of inorganic nitrogen and total nitrogen in the soil–sediment samples from the inland sea lagoon were positively correlated, but the correlation was not strong. This was probably because the inorganic nitrogen proportion of the total nitrogen was small, and the inorganic nitrogen content in aquatic sediments was lower than that in terrestrial soils. According to Figure 4c, the total nitrogen in the sediment was positively correlated with the ammonium content. Although the nitrogen and ammonium contents in the soil were also positively correlated, the correlation was not strong. The competition ability for ammonium ions in the sediment was weak, so it was challenging to desorb ammonium, and ammonium could be preserved in the sediment. Figure 4d shows that the total nitrogen in the soil was positively correlated with nitrate, and the total nitrogen in the sediment was also positively correlated with nitrate. Be-cause the grain size of the soil is coarser than that of sediment, the aeration was good, nitrification was easy to carry out and ammonium was easily oxidized to nitrate and nitrite. Figure 4e shows that the total nitrogen was positively correlated with the nitrite in the sediment, the total nitrogen was negatively correlated with the nitrite in the soil and the nitrite in the sediment was more abundant than that in the soil. This may be because the studied waters are mainly in a reducing environment, and in an anaerobic sedimentary environment, nitrate is easily reduced to nitrite.

3.

Organic nitrogen and inorganic nitrogen

Organic nitrogen and inorganic nitrogen jointly constitute the total nitrogen in soil and sediment, but their correlation varies greatly among the different soil and sediment samples. As shown in Figure 5a, inorganic nitrogen in soil shows a negative correlation with organic nitrogen, while inorganic nitrogen in sediment shows a positive correlation with organic nitrogen. This indicates that in inland and sandbar soils, there is a competitive effect between organic and inorganic nitrogen, but in lagoons and nearshore areas, there is a synergistic evolution effect between organic and inorganic nitrogen. This may be due to the strong conversion between inorganic nitrogen and organic nitrogen in soil because of the large biomass in the soil. The amount of organic nitrogen increases through the transformation of inorganic nitrogen by organisms. In addition, inorganic nitrogen also forms organic nitrogen-containing compounds through chemical interactions with soil organic matter. Due to the low terrain of the lagoon, the nitrogen carried by the rivers entering the lake accumulates in the sediment, making the sediment a “nitrogen sink”.

4.

Distribution of inorganic nitrogen

Ammonium includes ammonium and ammonia, and there is a dynamic equilibrium between ammonium ions and ammonia. In the core soil layer, fixed ammonium is the main form of inorganic nitrogen. From Figure 5b, overall, ammonium and inorganic nitrogen showed a significant positive correlation. In terms of classification, the correlation between ammonium and inorganic nitrogen in lagoons and offshore sediments was stronger, and the content was higher. This may be because the closer the soil water is to saturation, the weaker the nitrification, while the lagoon and offshore sediments feature a hypoxic environment with weak nitrification and the accumulation of ammonium. Figure 5c shows that, overall, nitrate and inorganic nitrogen had a significant positive correlation. In terms of classification, the correlation between nitrate and inorganic nitrogen in inland and sand bar soils was stronger, and the content was higher. This may be because nitrate mainly comes from the erosion and scouring of the surface by the three rivers entering the lake in the north, west and south. In addition, because of the oxidative environment both inland and in the sand bars, nitrification is strong, and nitrate can accumulate. In addition, denitrification easily occurs under the insufficient oxygen supply in the offshore area, and nitrate is easily reduced to nitrogen, nitrogen oxides and other gases, resulting in the loss of nitrate from the sediments. According to Figure 5d, overall, nitrite was significantly and positively correlated with inorganic nitrogen. In terms of classification, the correlation between nitrite and inorganic nitrogen in lagoons and offshore sediments was strong, and the content of nitrous nitrogen and inorganic nitrogen was high. This may be because in lagoons and offshore areas, ammonium may generate nitrite through the ammonia oxidation process, but there are insufficient oxidation conditions to further generate nitrate, although the originally accumulated nitrate is easily reduced to nitrite.

4.2. Correlation between the Soil–Sediment Particle Size and Nitrogen

It can be seen from the correlation analysis in

Table 7. Correlation analysis between the basic soil–sediment physical and chemical properties and nitrogen forms (69 samples).

	pH	Organic Matter	CEC	Clay	Total Nitrogen	Organic Nitrogen	Inorganic Nitrogen	Ammonium	Nitrate	Nitrite
pH	1.00
organic matter	−0.23	1.00
CEC	−0.23	0.828 **	1.00
clay	−0.295 *	0.560 **	0.670 **	1.00
total nitrogen	−0.271 *	0.240 *	0.18	0.14	1.00
Organic nitrogen	−0.267 *	0.23	0.17	0.13	1.000 **	1.00
inorganic nitrogen	−0.21	0.370 **	0.360 **	0.380 **	0.08	0.06	1.00
ammonium	0.18	0.514 **	0.388 **	0.360 **	−0.07	−0.08	0.413 **	1.00
nitrate	−0.284 *	0.253 *	0.274 *	0.301 *	0.11	0.09	0.962 **	0.15	1.00
nitrite	0.06	0.293 *	0.340 **	0.420 **	0.01	−0.01	0.714 **	0.533 **	0.596 **	1.00

* At the 0.05 level (two-tailed), the correlation is significant. ** At the 0.01 level (two-tailed), the correlation is significant.

that, overall, the soil–sediment clay content in the study area was weakly, positively correlated with total nitrogen and organic nitrogen and showed a significant positive correlation with all inorganic nitrogen. In terms of classification (Table 3, Table 4, Table 5 and Table 6), the clay content of inland soil and lagoon sediments was positively correlated with all nitrogen forms, but the correlation was not strong. The clay content in sand bar soil and offshore sediments was negatively correlated with total nitrogen and organic nitrogen and was significantly and positively correlated with all inorganic nitrogen. This is because clay minerals account for a large part of the clay particles, and the fixed ammonium generated by the fixation of nitrogen by clay minerals is the main form of soil inorganic nitrogen, especially in the core soil layer [21]. This also confirms that the more clay and organic matter in the soil, the lower the leaching rate of nitrate [22]. This shows that soil particle size is the main factor affecting soil–sediment inorganic nitrogen but not the main factor affecting total nitrogen and organic nitrogen, which is consistent with previous research results [23,24]. Previous studies have provided little discussion on the correlation between soil particle size and total nitrogen.

4.3. Correlation between Soil–Sediment Organic Matter and Nitrogen

It can be seen from the correlation analysis in Table 7 that, overall, organic matter was positively correlated with soil–sediment total nitrogen and nitrogen forms. In terms of classification (Table 3, Table 4, Table 5 and Table 6), the organic matter contents of inland soil, lagoon sediments and sand bar soil were positively correlated with all nitrogen forms and significantly and positively correlated with ammonium, which is similar to the conclusion of He Jun et al. [25] that offshore sediment organic matter was negatively, but not significantly, correlated with all nitrogen species and was significantly and positively correlated with nitrous nitrogen. This is because most of the nitrogen in the soil–sediment surface exists in the organic matter, and the colloidal adsorption by the organic matter can also improve the nitrogen preservation ability in the soil–sediment. The ammonium obtained by the ammonification of organic matter is the substrate source of the nitrification process and affects the activity of nitrifying bacteria in soil. In general, soil organic matter helps to improve the adsorption and retention capacity of ammonium ions and ammonia and reduce the volatilization of soil nitrogen. This result shows that organic matter is the main factor influencing the soil–sediment total nitrogen and organic nitrogen, which is consistent with the research results of Li Hui et al. [26].

4.4. Correlation between the Soil–Sediment pH and Nitrogen

It can be seen from the correlation analysis table (Table 7) that, overall, pH was negatively correlated with soil–sediment total nitrogen, organic nitrogen, total inorganic nitrogen and nitrate, but the correlations were not strong. Ammonium and nitrite showed a weak positive correlation, and the effect of pH on nitrogen forms in sediments was not obvious. The specific pattern (Table 3, Table 4, Table 5, Table 6 and Table 7) showed that inland soil pH and all nitrogen forms were negatively correlated, but not significantly. Lagoon sediment pH was positively correlated with total nitrogen, organic nitrogen, inorganic nitrogen, nitrate and nitrite and negatively correlated with ammonium, but not significantly. The soil pH in the sand bar was weakly, positively correlated with total nitrogen and organic nitrogen and negatively correlated with inorganic nitrogen. Offshore sediment pH was negatively correlated with all nitrogen species, but not significantly. This may be because pH is related to whether nitrogen compounds can undergo redox changes, and in soil with a high pH, clay has a stronger ability to fix nitrogen [21]. The pH of the study area ranges from weakly acidic to weakly alkaline from west to east, the suitable pH range for denitrification is 6.50–7.50 and the suitable pH range for nitrification is 7.50–8.50 [26]. However, in the normal pH range, the change in pH will not have an important effect on the mineralization, nitrification and denitrification of organic nitrogen [27]. This indicates that pH is not the main factor influencing soil–sediment nitrogen.

4.5. Correlation between the Soil–Sediment Cation Exchange Capacity and Nitrogen

From the correlation analysis table (Table 7), it can be observed that the CEC was positively correlated with all nitrogen forms in the soil–sediment samples and had a significant positive correlation with all inorganic nitrogen. The specific performance of different types of environments was as follows (Table 3, Table 4, Table 5, Table 6 and Table 7): inland soil CEC was positively correlated with all nitrogen forms and significantly and positively correlated with ammonium; lagoon sediment CEC was negatively correlated with all nitrogen forms but not significantly correlated with nitrite; sand bar soil CEC was negatively correlated with total nitrogen and organic nitrogen and significantly and positively correlated with all inorganic nitrogen and offshore sediment CEC was negatively correlated with all nitrogen forms but not significantly correlated with nitrous nitrogen, similar to the findings of Zhao Ziwen [28] and Geng Ruonan [29]. Because the higher the soil CEC is, the easier it is to adsorb and fix ammonium ions in the soil, and it is not easy to nitrify; the above analysis showed that the cation exchange CEC exhibited the same pattern as the clay content in the soil–sediment samples. The main factors influencing inorganic nitrogen identified in this work are consistent with previous research results [30].

4.6. Analysis of the Main Factors That Influence the Spatial Variations in Soil–Sediment Nitrogen Forms

To further understand nitrogen in the soil and sediment, the relationship between morphospatial differentiation and environmental factors [31,32] must be studied. Thus, an RDA study on soil–sediment nitrogen content and environmental factors was carried out, and nitrogen content in the soil and sediment was selected as the response variable, and particle size Caly, organic matter OM, pH and cation exchange capacity CEC were selected as explanatory variables. The RDA sorting results showed that the gradient length of the first sorting axis was 1.2, indicating that linear sorting was suitable for further RDA. The results showed that the first 2 RDA axes explained 18.86% and 2.39% of the total variance in the soil–sediment nitrogen content, and 21.2% of the variation in the soil–sediment nitrogen content could be explained by environmental factors, which indicated that environmental factors had a certain influence on the soil–sediment nitrogen content. The explanatory degrees of environmental factors from large to small were 17.2% for organic matter, 2.1% for pH, 1.8% for cation exchange capacity and 0.2% for particle size. The cation exchange capacity was 8.5%, and the particle size was 0.8% (

Table 8. Redundancy analysis results between the soil–sediment nitrogen content and environmental variables.

Environmental Variable	Explanation %	Contribution %	Pseudo-F	p
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

). This showed that among the four explanatory variables, particle size, organic matter, pH and cation exchange capacity, organic matter had the largest contribution to the nitrogen content in soil–sediment and was the main factor affecting the distribution of nitrogen in soil and sediment in the Shamei Inland Sea.

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.