Distribution Characteristics and Factors Controlling Different Phosphorus Fractions in the Soils and Sediments of an Inland Lagoon

Abstract

Lagoons, significant coastal wetlands, stand out for their vital role in the cycles and transformations of phosphorus. However, the relationship between the spatial distributions of various phosphorus forms in the soil and sediments of lagoon areas remains unclear. This study investigated the phosphorus fractions and distribution patterns in the soil and sediments from Shimei Inland Sea, Hainan Province, China, and identified the factors controlling phosphorus distribution and its relationship with lagoon eutrophication. The results revealed significant enrichment of inorganic phosphorus (IP) in the Shimei Inland Sea (64.1–679.0 mg/kg) and offshore areas (56–627.2 mg/kg), while organic phosphorus (OP) was relatively concentrated in land (60–744.3 mg/kg), suggesting that the productivity of the terrestrial ecosystem controlled the OP content and IP formed from the mineralization of OP was more likely to leach into the water and combine with minerals. Total phosphorus (TP) content was negatively affected by pH, and organic matter, cation exchange capacity and iron/aluminum oxides showed significant positive correlations with TP content. These findings highlighted the coupled impact of soil and sediment physicochemical properties on phosphorus enrichment and transformation, providing theoretical support for the ecological restoration of ecosystems in tropical coastal wetlands.

1. Introduction

Coastal wetlands, crucial for studying environmental changes, serve as an essential link between land and sea, responding sensitively to climatic shifts [1]. Lagoons, fundamental elements within coastal wetlands, are defined as nearly enclosed lakes formed via the deposition of sand spits or bars that block the mouth of a shallow bay, and they constitute approximately 13% of the world’s coastlines [2]. Coastal lagoons are located in the transitional zone between sea and fresh/saltwater and represent a dynamic and complex ecosystem regarding hydrological variation, ecotone environment, ever-changing morphology, and biotic interactions [3,4]. However, these environments are susceptible to the influences of both natural environmental factors and human activities, rendering them vulnerable to ecological degradation.

Phosphorus, an essential element in organisms, is categorized into organic and inorganic forms, specifically as iron–aluminum-bound phosphorous (Fe/Al-P) and calcium-bound phosphorous (Ca-P), in soils and sediments. Organic phosphorus (OP) is prone to decomposition and exhibits strong activity. Conversely, Fe/Al-P is one of the most important phosphorus forms influencing the phosphorus cycle in lagoons. It exhibits high reactivity during the processes of phosphorus adsorption and release [5]. Ca-P, while stable and sparingly soluble in water, is resistant to release and biological utilization [6]. However, an increase in CO₂ content in water can lead to a decrease in pH, causing the dissolution and release of Ca-P in sediments. Ongoing research into phosphorus cycling in lagoons emphasizes spatial distribution, speciation, transformation mechanisms, and eutrophication status in water bodies [7,8,9,10]. Hayn et al. [11] analyzed the exchange in nitrogen and phosphorus between West Falmouth Harbor Lagoon and coastal waters in the United States and found that when sufficient nitrogen and phosphorus supplies were exchanged with coastal waters, shallow water systems dominated by benthic producers could retain a significant amount of terrestrial nitrogen load. Nazneen et al. [12] found in their study of the largest lagoon in Asia that the factors affecting phosphorus spatial distribution were complex, such as sediment particle size, sediment type, and hydrodynamic environment. While existing studies provide valuable insights, a limitation arises from their focus on single-phase sediment analysis, lacking integrated comparative studies of the soil, river, and marine sediments around lagoons. Such limitations hinder a comprehensive understanding of the differentiation and transfer of biogenic elements under different physicochemical backgrounds in coastal areas. Therefore, it is important to explore the presence of phosphorus in the sediments and soils surrounding a coastal lagoon.

Shamei Inland Sea, a key area of coastal wetland in Hainan province, China, has been progressively enclosed since 1800 AD. This alternating sedimentation and soil formation cycle created significant physicochemical gradients, offering an excellent site for studying phosphorus distribution in coastal soil–sediment systems. The primary aims of this study were to understand the influence of the physicochemical properties of lagoon soil and sediments on phosphorus distribution and clarify the factors controlling the distribution of phosphorus fractions in a typical lagoon sediment ecosystem, providing a theoretical basis and support for the protection of the environment of the Shamei Inland Sea.

2. Study Area and Methods

2.1. Study Area

Shamei Inland Sea (110°32′–110°34′ E, 19°05′–19°09′ N), a naturally formed lagoon in the southeastern part of Qionghai city, Hainan Province, China, features a flat topography and a tropical monsoon climate, nourished by the Wanquan River, Jiuqu River, and Longgun River (Figure 1). This lagoon, approximately 6 km long in the south and 2.5 km long in the north, exhibited salinity ranging from <9.77 to 34.24 psu, averaging 22.16 psu. Influenced by freshwater inflows, the salinity gradient ranged from low in the south to high in the north. Surface water temperatures varied from 26.1 to 31.2 °C, with an average of 29.2 °C, showing a decrease from the south to the north within the lagoon and from upstream to downstream at the mouth of the Wanquan River.

2.2. Sample Collection

Sampling campaigns were conducted from 20 June to 20 August 2021, with a working scale of 1:50,000 and a sampling resolution of 2 samples/km². A total of 69 samples were gathered, comprising 41 soil samples and 28 sediment samples. Among them, there were 25 inland soil samples, 16 bar soil samples, 19 lagoon sediment samples, and 9 offshore sediment samples, as shown in Figure 1b. The soil samples were collected at a depth of 20–40 cm and sieved on site through a 2 mm nylon sieve, with a sample weight of ≥0.5 kg [13]. Sediment samples were collected using a grab sampler, visible foreign matter was removed, and the samples were sieved on site through a 2 mm nylon sieve, with the sieved sediment weight being ≥1.5 kg. The wet sediment was sieved through a 60-mesh nylon mesh to drain as much moisture as possible. The replicate samples were collected from each sampling site and pooled to mitigate the influence of the heterogeneous distribution of analytes.

Samples were air-dried and sieved through a 2 mm nylon sieve. Sieved samples were transferred to polyethylene plastic bags and stored at 4 °C.

2.3. Experimental Methods

2.3.1. Total Phosphorus (TP) and Phosphorus Fractions

Fe/Al-P: Firstly, 0.2 g of the sample was added to 20 mL of NaOH (1 mol/L) and agitated at room temperature for 16 h, and the extraction solution A was obtained after centrifugation (the residue was used for Ca-P analysis). Next, 10 mL of the above extraction solution A was added to 4 mL of HCl (3.5 mol/L) and left undisturbed at room temperature for 16 h to obtain extraction solution B. The phosphorus concentration in extraction solution B was determined using the Mo-Sb anti-spectrophotometric method [14,15].

Ca-P: The residue of the above solution was added to 20 mL of HCl (1 mol/L) and left undisturbed at room temperature for 16 h to obtain extraction solution C. The phosphorus concentration in extraction solution C was determined using the molybdenum–antimony method [16].

Inorganic phosphorus (IP): Firstly, 0.2 g of the sample was added to 20 mL of HCl (1 mol/L) and left at room temperature for 16 h to obtain the extraction solution. The phosphorus concentration in the extraction solution was determined using the molybdenum–antimony method, and the residue remaining was used for OP analysis [17].

OP: After IP extraction, the residue was transferred to a porcelain crucible, dried, calcined in a muffle furnace at 450 °C for 1 h, cooled, and added to 20 mL of HCl (1 mol/L). After the residue was left at room temperature for 16 h, the extraction solution was obtained, and the phosphorus concentration in the extraction solution was measured [14,17].

2.3.2. Organic Matter (OM)

Upon heating, soil organic carbon underwent oxidation using an excess of potassium dichromate–sulfuric acid solution. The surplus potassium dichromate was titrated using a ferrous sulfate standard solution. The organic carbon content was determined by multiplying the amount of potassium dichromate consumed by an oxidation correction coefficient and then multiplying it by the constant 1.724 to obtain the OM content [18].

2.3.3. pH

The air-dried soil sample was sieved through a 2 mm sieve. A solution was prepared by adding deionized water at a soil-to-water ratio of 1:2.5. The pH of the solution was measured using a Multi 3510 portable multiparameter water quality analyzer, calibrated at room temperature with three standard buffer solutions (benzoate, phosphate, and borate) [19].

2.3.4. Cation Exchange Capacity (CEC)

Air-dried soil samples were weighed and sieved through a 2 mm sieve and repeatedly treated with ammonium acetate solution, excess ammonium acetate was rinsed away with 95% ethanol, and the remaining residue was placed in a NKB-3200 automatic nitrogen determinator digestion tube with deionized water for distillation. Finally, the absorption solution was titrated with 0.05 mol/L hydrochloric acid standard solution, and a blank test was simultaneously performed [20].

2.3.5. Iron and Aluminum Oxides

Al₂O₃ and Fe₂O₃ were determined via X-ray fluorescence (XRF) spectroscopy. A sample was taken, dried at 105 °C, accurately weighed, and placed in a platinum crucible. A mixed melting agent consisting of lithium tetraborate, metaborate, and lithium nitrate was added, and the sample was melted at 1050 °C using a high-precision melting instrument to form a melt. Measurements were performed with the instrument. At the same time, another dried sample was taken, and the loss on ignition (LOI) was determined by burning it in an oxygen-filled muffle furnace at 1000 °C. The sum of the LOI and the measured XRF values were expected to equal approximately 100% [21].

2.3.6. Statistical Analysis

Kriging interpolation analysis to generate spatial distribution maps of phosphorus level in various forms for sampled area was performed using ArcGIS 10.0 software. Correlation analysis and significance testing were conducted with SPSS Statistics 29.0. Non-parametric tests (Kruskal–Wallis H test) were applied for significance analysis, while the sample data did not follow a normal distribution. Otherwise, an independent-samples t test was employed to detect differences between the samples. Bivariate correlation analysis using Pearson’s correlation coefficient was conducted to examine the correlation between samples.

3. Results and Discussion

3.1. Phosphorus Distribution

Table 1. Description of soil–sediment phosphorus fractions.

Type	Samples Quantity	Index	Fe/Al-P mg/kg	Ca-P mg/kg	IP mg/kg	OP mg/kg	TP mg/kg
Lagoon sediment	19	Maximum	503.0	171.9	679.0	196.7	1090.45
		Minimum	2.4	16.0	64.1	33.4	327.83
		Average value	192.6	101.6	323.0	99.9	776.56
		Standard deviation	154.9	50.5	207.1	51.2	227.90
		Coefficient of variation	0.8	0.5	0.6	0.5	0.29
Offshore sediment	9	Maximum	379.0	246.5	627.2	191.4	1103.44
		Minimum	12.5	16.9	56.0	26.7	261.87
		Average value	79.3	111.7	204.5	56.8	515.96
		Standard deviation	116.7	77.1	180.6	54.3	311.27
		Coefficient of variation	1.5	0.7	0.9	1.0	0.60
Bar soil	16	Maximum	258.8	70.0	272.5	309.5	967.51
		Minimum	12.1	0.0	14.2	20.0	133.93
		Average value	77.3	22.5	81.2	93.9	362.70
		Standard deviation	64.4	16.1	65.4	83.6	208.96
		Coefficient of variation	0.8	0.7	0.8	0.9	0.58
Land soil	25	Maximum	540.3	560.6	787.1	744.3	2589.70
		Minimum	45.9	0.0	19.0	60.0	278.86
		Average value	152.4	49.0	119.5	198.3	652.92
		Standard deviation	139.8	136.9	189.1	169.5	615.17
		Coefficient of variation	0.9	2.8	1.6	0.9	0.94
Total	69	Maximum	540.3	560.6	787.1	744.3	2589.70
		Minimum	2.4	0.0	14.2	20.0	133.93
		Average value	126.7	62.0	172.8	114.9	563.95
		Standard deviation	127.6	83.5	186.5	110.2	392.08
		Coefficient of variation	1.0	1.3	1.1	1.0	0.70

shows that TP content across the study area ranged from 133.9 to 2589.70 mg/kg, with an average of 563.95 mg/kg. Similar TP contents in the sediment were observed in other regions. Spooner et al. [22] reported the average TP content of 670 ± 0.04 mg/kg in sediment from Lake Colac, Australia, and Gao et al. [23] found TP content ranged from 79.15 to 565.12 mg/kg in sediment from Swan Lake in Shandong, China. In addition, comparable TP content was detected in sediment from Lake Edku, Egypt (549 to 1198 mg/kg) [24], and content in El Mex Bay and Lake Mariut in Egypt ranged from 332 to 2171 mg/kg [25]. Lagoon sediment was significantly higher than that in the bar (Kruskal–Wallis 1-way ANOVA, p < 0.001), with no significant differences between the other regions. The majority of TP was primarily distributed in the lagoon and its western inland region, exhibiting characteristics of extension toward the lagoon and suggesting a process of land-derived phosphorus diffusion into the water.

OP, which varied from 20.0 to 744.3 mg/kg, had a consistent distribution pattern for TP (Figure 2a,e), decreasing from the western inland to the eastern offshore. However, significantly different distributions of IP (14.2–787.1 mg/kg) and TP were found (Figure 2b,e). IP was mainly distributed along the lagoon, with a high-value area appearing in the middle, at the mouth, and at the junction of the lagoon and the open sea. This indicated that, compared to TP and OP, the migration process of IP with water flow was more pronounced. Fe/Al-P was the predominant form of IP and relatively high in the sediments (Table 1), and it was concentrated in the western coastal area and the central lagoon (Figure 3c), which may be associated with the distribution of Al₂O₃. Similarly, enriched Ca-P was discovered in the sediments (Table 1). The southwest river inlet and its adjacent areas were characterized by high Ca-P values, and the northeast river inlet to the sea also exhibited a relatively high Ca-P zone. The former could be attributed to the upstream terrestrial supply of Ca-P by the river, while the latter might be associated with the accumulation of carbonate rocks as a result of marine biological activities.

3.2. Correlation between Different Phosphorus Fractions

The proportions of different forms of phosphorus to TP varied between different soil–sediment types. With an increase in TP, OP in the lagoon sediment, offshore sediment, bar soil, and land soil all increased (lagoon sediment: r = 0.754 **, p < 0.01; offshore sediment: r = 0.943 **, p < 0.01; bar soil: r = 0.599 **, p < 0.01; land soil: r = 0.621 *, p < 0.05;

Table 2. Correlation analyses of different fractions of phosphorus.

Type	Phosphorus Fractions	Fe/Al-P	Ca-P	IP	OP	TP
Lagoon sediment	Fe/Al-P	1
	Ca-P	0.456	1
	IP	0.575 *	0.723 **	1
	OP	0.287	0.619 **	0.767 **	1
	TP	0.464	0.639 **	0.798 **	0.754 **	1
Offshore sediment	Fe/Al-P	1
	Ca-P	0.675	1
	IP	0.923 **	0.884 **	1
	OP	0.994 **	0.721 **	0.954 **	1
	TP	0.865 **	0.767 **	0.924 **	0.943 **	1
Bar soil	Fe/Al-P	1
	Ca-P	−0.125	1
	IP	0.655 **	0.243	1
	OP	0.478 *	−0.057	0.089	1
	TP	0.923 **	0.005	0.612 **	0.599 **	1
Land soil	Fe/Al-P	1
	Ca-P	0.769 **	1
	IP	0.812 **	0.977 **	1
	OP	0.664 **	0.089	0.078	1
	TP	0.978 **	0.865 **	0.845 **	0.621 *	1

* correlation is significant at the 0.05 level (two-tailed test); ** correlation is significant at the 0.01 level (two-tailed test).

). However, the rate of increase in OP in the terrestrial environment was generally higher than that in the sediments (Figure 3a). This was because although the OP content correlated with OM, the low pH of soils promoted the absorption of phosphorus by OM, which helped to protect the phosphorus from microbial degradation [26]. Additionally, OP input from plants to the soil was a significant contributor to the increase in OP in the soil [27], whereas inputs from plants to the water were relatively limited.

Considering TP is composed of OP and IP, the rate of increase in IP in different soil–sediment types was roughly opposite to that of OP (Figure 3b). However, in the sediments and soils, IP was significantly positively correlated with TP. Furthermore, OP and IP in the soils and sediments both originated from the same sources, such as plant and animal remains and feces [28], leading to significant correlations between them (Table 2).

From an overall perspective (Figure 4), the proportions of the main forms of phosphorus in different soil–sediment types, except for some extreme values, ranged from 25% to 75% for Fe/Al-P, 0% to 50% for Ca-P, and 12.5% to 75% for OP, respectively. In the lagoon sediment, except for a few outliers, the majority of the data points were concentrated on the Fe/Al-P to Ca-P side of the plot. Combined with Figure 5, it was noted that the high proportion of Fe/Al-P in lagoon sediment may be attributed to the high content of iron and aluminum in the lagoon [29], followed by the comprehensive effects of various chemical and biological processes in the tropical water body and sedimentation. Al/Fe-P, Ca-P, and OP were not significantly correlated, while there was a significant positive correlation between Ca-P and OP (r = 0.619 **, p < 0.01), indicating that there was both competition and synergy among OP, Fe/Al-P, and Ca-P in the study area.

In offshore sediments, the distribution of data points was relatively uniform. The proportion of Fe/Al-P ranged from 0% to 50%, and the proportion of Ca-P ranged from 50% to 100%, with no significant correlation between the two (r = 0.675, p > 0.05). This suggests that the sources and formation conditions of Fe/Al-P and Ca-P were different. The proportion of Ca-P was greater than that of Fe/Al-P (independent-samples t test: p = 0.496), which may be due to the substantial accumulation of carbonate that occurred due to marine biological activities in the offshore sediment; as a result, phosphorus was predominantly present in the form of Ca-P [30,31,32]. Overall, IP predominated in the sediments, indicating that the IP formed from the mineralization of OP was mostly lost to the water and combined with the corresponding minerals.

As shown in Figure 4, in both the bar soil and land soil, all data points were mainly concentrated on the Fe/Al-P to OP side of the plot. This indicated that in terrestrial soil, Fe/Al-P and OP dominated, and there was a certain synergistic relationship between these two compounds. Al₂O₃ was highest in the western terrestrial area and migrated from the land to the sea (Figure 5b). Therefore, the high content of Al₂O₃ resulted in the dominance of Fe/Al-P in the terrestrial soil and increased the content of complexed ferric oxides in the soil [33]. OM can combine with these complexed ferric oxides in the soil, forming an organic film on the surface of Al₂O₃, which not only prevented the adsorption of phosphorus by Al₂O₃ but also prevented the loss of Fe/Al-P. Consequently, there was a certain synergistic relationship between Fe/Al-P and OP.

3.3. Relationship between Iron/Aluminum Oxides and Phosphorus

Many studies have indicated that inorganic colloids such as those consisting of Al₂O₃ strongly adsorb phosphorus [34,35]. Generally, the higher the content of Al₂O₃, the higher the soil phosphorus content. Figure 5a,b show that the TP in both soils and sediments increased with an increase in the Al₂O₃ content. Overall, the correlation between Al₂O₃ and various forms of phosphorus was highly significant (Table 3). It was, however, found that in land soil, the correlation between Al₂O₃ and other forms of phosphorus and TP was not strong, except for OP. The reason may be that the OM in the land soil formed an organic film on the surface of colloidal particles such as Al₂O₃, which hindered the adsorption of phosphorus by these colloids [36]. Nevertheless, due to the high content of Al₂O₃ in the western inland and lagoon, the content of Fe/Al-P was higher in these areas than it was in the eastern bar and offshore areas.

3.4. Relationship between OM and Phosphorus

The average OM content in the soil was 13,080 mg/kg, which was significantly lower than that in the sediments, measuring 23,580 mg/kg (independent-samples t test: p < 0.05). This disparity may occur because the favorable soil aeration conditions promoted OM mineralization and decomposition. Additionally, OM might erode from terrestrial soil and subsequently settle in the lagoon. The enrichment of OM near the land and lagoon reflected the process of terrestrial OM erosion and supply to the lagoon, while the enrichment of OM in the lagoon may be related to the aggregation and settling of OM in the high-ion-concentration background of the river–sea transition zone.

OM is an important factor affecting phosphorus adsorption [37]. Various pieces of evidence suggest that OM can both promote and inhibit phosphorus adsorption and fixation. There was a significant positive correlation between OM and TP for soils and sediments, indicating an overall promoting effect of OM in the lagoon area. For the different soil types and different phosphorus fractions (Table 3,

Table 4. Correlation analysis matrix for basic chemical properties and phosphorus speciation in lagoon sediments.

	pH	OM	CEC	Fe/Al-P	Ca-P	IP	OP	TP	X2O3 (1)
pH	1
OM	−0.485 *	1
CEC	−0.313	0.878 **	1
Fe/Al-P	−0.029	0.147	0.419	1
Ca-P	−0.145	0.624 **	0.714 **	0.440	1
IP	−0.171	0.586 **	0.663 **	0.565 *	0.705 **	1
OP	−0.359	0.893 **	0.887 **	0.294	0.590 **	0.750 **	1
TP	−0.578 **	0.662 **	0.582 **	0.440	0.633 **	0.790 **	0.735 **	1
X2O3 (1)	−0.350	0.710 **	0.791 **	0.411	0.826 **	0.626 **	0.722 **	0.715 **	1

* correlation is significant at the 0.05 level (two-tailed test); ** correlation is significant at the 0.01 level (two-tailed test). ⁽¹⁾ iron/aluminum oxide.

,

Table 5. Correlation matrix for basic chemical properties and phosphorus speciation in offshore sediments.

	pH	OM	CEC	Fe/Al-P	Ca-P	IP	OP	TP	X2O3 (1)
pH	1
OM	−0.550	1
CEC	−0.519	0.996 **	1
Fe/Al-P	−0.577	0.997 **	0.994 **	1
Ca-P	0.007	0.687 *	0.690 *	0.665	1
IP	−0.384	0.942 **	0.935 **	0.930 **	0.879 **	1
OP	−0.563	0.992 **	0.983 **	0.990 **	0.701 *	0.952 **	1
TP	−0.523	0.868 **	0.832 **	0.852 **	0.755 *	0.930 **	0.904 **	1
X2O3 (1)	−0.566	0.862 **	0.828 **	0.844 **	0.721 *	0.913 **	0.899 **	0.993 **	1

* correlation is significant at the 0.05 level (two-tailed test); ** correlation is significant at the 0.01 level (two-tailed test). ⁽¹⁾ iron/aluminum oxide.

,

Table 6. Correlation matrix for basic chemical properties of bar soil and phosphorus species.

	pH	OM	CEC	Fe/Al-P	Ca-P	IP	OP	TP	X2O3 (1)
pH	1
OM	−0.490 *	1
CEC	−0.354	0.609 **	1
Fe/Al-P	−0.486 *	0.290	0.591 **	1
Ca-P	0.360	−0.242	−0.291	−0.159	1
IP	−0.233	0.012	−0.030	0.647 **	0.282	1
OP	−0.295	0.527**	0.675 **	0.448 *	−0.086	0.085	1
TP	−0.448 *	0.467*	0.724 **	0.906 **	0.006	0.590 **	0.587 **	1
X2O3 (1)	−0.277	0.544**	0.971 **	0.586 **	−0.236	−0.058	0.652 **	0.726 **	1

* correlation is significant at the 0.05 level (two-tailed test); **: correlation is significant at the 0.01 level (two-tailed test). ⁽¹⁾ iron/aluminum oxide.

and

Table 7. Correlation matrix for basic soil chemical properties and phosphorus species in land soil.

	pH	OM	CEC	Fe/Al-P	Ca-P	IP	OP	TP	X2O3 (1)
pH	1
OM	−0.417	1
CEC	−0.368	0.693 **	1
Fe/Al-P	−0.230	0.155	0.117	1
Ca-P	−0.070	0.261	0.010	0.754 **	1
IP	−0.075	0.203	0.071	0.790 **	0.960 **	1
OP	−0.061	0.018	0.124	0.644 **	0.102	0.090	1
TP	−0.122	0.188	0.094	0.960 **	0.840 **	0.829 **	0.597 *	1
X2O3 (1)	−0.062	−0.114	0.223	0.412	−0.011	0.066	0.590 *	0.327	1

* correlation is significant at the 0.05 level (two-tailed test); ** correlation is significant at the 0.01 level (two-tailed test). ⁽¹⁾ iron/aluminum oxide.

), the correlation between OM and OP was the highest in the lagoon sediments, offshore sediments, and bar soil, with correlation coefficients of 0.893, 0.992, and 0.527, respectively, and these values indicated highly significant positive correlations. However, the correlation coefficient of OM with OP in the terrestrial soil was significantly lower than that of the other phosphorus fractions, i.e., only 0.018, and the correlation between OM and all phosphorus fractions in this soil type was not significant. This might occur because OM played an inhibitory role in phosphorus adsorption in land soil. It was possible that the OM formed a film on soil colloid surfaces, inhibiting the adsorption and fixation of phosphorus on colloids [36]. Additionally, since both organic molecules and phosphate ions can adsorb at the same adsorption sites on soil colloids, organic molecules can compete with phosphate ions for adsorption sites.

Table 3. Correlation matrix between basic chemical properties and phosphorus species in soils and sediments from the study areas.

	pH	OM	CEC	Fe/Al-P	Ca-P	IP	OP	TP	X2O3 (1)
pH	1
OM	−0.233	1
CEC	−0.231	0.828 **	1
Fe/Al-P	−0.166	0.403 **	0.553 **	1
Ca-P	0.241 *	0.404 **	0.317 **	0.543 **	1
IP	0.155	0.533 **	0.536 **	0.697 **	0.792 **	1
OP	−0.390 **	0.319 **	0.439 **	0.465 **	0.080	0.136	1
TP	−0.130	0.507 **	0.517 **	0.773 **	0.744 **	0.746 **	0.560 **	1
X2O3 (1)	−0.238 *	0.504 **	0.766 **	0.535 **	0.239 *	0.363 **	0.588 **	0.592 **	1

* correlation is significant at the 0.05 level (two-tailed test); ** correlation is significant at the 0.01 level (two-tailed test). ⁽¹⁾ iron/aluminum oxide.

Table 3. Correlation matrix between basic chemical properties and phosphorus species in soils and sediments from the study areas.

	pH	OM	CEC	Fe/Al-P	Ca-P	IP	OP	TP	X2O3 (1)
pH	1
OM	−0.233	1
CEC	−0.231	0.828 **	1
Fe/Al-P	−0.166	0.403 **	0.553 **	1
Ca-P	0.241 *	0.404 **	0.317 **	0.543 **	1
IP	0.155	0.533 **	0.536 **	0.697 **	0.792 **	1
OP	−0.390 **	0.319 **	0.439 **	0.465 **	0.080	0.136	1
TP	−0.130	0.507 **	0.517 **	0.773 **	0.744 **	0.746 **	0.560 **	1
X2O3 (1)	−0.238 *	0.504 **	0.766 **	0.535 **	0.239 *	0.363 **	0.588 **	0.592 **	1

* correlation is significant at the 0.05 level (two-tailed test); ** correlation is significant at the 0.01 level (two-tailed test). ⁽¹⁾ iron/aluminum oxide.

3.5. Relationship between Acidity and Phosphorus

The average pH of soil samples was 5.1, signifying acidic conditions. This suggests notable leaching of base cations like K, Na, Ca, and Mg, coupled with the enrichment of acid-forming ions such as H⁺ and Al³⁺ [38]. Sediment samples exhibited pH values of 7.3, indicating neutral conditions, largely due to the introduction of terrestrial base cations, reflecting cation accumulation. Spatially, as shown in Figure 5d, soil pH was lowest and slightly acidic in the western land area, while the pH was highest and neutral in the eastern sea area. The lagoon conditions, on the whole, were slightly acidic to neutral, showing transitional features. Overall, the different pH values reflected the accumulation and migration processes of base cations.

According to Figure 2e and Figure 5d, the trends in the distribution of soil–sediment pH was roughly opposite to that of TP. Specifically, soil–sediment pH and TP were negatively correlated (Table 3, Table 4, Table 5, Table 6 and Table 7). For different soil–sediment types, a significant correlation was observed between pH and TP in the lagoon sediments (r = −0.578). The reason may be that under low pH conditions on the sediment surface, OH⁻ was easily replaced with phosphate ions, increasing the phosphorus content in the sediments [39]. Among the different fractions of phosphorus, the correlation with pH was more significant for Al/Fe-P, with bar soil showing a significant negative correlation with Fe/Al-P compared to the other three types of soil–sediment samples. This was because this soil was acidic, the activity of iron/aluminum compounds was high, and displaced OH⁻ was quickly neutralized [39]. Furthermore, as the pH decreased, the anion-exchange adsorption capacity of the soil increased.

3.6. Relationship between CEC and Phosphorus

CEC is a crucial factor involved in characterizing sediments. The average CEC of the soil samples was 8.97 cmol/kg, whereas the average CEC of the sediments was 12.52 cmol/kg, with the former being significantly lower than the latter, reflecting the desalination of the soil in the land area. Regarding spatial distribution (Figure 5e), the distribution pattern of CEC closely mirrored that of OM, demonstrating peak values in the southwestern land, adjacent water areas, and the lagoon mouth region. This indicated that the migration of cations was related their adsorption to OM. In addition, it can be observed that compared to diffusion from land to lagoon, the diffusion of ions in the lagoon mouth area was more pronounced, which might occur due to the lower OM content in this region and because the open water area allowed ion diffusion.

There was a significant positive correlation between CEC and the contents of various fractions of phosphorus (Table 3). In the correlation analysis (Table 3, Table 4, Table 5 and Table 6), CEC in the lagoon sediments and offshore sediments showed a significant positive correlation with various fractions of phosphorus; in bar soil, CEC had a negative correlation with Ca-P and total IP but a significant positive correlation with Fe/Al-P, OP, and TP. Compared to the other three types of soil–sediment samples, the correlation between CEC and phosphorus content in land soil was not significant, and the phosphorus content was only slightly positively correlated with CEC. The distribution pattern of CEC and OM was similar, and their correlation was also significant (r = 0.828), indicating that the migration of cations was related to their adsorption to OM. The OM in land soil might occupy adsorption sites for phosphate ions, thus inhibiting phosphorus adsorption [36,37]. In conclusion, although CEC can promote the absorption of phosphorus on soil–sediment surfaces, the relationship between CEC and OM, as well as the relationship between OM and phosphorus, suggested that the inhibitory effect of OM in land soil was stronger than the promoting effect of CEC. Therefore, the correlation between phosphorus fractions in land soil and CEC was not as significant as that in the other three soil–sediment types.

4. Conclusions

This study studied the Shimei Inland Sea Lagoon, revealing distinct spatial patterns of nitrogen and phosphorus between zones. Primary findings highlighted the enrichment of IP in lagoon and marine areas, contrasting with higher concentrations of OP in terrestrial regions.

Nuanced variations in the ratio between Fe/Al-P and Ca-P were uncovered, linking them to primary and secondary carbonate sedimentation in specific locales. The relationship between pH and phosphorus distribution, influenced by OM, demonstrated complexities in these dynamics.

Distinct relationships between phosphorus forms emerged across environments. Lagoon sediments favored Al/Fe-P, offshore sediments favored Ca-P, and terrestrial soils demonstrated synergy between Al/Fe-P and OP.

Future research should explore temporal and broader geographical variations, considering additional environmental factors. This study serves as a foundational step, emphasizing the ongoing research used to guide ecological conservation.