Geochemical Behavior of Sedimentary Phosphorus Species in Northernmost Artificial Mangroves in China

Abstract

Mangroves are typically found in tropical coastal areas, and these ecosystems face deterioration and loss due to threats from climate and human factors. In this study, sediment cores were collected from human-planted mangroves in sub-tropical Ximen Island, China, and were determined for sedimentary phosphorus (P) species. The objective was to investigate the ability of mangroves planted in a zone bordering their temperature limit to preserve and regulate P. Our results showed that bioavailable P (BAP), which includes exchangeable-P (Ex-P), iron-bound P (Fe-P), and organic P (OP), accounted for approximately 64% of total P (TP). Apatite P (Ca-P), which accounted for 24% of TP, most likely originated from aquaculture activities surrounding the island. The vertical distribution of sedimentary P species along the sediment cores showed a rather constant trend along the salt marsh stand but considerable fluctuations for the mangroves and bare mudflat. These results indicate that mangroves accumulated P when there was a high P discharge event, and that this P was eventually released during organic matter decomposition and contributed to Ca-P formation. Nevertheless, old and young mangroves accumulated higher sedimentary P species, OP, and BAP compared to the salt marsh stand and bare mudflat areas. This study showed the potential of mangroves planted outside their suitable climate zone to preserve and regulate P.

1. Introduction

Mangroves are intertidal ecosystems typically found in shallow-area transition zones from land to sea. Mangrove forests are important blue carbon ecosystems that not only serve as carbon reservoirs [1,2,3], but also provide ecosystem services by regulating organic pollutants, heavy metals [4,5,6], nutrients [7,8,9], and microplastics [10,11] along coastal zones. In addition, these ecosystems are habitats for many animals [9,12] and provide important means of livelihood, such as fishing [13,14] and tourism industries [15,16]. However, mangrove forests are seriously threatened by climate change factors, such as sea level rise [17,18]; human activities, such as land use and deforestation [19,20]; pollution [21,22]; and debris problems [23]. Realizing the fact that mangroves are facing serious degradation and loss has prompted various restoration efforts, such as hydrological and community-based ecological restoration and sowing of seedlings and propagules [24,25]. These restoration and conservation efforts have enhanced the tree density and maintained species diversity in mangrove ecosystems [26,27]. Mangrove forests took only five years after restoration to obtain their maximum protection capacity [28]. In addition, the values of carbon sequestration, ecotourism, and forest and fishery products exceeded the cost of rehabilitation [27,29], supporting the importance of restoration, rehabilitation, and conservation works on mangrove forests.

The content, form, and distribution of biogenic elements in sediments are influenced by atmospheric deposition, terrestrial input, and human activities [30,31], and sediments provide records of these past events. The enrichment and release of phosphorus (P) in mangroves is a key factor leading to eutrophication in coastal waters. Meanwhile, an increasing awareness of environmental protection has somewhat reduced the input of P from external sources; however, the problems related to P pollution remain serious. Studies of different sedimentary P species in lake sediments have demonstrated the occurrence of P release from sediments [32,33]. In addition, a few P species, such as the exchangeable-P (Ex-P), iron-bound P (Fe-P), and organic P (OP), can be released from sediments into overlying water and become bioavailable [34,35]. Therefore, the understanding of the dynamics of sedimentary P species needs to be improved. Although sedimentary P species have been extensively studied in lakes and coastal areas [36,37,38,39,40,41], only a few studies have been conducted on mangrove systems [42,43], especially on human-planted mangroves in a sub-tropical zone.

The natural distribution of Chinese mangroves is from Yulin Harbour (18°09′ N) in Hainan Province to Fuding (27°20′ N) in Fujian Province, and the northernmost limit of artificial mangroves is Ximen Island (28°21′ N), Wenzhou, Zhejiang Province [44,45]. The mangrove ecosystem of Ximen Island is currently the northernmost artificial mangrove in China that can self-reproduce. The mangrove was introduced from Fujian in 1957 and planted with Kandelia obovata. Previous studies on the mangroves in Ximen Island involved the determination of microbial distribution and environmental characteristics of mangrove sediments [46,47], investigation of the annual growth of different mangrove species in experimental plots [48,49], and studying the ecological relation between the benthic community and mangrove forest [50,51]. However, little is known about the feasibility of these northernmost human-planted mangrove systems to regulate P. For this purpose, sediment cores were collected from two mangrove systems on Ximen Island: younger (approximately 4 years old) and older (approximately 60 years old) mangrove forests. To our knowledge, this is the first study conducted to determine the sedimentary P species in the northernmost human-planted mangroves in China. This study aimed to determine how well artificial mangroves situated in the coldest temperature limit to their growth fare in preserving and regulating P in their sediments. The study results will provide important information to policy makers on the potential of mangrove planting.

2. Materials and Methods

2.1. Study Area and Sampling Locations

Ximen Island is located off Wenzhou, Zhejiang Province, China. It is the largest island in the northern part of Yueqing Bay, Zhejiang Province, with a land area of 6.97 km², a beach area of 19.20 km², and a coastline length of 11.81 km [44,52]. The low-temperature-resistant mangrove plant Kandelia obovata was introduced from Fujian into Ximen Island in 1957, and since then, the mangrove area has reached approximately 0.33 km² through natural breeding [53,54]. The island has a subtropical monsoon climate, with an average annual temperature of 17–18 °C and an average annual precipitation of 1700 mm. The tides in Yueqing Bay showed irregular and semi-diurnal variation, with an average variation of 4.2 m and a maximum variation of 8.34 m [44,47]. The mangrove wetland of Ximen Island is rich in biodiversity and there are no streams on the island [48,52].

Four sediment cores were successfully collected from Ximen Island mangroves on 28 and 29 May 2021. The cores were collected from two mangrove areas (M1 and M2), where M1 was the older mangrove system of approximately 60 years old and M2 was the younger mangrove system of approximately 4 years old. Two locations were sampled in each mangrove area. In M1, L1 was located on the landward side and had mangrove vegetation, and S1 was located at the fringe of the mangrove coastal area and had salt marsh plants Spartina alterniflora. In M2, L2 was the landward location with younger mangrove plants, and S2 was a bare mudflat near the coastal area (

Table 1. Sampling locations and samples information.

Sampling Sites	Locations		Core Length (cm)	Number of Samples	Other Information
	Latitude	Longitude
M1-L1	28°21′05″ N	121°11′26″ E	100	26	Covered by an old mangrove forest and with the effects from the cold.
M1-S1	28°21′04″ N	121°11′25″ E	100	26	In the seaward vegetation transition zone which has Spartina alterniflora.
M2-L2	28°20′39″ N	121°10′19″ E	100	26	Covered by young Kandelia obovata plant.
M2-S2	28°20′39″ N	121°10′18″ E	96	25	In mudflat on the seaward side.

; Figure 1). Sediment cores were collected using a peat auger with a length of 1 m and an inner diameter of 6 cm. The cores were of lengths 100 cm (L1, S1, and L2) and 96 cm (S2). At each location, three cores were collected, and samples of sediment slices at 2-cm intervals were pooled from the three cores and then immediately placed in an incubator. The sediments were dried at approximately 45 °C for three days in the laboratory. Debris, such as dead branches and leaves, were removed. The sediments were then ground with a mortar and pestle until homogenous and ready for subsequent analyses.

2.2. Sequential P Extraction

Different sedimentary P species were extracted using Ruttenberg’s (1992) sequential extraction method [55,56]. In the first step, 0.3 g of dry sediment was weighed into a 50-mL centrifuge tube, to which 20 mL of 1 M MgCl₂ was added, and the solution pH was adjusted to 8 with 10 M NaOH solution. Extraction was performed by shaking the solution for 2 h at room temperature (RT), after which the content was centrifuged and the supernatant was decanted and set aside. Another 20 mL of 1 M MgCl₂ was added to the residue, and the process was repeated. The residue was washed with 10 mL of H₂O for 2 h and centrifuged. All supernatants were saved to determine their Ex-P content. In the second step, 20 mL of citrate–dithionite–bicarbonate solution (32.4 g C₆H₅Na₃O₇ · 2H₂O + 42 g NaHCO₃ + 2.87 g Na₂S₂O₄ in 500 mL distilled water) was added to the residue, and the solution was shaken for 8 h at RT. The content was centrifuged, and the supernatant was decanted and saved. Then, 20 mL of 1 M MgCl₂ was added to the residue and shaken for 2 h, followed by centrifugation and supernatant extraction. Subsequently, the residue was washed with 10 mL of H₂O for 2 h, centrifuged, and then all of the supernatant was removed and saved for Fe-P determination. For the next fraction, 20 mL of 0.2 M acetate buffer (pH = 4) was added to the remaining residue and shaken for 6 h at RT, after which the mixture was centrifuged and the supernatant was saved. The residue was then washed twice with 1 M MgCl₂, centrifuged again, and the supernatant was removed and set aside. The residue was finally washed with 10 mL of H₂O, centrifuged once more, and the supernatant was removed and saved. The supernatants obtained from this step were determined for authigenic P (Ca-P) content. In the next step, 1 M HCl was added to the residue and shaken for 16 h, after which the content was centrifuged, and the supernatant was saved for the analysis of detrital apatite P (De-P). The residue was then moved to a crucible and dried in an oven at 80 °C for one day, followed by combustion at 550 °C for 5 h. The residue was cooled, added with 1 M HCl, and shaken for 16 h, and the supernatant was determined for OP content. All P concentrations were determined colorimetrically as molybdenum blue complexes using a UV-visible spectrophotometer UV-8000 (METASH, Shanghai, China). Absorbance was measured at a wavelength of 885 nm. Inorganic P (IP) is the sum of Ex-P, Fe-P, Ca-P, and De-P; total P (TP) is the sum of all P fractions (or the sum of IP and OP); and bioavailable P (BAP) is the sum of Ex-P, Fe-P, and OP.

2.3. Statistics and Visualization

Pearson correlation analysis and two-tailed significance tests were conducted to determine the correlation between the different parameters. Correlations were considered significant at p < 0.05. Data analyses were conducted using IBM SPSS for Windows (Version 26.0, Armonk, NY, USA), and the “±” symbol represents the standard deviation. The maps and diagrams were mainly produced using Origin (Version 2018, Northampton, MA, USA).

3. Results

The results of sedimentary P forms and percentages of OP, IP, and BAP to TP are presented in Appendix A.

Table 2. Ranges and mean, and percentage ranges and mean, of each sediment P to TP.

	M1-L1	M1-S1	M2-L2	M2-S2	All
Ex–P (mg/kg)	28.01–49.21	30.24–46.31	31.67–46.23	27.95–41.82	27.95–49.21
Mean	39.19	38.66	36.30	32.83	36.75
Ex–P/TP	29.30–49.92%	38.10–58.22%	28.95–47.21%	39.34–69.34%	29.30–69.34%
Mean	42.06%	50.98%	37.05%	48.61%	44.68%
Fe–P (mg/kg)	0.46–15.80	2.33–9.68	2.30–8.13	0.49–16.30	0.46–16.30
Mean	4.85	5.86	3.84	4.30	4.71
Fe–P/TP	0.51–15.49%	3.05–12.09%	2.20–8.04%	0.73–21.66%	0.51–21.66%
Mean	5.21%	7.73%	3.92%	6.37%	5.81%
Ca–P (mg/kg)	20.68–41.53	12.29–36.16	12.71–56.27	1.89–17.42	1.89–56.27
Mean	27.80	18.57	29.65	7.18	20.80
Ca–P/TP	22.76–45.82%	18.66–45.56%	12.82–46.02%	2.64–23.85%	2.64–45.82%
Mean	29.84%	24.48%	30.27%	10.63%	23.81%
De–P (mg/kg)	1.49–14.67	2.64–10.47	4.95–31.57	1.89–26.29	1.49–31.57
Mean	6.05	5.24	15.62	14.63	10.39
De–P/TP	1.64–15.35%	2.94–16.10%	5.45–30.93%	3.73–32.64%	1.64–21.64%
Mean	6.49%	6.90%	15.95%	21.66%	12.75%
OP (mg/kg)	7.50–21.56	3.23–17.97	3.91–25.14	1.95–24.58	1.95–25.14
Mean	15.28	7.51	12.55	8.96	11.08
OP/TP	8.13–23.35%	4.84–19.99%	4.24–27.64%	4.37–33.76%	4.24–33.76%
Mean	16.46%	9.65%	12.73%	13.15%	13.00%
IP (mg/kg)	66.83–97.68	58.85–75.65	63.94–111.31	42.56–79.49	42.56–111.31
Mean	77.89	68.33	85.41	58.58	72.55
IP/TP	76.65–91.87%	80.01–95.16%	72.36–95.76%	66.24–95.63%	66.24–95.76%
Mean	83.54%	90.35%	87.27%	86.85%	87.00%
BAP (mg/kg)	43.25–80.39	40.19–62.53	42.11–70.51	34.19–62.52	34.19–80.39
Mean	59.32	52.04	52.69	45.73	52.45
BAP/TP	47.72–71.30%	50.99–74.51%	43.19–77.51%	57.48–85.86%	43.19–85.86%
Mean	63.64%	68.71%	54.20%	68.22%	63.69%
TP (mg/kg)	79.45–113.99	63.91–89.90	77.24–125.34	44.58–84.36	44.58–113.99
Mean	93.16	75.84	97.95	67.53	83.62

shows the ranges and mean values of the different sedimentary P species and TP. The ranges of TP concentrations for the old mangrove (L1), salt marsh (S1), young mangrove (L2), and bare mudflat (S2) were 79.45–113.99, 63.91–89.90, 77.24–125.34, and 44.58–84.36 mg/kg, respectively. The young mangrove had the highest mean TP (97.95 mg/kg), followed by the old mangrove (TP = 93.16 mg/kg), zone with salt marsh plants S. alterniflora (TP = 75.84 mg/kg), and bare mudflat (TP = 67.53 mg/kg). The old mangrove and S. alterniflora stand accumulated the highest Ex-P and Fe-P concentrations, followed by the young mangrove and mudflat; the old and young mangroves accumulated higher Ca-P, IP, OP, and BAP concentrations than salt marsh and mudflat. Overall, the old and young mangroves accumulated higher P in their sediments than salt marshes and mudflats.

Based on the ranges of concentrations of different sedimentary P species along the four cores in both mangroves, the contribution of each P form to TP were in the following order: Ex-P > Ca-P > OP > De-P > Fe-P. The order of the different P species varied slightly for each location: L1 (Ex-P > Ca-P > OP > De-P > Fe-P), S1 (Ex-P > Ca-P > OP > Fe-P > De-P), L2 (Ex-P > Ca-P > De-P > OP > Fe-P), S2 (Ex-P > De-P > OP > Ca-P > Fe-P). The order of BAP concentrations for the four locations was as follows: L1 > L2 > S1 > S2 (Table 2). Comparison of the sedimentary P species in the surface sediment at different locations also showed a similar order as the overall ranges of different sedimentary P species and BAP, as follows: L1 > L2 > S1 > S2 (Figure 2).

Fe-P, De-P, and OP showed a general trend of decrease from 100 cm to the surface sediment along L1, L2, and S2 cores. These P species showed a rather constant trend along S1, except for the highest OP observed at 4–12 cm. Ex-P along L1 decreased from 100 cm to approximately 74–76 cm and then started to increase toward the 22–24 cm layer, followed by decreasing trend toward the surface sediments. Ex-P along S1 remained constant from 100 to 20 cm and then decreased from approximately 20 cm toward the surface sediments. Ex-P along the M2 cores (L2 and S2) showed a decrease from 100 cm to the surface sediments. In contrast, Ca-P along the M1 (L1 and S1) and S2 cores showed an increasing trend toward the surface sediment, while Ca-P along L2 showed an overall decreasing trend toward the surface sediments, except for a drastic increase at approximately 30 cm. Both IP and TP exhibited the same trend; they were relatively lower before 30 cm, showed the highest value at approximately 30 cm, and then a decreasing trend from approximately 30 cm toward the surface sediments, except for the salt marsh, which showed a gradual increase in IP and TP from approximately 30 cm toward the surface sediments (Figure 3). The BAP concentration was rather constant along L1 below the 40 cm layers, showed the highest concentrations at approximately 25–40 cm, and then decreased toward the surface sediments. The BAP concentration remained constant throughout S1, except for the drastic decrease in the surface sediment. It exhibited a decreasing trend from the bottom to the surface layer of L2 and showed a decreasing trend from 36 cm toward the surface sediment along the core S2 (Figure 4).

4. Discussion

4.1. Sedimentary P Species

Different sedimentary P species can indicate the sources and bioavailability of P in the sediment. For instance, Ex-P has been related to wastewater discharge [39,57], and sewage and shrimp farms have affected mangroves [21,42]. Fe-P is also an indicator of anthropogenic pollution. A low Fe-P concentration is usually associated with oxygen depletion, as Fe-P can be released from sediments in reducing environments; high pH and bacterial activity also enhances the release of Fe-P [58,59,60]. OP can originate from wastewater and mariculture activities [39,61] and can be released by mineralization and become bioavailable [61,62]. In contrast, De-P is an inert and stable fraction that usually originates from land sources such as rivers [39,63]. The Ca-P sources include carbonate-associated P; biogenic apatite, such as bones and teeth; and authigenic fluorapatite [39,64,65]. The mineralization of organic matter can release phosphate, which is then adsorbed by clay minerals and the surface of Fe oxides. Some of the OP released upon organic matter decomposition and the P released from the Fe-P fraction result in the formation and accumulation of Ca-P in sediments [62,63,66,67].

The sediment cores from the mangroves on Ximen Island exhibited the highest proportion of Ex-P, which accounted for an average of 45% of TP. The proportion of OP accounted for 13% of TP, and that of Fe-P accounted for 5.81% of TP in the mangroves on Ximen Island. We presume that wastewater from household and farming activities along the coastal areas surrounding the island could be the main contributor to the high Ex-P, Fe-P, and OP proportions in these mangroves. The second highest Ca-P species, which accounted for a mean of approximately 24% of TP, was likely attributed to the aquaculture activities conducted along the coastal zones surrounding the island. Shrimp farm-impacted mangroves usually have a high Ca-P content due to fertilization with inorganic phosphate and liming at shrimp ponds [34,42,68]. The low Fe-P content in these mangroves is attributable to the release of some of this P from sediments under anoxic conditions after a period of prolonged inundation. As the rivers discharged from the mainland surrounding Yueqing Bay contribute fewer materials to the bay compared to the contributions from the Changjiang River [69,70], the low De-P concentration in these mangrove systems (12.75% of TP) indicates a lesser contribution of P from the adjacent rivers.

Comparison with the sedimentary TP contents in other mangroves such as the Jiulong estuary mangrove (TP = 821–1689 mg/kg; Jiang et al., 2018) and Quanzhou Bay mangrove (TP = 492–726 mg/kg; Le et al., 2022) in China, and the surface sediment (TP = 451–552 mg/kg) and sediment cores (TP = 459–530 mg/kg) of the Pichavaram mangrove in India [71], shows that the mangroves in Ximen Island have a relatively low TP (44.58–113.99 mg/kg;

Table 3. Comparison of the concentrations of sedimentary P species in the present study with those in other mangrove areas reported in the literature.

Study Area	Concentrations (mg/kg)							References
	Ex-P	Fe-P	Ca-P	De-P	OP	IP	TP
The Jiulong Estuary mangrove, China	-	223–793	-	100–200	100–200	721–1489	821–1689	Jiang et al., 2018 [43]
Surface sediments in the Pichavaram mangrove, India	49.6	172	122	92	58	403–473	451–552	Prasad and Ramanathan, 2010 [71]
Core sediments in the Pichavaram mangrove, India	29	126	144	138	450	-	459–530	Prasad and Ramanathan, 2010 [71]
The Ximen Island mangrove, China	36.75	4.71	20.80	10.39	11.08	42.56–111.31	44.58–113.99	This study
	HCl–P	NaOH–P	-	BAP	OP	IP	TP
Quanzhou Bay mangrove wetland, China	60–96	107–151	-	63–106	144–192	297–530	492–726	Le et al., 2022 [72]

). Similar to Jiulong estuary and Quanzhou Bay mangroves, the mangroves in Ximen Island showed high contents of BAP, thus indicating that these mangroves have a potential to release BAP to the coastal water. The surface sedimentary P species of the Pichavaram mangroves are in this order: Fe-P > Ca-P > De-P > OP > Ex-P. Thus, the sediments of the Pichavaram mangroves could be releasing P under anoxic conditions whilst the sediments of the Ximen Island mangroves could be releasing P under normal perturbations and during organic matter decomposition.

4.2. Different Wetland Ecosystems

The accumulation of mangrove plants, litterfall, root, algae, seagrass, and phytoplankton materials can enhance soil carbon, nutrients, and OP in mangrove sediments [43,73]. L1 and L2 exhibited higher contents of sedimentary P species, as well as IP, BAP, and TP, followed by S1 and S2, indicating that mangroves can accumulate P better than the salt marsh stand and bare mudflat. Other studies have also demonstrated that the Spartina alterniflora stand has a lower TP content than the natural Kandelia obovata mangrove forest, indicating the effectiveness of mangrove sediments to trap nutrients. Accordingly, the low BAP concentration in Spartina alterniflora sediments was attributed to the transfer of P from the sediment to the tissues of Spartina alterniflora for their growth [74,75]. The lowest BAP and TP concentrations, as well as most of the P species in S2, further proves that non-vegetated wetland ecosystems are less efficient in trapping nutrients than vegetated wetland ecosystems. Mangrove forests have been found to be able to remove nutrients from tidal water [8,76]. Besides, nutrients produced during mangrove litter decomposition can be re-absorbed by mangrove forests [73,77]. Thus, the efficiency of mangrove to trap nutrients results in increased P content with mangrove restoration [74].

4.3. Vertical Distribution of Sedimentary P Species

Some studies have shown a trend of decreasing P with increasing depth due to the degradation process over time and the loss of sedimentary P to the overlying water column [78,79]. Other studies have also shown that a period of stronger P enrichment can reflect the influence of human activities and untreated sewage [39,61,79]. The sedimentary P species, IP, and TP along the cores of both mangrove systems in Ximen Island showed considerable fluctuations. The trends of OP and Ex-P were quite similar to that of BAP (based on significant correlation results, p < 0.01;

Table 4. Pearson’s correlations between phosphorus fractions from the sediment cores of Ximen Island mangrove.

	Ex-P	Fe-P	Ca-P	De-P	OP	IP	BAP	TP
(a) M1-L1 (n = 26)
Ex-P	1
Fe-P	0.218	1
Ca-P	−0.160	−0.280	1
De-P	−0.232	0.132	−0.060	1
OP	−0.014	0.127	−0.457 *	0.055	1
IP	0.550 **	0.496 *	0.429 *	0.260	−0.242	1
BAP	0.737 **	0.685 **	−0.430 *	−0.059	0.468 *	0.494 *	1
TP	0.552 **	0.556 **	0.241	0.288	0.180	0.911 **	0.700 **	1
(b) M1-S1 (n = 26)
Ex-P	1
Fe-P	−0.195	1
Ca-P	−0.187	−0.35	1
De-P	−0.286	0.378	−0.388	1
OP	0.128	−0.131	0.322	−0.348	1
IP	0.311	0.004	0.696 **	−0.047	0.25	1
BAP	0.646 **	0.095	0.015	−0.315	0.778 **	0.377	1
TP	0.284	−0.075	0.657 **	−0.238	0.760 **	0.819 **	0.713 **	1
(c) M2-L2 (n = 26)
Ex-P	1
Fe-P	−0.097	1
Ca-P	−0.596 **	0.273	1
De-P	0.375	−0.265	−0.239	1
OP	0.233	−0.169	−0.209	0.11	1
IP	0.007	0.197	0.649 **	0.513 **	−0.072	1
BAP	0.702 **	0.025	−0.426 *	0.233	0.828 **	−0.009	1
TP	0.111	0.106	0.507 **	0.525 **	0.383	0.894 **	0.365	1
(d) M2-S2 (n = 25)
Ex-P	1
Fe-P	−0.331	1
Ca-P	0.212	−0.01	1
De-P	0.157	0.265	−0.31	1
OP	0.157	−0.249	−0.214	−0.029	1
IP	0.430 *	0.446 *	0.167	0.805 **	−0.128	1
BAP	0.515 **	0.222	−0.036	0.254	0.684 **	0.479 *	1
TP	0.480 *	0.314	0.063	0.754 **	0.325	0.896 **	0.763 **	1

Pearson correlation is used to determine the relationships among the P fractions. Results of correlation coefficient (r) are presented in the table; n = sample size; * indicates p is significant at the 0.05 level, ** indicates p is significant at the 0.01 level, the numbers not marked by * or ** indicate no significant correlation between the two parameters.

), all showing a decreasing trend, followed by an increasing trend and then an overall decreasing trend from the core bottom toward the present. These mangroves most probably received P from the various activities conducted in the surrounding areas, such as farming, mariculture, and household waste. In addition, they were subjected to replanting and pesticide pollution. Unlike the rather constant BAP in the salt marsh stand, which indicated a steady absorption of P by S. alterniflora plants, the sudden increases in the sedimentary P species and BAP, IP, and TP at the mangrove locations indicate the potential of mangrove systems to accumulate P under the occurrences of high P discharge events. Peak sedimentary P species are usually followed by decreasing trends, indicating the occurrence of the processes of organic matter decomposition and absorption of P by mangrove plants. Studies have found that variations of the concentrations of Ex-P, Fe-P, Ca-P and OP could be due to transformation of these sedimentary P species during burial [73]. In the Ximen Island mangrover, some of the bioavailable P released from the sediments are then transformed into Ca-P. Thus, a simultaneous increasing trend in Ca-P (significant negative correlation with BAP, p < 0.05; Table 4) further demonstrated that besides the contribution from shrimp farming activities, some of this Ca-P is attributable to the release of the BAP fraction. Mangrove systems usually show stronger P deposition in summer and autumn due to the enhanced inputs of labile organic matter. The sediments then release P into the overlying water [35], especially during the flood season in the subsequent spring and summer, when oxygen depletion most likely occurs and the microbial iron reduction process is predominant within the top layer of the sediment [59,80].

5. Conclusions

The dynamics of sedimentary P species in human planted mangrove systems in Ximen Island, China, were investigated in this study. Sediment cores were collected from two locations in the old mangrove system (M1)—the landward side with an old mangrove of 60 years old (L1) and the seaward side with Spartina alterniflora marsh plants (S1)—and two locations from the young mangrove system (M2): the landward side with a young mangrove of 4 years old (L2) and the seaward side, which was a bare mudflat (S2). The order of different sedimentary P forms was as follows: Ex-P (44.68% of TP) > Ca-P (23.81% of TP) > OP (13.00% of TP) > De-P (12.75% of TP) > Fe-P (5.81% of TP). The overall high concentrations of BAP, which was composed of Ex-P, OP, and Fe-P, indicated that these mangrove systems received P input from pollution in the surrounding areas, and that these sediments could release P during organic matter decomposition and under prolonged inundation. The second most abundant Ca-P might have originated from the aquaculture and shrimp farming activities surrounding the island. Some of this Ca-P could have been contributed by the release of sedimentary BAP. The higher sedimentary P content in the mangroves indicates that mangroves preserve P better than the salt marsh stand and bare mudflat. The salt marsh stand showed a constant distribution of sedimentary P species along the core, indicating the continuous absorption of P by the marsh plants. The sediment cores in the old and young mangrove systems showed an increase in sedimentary P contents during periods of high P discharge; this P was eventually released from the sediments during organic matter decomposition, and some contributed to the Ca-P formation.