Seasonal Dynamics of Nitrogen and Phosphorus in Wetland Plants: Implications for Efficient Eutrophication Control

Abstract

Eutrophication challenges aquatic ecosystems, with wetland plants serving as sustainable sources of in situ remediation for nitrogen (N) and phosphorus (P) removal. The limited understanding of seasonal nutrient dynamics hinders sustainable management development. This review classifies wetland plants by growth patterns: spring–summer growth plants (SSPs), spring–summer–autumn growth plants (SSAPs), and all-year-round growth plants (APs). SSPs exhibit peak N/P in spring–summer, SSAPs sustain high levels in autumn, while APs maintain stable contents through cold-tolerant sustainability. Perennials achieve sustainable cycling by seasonally storing N/P in tissues and redistributing them in spring. We propose the following sustainable harvesting protocols: harvest SSPs pre-September, SSAPs by November, and retain APs over winter. Height-controlled harvesting during growth peaks enhances removal while preserving regeneration. Strategic combinations of SSPs, SSAPs, and APs ensure sustainable, efficient nutrient removal across hydrological/climatic conditions, supporting wetland conservation.

1. Introduction

The escalating roles of nitrogen (N) and phosphorus (P) in human activities, including food production, energy supply, metal smelting, animal husbandry, and urban construction, has led to a significant increase in the influx of nutrients into aquatic environments since the 20th century [1,2]. From 2018, the influx of N and P from human daily life into water bodies has been estimated to be 1.48 × 10⁵ kg of N and 1.1 × 10³ kg of P annually worldwide [3,4]. In Dianchi Lake, which is highly impacted by human activities, the concentrations of N and P levels once reached a maximum of 16.79 mg/L and 1.46 mg/L, respectively, during the most severe eutrophication period, far exceeding the thresholds specified in the “Technical regulations for surface water quality assessment” (SL395-2007) [5]. The rising nutrient load has intensified eutrophication and caused severe damage to aquatic ecosystems [6]. For instance, Xuanwu Lake in Nanjing experienced severe eutrophication, leading to excessive algal proliferation, which significantly diminished the dissolved oxygen (DO) levels in the water, resulting in the mass mortality of fish and other aquatic organisms [7]. Furthermore, the excessive proliferation of algae can produce harmful substances, such as microcystins and cylindrospermopsin, which exert toxic effects on both aquatic organisms and human health [8,9].

Wetland systems provide nature-based solutions for the sustainable regulation of N and P in water. As shown in Figure 1, compared with other natural water bodies, wetland sediment has a smaller specific surface area and more functional groups, which promotes N and P adsorption. Moreover, abundant organic matter supports the microbial communities promoting sustainable nutrient cycling, including nitrifiers, denitrifiers, and P-accumulating organisms. In addition, the rich diversity of wetland plants contributes to greater N and P removal [10,11,12]. Sun et al. [13] found that the Panhe constructed wetland (CW), primarily planted with Acorus calamus, was capable of removing up to 49% of N in the autumn; Nandakumar et al. [14] reported that the CW planted with Brachiaria mutica achieved a maximum P removal efficiency of 85.6% over the course of a year; and Zhao et al. [15] demonstrated that the CW, which included nine wetland plant species, achieved an annual average N and P removal rate of 45.1% and 43.3%, respectively. Notably, when wetland systems remove N and P from water bodies, wetland plants always play a central role, driving the sustainable removal of N and P [16]. The contribution of wetland plants to N and P removal accounts for 15–80% and 24–80%, respectively, of the total removal in wetland purification systems [17]. Wetland plants directly reduce the N and P concentrations in water by absorbing N and P, promoting the settlement of suspended matter and mitigating the release of N and P from sediments through their robust root systems [18]. In addition, wetland plants provide attachment sites and nutrients for microbes, while their roots secrete oxygen to improve redox conditions, thereby enhancing microbial processes such as nitrification and denitrification, which further alleviate the eutrophication of water bodies [19,20]. The efficacy of wetland plants in mitigating water eutrophication by reducing N and P is well established. Wang et al. [21] documented the sustainable performance of Myriophyllum elatinoides verticillatum, with water trophic index declines of 4.01–10.85, after being used for eutrophic water remediation. Algal growth is also a key eutrophication indicator. Xiang et al. [22] found that wetland plant-based remediation cut off the algal nutrient supply by reducing nitrogen and phosphorus, achieving an algae inhibition rate of over 90% and effectively controlling eutrophication.

Remarkably, nutrient absorption by wetland plants is always the primary process in the removal of N and P from water, contributing to up to 50% of the total reduction [23,24]. Plants absorb and utilize N and P to synthesize nucleic acids to meet various growth requirements, including the synthesis of nucleic acids for cell division, the production of amino acids and phospholipids for metabolism and cell structure, and the formation of chloroplasts and proteases, which are essential for photosynthesis, energy conversion, and biosynthesis [25]. Previous studies have shown the close relationship between the nutrient content of plants and their removal performance. For instant, Lan et al. [26] found that Hibiscus rosa-sinensis had both a higher nutrient content and removal efficiency for N and P compared to Duranta repens ‘Variegata’ and Nerium oleander. Notably, the accumulation content of N and P inside the plants exhibited seasonal variation; for example, Wang et al. [27] also found that Canna glauca obtained higher N and P contents in summer compared to winter, while the corresponding removal rate is 7–8 times higher in summer than in winter. Thus, assessing the N and P contents in wetland plants provides valuable insights into their capacity to absorb and accumulate these nutrients from water. In other words, the levels of these nutrients in wetland plants exhibit clear and distinct seasonal variations, driven by fluctuations in plant nutrient demands and absorption patterns, thereby critically influencing the efficiency of N and P removal from water. Ye et al. [28] also found that the P content in M. elatinoides in summer was 1.74 times that in winter, and the P removal rate from water was 22.34% more efficient in summer. However, there is limited research recognizing the seasonal N and P content variations in different wetland plants, which hinders a comprehensive assessment of their role in nutrient removal.

Perennial plants are prioritized for sustainable water management due to their inherent nutrient recycling through seasonal regeneration. Due to the focus on seasonal variations in N and P contents in perennial wetland plants, researchers have further discovered the occurrence of the seasonal transfer of these nutrients. Nutrient transfer is a hallmark of perennial plants, characterized by the reallocation of N and P from senescent tissues to storage organs, which are later mobilized to regenerate new growth in the spring. This nutrient allocation strategy ensures winter survival and promotes the long-term growth of the plant [29]. Previous studies have found that over 50% of the N in Rumex acetosa’s withering leaves is transferred to the storage roots and stems, with more than 70% of these reserves subsequently fueling the development of new branches and leaves in the following year [30]. Active nutrient translocation from roots to shoots has also been confirmed in numerous wetland plant species. Garcia et al. [31] found that the nutrient content in roots was significantly higher during dormancy; upon the end of winter dormancy, Pontederia cordata and Juncus effusus transferred these nutrients from their roots to new shoots, with the levels in the shoots exceeding those in the roots by more than twofold. Similarly, Guo et al. [32] discovered that as the wilting season drew near, the contents of N and P in Typha orientalis exhibited a downward trend; nevertheless, due to the transfer of N and P from the shoots to the roots, the reduction in the contents of N and P in the roots was notably less pronounced compared to that in the shoots. Although the phenomenon of seasonal nutrient translocation in perennial plants has been widely documented and studied, a comprehensive understanding of its underlying mechanisms remains elusive, including the forms of translocated elements, the physiological processes involved, and the factors influencing translocation.

Additionally, the effective management of wetland plants is crucial for optimizing their capacity to remove N and P from water. Tang et al. [33] found that up to 90% of the N and P in the litter of Phragmites australis and T. orientalis was re-released into the water without proper harvest management. Ye et al. [34] also demonstrated that 6.19 t of N and 1.12 t of P would be released after wetland plants had withered for 21 days, potentially leading to secondary pollution. Thus, sustainable harvesting protocols aligned with plant phenology are critical for maximizing N/P sequestration [35]. Specifically, it is necessary to harvest the above-ground parts before N and P are transferred to the roots [36]. For instance, Wang et al. [36] found that harvesting P. australis prior to the onset of N and P transfer to the roots significantly enhances the removal of these nutrients from the water, with N removal increasing by 40.62–88.74% and P removal by 71.07–79.20%, compared to harvesting after the transfer has occurred. In addition, the proper configuration of wetland plants is crucial for ensuring their efficient and sustainable purification efficiency. Chang et al. [37] indicated that the proper configuration of wetland plants exhibits superior N and P removal efficiencies compared to single plants, with improvements spanning from 10.10 to 17.01% for N and from 6.36 to 12.02% for P.

Despite the recognized role of wetland plants in nutrient removal, the seasonal dynamics of N and P in different plant types, especially the relationship between the growth patterns of different plants and their seasonal variations in N and P contents, as well as the specific processes and mechanisms of N and P seasonal transfer, remain poorly understood. This impedes the development of effective management strategies, restricting the efficient and stable control of eutrophic water bodies via wetland plants. In this study, we aim to address this gap by categorizing wetland plants based on their growth patterns, analyzing the seasonal variations in N and P contents and transfer mechanisms, and proposing more scientific strategies for the harvesting and pairing of wetland plants, providing a foundation for the smart and sustainable remediation of eutrophic water bodies.

2. Research Methodology

2.1. Data Collection and Research Status

We retrieved the relevant literature from 2000 to 2024 through the “Web of science core collection” and “Chinese science citation database^SM” on the topic of “Uptake and removal efficiency of nitrogen and phosphorus in wetland plants” and “Seasonal variation in nitrogen and phosphorus content of wetland plants”, while the search string was specifically on “wetland plants” or “aquatic plants” or “artificial wetland” and “nitrogen” or “phosphorus” and “seasonal change” or “seasonal dynamics” or “content change” or “reserve change” or “removal efficiency” or “growth pattern” or “biomass”. In total, 845 papers were collected, but most studies focused on the short-term removal efficiency of N and P by different wetland plants, while only 114 papers were about the effects of seasonal variation on phytoremediation potential and content inside wetland plants. There is a lack of a comprehensive summary on the influential factors and regulatory rules on the seasonal dynamics of wetlands plants that gives enough information for guidance on plant selection in treating eutrophic water in different seasons with higher cost-effectiveness.

Based on the above limitation, we tried to search for studies on the “Seasonal dynamics of nitrogen and phosphorus transfer in perennial wetland plants”, with a special research string of “perennial plants” or “perennial wetland plants” or “perennial aquatic plants” or “perennial” and “nitrogen” or “phosphorus” and “seasonal migration” or “seasonal cycle” or “seasonal dynamics” or “seasonal transfer” or “migration” or “cycle” or “transfer”, retrieving a total of 179 papers. Notably, the existing studies on the seasonal transfer characteristics and mechanisms of N and P in perennial plants predominantly focus on terrestrial species instead of perennial wetland plants. Limited research has specifically confirmed the seasonal dynamics of N and P in above- and below-ground parts, which hinders the development of harvesting management strategies based on N and P transfer in wetland plants.

Furthermore, we explored the research related to “Strategic wetland plant management for efficient nutrient removal”, with a string of “wetland plants” or “aquatic plants” or “constructed wetlands” and “nitrogen” or “phosphorus” and “mixed planting” or “plant configuration” or “combined planting” or “harvesting” or “cutting”, yielding 240 relevant papers. Most of the studies published primarily emphasize the benefits of wetland plants, comparing the difference between harvesting and non-harvesting; there is a lack of optimal part-harvesting strategies based on wetland plant growth patterns and seasonal N-P dynamics for different types of wetland plants under diverse environmental conditions.

2.2. Data Processing and Research Contributions

After conducting a comprehensive analysis of the relevant studies on “Uptake and removal efficiency of nitrogen and phosphorus in wetland plants” and “Seasonal variation in nitrogen and phosphorus content of wetland plants”, we found that growth patterns impact the seasonal biomass fluctuations and nutrient transfer. Then, we categorized commonly used wetland plants into three distinct groups: spring–summer growth plants (SSPs), spring–summer–autumn growth plants (SSAPs), and all-year-round growth plants (APs) and summarized the seasonal variation patterns of their respective nutrient contents and the reasons for the differences. Additionally, the mechanisms for the disparities in the N and P content variation patterns in SSPs, SSAPs, and APs were further investigated. Firstly, we summarized the general rules of N and P transfer in perennial terrestrial plants, which provided abundant information for the hypothesis of similar transfer manners in perennial wetland plants. Then, discussions on the transfers and storage forms of N and P in perennial wetland plants, the specific transfer pathways, key transfer enzymes, and environmental influencing factors were detailed for uncovering the regulatory rules. After that, the optimal harvesting seasons and frequencies for SSPs, SSAPs, and APs, as well as the best wetland plant configuration strategies under different environmental conditions, were proposed based on the above-mentioned studies on the growth patterns and seasonal N-P dynamics of the three types of wetland plants.

3. Results and Discussion

3.1. Uptake and Removal Efficiency of Nitrogen and Phosphorus in Wetland Plants

3.1.1. Seasonal Growth Rhythms and Nutrient Requirements

Previous studies have found that the different growth patterns of wetland plants result in varying seasonal demands for N and P [38]. Based on the differences in the growth patterns of wetland plants in previous studies, this paper classifies them into three types: SSPs, SSAPs, and APs. SSPs include Sparganium stoloniferum, Halerpestes sarmentosa, Scirpus mariqueter, Hydrocotyle vulgaris, T. orientalis, Sesuvium portulacastrum, P. cordata, Zizania latifolia, Eichhornia crassipes, Lemna minor, Pistia stratiotes, and J. effusus et al. SSAPs include P. australis, Spartina alterniflora, C. glauca, Scirpus validus, Thalia dealbata, Trapa bispinosa, Hydrilla verticillata, Lythrum salicaria, Pennisetum purpureum Schum, Alternanthera philoxeroides, Vallisneria natans, and M. elatinoides et al. APs include Iris tectorum, Acorus calamus, Cyperus alternifolius, Elodea nuttallii, and Vetiveria zizanioides et al.

The growth period of SSPs extends from spring to summer or early autumn, with the highest biomass accumulation peaking at the end of spring or the beginning of summer, while they quickly wither after autumn approaches. Specifically, typical SSP species, such as P. cordata, S. mariqueter, E. crassipes, and T. orientalis, can achieve a maximum biomass of 1541.77 ± 102.37 g/m², 482.52 ± 58.32 g/m², 1514.00 ± 124.57 g/m², and 2633.91 ± 264.21 g/m², respectively, in spring and summer, while their maximum biomass in autumn and winter drops to 1119.39 ± 69.57 g/m², 153.49 ± 8.15 g/m², 1198.00 ± 97.83 g/m², and 2049.74 ± 184.65 g/m², respectively (

Table 1. Seasonal variation in biomass of wetland plants.

Wetland Plants	Biomass (g/m2)				Reference
	Spring	Summer	Autumn	Winter
Pontederia cordata	893.53 ± 56.32	1541.77 ± 102.37	1119.39 ± 69.57	/	[39]
Scirpus mariqueter	44.30 ± 6.17	482.54 ± 58.32	153.49 ± 8.51	/	[40]
Eichhornia crassipes	451.36 ± 28.54	1514.00 ± 124.57	1198.00 ± 97.83	/	[39]
Typha orientalis	584.54 ± 63.26	2633.91 ± 264.21	2049.74 ± 187.65	/	[39]
Scirpus validus	953.19 ± 89.47	1514.89 ± 88.63	1140.42 ± 74.35	748.93 ± 54.34	[42]
Phragmites australis	891.85 ± 54.36	3726.71 ± 321.75	3329.10 ± 302.17	2122.34 ± 176.62	[40]
Myriophyllum elatinoides	829.91 ± 43.77	871.79 ± 56.84	940.17 ± 68.55	649.57 ± 63.54	[43]
Spartina alterniflora	2402.38 ± 366.52	2784.90 ± 261.57	2991.87 ± 231.98	741.28 ± 54.28	[40]
Thalia dealbata	1572.65 ± 124.25	2358.97 ± 203.42	2615.38 ± 164.33	2034.19 ± 189.67	[43]
Iris tectorum	1042.73 ± 89.33	1367.52 ± 96.57	1692.30 ± 121.94	1196.58 ± 107.53	[42]
Acorus calamus	871.79 ± 67.54	1709.40 ± 102.73	1931.62 ± 153.67	1384.61 ± 157.35	[42]

). Correspondingly, their requirements of N and P are significantly higher in spring and summer. Similarly, the nutrient storage of H. vulgaris, J. effusus, P. cordata, S. stoloniferum, and S. mariqueter in spring and summer ranges from 2.55 ± 0.17 to 40.13 ± 3.25 g/m² and 0.5 ± 0.03 to 5.89 ± 0.65 g/m², significantly higher than in autumn and winter, which is 1.83 ± 0.07–16.64 ± 1.42 g/m² for N and 0.40 ± 0.02–3.94 ± 0.41 g/m² for P [39,40,41].

According to the statistics, we found that the growth period of SSAPs extends from spring to autumn, with a significantly higher biomass during the growing season (829.91 ± 43.77–3726.71 ± 321.75 g/m²) compared to winter (649.57 ± 63.54–2122.34 ± 176.62 g/m²) (Table 1). Similarly, N and P storage during the growing season can reach 5.02 ± 0.48–75.23 ± 5.36 g/m² and 0.92 ± 0.04–13.30 ± 2.17 g/m², respectively, while in winter, they are reduced to 7.65 ± 0.24–25.87 ± 3.56 g/m² (N) and 1.68 ± 0.01–2.39 ± 0.01 g/m² (P) [40,41,42,43]. Notably, the rapid growth period of SSAP biomass varies. P. australis, S. validus, and T. bispinosa exhibit rapid biomass growth in summer, reaching 1514.89 ± 88.63–3726.71 ± 321.75 g/m², compared to 748.93 ± 54.34–3329.10 ± 302.17 g/m² in other seasons (Table 1). Their N and P storage in summer is significantly higher, at 15.54 ± 1.42–75.23 ± 6.38 g/m² and 1.48 ± 0.01–13.30 ± 1.27 g/m², respectively, versus 5.02 ± 0.61–37.61 ± 2.15 g/m² and 0.92 ± 0.07–2.51 ± 0.14 g/m² in other seasons [40,41,42]. Some SSAPs, such as M. elatinoides, S. alterniflora, and T. dealbata, enter their rapid growth period earlier, with their biomass reaching 56.36–80.69% of its maximum in spring. Their N and P demands in spring approach the maximum, at 8.19 ± 0.76–25.07 ± 3.51 g/m² and 1.53 ± 0.01–2.39 ± 0.3 g/m², compared to peaks of 9.87 ± 0.07–27.77 ± 1.56 g/m² and 2.31 ± 0.01–2.79 ± 0.04 g/m² [40,43]. During autumn, SSAPs enter a reproductive phase, with slower biomass growth but sustained high N and P demands to support rRNA and ATP synthesis [44]. For instance, S. validus, P. australis, S. alterniflora, and T. dealbata exhibit maximum N and P storage levels of 15.55 ± 2.07–35.31 ± 1.42 g/m² and 1.48 ± 0.02–2.79 ± 0.03 g/m², while autumn levels remain at 9.38 ± 0.07–30.17 ± 4.56 g/m² and 1.19 ± 0.01–2.79 ± 0.03 g/m² [40,42,43].

APs grow throughout the year and exhibit high cold tolerance, so they do not significantly wither during winter. This type of plant is relatively rare. According to the statistics, we found that the typical AP species, such as I. tectorum and A. calamus, can maintain a biomass of 1196.58 ± 107.53 g/m² and 1384.61 ± 157.35 g/m² in winter, respectively, while in other seasons, their biomass ranges from 1042.73 ± 89.33 to 1692.31 ± 121.94 g/m² and 871.79 ± 67.54 to 1931.63 ± 153.67 g/m² (Table 1). Accordingly, their demand for N and P in winter does not show a significant decrease compared to other seasons. In winter, the N and P storage of I. tectorum and A. calamus can reach 20.25 ± 3.12–25.70 ± 2.57 g/m² and 1.97 ± 0.02–2.51 ± 0.03 g/m², respectively, while in other seasons, it ranges from 8.20 ± 0.77 to 35.74 ± 2.56 g/m² and 0.90 ± 0.04 to 3.28 ± 0.27 g/m² [42,43].

3.1.2. Removal Efficiency of Nitrogen and Phosphorus

Wetland plants, renowned for their robust root systems and exceptional nutrient absorption capacity, exhibit remarkable efficiency in removing N and P. For example, species such as P. australis, T. orientalis, C. glauca, and T. dealbata achieve average annual removal rates of 38.70–97.77% for N and 24.70–80.00% for P [43,45,46]. However, the N and P removal efficiency of wetland plants varies significantly across seasons, with rapid plant growth during the growing season boosting nutrient assimilation and rhizosphere microbiome activity, leading to substantially higher N and P removal rates compared to the withering season [36,43]. Notably, the seasonal difference in P removal was less pronounced than that of N, as the physicochemical adsorption of the substrate played a key role in P removal [23].

We found that the seasonal growth rhythms and nutrient requirements of wetland plants are key factors driving their ability to remove N and P from water bodies across different seasons. Based on comprehensive analysis (

Table 2. Variable N and P removal capabilities of wetlands plants in different seasons.

Wetland Plants	Total Nitrogen Removal Rate %				Total Phosphorus Removal Rate %				References
	Spring	Summer	Autumn	Winter	Spring	Summer	Autumn	Winter
P. australis	40.90 ± 3.27–90.24 ± 8.51	37.59 ± 2.73–94.26 ± 5.45	38.70 ± 4.51–68.74 ± 6.39	1.37 ± 0.01–58.39 ± 3.57	24.70 ± 3.36–88.89 ± 11.22	37.40 ± 4.07–92.22 ± 8.36	30.90 ± 2.55–84.34 ± 6.73	23.27 ± 2.31–85.83 ± 7.54	[45,46,50]
Canna glauca	52.47 ± 7.54–92.39 ± 11.34	54.74 ± 4.26–90.27 ± 7.63	62.73 ± 5.47–67.28 ± 4.32	38.85 ± 4.84–62.26 ± 8.69	63.12 ± 7.33–83.36 ± 11.27	68.17 ± 5.64–90.96 ± 12.36	75.14 ± 6.18–82.61 ± 9.66	29.76 ± 1.57–84.36 ± 6.48	[43,46,48,51,52]
T. orientalis	88.28 ± 9.17	97.77 ± 11.32	40.72 ± 3.52	31.12 ± 5.46	52.99 ± 3.21	80.00 ± 12.33	72.17 ± 9.67	81.99 ± 6.42	[46]
Vetiveria zizanioide	84.96 ± 7.51	94.54 ± 8.34	45.39 ± 3.25	41.84 ± 3.36	71.79 ± 8.84	94.44 ± 10.27	91.30 ± 7.33	90.05 ± 12.69	[46]
Sesuvium portulacastrum	72.26 ± 5.63	90.75 ± 8.47	/	46.57 ± 1.58	46.57 ± 3.75	57.87 ± 4.26	/	21.92 ± 1.38	[17]
E. crassipes	/	81.92 ± 9.33	52.31 ± 4.12	15.95 ± 1.25	/	87.88 ± 7.88	50.76 ± 6.57	43.07 ± 4.14	[47]
T. dealbata	51.30 ± 11.24	72.37 ± 9.36–72.81 ± 11.33	57.82 ± 4.58	42.78 ± 3.37–53.02 ± 4.27	51.84 ± 8.61	58.06 ± 5.43–60.55 ± 8.96	71.85 ± 12.36	47.83 ± 6.17–80.51 ± 9.67	[43,48]
A. calamus	58.99 ± 8.23–59.66 ± 4.47	73.43 ± 6.36–75.80 ± 6.15	47.72 ± 3.28–71.66 ± 9.67	41.93 ± 3.56–59.58 ± 6.14	60.09 ± 7.33–67.61 ± 6.57	73.00 ± 11.24–82.15 ± 14.58	67.14 ± 6.51–79.52 ± 9.42	42.50 ± 3.36–57.27 ± 6.17	[43,53]
Cyperus alternifolius	44.15 ± 3.28–58.14 ± 3.66	56.98 ± 4.17–63.83 ± 5.62	52.35 ± 4.33	38.48 ± 2.87–49.07 ± 5.57	39.70 ± 3.16–48.58 ± 4.50	42.62 ± 3.92–60.85 ± 7.49	53.56 ± 4.30	31.38 ± 2.56–46.12 ± 7.33	[26,43,52]
S. validus	47.92 ± 3.27	69.32 ± 8.65	50.71 ± 5.24	38.64 ± 4.33	57.92 ± 6.92	60.76 ± 7.36	66.73 ± 5.48	55.77 ± 6.33	[43]
M. elatinoides	44.85 ± 3.16	60.11 ± 7.54	57.89 ± 9.42	31.45 ± 5.17	67.46 ± 3.31	83.76 ± 7.52	73.49 ± 4.63	60.87 ± 6.78	[43]
Alternanthera philoxeroides	/	69.35 ± 5.24	63.50 ± 6.71	20.71 ± 3.28	/	66.67 ± 7.78	61.53 ± 5.33	53.84 ± 4.52	[47]
I. tectorum	54.79 ± 5.27–59.47 ± 4.36	67.41 ± 6.98–84.43 ± 9.33	45.92 ± 3.15–75.66 ± 6.48	33.86 ± 4.23–64.14 ± 7.15	52.71 ± 5.12–86.41 ± 9.66	66.24 ± 4.27–94.76 ± 13.78	75.69 ± 6.36–85.15 ± 9.12	47.80 ± 3.57–77.97 ± 2.18	[43,47,48,49]
Lythrum salicaria	47.51 ± 3.36	66.46 ± 5.42	59.03 ± 6.87	49.77 ± 4.33	78.40 ± 8.94	87.32 ± 9.66	77.46 ± 5.26	68.07 ± 7.38	[53]
Elodea nuttallii	/	58.36 ± 7.58	80.77 ± 9.34	41.41 ± 54.36	/	59.09 ± 11.23	72.31 ± 8.69	58.46 ± 6.57	[47]
Carex	38.40 ± 2.37	40.20 ± 3.87	36.90 ± 2.56	47.00 ± 5.45	13.00 ± 2.36	39.00 ± 4.28	30.70 ± 3.77	18.40 ± 1.05	[54]
Zizania latifolia	/	20.41 ± 1.56–61.97 ± 5.74	/	25.56 ± 3.18–34.82 ± 1.98	/	28.61 ± 2.74–72.29 ± 9.63	/	45.02 ± 6.33–49.93 ± 2.79	[48,52]
P. cordata	/	50.10 ± 4.32	/	18.32 ± 3.74	/	30.69 ± 1.25	/	26.28 ± 4.62	[48]

), it was found that during spring and summer, SSPs such as T. orientalis, S. portulacastru, and E. crassipes enter their growth and maturity phases, characterized by a high biomass and elevated N and P demands, achieving removal rates exceeding 80%. Moreover, certain SSAPs, such as P. australis, C. glauca, and V. zizanioides, achieve high removal rates during summer, driven by substantial biomass increases [17,46,47]. Most SSAPs, including C. alternifolius, T. dealbata, and A. philoxeroides, sustain robust growth into autumn, with their biomass gradually peaking during this season [26,43,47,48]. Based on comprehensive analysis (Table 2), it was found that N and P removal by these plants declined in the autumn compared to the summer, but the reduction was generally of less than 15% and significantly smaller than that observed in SSPs (Table 2). Most APs, such as I. tectorum, E. nuttallii, and A. calamus, exhibit strong cold resistance, allowing them to sustain biomass growth during winter. This adaptability results in relatively high N and P removal rates, even in colder seasons [47,49]. Notably, certain SSPs and SSAPs, such as P. australis, T. orientalis, and V. zizanioide, maintain active root systems, even as their above-ground parts wither in autumn and winter. These active roots continue to support microorganism-mediated nutrient removal and sediment nutrient retention [36,43,46]. As a result, we found that these plants achieve N removal rates of 60–80% and P removal rates exceeding 70% during these colder seasons (Table 2).

3.2. Seasonal Variations in Nitrogen and Phosphorus Contents of Wetland Plants

3.2.1. Nitrogen and Phosphorus Accumulation Inside the Whole Plant

As shown in Figure 2, the N and P contents of wetland plants exhibit notable seasonal variations, varying across plant growth types [32,36,39,40,43,55,56,57,58,59,60,61,62,63,64,65]. We found that SSP-type plants such as T. orientalis, P. cordata, and Z. latifolia exhibit significantly higher N and P contents during spring and summer, with the N content ranging from 14.85 ± 2.17 to 28.56 ± 3.36 mg/g and P content from 2.43 ± 0.12 to 5.50 ± 0.76 mg/g, which are 1.11–2.18 times and 1.26–3.01 times greater than those observed in autumn and winter, respectively. Additionally, we found that the nutrient contents of wetland plants of the SSAP type are consistently higher in spring, summer, and autumn than in winter (Figure 2). Notably, as described by the previously outlined growth characteristics, SSAPs such as P. australis, S. validus, T. bispinosa, and H. verticillata, which experience relatively rapid growth during the summer, exhibit a “dilution effect” in their nutrient contents, leading to a slight decrease during this period [36,43]. As shown in Figure 2, their corresponding N contents in summer range from 6.70 ± 0.63 to 20.80 ± 3.52 mg/g, while in spring and autumn, it ranges from 9.77 ± 1.21 to 26.50 ± 2.33 mg/g and 7.60 ± 1.37 to 37.20 ± 3.54 mg/g, respectively. Similarly, their P contents in summer fluctuate between 0.40 ± 54.36 and 5.20 ± 54.36 mg/g, and range from 0.50 ± 0.04 to 9.10 ± 0.71 mg/g in spring and 0.60 ± 0.03 to 17.80 ± 2.17 mg/g in autumn. Moreover, we discovered that for the APs (C. alternifolius, I. tectorum, A. calamus, and V. zizanioides), the N and P contents remain consistent throughout the year, ranging from 11.11 ± 1.46 to 23.32 ± 3.52 mg/g and 0.94 ± 0.07 to 3.32 ± 0.02 mg/g, respectively (Figure 2).

Notably, we found that distinct variations in the N and P concentrations among the three plant types emerge in autumn and winter, attributable to differences in growth periods and cold resistance. As shown in Figure 2, in autumn, both SSAPs and APs exhibit significantly higher N and P contents compared to SSPs. In winter, APs have significantly higher N and P contents compared to SSAPs and SSPs. Research has shown that low-temperature exposure damages plant membranes, increasing permeability and nutrient depletion (N and P) [66]. This disrupts photosynthesis, triggers lipid peroxidation, and leads to reactive oxygen species accumulation, impairing plant physiology [67]. SSPs, such as L. minor and E. crassipes, lack cold stress adaptation capabilities, such as the synthesis of antifreeze proteins, membrane regulation, and accumulation of osmoprotectants. Consequently, they perish in the autumn due to their inability to withstand freezing temperatures. These species choose to exhibit rapid growth and reproduction during spring and summer, producing seeds that can endure cold temperatures through dormancy, ensuring species continuity. Post autumn, SSPs quickly die off, resulting in a sharp decline in N and P levels [68]. In contrast, SSAPs and APs have evolved better cold resistance mechanisms and regulatory processes, enabling them to maintain normal N and P levels. They have smaller specific leaf areas, thicker cuticles, and more developed root systems, which help in minimizing water and heat loss and enhancing nutrient storage, thereby sustaining their activity during winter [69]. Additionally, these plants activate antifreeze proteins to lower the freezing point of cell sap, increase the proportion of unsaturated fatty acids to maintain membrane stability, and accumulate osmoprotectants like mannitol and proline to regulate osmotic pressure and prevent cellular dehydration, thus preserving cellular functions and minimizing N and P loss [67]. Notably, we found that APs appear to possess stronger cold adaptability compared to SSAPs. For instance, when temperatures drop below 5 °C, the root vitality and membrane stability of the typical AP, I. tectorum, remain four times higher than those of the SSAP, T. dealbata, allowing I. tectorum to continue growing, while T. dealbata shows significant wilting [70]. This explains why APs can survive colder winters, whereas SSAPs begin to wilt under the same conditions.

The seasonal variations in the N and P contents of wetland plants are primarily driven by fluctuations in the N and P requirements at different growth stages, as well as the influence of seasonal environmental conditions on nutrient uptake and metabolic processes [43,57,58]. During periods of vigorous growth, plants exhibit a significantly higher demand for N and P. They efficiently absorb and utilize N and P to synthesize nucleic acids for rapid cell division, produce amino acids and phospholipids supporting metabolism, and generate the chloroplasts and proteases essential for photosynthesis, energy conversion, and biosynthesis. Concurrently, ATP production peaks, fueling cellular processes. As a result, the N and P contents in plants are significantly higher during the growing season, reflecting their heightened demand for these essential elements [25]. After entering the senescence period, the requirements of N and P for these activities weakens and diminishes, leading to a noticeable decrease in the N and P contents.

We hold the view that changes in the seasonal environmental conditions also affect the absorption and utilization of N and P by wetland plants, thus influencing their N and P contents. Previous research has shown that stronger light during spring and summer accelerates the two main rate-limiting steps of photosynthesis, thereby expediting carbon skeleton synthesis and promoting subsequent metabolic processes and photophosphorylation, which in turn enhance the N and P uptake of plants [71,72]. Meanwhile, favorable environmental conditions also increase root exudates in plants and enhance the activity of key enzymes and N and P transporters, such as nitrogenase, phosphatase, and nitrate reductase [73,74,75]. Moreover, moderate temperatures and abundant precipitation during the growing season significantly boost the abundance and activity of N- and P-associated microorganisms, such as arbuscular mycorrhizal fungi [76,77,78,79,80]. We hold that these factors increase nutrient availability and facilitate SSPs and SSAPs’ uptake of N and P in spring and summer. Remarkably, under extreme cases of high temperatures and drought in summer, microorganisms tend to be more efficient at acquiring N and P, intensifying competition for nutrients with plants. This could potentially explain the observed decrease in the N and P contents in SSAPs during the summer [81,82,83,84,85]. Conversely, during autumn and winter, the N and P levels significantly decline due to low temperatures and light. These conditions reduce plant metabolic activity, causing a drop in the enzyme activities of phosphatase and nitrate reductase, along with a decrease in chlorophyll and photosynthetic efficiency, which suppresses N and P assimilation and utilization [86,87]. Additionally, cold environments lower the rhizosphere microbial activity, reducing organic matter decomposition and soil N and P availability, thus affecting plant nutrient uptake [88]. Notably, as mentioned earlier, APs have an excellent strategy for coping with cold conditions. We are of the opinion that this allows APs to maintain stable cell structures, ensuring that enzymes like phosphatase and urease remain highly active, allowing for efficient N and P assimilation. Additionally, APs continue to produce root exudates during winter, providing energy for microorganisms. Their more developed root system also helps microorganisms to reside deeper in the soil, maintaining soil warmth and microbial activity, thus maintaining the supply of N and P. These functions allow APs to maintain higher levels of N and P even during winter [57,89,90].

3.2.2. Nitrogen and Phosphorus Contents in Shoots and Roots

Wetland plants have developed specialized structures and processes for efficient nutrient acquisition. The roots and underground rhizomes of wetland plants are responsible for nutrient absorption and storage, while the shoots regulate nutrient distribution within the plant through processes such as photosynthesis, gas exchange, transpiration, and the transport of nutrients [57]. We found that in all three categories of plants, during their respective growing seasons, plants transport absorbed N and P to shoots for growth and development, with shoots’ concentrations significantly higher than in roots. Conversely, during the dormant period, roots store these nutrients, resulting in roots’ concentrations equaling or exceeding those in shoots (Figure 3). For SSPs, because the growth season is in spring and summer, the N and P contents in the shoots are significantly higher in spring and summer compared to in the roots. Contrastingly, the shoots in SSAPs show notably higher N and P contents than their roots during spring, summer, and autumn. APs carry out photosynthesis and growth activities throughout the year, while nutrients are actively transported upwards to shoots after the primary adsorption of roots, resulting in a higher annual accumulation of nutrients in shoots than in roots. Notably, within the aboveground parts, the leaves generally contain higher levels of N and P than the stems during the growing season, as these nutrients are transported to the leaves during physiological processes such as photosynthesis, where they are synthesized into amino acids and ATPs [57]. For instance, it has been noted that the N and P contents in the leaves of P. australis and T. orientalis range from 12.35 ± 2.13 to 41.65 ± 3.88 mg/g and 0.98 ± 0.06 to 6.38 ± 0.81 mg/g, respectively, while the N content in the stems only ranges from 8.12 ± 1.54 to 11.97 ± 2.77 mg/g and 0.40 ± 0.02 to 5.54 ± 0.63 mg/g [62,91]. Notably, previous research has found that in SSPs and SSAPs, annual plants like E. crassipes and J. effusus exhibit higher shoot than root N and P contents during their growing period. Upon withering and death, their roots lose the ability to store N and P, maintaining consistently higher shoot N and P contents throughout their lifespan [31].

Additionally, the seasonal variations in N and P contents across different plant organs are inconsistent due to the varying growth patterns of different plants [36,39,43,62,91,92]. We found that for SSPs, shoot growth is more intense in spring and summer, resulting in higher N and P contents during these seasons compared to autumn and winter. C. esculenta, T. orientalis, S. mariqueter, and P. cordata have shoot N and P contents of 9.16 ± 54.36–40.62 ± 54.36 mg/g and 1.09 ± 54.36–6.96 ± 54.36 mg/g, respectively, during spring and summer, which decrease to 2.61 ± 54.36–18.12 ± 54.36 mg/g and 0.80 ± 54.36–3.75 ± 54.36 mg/g in autumn and winter (Figure 3). For SSAPs, shoots grow from spring to autumn, but the rapid increase in shoots’ biomass during the summer leads to a “dilution effect”, resulting in higher N and P contents in the shoots during the spring and autumn compared to the summer and winter. C. glauca, T. dealbata, P. purpureum Schum, S. validus, and P. australis have N and P contents in their shoots of 12.85 ± 54.36–54.52 ± 54.36 mg/g and 1.00 ± 54.36–10.15 ± 54.36 mg/g, respectively, during the spring and autumn, while those contents are 5.93 ± 54.36–19.18 ± 54.36 mg/g and 0.50 ± 54.36–3.39 ± 54.36 mg/g during the summer and winter, respectively (Figure 3). For APs, the growth rate of shoots exhibits no significant variation throughout the year, leading to relatively stable N and P contents in the shoots across seasons. I. tectorum, A. calamus, C. alternifolius, and S. alterniflora have N and P contents in their above-ground parts of 5.51 ± 0.45–24.25 ± 3.28 mg/g and 1.12 ± 0.02–1.93 ± 0.01 mg/g, respectively, during the winter, with only slight variations during other seasons, ranging from 3.38 ± 0.04 to 24.96 ± 2.98 mg/g and 0.95 ± 0.07 to 3.20 ± 0.42 mg/g (Figure 3).

Moreover, we discovered that the N and P contents in the roots of the three plant types follow a similar “single curve” pattern, with the lowest N content occurring during the rapid growth stage and the lowest P content during the reproductive stage. This is because during the rapid growth and reproductive phases, compared to other stages, plants require a large amount of N and P to be transported to the shoots for biomass growth and genetic activities, thereby reducing the N and P contents in the roots. For SSPs, spring and summer are its rapid growth and reproductive stages, respectively, so the N content is lowest in spring, and the P content is lowest in summer. For SSAPs and APs, both their rapid growth stages and reproductive stages are in summer and autumn, respectively; correspondingly, their lowest values of N and P contents both occur in summer and autumn (Figure 3).

3.3. Seasonal Dynamics of Nitrogen and Phosphorus Transfer in Perennial Wetland Plants

3.3.1. Nitrogen

Currently, perennial plants are commonly used for the remediation of eutrophic water bodies over annual plants, with considerations of higher cost-effectiveness and achieving long-term phytoremediation over the years, for example, like T. orientalis and P. cordata et al. (SSPs); P. australis, C. glauca, and T. dealbata et al. (SSAPs); and I. tectorum, A. calamus, and C. alternifolius et al. (APs). In perennial plants, the seasonal pattern of N allocation and reutilization plays a crucial role in enhancing nutrient use efficiency, thereby meeting the varying growth demands at different stages (Figure 4). Specifically, during autumn and winter, as the leaves undergo senescence, N compounds such as proteins and chlorophyll are broken down and transported via the phloem to the stems, rhizomes, and roots, where they are stored as vegetative storage proteins (VSPs), amino acids, and amides. In the following spring, these stored N compounds are mobilized and reutilized for the development of new tissues [93].

Previous studies have found that amino acids and nitrate are the two primary forms of N movement in plants [94]. Notably, the proportion of nitrate is significantly lower, ranging from 1/100 to 1/10 that of amino acids, making amino acids the predominant form of N transfer [95]. In perennial trees, N primarily undergoes seasonal transfer as glutamine (Gln) and asparagine (Asn). Enzymes such as serine protease, aspartic protease, cysteine protease, glutamine synthetase (GS), and asparagine synthetase (AS) play key roles in this process, with their expression levels significantly increasing in response to seasonal N transfer [96]. Additionally, in some perennial trees, such as Betula pendula, Juglans nigra × regia, and Malus domestica Bork, N undergoes seasonal transfer in the forms of arginine (Arg), citrulline (Cit), and aspartame (Asp) [97]. And in perennial herbaceous plants, N primarily undergoes a seasonal transfer, also in the forms of Gln, Asn, and some Arg [98,99]. Specifically, during plant seasonal transfer, various proteases and structural proteins are decomposed to produce amino acids [100]. Notably, over 80% of the N content in leaves is localized within the chloroplasts, specifically within RuBisCO and other enzymes and proteins integral to photosynthesis, serving as pivotal sources of amino acids [101]. These proteins are predominantly cleaved by serine proteases, aspartic proteases, and cysteine proteases to yield peptides, which are subsequently hydrolyzed into amino acids by aminopeptidases. Among the amino acids generated, glutamate and aspartate are the predominant products. These amino acids are then exported from the chloroplasts into the cytoplasm via specific transporters, where they are further metabolized by GS and AS into Gln and Asn, respectively, before being transferred [100]. Upon reaching the storage organs, Gln and Asn may be directly stored or converted into amino acids, or VSPs [96]. In spring, reconverted Gln and Asn, derived from amino acids and VSPs, are transported back to the newly developing tissues. Similarly to the autumn transfer process, the expression of asparaginase and cysteine proteases, as well as AS and GS, is significantly upregulated, playing a crucial role [102].

Moreover, we found that the transfer of amino acids from source to sink requires complex steps that include loading, transfer, and unloading through the phloem. The sieve element–companion cell complex (SE-CC) is an important molecular structure for organic matter from the source–phloem–sink. Specifically, amino acids are transported via phloem sieve tubes, which consist of interconnected sieve tube elements surrounded by companion cells forming the SE-CC. Amino acids are initially transferred into companion cells from various source cells, then loaded into sieve tubes. Upon reaching the target organs, they are unloaded into local companion cells, then transferred into the sink cells [103]. Notably, sieve tube elements lack mitochondria and the endoplasmic reticulum, relying on companion cells for energy and support in maintaining transport [104]. However, the specific mechanisms of amino acid loading and unloading in the SE-CC are not fully understood. Babst and Colemen proposed that the mechanism of amino acid loading and unloading in the phloem may be similar to that of carbohydrate loading and unloading, involving the apoplastic pathway, symplastic pathway, and polymer-trapping [105]. However, Couturier et al. [106] found that during plant senescence, the amino acid concentrations in the veins and petioles are higher than in the mesophyll, which is inconsistent with the symplastic pathway that relies on concentration gradients. Therefore, they speculated that the apoplastic pathway or polymer-trapping may play a dominant role.

Previous research has shown that amino acid transporters transfer amino acids across cell membranes, enabling their transport between different cells and tissues [107]. Notably, whether transporting amino acids from source cells to companion cells or loading and unloading in the SE-CC, both involve processes occurring across cell membranes. Therefore, we believe that amino acid transporters play a crucial role in the seasonal transfer of N. In perennial trees like Populus tremula, the cationic amino acid transporter family (CAT) within the amino acid polyamine choline transporter family (APC) includes proteins that transfer Gln, and their gene expression is notably upregulated when seasonal transfers occur in spring and autumn [106,108]. However, research on N seasonal transfer transporters in wetland plants is still limited. Studies on Arabidopsis have suggested that the amino acid permease family (AAP) may be responsible for the long-distance transfer of various amino acids in herbaceous plants, but whether the AAP plays a role in N seasonal transfer in wetland plants remains to be further explored [109].

Moreover, the seasonal transfer of N in plants is indirectly influenced by environmental factors such as light and temperature, primarily through their regulation of plant growth [110,111]. Short day length and low temperatures in autumn and winter halt plant growth and induce the expression of VSPs. Meanwhile, elevated levels of abscisic acid (ABA) drive the plant’s transition into dormancy, facilitating the transfer of N from the leaves to the roots. This process is essential for storing N to support future growth [108,112,113,114]. As spring arrives, rising temperatures and longer day lengths signal plants to emerge from dormancy. Photoreceptors like phytochromes detect the extended light period, leading to an increase in florigen and gibberellin-like plant growth hormones. These hormonal changes, in turn, promote the remobilization of N from storage organs to above-ground structures, including leaves, flowers, and fruits [111,115,116].

3.3.2. Phosphorus

In previous studies, the seasonal transfer of P in perennial plants was observed to be consistent with N [117]. During periods of slow growth in autumn, P is stored in organs as reserves and subsequently remobilized during the growing season to support new growth. In P. tremula, nearly 80% of P was reported to be transported to storage organs, with organic phosphorus (Po) identified as the primary storage form in stems, rhizomes, and roots [118]. Interestingly, the specific storage form of P may vary among plant species. Kurita et al. [119] identified inositol hexaphosphate as the predominant storage form of P in P. tremula during winter. In contrast, Chapin and Kedrowski [120] found that phospholipids, nonhydrolyzable ester phosphorus, and nucleic acid phosphorus were the main storage forms in Larix laricina, Picea mariana, and Betula papyrifera. However, research on the storage forms of P in perennial plants, especially in perennial wetland species, remains limited.

Additionally, we infer from a great deal of research that inorganic phosphorus (Pi) is the primary form of P transfer; P transfer begins with the breakdown of Po into Pi, which is then transported to target organs, where it is subsequently resynthesized into Po. For example, studies on P. tremula have shown that Pi levels in shoots decrease from summer to winter, then increase again in spring, while Po levels gradually rise from summer to winter and decline in spring [119]. Similarly, research on beeches also indicates that the phospholipid content in leaves decreases in autumn and winter, accompanied by an increase in the lysoPC (a phospholipases) content, while the stem phospholipid content increases, along with other phosphorus-containing organic compounds [121]. Furthermore, studies have also suggested that Po may play a role in P transfer, with isotope tracing techniques revealing the involvement of both the phloem and xylem [122]. In addition, the P transporter family (PHT), including AtPHO and OsPHO in Arabidopsis and rice, has been found to play a key role in P transfer [123,124]. However, research on the seasonal transfer of P in plants remains relatively limited. The specific roles of P transporters are not yet well understood, and further investigation is required to elucidate the mechanisms of P form conversion and transport, especially in wetland plants.

3.4. Strategic Wetland Plant Management for Efficient Nutrient Removal

3.4.1. Harvesting Strategies

Harvesting is a necessary measure to completely remove the N and P absorbed by wetland plants from the water body. Numerous studies are of the opinion that rational harvesting practices, when implemented effectively, can significantly enhance N and P removal from water by promoting plant regeneration and increasing biomass, thereby preventing secondary pollution. After plant harvesting, the activities of urease, phosphatase, and other N and P metabolic enzymes are enhanced. Additionally, it promotes plant tillering and enhances physiological activities such as photosynthesis and respiration, leading to faster growth rates and increased biomass, which in turn accelerates the absorption and utilization of N and P [125,126].

Currently, research on the harvesting of wetland plants primarily focuses on comparing the water purification efficiency of harvested and unharvested conditions. It has been well established that harvesting all types of wetland plants can enhance the removal of N and P from water bodies. As shown in

Table 3. Effect of harvesting on nitrogen and phosphorus removal in water bodies.

Plant Types	Species	Harvesting Method	Harvest Time	Enhancement Effects			Reference
				Nitrogen Removal	Phosphorus Removal	Control
Spring–summer growth plants (SSPs)	T. orientalis	Single harvest	October	22.67 ± 3.17 g/m2	5.22 ± 4.12 g/m2	non-harvesting	[128]
	E. crassipes		October	22.17 ± 2.14 g/m2	3.02 ± 0.42 g/m2	non-harvesting	[128]
	Nelumbo nucifera		October	12.88 ± 2.36 g/m2	2.69 ± 0.31 g/m2	non-harvesting	[128]
	Nymphaea tetragona		October	12.05 ± 1.78 g/m2	2.39 ± 0.25 g/m2	non-harvesting	[128]
	Potamogeton filiformis		July	39.40 ± 5.41 g/kg	2.30 ± 0.17 g/kg	non-harvesting	[127]
	Hydrilla verticillata		July	12.05 ± 2.31 g/kg	2.39 ± 0.28 g/kg	non-harvesting	[127]
	P. cordata		October	20.31 ± 2.33 g/m2	2.87 ± 0.33 g/m2	non-harvesting	[128]
Spring–summer–autumn growth plants (SSAPs)	P. australis		October	37.50 ± 4.57 g/m2	6.90 ± 1.21 g/m2	non-harvesting	[128]
	P. australis		October	48.00 ± 4.12 g/m2	4.20 ± 0.05 g/m2	non-harvesting	[129]
	Z. latifolia		October	33.88 ± 3.78 g/m2	6.97 ± 6.53 g/m2	non-harvesting	[128]
	C. papyrus		October	22.74 ± 4.77 g/m2	6.04 ± 1.22 g/m2	non-harvesting	[128]
	Ceratophyllum demersum		July	49.60 ± 5.96 g/kg	4.02 ± 0.05 g/kg	non-harvesting	[127]
	M. elatinoides		November	53.33 ± 6.87 g/m2	16.00 ± 2.55 g/m2	non-harvesting	[129]
All-year-round growth plants (APs)	C. alternifolius		July	50.56 ± 9.62 g/m2	10.48 ± 2.18 g/m2	non-harvesting	[126]
	A. calamus		January	38.50 ± 4.33 g/m2	5.00 ± 0.06 g/m2	non-harvesting	[129]
SSPs	Hydrocotyle vulgaris	Multiple harvests	May; September; November	1.78 ± 0.22 g/m2	0.26 ± 0.03 g/m2	single harvest in November	[41]
	H. vulgaris		September; November	590.00 ± 71.58 g	50.00 ± 8.36 g	single harvest in September	[130]
	Sparganium stoloniferum		May; September; December	2.08 ± 0.35 g/m2	0.46 ± 0.04 g/m2	single harvest in November	[41]
SSAPs	C. glauca		June; September; November	5.91 ± 0.76 g/m2	0.88 ± 0.11 g/m2	single harvest in November	[41]
	C. glauca		September; November	3550.00 ± 412.69.00 g	100.00 ± 18.25 g	single harvest in September	[130]
	M. elatinoides		September; November	710.00 ± 74.36 g	60.00 ± 4.33 g	single harvest in September	[130]
	M. elatinoides		April; June; September; November	5.67 ± 1.17 g/m2	0.88 ± 0.12 g/m2	single harvest in November	[41]
	Juncus effusus		April; June; September; November	2.14 ± 0.21 g/m2	0.45 ± 0.05 g/m2	single harvest in November	[41]
	J. effusus		September; November	370.00 ± 27.45 g	10.00 ± 2.56 g	single harvest in September	[130]

, we found that harvesting SSPs such as T. orientalis, E. crassipes, and P. cordata increases the annual average removal rates of N and P by 12.05 ± 1.78–22.67 ± 3.17 g/m² and 2.39 ± 0.25–5.22 ± 4.12 g/m², respectively. Harvesting SSAPs such as P. australis, M. elatinoides, and C. papyrus increases the annual average removal rates of N and P by 22.74 ± 4.77–53.33 ± 6.87 g/m² and 4.20 ± 0.05–16.00 ± 2.55 g/m², respectively. Harvesting APs also increases the annual average removal rates of N and P by 38.50 ± 4.33–50.56 ± 9.62 g/m² and 5.00 ± 0.06–10.48 ± 2.18 g/m², respectively. Furthermore, harvesting wetland plants to reduce the N and P in water can notably alleviate water eutrophication. For instance, Zhang et al. [127] restored a eutrophic water body with wetland plants and then harvested them, causing the trophic status index to drop from 67.32 to 27.92, directly validating their effectiveness in relieving eutrophication.

Notably, existing research has found that the seasonal dynamics of N and P vary among different plants, leading to significant differences in the removal efficiency based on harvesting timing. For example, Zhang et al. [130] found that harvesting H. vulgaris and J. effusus (SSP) at the end of summer could achieve N and P removal rates of 6.611 ± 0.43 kg and 0.885 ± 0.05 kg, respectively. However, if harvesting was performed in autumn, the removal of N and P was only 5.279 ± 0.04 kg and 0.215 ± 0.03 kg, respectively. He et al. [129] compared the N and P accumulation in P. australis and M. elatinoides (SSAPs) and indicated that the optimal harvest period was the end of autumn. However, limited knowledge of seasonal N and P dynamics in wetland plants hinders research on optimal harvest timing for various species, often resulting in suboptimal harvest schedules. As shown in Table 3, we found that while SSPs’ N and P levels peak in summer, harvest occurs in autumn, when levels have declined. SSAPs retain high nutrient levels into autumn, yet are harvested prematurely in summer.

Moreover, a multiple-harvest approach, involving an early harvest during the peak growth period followed by a final harvest at the end of the growing season, has been shown to be more effective in removing N and P from water bodies compared to a single harvest at the growing season’s end [41]. As shown in Table 3, multiple harvests of SSPs and SSAPs result in better N and P removal efficiency compared to a single harvest. Yu et al. [41] found that five types of plants, including C. glauca, had N and P removal rates that were 3.62–6.72 times higher through multiple harvests in June, September, and November compared to a single harvest in November. The annual N removal of P. australis in the Hetao area, harvested twice in July and September, was 86.4 ± 4.69 kg/hm², significantly higher than the single harvest of 60.39 ± 5.83 kg/hm² [131]. However, due to the current lack of understanding of the rapid growth periods of different wetland plant types, harvesting often occurs only once, at the end of the growing season.

In our view, the optimal time to harvest wetland plants is when they are fully mature, but before the withering stage, in order to avoid both insufficient nutrient accumulation and the transfer of N and P to the roots, where the nutrients may be released. Additionally, since multiple harvests have been shown to significantly enhance N and P removal efficiency, an initial partial above-ground harvest should be conducted during the vigorous growth period, followed by a comprehensive above-ground harvest at the end of the growth period. In conclusion, partial above-ground harvesting of SSPs can be performed in late spring and early summer (May–July), with full harvest around September. For SSAPs, partial harvesting should occur during summer (June–August), with complete harvesting in November. APs, which survive year-round, can be partially harvested in summer and autumn, when favorable rain and heat conditions support their biomass recovery. The strategies above can maximally reduce N and P concentrations in water, cut off nutrient sources for algae, effectively control their growth, and mitigate water eutrophication.

3.4.2. Plant Species Selection for Wetland Configuration

Numerous studies have shown that the configurations of different plant species, with varying nutrient absorption characteristics and requirements, significantly enhance N and P removal efficiency, while also boosting biodiversity within the wetland system and improving ecosystem stability [132]. Emergent plants, known for their superior nutrient removal capabilities, are commonly used in conjunction with other plant life-forms, to further enhance the stability of N and P purification [37]. The combination of “emergent plants and submerged plants” can significantly compensate for the reduced capacity of single emergent plants to absorb N and P when dealing with water bodies carrying high N and P loads. For instance, in high-N and high-P wastewater, the combination of P. australis and V. natans showed a notable enhancement in the removal efficiency of N and P, achieving respective increases of 10.79% and 1.72% compared to P. australis alone [133]. Additionally, the combination of emergent and submerged plants exhibits a greater ability to enhance DO in water compared to emergent plants alone and provides food sources and habitats for aquatic animals, thereby further enhancing the stability and diversity of aquatic ecosystems [134]. The combination of “floating plants and emergent plants” can also compensate for the deficiency in emergent plants’ ability to remove N and P when facing fluctuations in N and P loads. For example, the combination of C. glauca and P. stratiotes improves N and P removal efficiency by over 15% compared to C. glauca alone in wastewater with fluctuating nutrient concentrations [135]. Additionally, compared to single emergent plants, the combination of emergent and floating plants can more stably remove nutrients under complex hydrological conditions with significant water level fluctuations and variable retention times, making it more adaptable to changing environments [136].

Building on life-form combinations, we believe that the reasonable combination of SSPs, SSAPs, and APs can ensure continuous pollutant absorption throughout all seasons, thus preventing fluctuations in purification efficiency. As mentioned earlier, N and P contents in SSPs, SSAPs, and APs vary seasonally. Specifically, during spring and summer, certain SSPs like T. orientalis, P. cordata, and Z. latifolia, SSAPs such as L. salicaria, P. purpureum Schum, C. martini, P. australis, S. validus, and P. arundinacea, and APs like I. tectorum, maintain high N and P contents alongside a high biomass. These plants have N and P contents ranging from 15.54 ± 2.31 to 30.17 ± 6.13 mg/g and 1.42 ± 0.02 to 5.50 ± 0.76 mg/g, respectively, whereas others have lower ranges of 5.05 ± 0.06–24.01 ± 3.17 mg/g and 0.60 ± 0.03–2.47 ± 0.06 mg/g, ensuring efficient water purification during the spring and summer. In autumn, SSAPs and APs exhibit significantly higher N and P contents compared to SSPs, which enter the senescence phase. By winter, cold-tolerant APs, particularly I. tectorum and C. alternifolius, maintain higher N and P contents at 16.64 ± 1.32–21.31 ± 3.57 mg/g and 1.53 ± 0.02–1.70 ± 0.03 mg/g, respectively, while other APs have ranges of 13.72 ± 2.56–14.20 ± 3.15 mg/g and 1.17 ± 0.03–1.42 ± 0.02 mg/g and can sustain water purification capacity better in winter. Therefore, to achieve year-round stable and efficient N and P removal from water bodies, we recommend selecting SSPs and SSAPs that maintain a high biomass and N and P contents during the growing season and pairing them with APs that maintain high N and P contents in winter. Similarly, the above strategy of the mixed planting of different wetland plants enables effective reductions in N and P in water under diverse nutrient and seasonal conditions, strengthens nutrient competition against algae, inhibits their growth, and controls water eutrophication. As shown in

Table 4. Harvesting and configuration strategies for wetland plants.

Harvesting Strategy
Plant Type	Specific Species	Harvesting Method
SSPs	S. stoloniferum, S. mariqueter, H. vulgaris, T. orientalis, S. portulacastrum, P. cordata, Z. latifolia, E. crassipes, Pistia stratiotes, and J. effusus et al.	Harvest some of the above-ground parts from May to July, and harvest all of the above-ground parts before September.
SSAPs	P. australis, S. alterniflora, C. glauca, S. validus, T. dealbata, H. verticillata, L. salicaria, Pennisetum purpureum Schum, A. philoxeroides, Vallisneria natans, and M. elatinoides et al.	Harvest some of the above-ground parts from June to August, and harvest all of the above-ground parts before November.
APs	I. tectorum, A. calamus, C. alternifolius, E. nuttallii, and V. zizanioides et al.	Harvest some of the above-ground parts in summer and autumn, and leave the rest to survive the winter.
Configuration Strategy
Plant Types	Specific Species	Target Water Area
Emergent plants and submerged plants	Emergent plants: P. australis, C. glauca, T. dealbata, T. orientalis, and P. cordata et al. Submerged plants: M. elatinoides, V. natans, C. demersum, and E. nuttallii et al.	Suitable for restoring water areas with extremely high nitrogen and phosphorus concentrations.
Emergent plants and floating plants	Emergent plants: P. australis, C. glauca, T. dealbata, T. orientalis, and P. cordata et al. Floating plants: P. stratiotes, E. crassipes, and Lemna minor et al.	Suitable for restoring water areas with large variations in nitrogen and phosphorus concentrations and complex hydrological conditions.
SSPs and SSAPs and APs	SSPs: T. orientalis, P. cordata, S. stoloniferum, and S. mariqueter et al. SSAPs: P. australis, C. glauca, T. dealbata, and M. elatinoides et al. APs: I. tectorum, A. calamus, and V. zizanioides et al.	Achieve stable purification effects on the target water area throughout the year.

, we summarize the proposed harvesting and configuration strategies for various wetland plants.

4. Conclusions

In this study, common wetland plants were catalogized into three groups based on their growth characteristics and N and P requirements: SSPs, SSAPs, and APs. The seasonal variations in N and P contents, transfer patterns, and underlying mechanisms were thoroughly analyzed, and management strategies for effective eutrophication control were proposed. SSPs exhibit robust growth during spring and summer, characterized by high N and P levels and absorption capacities. In contrast, SSAPs maintain high N and P levels and absorption capabilities even in autumn. APs, which are cold-tolerant, demonstrate high N and P levels and absorption throughout winter. The seasonal variation in N and P contents within wetland plants is inconsistent between shoots and roots. Shoots typically exhibit higher N and P contents during the growth seasons, while roots store higher levels during dormancy periods. Perennial plants exhibit distinct seasonal transfer patterns for N and P. In autumn and winter, leaves convert N into amino acids, which are then stored in storage tissues and remobilized in spring to support new growth. A similar process occurs for P, involving the transfer of both Po and Pi forms. Key enzymes such as AS, GS, and phospholipases, as well as transporters like CAT and PHT, play crucial roles in these processes. Based on these findings, we propose management strategies to optimize nutrient removal and ensure sustainable wetland management. These strategies include conducting early partial harvests during peak growth phases and final harvests at the end of each season. Additionally, we recommend the mixed planting of different wetland plant species to achieve stable N and P removal under variable environmental conditions.

5. Future Perspectives

At present, research on the seasonal dynamics and management of wetlands remains limited. A key area that requires further exploration is the seasonal transfer of N and P in wetland perennial plants. Specifically, the mechanisms underlying amino acid transport, as well as the roles of N and P transfer-related proteins and genes, are not well understood. Future studies should focus on elucidating the mechanisms of amino acid transport via symplastic and apoplastic pathways, as well as the changes in P forms and the expression of transport proteins and their related genes in wetland plants during both dormancy and active growth periods. This research will help to clarify the seasonal transfer mechanisms of N and P. Additionally, there is a need for more research on how different harvesting seasons, frequencies, and intensities impact the N and P removal efficiency of wetland plants. Future studies should aim to identify the optimal harvesting periods, frequencies, and heights for various wetland plant species. By doing so, we can develop more effective management strategies that maximize nutrient removal while ensuring the sustainability of wetland ecosystems.