Method for repairing mangrove plants in high-beach difficult land

Abstract

The invention discloses a high-beach difficult land mangrove plant restoration method, and relates to the technical field of wetland ecological restoration. Through the design of the concave planting pits and the improvement technology of the compound matrix, the problems of insufficient tide immersion and soil hardening in existing high beach remediation are solved. According to the method, planting pits with specific sizes are dug, 2 / 3 of original soil is removed, and a composite matrix of a coco coir layer, a humus layer and a crushed soil layer is backfilled, so that the concave microhabitat with the surface layer 10-20 cm lower than the native mud flat is formed. The coco coir layer improves the air permeability of the soil, the humus layer provides organic nutrients, and the crushed soil layer enhances the stability of the root system. The concave structure prolongs the residence time of tidal water, promotes benthic organisms such as crabs to move and improves the soil quality. By means of the method, the survival rate of mangrove plants in the kaolin mud flat at the tail of the pond under the xiamen reaches 85% or above, and the forestation problem caused by high-beach soil hardening and insufficient tide flooding is solved.

Description

Technical Field

[0001] The present invention relates to the technical field of wetland ecological restoration, and particularly relates to a method for restoring mangrove plants in difficult high-tidal areas by improving the survival rate of mangrove plants at high-tidal positions through micro-topography transformation and substrate improvement. Background Technique

[0002] As one of the most recognized ecosystem services under the background of climate change, the mangrove wetland ecosystem has great potential in carbon storage. The "blue carbon" fixed by organisms in the world's oceans accounts for 55% of the "green carbon" stored by photosynthesis in the whole world (Wang Shanshan, Xu Mingwei, Han Yu, et al. Multi-year blue carbon analysis and scenario prediction of the tidal flat wetland on the south bank of Hangzhou Bay [J]. China Environmental Science, 2022, 42(9): 4380-4388), and it plays a key role in coping with climate change. The global mangrove area is disappearing at a rate of 1-2% per year. Despite years of restoration, the existing mangroves still face serious problems of habitat fragmentation and functional degradation. Research shows that due to the expansion of aquaculture, coastal zone development, and pollution input, etc., the global mangrove habitat fragmentation index has increased by 58% within 35 years (Bryan-Brown, D.N., Connolly, R.M., Richards, D.R. et al. Global trends in mangrove forest fragmentation [J]. Global Ecology and Biogeography. 2020, 29(5): 745-760.), and the soil organic carbon storage per unit area of fragmented mangroves has decreased by 30-40% compared with intact habitats (Friess D A, Rogers K, Lovelock C E, et al. The State of the World's Mangrove Forests: Past, Present, and Future [J]. Annual Review of Environment and Resources, 2019, 44(1):1-27.).Mangroves are home to a large number of endangered and endemic species and are of great value in maintaining biodiversity in coastal areas (Su J, Friess D, Gasparatos A. A meta-analysis of the ecological and economic outcomes of mangrove restoration[J]. Nature Communications, 2021, 12: 5050.). The fragmentation of mangrove habitats has a significant impact on benthic animal communities, mainly manifested as a decline in biodiversity and ecological functions. The benthic biodiversity in disturbed mangrove areas decreases by 20% (Carugati L, Gatto B, Rastelli A, et al. Impact of mangrove forests degradation on biodiversity and ecosystem functioning[J]. Scientific Reports, 2018, 8(1): 13298.). Menéndez et al. (2020) found through global modeling that the mangrove ecosystem can provide effective flood buffer protection for more than 15 million people every year (Menéndez P, Losada I J, Torres-Ortega S, et al. The Global Flood Protection Benefits of Mangroves[J]. Scientific Reports, 2020, 10: 4404.). The vegetation density and distribution of mangroves have a significant impact on tidal flow patterns. The reduction or discontinuous distribution of vegetation (i.e., mangrove habitat fragmentation) may lead to changes in tidal flow patterns, thus affecting the overall flood control function of mangroves (Horstman E M, Dohmen-Janssen C M, & Hulscher S J M H. Modeling tidal dynamics in a mangrove creek catchment in Delft3D [J]. Coastal Engineering, 2015, 103, 94-111.). Therefore, the scientific restoration of fragmented mangrove wetland habitats can not only enhance the resilience of the ecosystem, but also improve biodiversity and carbon sequestration functions (Zhang Yun, Liao Baowen. Analysis of the current research status of mangrove wetland ecological restoration technology in China. Bulletin of National Natural Science Foundation of China, 2022, 36(3): 412-419.), thus achieving a win-win situation for both ecology and economy.

[0003] The key to the ecological restoration project of mangrove wetlands is the selection criteria for suitable forest land. The selection of the site beach position is particularly important for improving the survival rate and forest formation rate of mangrove plants. Too high a beach elevation will lead to phenomena such as the death or extreme dwarfing of mangrove plant seedlings. The optimal growth elevation of mangrove plants ranges from 1.6 m to 2.1 m above the Huanghai Elevation. In ecological engineering afforestation, being lower than 1.6 m or higher than 2.1 m above the Huanghai Elevation is not conducive to the growth of mangrove plants.

[0004] The habitats where mangrove plants in difficult areas cannot grow normally have the following problems: ① The beach elevation is too high (higher than the optimal habitat elevation for the growth of mangrove plants), resulting in a short periodic inundation time by the tide, and even during neap tides, the tide cannot inundate this area; ② The substrate filled in the beach area during wetland engineering restoration is kaolin, resulting in soil compaction, extremely poor air permeability, barren soil, and lack of nutrients such as organic matter in this area; ③ Due to the high beach elevation and the soil being kaolin, the surface of this area is flat during neap tides (as hard and flat as a cement slab), and during the flood tide, the tide cannot strand the natural organic debris washed up by the rising tide, such as fallen leaves, branches, and other natural organic nutrients. These two major problems seriously affect the planting survival rate and forest formation effect of mangrove plants in wetland ecological engineering restoration.

[0005] To solve the problem of mangrove plant restoration in difficult areas of this region, the problems to be solved are: (1) Creating a beach elevation suitable for the normal growth of mangrove plants; (2) Improving the soil substrate and enhancing the air permeability of the soil; (3) Creating conditions to improve the barren soil quality and increase the soil fertility in the microhabitat of the planting pit by retaining natural fallen leaves and branches and creating microhabitat habitats for marine organisms such as crabs, snails, and shellfish. Summary of the Invention

[0006] The purpose of the present invention is to provide a method for effectively afforesting mangrove plants on high beach position difficult lands, which can reduce the beach elevation of local positions in the planting area, extend the water storage time of the microhabitat in the concave position of mangrove plant seedlings during high tide, and at the same time improve the compactness characteristics of kaolin, thereby improving the planting survival rate and forest formation rate of mangrove plants.

[0007] To achieve the above invention purpose, the present invention provides the following technical solutions.

[0008] A method for restoring mangrove plants on high beach position difficult lands, comprising the following steps:

[0009] 1) Microtopography transformation: Excavate a concave planting pit in the beach, remove 2 / 3 of the original soil in the pit, and leave 1 / 3 of it broken beside the planting pit as broken soil for standby, which is used as backfill soil after planting mangrove seedlings.

[0010] 2) Construction of layered substrate: The substrate and crushed soil are backfilled into the sunken planting pit, and from bottom to top are the bottom water retention layer, transition layer, nutrient layer, and surface protection layer in sequence; the bottom water retention layer uses coconut coir, the nutrient layer uses humus, and the transition layer and surface protection layer use crushed soil;

[0011] 3) Planting of mangrove plants: The 1 - 3 - year - old mangrove plant seedlings are implanted into the planting pit to ensure that the roots completely cover the humus layer, and the substrate is compacted layer by layer;

[0012] 4) Activation of the ecological chain: The sunken structure naturally retains the tide water and natural litter, promotes the activities of benthic animals such as crabs and snails, and forms a symbiotic system of "plant - animal - microorganism".

[0013] In step 1), the length and depth of the planting pit are determined according to the specifications of the seedling age. For the mangrove plant seedlings with a seedling age of 1 a - 3 a, the diameter width of the planting pit is controlled between 25 cm and 50 cm, and the depth is controlled in the range of 40 cm to 60 cm.

[0014] In step 2), the construction of the layered substrate includes, from bottom to top in sequence: the bottom water retention layer, transition layer, nutrient layer, and surface protection layer; specifically:

[0015] Bottom water retention layer (h1): A 5 - 10 cm coconut coir layer can be laid at the bottom of the pit as the bottom water retention layer, with a bulk density of 0.06 - 0.15 g / cm³ and a porosity of ≥70%;

[0016] Transition layer (h2): 3 - 5 cm of crushed soil can be backfilled on the coconut coir layer as the transition layer, and the particle size of the crushed soil can be ≤2 cm;

[0017] Nutrient layer (h3): 5 - 8 cm of organic humus can be added on the transition layer as the nutrient layer. The organic humus is made by fermenting the natural litter of mangrove plants, and the organic matter content is ≥40%; the organic matter humus is the natural fallen branches and leaves of the surrounding mangrove plants, which is a waste utilization and will not cause an environmental burden on the marine environment.

[0018] Surface protection layer (h4): 3 - 5 cm of crushed soil can be covered on the nutrient layer as the surface protection layer, and the particle size of the crushed soil can be ≤2 cm; Since both coconut coir and humus belong to loose substrates with poor stability, adding crushed soil to the root planting layer of mangrove plant seedlings can greatly increase the stability of the substrate, and can avoid damage to the roots of mangrove plant seedlings caused by the ebb and flow of sea water and the erosion of sea waves during high tide.

[0019] In step 4), the natural retention of tidal water and natural litter through the concave structure, the height h5 of the topsoil in the planting pit from the normal non-concave topsoil with difficulty is equal to ΔH, that is, ΔH = H1 - H2. The height ΔH of the topsoil from the edge of the planting pit is controlled at about 10 cm to 20 cm, and the specific value can be adjusted according to local tidal laws, flood tolerance characteristics of mangrove plants and other factors. Ensure that the elevation of the tidal flat where the microhabitat of the mangrove plant seedlings is located is lower than the elevation of the original habitat, so that each mangrove plant seedling grows under the microhabitat conditions of this optimal tidal flat elevation in its respective concave shape; the volume of the planting pit from the ground surface of the topsoil of the planting pit can very well play the role of accumulating tidal water. Therefore, the seawater accumulated through the planting pit can naturally improve the originally compact and hard soil quality of the high tidal flat around the planting pit. Over the years, due to the penetration of the seawater accumulated in the planting pit, the space d0 for the horizontal and vertical expansion growth of the roots of the mangrove plant seedlings can be greatly increased. Moreover, the concave microhabitat can well retain the natural fallen leaves and branches of mangrove plants, and promote the growth and activities of marine organisms such as crabs and snails in the soil matrix of the concave microhabitat of the mangrove seedlings, improving the soil air permeability and soil fertility of the microhabitat of the mangrove plant seedlings, and thus enabling the mangrove plant seedlings on the difficult high tidal flat to achieve high survival rate and fast forest formation. Its action mechanism and effects cover but are not limited to aspects such as improving the physical and chemical properties of the soil through biological activities, and the microhabitat construction method can be reasonably adjusted and expanded according to the characteristics of the local ecosystem.

[0020] In the present invention, the materials used for the bottom water retention layer, nutrient layer, transition layer and surface protection layer, through reasonable matching and laying, form a stable structure in the root planting layer of the mangrove plant seedlings, effectively increasing the matrix stability, resisting external forces such as the scouring of sea waves and the impact of water flow during high tide, and avoiding damage to the roots of the mangrove plants. The material selection and laying method can be equivalently replaced and optimized according to the actual engineering requirements and local environmental conditions.

[0021] Compared with the prior art, the present invention has the following outstanding technical effects:

[0022] The present invention involves removing two-thirds of the soil volume dug out from the planting pit, crushing the remaining one-third of the soil volume, and leaving it beside the planting pit as the backfill soil after planting mangrove seedlings. An appropriate amount of coconut coir and humus is added to the planting pit to replace part of the backfill soil. When planting mangrove plant seedlings, the total height of the backfill substrate (including one-third of the original soil volume dug out, coconut coir, and organic matter) in the planting pit must be lower than the height of the planting pit. Therefore, after filling the surface backfill soil of the planting pit, the height ΔH of the surface soil from the edge of the planting pit should be controlled at about 10 - 20 cm. The concave microhabitat enhances the tidal retention time of the soil at the roots of mangrove plant seedlings during high tide, improving the problem of short tidal inundation time and soil compaction and poor air permeability at the original high-tidal flat position. The tidal water retained in the microhabitat of the planting pit, through long-term water storage and infiltration, makes the originally hard and compact soil around the planting pit become soft, thus making it easier for the root systems of mangrove plant seedlings to penetrate the soil layer. Moreover, the concave planting pit can well retain natural mangrove plant litter and leaves, and can promote the growth and activities of marine benthic animals such as crabs and snails in the concave microhabitat of the mangrove seedlings, improving the soil air permeability and soil fertility of the microhabitat of mangrove plant seedlings, and thus increasing the survival rate of planted mangrove plant seedlings and achieving the effect of fast forest formation. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 is Xiatanwei Mangrove Park.

[0024] Figure 2 is the technical model diagram of the accumulation of natural organic debris on the surface soil of the concave planting pit.

[0025] Figure 3 is the early-stage engineering afforestation effect diagram of mangrove plant seedlings in the concave microhabitat planting mode in the difficult-to-plant high-tidal flat area with kaolin.

[0026] Figure 4 is the schematic diagram of the principle of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0027] In order to make the objectives, technical solutions, and advantages of the present invention clearer, the following embodiments will further illustrate the present invention in conjunction with the accompanying drawings. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention. On the contrary, the present invention covers any alternatives, modifications, equivalent methods, and solutions made within the spirit and scope of the present invention defined by the claims.

[0028] The embodiments of the present invention provide a method for reducing the elevation of the tidal flat at a local position of the planting area, extending the water storage time of the microhabitat at the concave position of mangrove plant seedlings during high tide, simultaneously improving the compactness characteristics of kaolin, and thus increasing the planting survival rate and forest formation rate of mangrove plants, for effective afforestation of mangrove plants on difficult high-tidal flat areas.

[0029] Example 1: The mangrove plant restoration project on Island No. 5, Xiamentail, Xiamen City, Fujian Province

[0030] As Figure 1 , due to reclamation and development, the elevation of the intertidal zone on Island No. 5, Xiamentail, Xiamen City is relatively high, with the Huanghai elevation reaching 2.6 - 3.0 m. The soil is kaolin, with a large bulk density, low porosity, and little organic matter content. The growth environment of mangrove plants is harsh, the survival rate of the original mangrove plants is low, and the ecosystem function degenerates. Therefore, restoration work is imperative.

[0031] The key to the mangrove wetland ecological restoration project is the selection criteria for suitable forest land. The selection of the site intertidal zone is particularly important for improving the afforestation survival rate and forest formation rate of mangrove plants. Too high an elevation of the intertidal zone will lead to phenomena such as the death or extreme dwarfing of mangrove plant seedlings. The optimal growth elevation of mangrove plants is in the range of 1.6 - 2.1 m of the Huanghai elevation. In ecological engineering afforestation, being lower than 1.6 m or higher than 2.1 m of the Huanghai elevation is not conducive to the growth of mangrove plants. The habitats where mangrove plants in the difficult areas of Island No. 5, Xiang'an District, Xiamen City cannot grow normally have the following problems: ① The elevation of the intertidal zone in this area is on the high side (higher than the optimal habitat elevation for mangrove plant growth). Through RTK equipment monitoring, it can be seen that the Huanghai elevation of the intertidal zone in the difficult areas of Island No. 5, Xiamentail reaches 2.6 - 3.0 m. The too high elevation of the intertidal zone results in a short periodic inundation time of the tide in this area, and even during neap tides, the tide cannot inundate this area; ② In the intertidal zone landfill project in 2013 in the Xiamentail mangrove wetland, most of the substrates filled in this intertidal zone area are kaolin, resulting in soil compaction, extremely poor air permeability, infertile soil, and lack of nutrients such as organic matter in this area; ③ Due to the relatively high elevation of the intertidal zone and the soil quality being kaolin, Figure 1 As shown by the display and long-term tracking observations, the surface of this area is flat during neap tides (as hard and flat as a cement board), and during the flood tide period, it is impossible to strand the natural organic debris washed up by the rising sea water, such as fallen leaves, branches, and other natural organic nutrients. These two major problems seriously affect the planting survival rate and forest formation effect of mangrove plants in the wetland ecological engineering restoration.

[0032] To solve the problem of restoring mangrove plants in the difficult areas of this region, the problems that need to be solved are: (1) creating a suitable intertidal zone elevation for the normal growth of mangrove plants; (2) improving the soil substrate and enhancing the air permeability of the soil; (3) creating conditions to retain natural fallen leaves and branches, and creating microhabitat habitats for marine organisms such as crabs, snails, and shellfish to improve the infertile soil quality and increase the soil fertility in the planting pits.

[0033] The restoration steps of the embodiment of the present invention are as follows:

[0034] A. Design of the planting pit specifications:

[0035] Due to the fact that Island No. 5 is a difficult area filled with kaolin, with a high elevation of the tidal flat, less inundation by tides, and hard soil, the width and depth of the planting pits dug in the difficult area should be larger and deeper than those in conventional tidal positions. This is to ensure that after planting mangrove seedlings, the elevation of the tidal flat where the mangrove plant seedlings are located in the planting pits shows a concave pattern. The length and depth of the planting pits are determined according to the specifications of the seedling age. Generally, for mangrove plant seedlings with a seedling age of 1 a - 3 a, the diameter width of the planting pits is controlled between 25 - 50 cm, and the depth is controlled within the range of 40 - 60 cm. Also, 2 / 3 of the soil volume dug out from the planting pits is transported away, and the remaining 1 / 3 of the soil volume is fragmented and left beside the planting pits as the backfill soil after planting the mangrove seedlings.

[0036] B. Water Retention Technology

[0037] Put the mangrove plant seedlings into the planting pits, and add coconut coir to the planting pits, as shown by the substrate layer h1 in Figure 2 . As can be seen from the above, the soil permeability in this area is extremely poor. Relying only on fragmented soil backfill cannot well solve problems such as the long-term deposition and hardening of kaolin in the later stage. Therefore, during the implementation of this project, the permeability of the existing soil is essentially improved. Coconut coir plant fiber has good air permeability and adsorption characteristics. It can adsorb nutrient elements in the surrounding environment, which is beneficial to the growth of plants; coconut coir belongs to the reuse of waste resources, is environmentally friendly and harmless, and will not affect the marine habitat in the mangrove wetland ecological restoration project.

[0038] C. Root Stabilization Measures

[0039] After adding coconut coir to the bottom of the planting pits, backfill crushed soil with a thickness of about 3 - 5 cm is needed, as shown by the substrate layer h2 in Figure 2 . The main function is: Since coconut coir is a loose substrate with poor stability, adding 3 - 5 cm of crushed soil to the seedling root planting layer can greatly increase the stability of the substrate, and can avoid the damage to the roots caused by the scouring force brought by the ebb and flow of the sea water and the waves when the tide rises after the seedlings are planted.

[0040] D. Technology for Improving Soil Fertility

[0041] Add humus to the planting pits, as shown by the substrate layer h3 in Figure 2 . As mentioned above, adding organic humus nutrients to the root layer of the planted seedlings. Since the original soil in this area belongs to kaolin and is very barren, and the nutrients adsorbed by coconut coir alone from the sea water scouring are limited, one of the very important technical points of the present invention is to add a humus layer to the root layer above the above-mentioned backfill soil when planting mangrove seedlings. The thickness of the humus can be controlled at about 5 - 8 cm. The humus material is mainly natural plant fallen leaves and branches. The fallen leaves and branches as organic humus belong to natural plant materials, which are waste utilization and will not cause an environmental burden on the marine environment.

[0042] E. Sunken planting pattern design

[0043] The organic humus layer added to the root layer of the mangrove seedlings is added with a layer of crushed soil, such as Figure 2 As shown in the matrix layer h4 in the figure, as mentioned above, although coconut bran and humus have very good air permeability, their soil is loose and the root system can hardly resist the invasion of waves. Therefore, in addition to providing certain nutrients, the most important thing about adding crushed soil to the surface is that the root system of mangrove seedlings can be further stabilized by adding the crushed soil layer h4, which plays a role in protecting the root system and stabilizing the seedlings. In addition, the value of the distance h5 between the crushed soil layer and the top of the planting pit is very important, because the difficult area belongs to a high beach, and the original tidal inundation time is short, and even during low tides, the tide cannot inundate the soil in this area at all. This causes the soil in this area to be severely compacted, with extremely poor air permeability and high hardness. Therefore, after the surface backfill soil of the planting pit is filled, the height of the surface soil from the edge of the planting pit is h5=ΔH, that is, ΔH=H 1 -H 2 It should be controlled at about 10~20 cm, so that the volume of the planting pit where the surface soil of the planting pit is above the ground can play a very good role in storing tidal water. Therefore, the seawater accumulated in the planting pit can naturally improve the soil quality around the planting pit. Over the years, due to the infiltration of seawater accumulated in the planting pit, the space for the horizontal and vertical expansion and growth of the roots of the mangrove seedlings can be greatly increased. Figure 2 As shown in d0, the problem that the roots of mangrove seedlings could not expand due to the originally hardened soil was solved. The sunken microhabitat can well retain the natural fallen branches and leaves of mangrove plants, and can promote the growth and activities of marine organisms such as crabs and snails in the sunken microhabitat of mangrove seedlings, thereby improving the soil permeability and soil fertility of the microhabitat of mangrove seedlings.

[0044] The existing survival rate of mangrove plants in the high beach of No. 5 Island, Xiatanwei Wetland Park, Xiang'an District, Xiamen, is extremely low, only 10% to 40%. Figure 3 As shown in the figure, after a period of monitoring, the results show that the survival rate of mangrove seedlings under the high-beach sunken planting method provided by the present invention is more than 85%, the soil bulk density is reduced from the original 1.5 ~ 1.9 g / cm³ to below 1.1~1.3 g / cm³, the porosity is increased from <25% to more than 40%, the soil permeability and water retention are significantly improved, and the ecosystem function is gradually restored. The sunken microhabitat conditions ensure that the mangrove seedlings grow normally at the optimal beach elevation, avoiding the death or dwarfing of the seedlings, thereby achieving the effect of rapid forest formation. .

[0045] The schematic diagram of the principle of the present invention is as follows Figure 4As shown, after the tide ebbs, seawater accumulates in various planting pits. There are two modes of seawater loss. One is slow longitudinal loss along the planting pits (as shown by the longitudinal arrow of the seawater layer in Figure 4 ), and the longitudinal loss will definitely not be too fast because the seawater accumulation in the underlying humus and coir dust has reached a saturated state, and a certain amount of water storage can be maintained longitudinally for a short time. The other is that the seawater accumulated in the planting pits can be lost laterally (as shown by the lateral arrow of the seawater layer in Figure 4 ). Through the lateral osmosis of seawater in the planting pits, the soil quality around the planting pits will slowly improve its hardness due to the increase in soil moisture content, expanding the root expansion area of mangrove seedlings. The concave microhabitat ensures the normal growth of mangrove seedlings.

[0046] The above embodiments are only preferred embodiments of the present invention and cannot be considered as limiting the scope of implementation of the present invention. All equivalent changes and improvements made according to the scope of the present invention application shall still fall within the scope covered by the patent of the present invention.

Claims

1. A method for restoring mangrove plants in difficult high-beach locations, characterized in that The following steps are involved: 1) Micro-topography transformation: dig a sunken planting pit on the beach, remove 2 / 3 of the original soil in the pit, and leave the remaining 1 / 3 crushed soil next to the planting pit as spare soil for backfill after planting mangrove seedlings; 2) Layered matrix construction: The matrix and crushed soil are backfilled into the concave planting pit, which is composed of a bottom water-retaining layer, a transition layer, a nutrient layer and a surface protective layer from bottom to top; the bottom water-retaining layer is made of coconut bran, the nutrient layer is made of humus, and the transition layer and the surface protective layer are made of crushed soil; 3) Mangrove Planting: Plant 1-3 year old mangrove seedlings into the planting pits, ensure that the roots are completely covered with the humus layer, and compact the substrate in layers; 4) Activation of the ecological chain: The sunken structure naturally retains tidal water and natural litter, promotes the activity of benthic animals, and forms a "plant-animal-microorganism" symbiotic system.

2. The method for phytoremediation of mangroves in difficult high-beach locations as claimed in claim 1, characterized in that In step 1), the length and depth of the planting pit are based on the specifications of the seedling age. For mangrove seedlings aged 1 to 3 years, the diameter of the planting pit is 25 to 50 cm and the depth is 40 to 60 cm.

3. The method for phytoremediation of mangroves in difficult high-beach locations as claimed in claim 1, characterized in that In step 2), the construction of the layered matrix includes, from bottom to top: Bottom water-retaining layer: Lay coconut bran layer at the bottom of the pit as the bottom water-retaining layer; Transition layer: Backfill the coconut husk layer with crushed soil as the transition layer; Nutrition layer: Add organic humus as the nutrition layer on the transition layer; Surface protective layer: Cover the nutrient layer with crushed soil as a surface protective layer.

4. The method for phytoremediation of mangroves in difficult high-beach locations as claimed in claim 3, characterized in that The bottom water-retaining layer has a depth of 5-10 cm, a bulk density of 0.06-0.15 g / cm³, and a porosity of ≥70%.

5. The method for phytoremediation of mangroves in difficult high-beach locations as claimed in claim 3, characterized in that The depth of the transition layer is 3-5 cm, and the particle size of the crushed soil is ≤2 cm.

6. The method for phytoremediation of mangroves in difficult high-beach locations as claimed in claim 3, characterized in that The depth of the nutrient layer is 5 to 8 cm; the organic humus is made by fermenting natural fallen leaves of mangrove plants.

7. The method for phytoremediation of mangroves in difficult high-beach locations as claimed in claim 3, characterized in that The depth of the surface protective layer is 3-5 cm, and the particle size of the crushed soil is ≤2 cm.

8. The method for phytoremediation of mangroves in difficult high-beach locations as claimed in claim 1, characterized in that In step 4), the sunken structure is used to naturally retain tidal water and natural litter, and the height h5 of the surface soil in the planting pit from the normal non-concave surface soil in the difficult area is equal to ΔH, that is, ΔH=H1-H2, and the height ΔH of the surface soil from the edge of the planting pit is 10~20 cm, ensuring that the elevation of the tidal flat where the microhabitat of the mangrove seedlings is located is lower than the elevation of the original habitat.

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