Study on overwintering growth characteristics and purification effect of floating bed cultivated plants

. Artificial floating bed technology has been used to repair eutrophic water. However, there are relatively few studies on the overwintering of economic plants on the floating bed. In order to realize the resource utilization of wastewater, celery, Artemisia selengensis, Garlic, Chinese yam, lettuce and other crops were selected as the test plants for the floating bed. Quantitative experimental study was conducted on the aquaculture wastewater in winter. The results showed that after 90 days of acclimation, the tested plants showed different growth characteristics, with the survival rates above 84.48%. The distribution of nitrogen and phosphorus in water and subsurface plants of 5 species were different. The contents of nitrogen and phosphorus in plants The accumulative amounts were 1.15 ~ 5.18 gꞏm-2 and 0.04 ~ 0.21 gꞏm-2, respectively, with the order of garlic > celery > seabuckthorn > sedum chrysanthemum> lettuce and celery> garlic> sesame oil> sesame oil> lettuce.


Introduction
Water resource is the foundation of human life and social progress.It is the central link of economic and social development, and also the prerequisite for achieving sustainable development and healthy ecosystem.At present, there are many phenomena of direct discharge of aquaculture wastewater in China, which makes the receiving water eutrophic and reduce the biodiversity.In terms of intensive aquaculture water bodies, total phosphorus and ammonia nitrogen pollution have become the main stress factors restricting the aquaculture environment [6][7].The research shows that aquaculture organisms only absorb 23% ~ 31% of N and 10% ~ 13% of P, while 14% ~ 53% of N and 39% ~ 67% of P accumulate in the sediment.57% of n is discharged into the environment.It can be seen that the nutrients released into the environment by breeding ponds are considerable.
In the mid-1990s, artificial floating islands were widely recognized by Japanese environmental and lake scientists.In the 1980s, the United States began to use a variety of fish culture wastewater to produce lettuce, tomato, strawberry, cucumber and other vegetables and hyacinth and other flowers.Nathalie et al.Used the commercial deep flow (NTF) hydroponic system to float and cultivate D.innoxia to purify and repair domestic sewage, and achieved good results.Biological floating island technology has also been rapidly promoted and applied.However, in the selection of biological floating island plants, the research mostly focuses on hygrophilic vegetables and flowers.Most of these plants are suitable for planting biological floating islands in spring, summer and autumn.For example, Canna has a good removal effect on pollutants in water, but it is not cold resistant.After frost beating, the stems and leaves will wither, and it is easy to freeze to death below 5℃.It is difficult to survive the winter under floating bed planting conditions, and it needs to be replanted in the second year.Therefore, the selection of overwintering plants on biological floating islands has become an important issue in current research.

Materials and methods
This experiment was conducted in a fish pond of a pig farm, which mainly raised four large fish.The fishpond area selected for the test and research is about 5661.5 square meters, with water quality of COD of 8 ~ 32mg/l, NH3-N of 0.06 ~ 0.48mg/l, TN of 2.2 ~ 4.7mg/l and TP of 0.17 ~ 0.23mg/l.
The research site has a subtropical monsoon climate with four distinct seasons, sufficient rainfall and mild climate.The annual average temperature is 16.9 ℃, the extreme maximum temperature is 41.4 ℃(July) and the extreme minimum temperature is -9.0 ℃ (January).The average annual sunshine duration in the area is 1669.2hours, and the average daily sunshine is 4.57 hours.

Experimental devices and materials
The extruded plate floating island carrier (1.8m long, 0.6m wide and 3cm thick) used in this study is shown in Fig. 1.Each plate is drilled with 27 holes with a diameter of 20mm, and the hole penetrations are reserved for fixing plants.Floating bed units are connected and fixed with iron wire to form 0.6m ×72m rectangular floating bed group is bound to the edge of the fish pond with nylon ropes.In the process design of plant floating bed, the selection of plants should be able to adapt to the local species growing under hydroponic conditions, with high survival rate, long evergreen period, fast sprouting and rapid growth; The plant root system is developed and the biomass is large; It has good adaptability to polluted water bodies, is resistant to pollution and has good pollution control potential.According to the screening principle of floating bed plants, celery, chrysanthemum, garlic, potherb mustard, oil lettuce and other crops were selected as floating bed test plants, and a quantitative experimental study was carried out on aquaculture wastewater in winter.

Data analysis and processing
Use Excel and SPSS software to process the experimental data, eliminate outliers and then calculate the average value, calculate the removal effect of pollutants in the water, the biological growth of plants and the concentration of nutrients, and compare the removal effects of various plants; Calculate the nitrogen and phosphorus content of plants, and speculate the purification effect of plants on water.

Growth status
Five kinds of vegetables show different growth characteristics after a period of growth: After 90 days of domestication and adaptation, celery shows strong ecological adaptability and can grow normally from beginning to end.Each plant has new leaves, leaves grow, plants grow tall, biomass increases, and plants constantly tiller out new plants.The root system can develop normally.During the test period, new root systems have been formed continuously, and the root system has gradually increased, forming a compact and developed root system.After 90 days of domestication and adaptation, chrysanthemum garland, lettuce, garlic and Potherb Mustard all showed strong ecological adaptability and could grow normally from beginning to end.However, in the early stage of the experiment, due to the direct weather reasons and the adaptation to water temperature, some of the original stems and leaves of the plant were withered and frostbitten, but most of the leaves were bright green and grew well, with new leaves emerging.The leaves grew up, the plant grew tall and the biomass increased.Among them, Chrysanthemum chrysanthemum sprouts new plants.The root system can develop normally, and there are new root systems, and the root system also grows gradually.However, in the later stage of the experiment, aphids appeared in the leaves of potherb mustard, which was easy to cause insect damage.

Changes of plant roots
Plant roots can adsorb suspended solids and colloids in water.When growing in heavily polluted and suspended solids rich water, with the continuous growth and expansion of roots, not only the adsorption of suspended solids, particles and colloids in water is increasing, but also provide a certain place for microorganisms, leading to the proliferation of foreign microorganisms, the ability to decompose organic suspended solids, particles and colloids is strengthened, and finally the transparency of water body is improved, Water quality has been improved.
When the organic nitrogen and phosphorus in sewage are transformed into inorganic ions, they can be directly absorbed and utilized by plant roots to promote plant growth.Most of the nitrogen absorbed by plant roots is in the form of nitrate ions.Part of the nitrogen is directly transported to the leaves in the form of nitrate ions and then reduced for utilization.The other part is reduced in the roots.Various amino acids and nitrogen-containing organic compounds are synthesized and transported to the upper part of the ground in the form of amino acids or other organic compounds through plant transpiration.The phosphorus absorbed by roots is mainly transported to the shoot in the form of phosphate ions, and a small amount of phosphorus is transported to the stems and leaves after the roots are synthesized into organic compounds.
The change of root length of the five plants is more obvious than that of height.Except that the net root growth of garlic is less than that of its plant height, the net root growth of the other four plants is higher than that of its plant height (Fig. 2).The roots of celery and Potherb Mustard grew the most, 14.0 cm and 11.3cml respectively; The second was the lettuce, whose root length increased by 5cm; The root growth of garland chrysanthemum and garlic were 3.9cm and 3.6cm respectively.The change of root length is mainly determined by the ecological adaptability of various plants and the environmental conditions during the experiment.The roots of celery and garland chrysanthemum belong to the radial structure of stem and root, and a large number of adventitious roots are formed in the rhizome; The root system of garlic is white and tender; Potherb mustard leaves its lateral roots from one main root; The root system of oilseed rape is relatively sparse, and there are adventitious roots in the rhizome.Celery and Potherb Mustard have developed roots.In the water environment, plants mainly rely on adventitious roots and their branch roots to complete the absorption of nutrients [7].Therefore, the absorbed surface area is increased through the root system to meet the needs of nutrient absorption.In this way, the root system can absorb a large amount of nutrients in the water to meet its growth, and its biomass is also large.The adaptability of oilseed lettuce is poor, and the upper leaves wither and fall off, which affects the growth and development of the plant.

Distribution of nitrogen and phosphorus content in plants
In terms of the distribution of five plant nutrients, the nitrogen content in the aquatic part ranged from 31.31 to 56.13 gꞏkg -1 , of which potherb mustard had the highest nitrogen content, 56.13 gꞏkg -1 , followed by garlic (42.45 gꞏkg -1 ), celery (40.66 gꞏkg -1 ) and oilwheat (34.37 gꞏkg -1 ), and chrysanthemum garland had only 31.31 gꞏkg -1 ; The contents of nitrogen in the five plants under water ranged from 23.69 to 55.89 gꞏkg -1 , among which garlic was the highest, followed by potherb mustard (46.45 gꞏkg -1 ) and celery (38.85 gꞏkg -1 ).There was little difference between chrysanthemum and oilwheat, which were 24.88 gꞏkg -1 and 23.69 gꞏkg -1 respectively.Celery (1.83 gꞏkg -1 ), potherb mustard (1.59 gꞏkg -1 ) and garland chrysanthemum (1.54 gꞏkg -1 ), garlic (1.26 gꞏkg -1 ) had the highest phosphorus content in the water part, followed by oilwheat (1.11 gꞏkg -1 ); The contents of phosphorus in the submerged parts of the five plants ranged from 1.19 to 1.73 gꞏkg -1 , among which potherb mustard had the highest phosphorus content, followed by celery (1.66 gꞏkg -1 ), garlic (1.44 gꞏkg -1 ) and chrysanthemum (1.33 gꞏkg -1 ), and the smallest was lettuce.The contents of total nitrogen and total phosphorus in celery and chrysanthemum are distributed in the water part > underwater part.Nitrogen and phosphorus are transported to the water part, and phosphorus absorbed by roots is transported upward in the form of ions [68]; The distribution of total nitrogen and total phosphorus in garlic was underwater part > underwater part; The content of total nitrogen in potherb mustard and oilseed lettuce showed the distribution of above water part > under water part.Nitrogen was transported to the above ground part, while the content of total phosphorus was under water part > above water part.

Nitrogen and phosphorus accumulation in plants
Combined with the relevant data of biomass and nitrogen and phosphorus content in the body of five plants, the nitrogen and phosphorus accumulation in the above water and under water parts of plants can be obtained.The accumulated amount of nitrogen and phosphorus in the aquatic part of the planting object is 0.68 ~ 2.44 gꞏm -2 and 0.022 ~ 0.110 gꞏm -2 respectively.The nitrogen content is celery > potherb mustard > garlic > garland chrysanthemum > oil wheat, and the phosphorus content is celery > garland chrysanthemum > garlic > potherb mustard > oil wheat.The accumulated amount of nitrogen and phosphorus in celery is the largest, indicating that it has the strongest ability to absorb nitrogen and phosphorus.The accumulated amounts of nitrogen and phosphorus in the underwater part were 0.47 ~ 2.94 gꞏm -2 and 0.024 ~ 0.100 gꞏm -2 , respectively.The nitrogen content was garlic > celery > potherb mustard > garland chrysanthemum > oilwheat, and the phosphorus content was celery > garlic > potherb mustard > garland chrysanthemum > oilwheat, in which garlic had the largest accumulated amount of nitrogen and had the strongest nitrogen absorption capacity on the surface; Celery has the largest phosphorus accumulation, indicating that it has the strongest ability to absorb phosphorus.Because the difference of nitrogen and phosphorus content between garland chrysanthemum and oilseed lettuce is not too large, but the nitrogen and phosphorus accumulation in plants is large, the main reason for the difference is the difference of biomass.
It can be seen that the distribution of nitrogen accumulation in each part of water celery shows a trend of 2.44 gꞏm -2 > 2.33 gꞏm -2 in the water part, and the distribution of phosphorus accumulation shows a trend of 0.11 gꞏm -2 > 0.10 gꞏm -2 in the water part.The nitrogen and phosphorus accumulation in the water part accounts for 51.2% and 52.4% of the total accumulation respectively; In the nitrogen accumulation of each part of chrysanthemum, the distribution of the above water part is 1.38 gꞏm -2 > the under water part is 1.10 gꞏm -2 , and the distribution of the phosphorus accumulation is 0.068 gꞏm - 2 > 0.059 gꞏm -2 .The nitrogen accumulation of the above water part accounts for 55.6% of the total accumulation, and the phosphorus accumulation of the above water part accounts for 53.5% of the total accumulation; The nitrogen accumulation of garlic above and under water was 2.94 gꞏm -2 > 2.24 gꞏm -2 in the water part, and the phosphorus accumulation was 0.076 gꞏm -2 > 0.066 gꞏm -2  in the water part.The nitrogen accumulation in the water part accounted for 43.2% of the total accumulation, and the phosphorus accumulation in the water part accounted for 46.5% of the total accumulation; The nitrogen accumulation of potherb mustard was 2.29 gꞏm -2 > 1.90 gꞏm -2 in the water part, and the phosphorus accumulation was 0.071 gꞏm -2 > 0.065 gꞏm -2 in the water part.The nitrogen accumulation in the water part accounted for 54.7% of the total accumulation, and the phosphorus accumulation in the water part accounted for 47.8% of the total accumulation; The accumulation of nitrogen in the water and underwater parts of oilseed lettuce was 0.68 gꞏm -2 > 0.47 gꞏm -2 in the water part, and the accumulation of phosphorus was 0.024 gꞏm -2 > 0.022 gꞏm -2 in the water part.The accumulation of nitrogen in the water part accounted for 59.1% of the total accumulation, and the accumulation of phosphorus in the water part accounted for 47.8% of the total accumulation; Although the total accumulation of nitrogen and phosphorus in the whole plant was the smallest among the five plants, the share of nitrogen accumulation in the water part was the highest, accounting for 59.1% of the total accumulation; The content of nitrogen and phosphorus in the underwater part of garlic was higher than that in the water part, and the proportion of nitrogen and phosphorus accumulation in the water part was the lowest.
It can be seen from the above analysis that the proportion of nitrogen and phosphorus accumulated in the water part of celery and chrysanthemum is more than 50%, indicating that the nitrogen and phosphorus absorbed by the two economic plants from the water through their roots migrate and accumulate more to the stems and leaves of the water part.This distribution is conducive to the removal of nitrogen and phosphorus by picking plants.There was a significant linear relationship between nitrogen and phosphorus accumulation and biomass (p<0.05).The correlation coefficients between nitrogen and phosphorus accumulation and biomass were 0.869 and 0.980 respectively, indicating that the correlation between nitrogen and phosphorus accumulation and biomass was good.

Conclusion and discussion
In this study, the growth status and the ability to absorb nitrogen and phosphorus of celery, chrysanthemum, garlic, potherb mustard and oilwheat were compared and analyzed.The main conclusions are as follows: (1) After about 90 days of acclimation, the five tested plants showed different growth characteristics, all of which were more than 84.48%.
(2) After 90 days' growth, the fresh weight of the five plants ranged from 232.74 gꞏm -2 to 418.26 gꞏm -2 , and the order was: Celery > garlic > garland chrysanthemum > potherb mustard > lettuce; In terms of the net increment of dry matter biomass, the biomass of the five plants ranged from -3.1 to 49.32 gꞏm -2 , celery > chrysanthemum > potherb mustard > garlic > lettuce.The moisture content of the five plants ranged from 82.49% to 88.60%, and the order was as follows: lettuce > chrysanthemum > celery > garlic > potherb mustard; The water content of each vegetative organ of the five plants increased from bottom to top, and the general rule was: stem and leaf > root.
(3) 5.The content of nitrogen in the above water part of the planting object was 31.31~ 56.13 gꞏkg -1 , potherb mustard > garlic > celery > oats > garland chrysanthemum, the content of nitrogen in the plant under water was 23.69 ~ 55.89 gꞏkg -1 , garlic > potherb mustard > celery > garland chrysanthemum > oats; The phosphorus content of the above water part of the five plants was 1.11 ~ 1.83 gꞏkg -1 , celery > potherb mustard > garland chrysanthemum > garlic > oatmeal, the phosphorus content of the under water part of the plants was 1.19 ~ 1.73 gꞏkg -1 , potherb mustard > celery > garlic > garland chrysanthemum > oatmeal.The distribution law of nitrogen and phosphorus content in the parts of plants above and under water is different,.The contents of nitrogen and phosphorus in celery and chrysanthemum showed the distribution of above water part > under water part, and nitrogen and phosphorus were transported to the above water part; The distribution of total nitrogen and total phosphorus in garlic was underwater part > underwater part; The content of total nitrogen in potherb mustard and Brassica oleracea showed the distribution of above water part > under water part.Nitrogen was transported to the aboveground part, while the content of total phosphorus was under water part > above water part, mainly concentrated in the root.
(4) 5 the accumulation of nitrogen and phosphorus in the planting objects ranged from 1.15 to 5.18 gꞏm -2 and 0.04 to 0.21gꞏm -2 , respectively.The order of magnitude was garlic > celery > potherb mustard > garland chrysanthemum > lettuce and celery > garlic > potherb mustard = garland chrysanthemum > lettuce.The accumulation of nitrogen and phosphorus of the five plants has a good correlation with the biomass.The biomass can reflect the accumulation of nitrogen and phosphorus, which is conducive to calculating the content of nitrogen and phosphorus absorbed by plants through harvesting and weighing.

Figure 1 .
Figure 1.Top view of floating body material (unit: mm)

Figure 2 .
Figure 2. Change of plant root length

Figure 3 .Figure 4 .
Figure 3. Distribution of nitrogen content in plants

Table 1 .
Physiological indexes of tested plants At the same time, two water samples from the fish pond were collected to measure their pH, water temperature, COD, TN, TP and other indicators.

Table 2 .
List of experimental water quality.

Table 3 .
List of plant composition analysis

Table 4 .
Statistics of monthly survival rate of floating bed cultivated plants