Effect of composite amendments on physicochemical properties of copper tailings repaired by herbaceous plants

Phytoremediation is considered to be the most environmentally friendly green restoration technology for dealing with mine waste. Adding amendments can improve the substrate environment for plant growth and enhance remediation efficiency. Herbaceous plants have become the preferred species for vegetation restoration in abandoned mines because of their fast greening and simple management. After 8 weeks of pot experiments in the early stage, it was shown that the plant height and fresh weight of the plants treated with 5% conditioner and 0.5% straw (C2S2) were significantly higher than those of other treatments. Considering that, in this paper, to explore the effect of composite amendments on physicochemical properties of copper tailings repaired by herbaceous plants, the untreated copper tailings were employed as the control group, whereas copper tailings repaired by ryegrass (Lolium perenne L.), vetiver grass (Chrysopogon zizanioides L.), and tall fescue (Festuca arundinacea) with or without conditioners and straw combination into the compound amendments were taken separately as the test group. After 6 months of planting, the pH, electrical conductivity, water content, available potassium, organic matter, total nitrogen, and available phosphorus in the main physical and chemical properties of copper tailings in each experimental area were analyzed. The results showed that the electrical conductivity, organic matter, and total nitrogen content of copper tailings were improved to a certain extent by planting plants without treatment. Meanwhile, compared with the control group, all indexes of planting plants showed an upward trend after adding composite amendments. Among them, pH, water content, and available potassium content of copper tailings were enhanced more obviously. Furthermore, as discovered from the gray correlation analysis results, vetiver grass planted with composite amendments has the best comprehensive effect of improving the physicochemical properties of copper tailings, followed by tall fescue and ryegrass.


Introduction
With the large-scale exploitation of copper resources, the discharge of copper tailings has increased rapidly (Yan et al. 2019). However, its comprehensive utilization rate is low, and its massive use is also difficult, resulting in a sharp increase in tailing stockpiles and a series of environmental problems, such as soil heavy metal pollution, dust pollution, and water pollution (Yi et al. 2019;Ayangbenro et al. 2018). Meanwhile, the tailing pond suffers from the risk of dam collapse, and there are serious hidden safety dangers (Jiang et al. 2020). In view of the above series of environmental problems, traditional physical, chemical, and biological treatment and utilization technologies have the disadvantages of high cost, difficult operation, producing secondary pollution easily, and being unable to effectively control the heavy metal pollution in the process of tailings utilization and efficiently utilize tailings on a large scale (Salazar-Ramírez et al. 2020;Gazitúa et al. 2021). Apart from that, the surface soil of tailings is deficient and barren and thus cannot provide an environment for vegetation to grow (Valentín-Vargas et al. 2018). It is impossible to restore the original vegetation coverage within 10 years only by natural restoration. In this case, the ecological restoration and management of mining areas gradually attract more attention (Valentín-Vargas et al. 2014). At present, the direct pollution Responsible Editor: Elena Maestri area caused by tailings in China alone has reached 100,000 hm 2 , and the indirectly polluted land is more than 1 million hm 2 . Therefore, it is of great significance to strengthen tailings management and habitat restoration.
For soil improvement in mine wasteland, soil covering, physical and chemical treatment, biological treatment, application of organic fertilizer, and other methods are mainly adopted (Sansupa et al. 2021;Zhang et al. 2018;Soldo et al. 2020). Although the use of these methods can improve the soil in abandoned mines, the effect is limited, and it is difficult to apply engineering. Different from that, phytoremediation is an in situ green remediation technology, which is considered as one of the most environmentally friendly methods of dealing with mine waste (Borowik et al. 2020). However, there are few successful cases concerning direct improvement with plants, especially for the improvement of copper tailings because the physical and chemical properties of copper tailings themselves are far from those of general mellow soils, thus being not suitable for the survival of most plants (Fu et al. 2015;Mohammadi et al. 2012). Therefore, its physical and chemical properties should be improved first for tailings pond reclamation and establishment of the ecological area. Adding a certain amendment alone to the treatment of contaminated soil often requires a large amount of modifier and should be performed repeatedly (Abbas et al. 2020). Too many amendments mixed into the soil may cause irreversible damage to the soil structure and even lead to the pollution of surface or groundwater sources (Eid et al. 2019). For example, the application of large amounts of phosphorus may result in the enrichment of nitrogen and phosphorus in the water body, increasing the risk of eutrophication (Vieira and Pazianotto 2016). Many studies have shown that the compound addition of two or more amendments can effectively enhance the physical and chemical properties of tailings. Kumar et al. (2008) added different organic amendments such as bio-sludge, domestic sludge, and bio-fertilizer to tailings, which promoted the growth of jatropha (Jatropha curcas) while enhancing the nutrition and water retention of the substrate. The application of green manure and sludge compost as composite amendments to extremely barren gold tailings improved tailings nutrients and significantly increased plant biomass and microbial biomass carbon in tailings after treatment (Ai et al. 2020).
Soil conditioner is the most effective method of ameliorating or eliminating the obstacles such as acid, clay, alkali, sand, and drought . It is mainly used to improve the physical, chemical, and biological properties of the soil that then becomes more suitable for plant growth (Ogawa et al. 2020). Its main effects are improving soil physical properties, soil structure, soil water-holding and air permeability, soil fertility, and soil water storage and fertilizer retention capacity, adjusting soil pH, cation adsorption and exchange performance, reducing the harm caused by acid soil and saline-alkali land, adsorbing and fixing heavy metal ions, weakening the migration ability of heavy metal ions in the soil, and enhancing soil biological properties (Zheng et al. 2020;Bi et al. 2017;Singh et al. 2018).
Straw is rich in N, P, K, and other nutrients (Fang et al. 2020). Studies have revealed that straw returning to the field can increase the essential elements and trace elements in soil for plant growth. Meanwhile, it can improve soil available nutrients, enhance soil fertility, change soil structure, and increase soil water storage and moisture retention capacity (Yang et al. 2018a). Therefore, this method can provide an economical and feasible option for tailing remediation. The application of crop straw in the soil cannot only improve soil structure and reduce soil erosion, but also prevent the loss of soil nutrients through leaching (Yang et al. 2018b). When crop straw is applied to soil, it changes soil microbial community and structure, increases soil organic carbon content, and reduces soil erosion (Jing et al. 2020).
In this study, copper tailings were taken as the experimental research object, and the composite amendment composed of conditioner and straw was added to copper tailings, and herbaceous plants such as ryegrass, vetiver grass, and tall fescue were sown to explore the composite amendment effects on the physical and chemical properties of copper tailings repaired by herbaceous plants. Then, findings in this study would help fully and rationally utilize sediments, save mine restoration funds, and provide scientific basis and technical support for rational utilization of amendments and artificial ecological restoration of copper tailings.

Experimental site
The test area is located in Chengmen Mountain Copper Mine, Chaisang District, Jiujiang City, Jiangxi Province (115° 50′ 2″ E, 29° 41′ 10″ N). The total mining area is about 26.7 km 2 , which is a transitional zone between mountains and lakes, to the south and east are Chengmen Mountain File Mountain. The highest elevation of the mining area is 147.8 m at the top of File Mountain, while the lowest elevation of the lake area is 13.0 m. Besides, the mining area is a subtropical humid and hot climate zone, humid and rainy, with four distinct seasons, dry and cold in winter, and hot in summer. The mean annual temperature is 17℃, when the highest monthly temperature appears in July (40.2℃), and the lowest monthly temperature is in January (− 9.7℃). Beyond that, the annual average precipitation is about 1400 mm, and the annual average relative humidity is 76-80%, belonging to the humid zone. Moreover, Chengmenshan Copper Mine currently has three tailing ponds, namely Xiongjiawa, Fengzuogou, and Liujiagou, which are adjacent to each other. The copper tailings in this experiment were taken from the Liujiagou tailings pond, whose southeast was the test area with a total area of about 160 m 2 .

Experimental materials
In view of the extremely low nutrient content and poor water retention capacity of copper tailing, peat, organic fertilizer, and water retention agent should be added. The pH of peat is acidic, which can effectively neutralize the alkalinity of copper tailing; the content of nutrient elements is rich; and the content of organic matter is high (Jarukas et al. 2021). The mesoporous structure inside of peat can provide a good "shelter" for microorganisms in copper tailing, which is conducive to microbial activities. Organic fertilizer is rich in a large number of beneficial substances, which can provide comprehensive nutrition for plants and increase the microbial content of copper tailings (Peng et al. 2021). Adding a water retaining agent can effectively inhibit water evaporation in copper tailing and increase the saturated water content of copper tailing, thereby slowing down the release rate of water from copper tailing and reducing the infiltration and loss of water in copper tailing (Wei et al. 2016). Medical stone has a porous sponge-like structure, which can effectively adsorb heavy metal elements and purify copper tailings (Ou et al. 2018). As a traditional and effective alkaline soil amendment, calcite can effectively improve the alkaline environment of copper tailings and add calcium to copper tailings (Randall et al. 2019). The master material with frankincense as copper tailing conditioner can catalyze all kinds of enzymes and make all kinds of minerals achieve conversion function (Zhang et al. 2013). Adding a binder can effectively enhance the viscosity of copper tailings (Pei and Fan 2021).
In summary, according to the hazardous composition detection report of copper tailings, the analysis of physical and chemical structure characteristics, combined with the basic nutrient requirements for plant growth, and the characteristics of local climate and environment, the research team obtained a kind of conditioner for fertilizer restoration of copper sulfide tailings after repeated experimental research in the early stage, and its components expressed in weight percentage are 50% peat, 5% medical stone, 5% calcite, 2% frankincense, 118% organic fertilizer, 2% waterretaining agents, and 1% binder. Therefore, the self-invented copper sulfide tailings conditioner was used in this study to improve the physical and chemical properties of tailings. In addition, a straw was taken from Manhu Village, Xinjian District, Nanchang City, Jiangxi Province. The three herbaceous plants selected for the experiment were ryegrass, vetiver grass, and tall fescue. This approach was based on the research group's previous experimental results (Guo et al. 2018).

Preliminary experiment
The pre-experiment was carried out on the rooftop of the 6th floor of the experimental building in the Nanchang Campus of Jiangxi University of Science and Technology for 8 weeks. Ryegrass was used as plant material for thinning 7 days after germination. Besides, every 10 seedlings were moved to a 2000-g copper tailings pot (top diameter: 23.5 cm, bottom diameter: 12.8 cm, height: 14 cm). After that, the selected plants were cultivated under six different soil treatments, namely CK (100% original tailings), C 1 (99% original tailings + 1% conditioner), S 1 (99.9% original tailings + 0.1% straw), C 1 S 1 (98.9% original tailings + 1% conditioner + 0.1% straw), C 2 S 2 (94.5% raw tailings + 5% conditioner + 0.5% straw), and C 3 S 3 (89% original tailings + 10% conditioner + 1% straw). Set 3 parallel for each group. When planting, water the fixed root thoroughly and adopt extensive management. Here, it should be noted that watering is only performed when there is no rain for a long time. After 8 weeks of growth, the ryegrass plants were harvested, and the length and fresh weight in roots and shoots were measured.

Field test
All treatments were arranged in the randomized complete block design with eight blocks (4 m × 5 m), marked as CK, CS, 1#, 2#, 3#, 4#, 5#, and 6#, respectively. Among them, CK was the copper tailing control group without taking any measures; CS was the unplanted plants with composite amendments added, and the copper tailings without any measures were planted with ryegrass, vetiver grass, and tall fescue, respectively, marked as 1#, 2#, and 3#, while the groups with composite amendments added to grow ryegrass, vetiver grass, and tall fescue were marked as 4#, 5#, and 6#, accordingly. According to the results of the previous pot experiment, the optimal chemical ratio of conditioner and straw was used to improve the original copper tailing. In late July 2021, the fermentation good conditioner was put into the copper tailing trench with a depth of 15 cm, and the conditioner was 1 kg/m 2 . After ploughing, the planting density was about 25 g/m 2 . After sowing, a straw with a length of about 50 cm was added, and the covering density was 0.1 kg/ m 2 , about 4 cm from the top of the copper tailing. Other than that, copper tailings samples (15-cm deep) from the test area were collected 6 months after sowing and sifted through a 1.2 × 1.2-cm sieve to remove larger stones. After being mixed evenly, the samples were put into a sealed sample bag by the quartering method and placed in a low-temperature incubator. The quality of copper tailings in the sealed sample bag was controlled at about 60 g, and the copper tailings were brought back to the laboratory to determine the content of water content, pH, electrical conductivity, total nitrogen, available phosphorus, available potassium, and organic matter.

Copper tailings analysis and statistical analysis
Copper tailings pH and electrical conductivity (EC) were measured using a soil-to-water ratio of 1:5 with a pH meter (Thermo, Orion 900A) and an Orion 162A conductivity meter, respectively (Ahmad et al. 2016). Water content (WC) was obtained by the drying method and the neutron deceleration method (Guimaraes et al. 2020), while available potassium (AK) was calculated using ammonium acetate extraction and flame photometry method (Shen et al. 2019). Besides, organic matter (SOC) was identified using the K 2 Cr 2 O 7 -H 2 SO 4 oxidation-reduction colorimetric method (Tong et al. 2017), whereas total nitrogen (TN) was acquired colorimetrically following micro-Kjeldahl digestion41 . Furthermore, available phosphorus (AP) was determined by molybdenum inverse colorimetry after extraction with sodium bicarbonate (Reijers et al. 2019). In addition, Excel 2010, SPSS 20 software and SPSSAU-online data analysis platform were employed for data analysis and processing, and Origin 9.1 software was used for drawing.
Then, the gray correlation analysis (Xu et al. 2019) was performed to determine the correlation between the physical and chemical properties of copper tailings and three herbaceous remediation modes, which was standardized by the following formula (1): The correlation coefficient was calculated by formula (2): The degree of association was determined by formula (3):

The plant growth responses
According to plant height ( Fig. 1a) and fresh weight ( Fig. 1b), the optimum chemical ratio of conditioner and straw as the copper tailings amendments was determined. A single application of conditioner had greater effects on growth indexes of ryegrass than a single application of straw, and the effect of a mixture application of conditioner and straw was better than a single application of conditioner or a single application of straw. Under the condition of single conditioner or straw application, the copper tailing in the C 1 treatment significantly enhanced plant growth, which were 2.21 (1.78) and 1.81 (2.05) times in plant shoots (roots) length and fresh weight, compared to the original The effects of CK (100% original tailings), C 1 (99% original tailings + 1% conditioner), S 1 (99.9% original tailings + 0.1% straw), C 1 S 1 (98.9% original tailings + 1% conditioner + 0.1% straw), C 2 S 2 (94.5% raw tailings + 5% conditioner + 0.5% straw), and C 3 S 3 (89% original tailings + 10% conditioner + 1% straw) on the length (a) and fresh weight (b) of tall fescue in shoots and roots. Error bars represented the standard deviation of three replicates. The same letter indicated no significant difference between treatments (P > 0.05) tailings (CK). The mixed application of conditioner and straw was better than the single application of conditioner or a single application of straw. Significantly, the plant height, root length, aboveground, and underground fresh weight of ryegrass, with average plant height 20.23 cm, root length 34.37 cm, aboveground fresh weight 14.83 g, and underground fresh weight 3.80 g were 237.17%, 244.73%, 248.37%, and 224.79% higher with C 2 S 2 application than the control group, followed by C 3 S 3 and C 1 S 1 . Therefore, in order to meet the demand for soil environmental quality without affecting the normal growth of plants, C 2 S 2 was used in the following field experiment.

Effects of phytoremediation modes with three different plants on the pH value of copper tailings
The pH value of the control group was 8.05, which was alkaline and not conducive to plant growth. Meanwhile, the pH value of copper tailings applied with composite amendments before planting was 7.46. The pH value of each herb planting area was measured after 6 months, and the results are shown in Fig. 2. It was found that there was a difference in the effect of the modifier compound on the pH value of copper tailings under different vegetation. After 6 months of planting ryegrass, vetiver grass, and tall fescue with composite amendments, the pH value of copper tailings decreased to about 7.0. However, the pH value of copper tailings basically did not change after planting three kinds of plants without composite amendments. The change of pH value of different plants with or without composite amendments was ranked as 2# > 1# > 3# > 5# > 4# > 6#, and the average pH values are 8. 18, 8.12, 7.91, 7.12, 7.06, and 7.03 separately.

Effects of three remediation modes with plants, on electrical conductivity and water content of copper tailings
It can be seen from Fig. 3 that the electrical conductivity in the three herb habitats without the application of composite amendments was in descending order: CK > 3# (reed fescue) > 1# (ryegrass) > 2# (vetiver grass), and the soil conductivity was 134.63 μs/cm, 120.20 μs/cm, 117.47 μs/cm, and 86.67 μs/cm, respectively. Thus, it can be seen that the electrical conductivity of the three herbaceous plant habitats was lower than that of the control group CK, indicating that herbal planting has a certain repairing effect on the electrical conductivity of copper tailings and effectively reduces the salt content of copper tailings. Moreover, the repairing Fig. 2 pH value of copper tailings in different habitats. CK (copper tailings control without taking any measures), CS (copper tailings with compound amendments added but no plants), 1# (copper tailings planted with ryegrass), 2# (copper tailings planted with vetiver grass), 3# (copper tailings planted with tall fescue), 4# (copper tailings with composite amendments added and planted with ryegrass), 5# (copper tailings with composite amendments added and planted with vetiver grass), and 6# (copper tailings with composite amendments added and planted with tall fescue). Values are means ± standard error. The same letter indicated no significant difference between treatments (P > 0.05) Fig. 3 Electrical conductivity and water content of copper tailings in different habitats. EC (electrical conductivity), WC (water content), CK (copper tailings control without taking any measures), CS (copper tailings with compound amendments added but no plants), 1# (copper tailings planted with ryegrass), 2# (copper tailings planted with vetiver grass), 3# (copper tailings planted with tall fescue), 4# (copper tailings with composite amendments added and planted with ryegrass), 5# (copper tailings with composite amendments added and planted with vetiver grass), and 6# (copper tailings with composite amendments added and planted with tall fescue). Values are means ± standard error. The same letter indicated no significant difference between treatments (P > 0.05). Values are means ± standard error. According to Duncan's multiple range test, values with different letters indicated a significant difference (P ≤ 0.05). The blue letters indicated a significant difference in the measured copper tailings water content experimental data, and the black letters indicated a significant difference in the measured electrical conductivity effect of ryegrass, vetiver grass, and tall fescue was more obvious after the application of composite amendments, which decreased by 29.61%, 31.46% and 34.34%, respectively, compared with the control group, and the electrical conductivity of the CS group decreased by 25.15%. As far as the conductivity improvement performance is concerned, the tall fescue planting after the application of the composite amendments is featured with the best effect in practical engineering practice.
As can be seen from Fig. 3, the water content of copper tailings in different habitats in descending order was 4# (ryegrass) > 6# (tall fescue) > 5# (vetiver grass) > CS > 2# (vetiver grass) > 3# (tall fescue) > 1# (ryegrass) > CK, and it was 16.16%, 15.74%, 15.57%, 14.52%, 4.59%, 4.19%, 3.80%, and2.97%, respectively. Without taking any measures to plant herbaceous plants, the water content of copper tailings increased slightly. However, after the application of composite amendments, the water content of copper tailings planted with three kinds of plants increased significantly, compared with CK: 444.10% (4#), 424.24% (5#), and 429.97% (6#), respectively, implying that the remediation by plants can play a certain role in regulating the water content of copper tailings, and the application of composite amendments is more conducive to the water content of copper tailings in the experimental area of herb remediation. Table 1 shows the available potassium, organic matter, total nitrogen, and available phosphorus content of copper tailings under the three herbal phytoremediation modes after 6 months of planting. Potassium is one of the three main nutrient elements required for plant growth, and the content of available potassium can directly reflect the potassium level provided by soil for plant utilization. The content of available potassium in copper tailings planted only with plant changed slightly, while that in the experimental group after the application of the modifier compound increased to a certain extent. Among them, the increase in the experimental area of tall fescue and vetiver grass was the most obvious, reaching 22.02 mg/ kg and 18.06 mg/kg, which were 2.66 and 2.18 times that of the control group, respectively. Apart from that, the organic matter content of the plant growth substrate is one of the important indicators that measure the nutrient content in the process of plant growth. The organic matter content of copper tailings increased after the restoration of the three kinds of plants, and the composite amendment group was more obvious, reaching 29.72 g/kg. Compared with the control group, that in the other experimental groups increased by 1.37 to 28.24%. Nitrogen content in plant growth substrates exerts an important effect on plant growth and development. After 6 months of planting, the total nitrogen content in each experimental area was 0.06 g/ kg, 0.37 g/kg, 0.09 g/kg, 0.09 g/kg, 2.92 g/kg, 2.84 g/kg, 2.63 g/kg, and 2.14 g/kg, respectively. Among them, the improvement effect of ryegrass was the most significant. The total nitrogen content of copper tailings featuring ryegrass planted with or without composite amendments was 2.84 g/ kg and 0.37 g/kg, which were 47.33 and 6.17 times that of the control group, respectively. With respect to soil available phosphorus content, it noticeably affects plant physiological activities, and low phosphorus or phosphorus deficiency environments have a direct impact on plant growth and morphology. The available phosphorus content of the three herb planting areas showed a downward trend and increased by 91.26%, 144.01%, and 173.40% in ryegrass, vetiver grass, Table 1 Nutrient content of copper tailings in different habitats CK copper tailings control without taking any measures, CS copper tailings with compound amendments added but no plants, 1# copper tailings planted with ryegrass, 2# copper tailings planted with vetiver grass, 3# copper tailings planted with tall fescue, 4# copper tailings with composite amendments added and planted with ryegrass, 5# copper tailings with composite amendments added and planted with vetiver grass, and 6# copper tailings with composite amendments added and planted with tall fescue. The results are expressed as the mean ± standard error. The same letter indicated no significant difference between treatments (P > 0.05) and tall fescue areas, accordingly, compared with that of the control group after the composite amendments were applied. In this case, it is shown that the application of composite amendments promoted the improvement of the available phosphorus content in copper tailings by plants in the experimental area, and tall fescue possessed the most obvious improvement effect.

Gray correlation analysis on physicochemical properties of copper tailings
After analyzing the data about the physical and chemical properties of the copper tailings after the restoration of the three plants, it is found that the changes of the measured indicators were different. pH (X1), electrical conductivity (X2), water content (X3), available potassium (X4), organic matter (X5), total nitrogen (X6), and available phosphorus (X7) were employed as the indicators. Then, gray correlation analysis was made to analyze the correlation degree of the data, and the correlation degree of each corresponding point was calculated (resolution coefficient = 0.5). The obtained results quantitatively evaluated the effect of each treatment on the physical and chemical properties of copper tailings. According to the principle of correlation degree analysis in grey system theory, the quality of copper tailings in "reference" series (the control group) is the lowest in the system. Therefore, the smaller the correlation degree of evaluation indicators, the better the physical and chemical properties of copper tailings. In addition, it can be seen from Table 2 that the correlation coefficient of the physicochemical properties of copper tailings after being repaired by each treatment method followed the order: 5# (0.8569) < 6# (0.8677) < 4# (0.8707) < CS (0.8722) < 1# (0.9700) < 2# (0.9910) < 3# (0.9922). The results demonstrate that the physicochemical effects of the three restoration modes with plants on the copper tailings were improved as follows: 5# > 6# > 4# > C S > 1# > 2# > 3#.

Correlation analysis of physical and chemical properties of copper tailings
On the basis of the single physical and chemical property analysis of copper tailings in the experimental area, the correlation between physical and chemical indicators of copper tailings added with composite amendments and planted with vetiver grass was further analyzed to explore the change mechanism. The correlation analysis results are shown in Fig. 4. It can be seen from the upper part of Fig. 4 that the electrical conductivity is strongly correlated with total nitrogen and organic matter, the water content is strongly correlated with total nitrogen, total nitrogen is highly correlated with available phosphorus, there is a strong correlation with organic matter, and there is a strong correlation between available potassium and available phosphorus, and also a strong correlation with total nitrogen. The lower left picture in Fig. 4 is a hierarchical clustering diagram. The correlation between the physical and chemical indicators of copper tailings in the test area is more intuitive after classification. The results showed that there was a stronger correlation between the four indicators of soil available potassium, organic matter, total nitrogen, and available phosphorus, while the remaining three indicators, namely, electrical conductivity Table 2 Initialization treatment and correlation coefficient of physicochemical properties of copper tailings CK copper tailings control without taking any measures, CS copper tailings with composite amendments added but no plants, 1# copper tailings planted with ryegrass, 2# copper tailings planted with vetiver grass, 3# copper tailings planted with tall fescue, 4# copper tailings with composite amendments added and planted with ryegrass, 5# copper tailings with composite amendments added and planted with vetiver grass, and 6# copper tailings with composite amendments added and planted with tall fescue. PH (X 1 ), electrical conductivity (X 2 ), water content (X 3 ), available potassium content (X 4 ), organic matter content (X 5 ), total nitrogen content (X 6 ), and available phosphorus content (X 7 ) were employed as the indicators. These data are calculated by the gray system theory (resolution coefficient = 0.5), which refers to Formulas (1), (2) and water content, had a strong correlation, but the correlation with pH was low. The angular order correlation matrix of the eigenvectors between the physical and chemical indicators in Fig. 4 shows that there is a stronger correlation between the physical and chemical indicators, and its indicating significance is similar to the previous sub-figures. Soil total nitrogen, available phosphorus, and available potassium were highly correlated, and water content and organic matter were strongly correlated with electrical conductivity. In order to further find out the dominant properties among the physical and chemical indexes of copper tailings in the test area, principal component analysis was carried out on the physical and chemical properties of copper tailings. The principal component analysis results show that the number of principal components is taken as two because the variance of the contribution of the third principal component is less than one. The gravel diagram of its principal components is shown in Fig. 5 It can be seen from the load coefficients of the two principal components that after adding composite amendments and planting vetiver grass, the first principal component is copper tailings total nitrogen, available phosphorus, and available potassium as the main components, while the second principal component is copper tailing organic matter, Fig. 4 Correlation coefficient matrix of basic physical and chemical properties of copper tailings in the test area. EC, electrical conductivity; WC, water content; AK, available potassium; SOC, soil organic matter content; TN, total nitrogen; AP, available phosphorus. The top left graph in Fig. 3 shows the correlation coefficients between physical and chemical indicators. In the upper right corner of Fig. 3, the correlation between the physical and chemical indicators is displayed in shades of color. The darker the color, the stronger the correlation. The sub-graph in the lower left corner of Fig. 3 shows the hierarchical clustering sequence correlation matrix between physical and chemical indicators, in which the color depth indicates the strength of the correlation. The lower right corner of Fig. 3 shows the angular order correlation matrix of the eigenvectors between the physical and chemical indicators, in which the color intensity and the size of the circle are proportional to the correlation coefficient conductivity, and pH are the main components. Among the two principal components, the first principal component tends to be dominated by the chemical properties of copper tailings, while the second principal component tends to be dominated by the physical properties of copper tailings.

Discussion
According to observations, the ryegrass planted on copper tailings were short, with withered stems and leaves, mainly due to the poor nutrients of the copper tailing matrix, low water holding capacity, and lacked organic matter, nitrogen, phosphorus, and other nutrients needed by plants, which was not conducive to the normal growth of plants (Machado et al. 2013). Compared with straw addition, the application of conditioner had a more obvious effect on the increase of plant height and fresh weight of ryegrass. The mixed application of conditioner and straw had the best effect, mainly because the organic fertilizer and peat in the conditioner contained a lot of nutrient elements, which could provide sufficient nutrients for plant growth and were beneficial to plant growth. On the other hand, the application of straw can increase the water storage capacity of a substrate and improve soil fertility (Nayan et al. 2019). However, the greater chemical proportions of conditioner and straw did not necessarily lead to better plant growth. It was in accordance with the research of Li et al. (2019) that 5%, 10%, and 15% attapulgite and biochar are added to the soil contaminated by complex heavy metals, and 10% attapulgite and 10% biochar combined application had the best growth promotion effect on ryegrass.
Soil pH value plays a significant role in the activities of soil microorganisms and in the appearance change, material metabolism, growth, and development of plants and is an extremely important factor in the physical and chemical properties of soil (Huang et al. 2021). Due to the addition of alkaline substances such as lime and calcium hydroxide during the beneficiation process for recovering Cu 2+ in mine wastewater, the copper tailings are alkaline (Dzurendova et al. 2020). The pH value of copper tailings increased a little after restoration by three kinds of plants, which might be caused by the large amount of Ca 2+ in copper tailings. When the Ca 2+ in the copper tailings is gradually saturated, the external Na + infiltrates into the soil through rainfall, etc., so that the electric charge in the copper tailings reaches a balance (Cai et al. 2021). After applying composite amendments and planting plants, the pH in the test area was between 7 and 7.5, which was neutral, and the soil nutrient elements were in the pH range with high availability. This situation is mainly because the conditioner contains a large amount of peat, and the pH of peat is acidic, which can effectively neutralize the alkaline degree of copper tailing and may be related to the selective enrichment of some basic ions in the rhizosphere of these plants to reduce soil alkalinity. . From another point of view, the application of composite amendments enhanced the improvement effect of herbaceous plants on the pH value of copper tailings.
Soil conductivity is the threshold that restricts the activity of plants and microorganisms in the soil, which directly affects soil nutrients, the transformation of pollutants in the soil, the state of existence, and availability (Sun et al. 2020). Conductivity measures the total concentration of soluble minerals in the water, while soluble minerals reflect the mineral content of the soil. The higher the concentration of soluble minerals, the higher the conductivity (Faria et al. 2018). After the application of composite amendment, the electrical conductivity of copper tailing decreased to a certain extent, which was likely due to the catalytic effect of frankincense in the conditioner on various enzymes in copper tailing, making various minerals achieve the conversion function, thus reducing the concentration of soluble minerals (Česonienė et al. 2019). Measuring the electrical conductivity of soil can directly reflect the total salt content of the soil, which is an important indicator for evaluating the degree of soil salinization and is widely used in soil salinization research (Li et al. 2016). Actually, there are significant differences in the conductivity of copper tailings under the three herbal phytoremediation modes, mainly because various salt ions dissolved in soil water will gradually migrate to the near-root zone of plants due to the absorption of roots, but considering different material absorption rates of roots, various degrees of deficit appeared in the soil mineral salt ions.
Plants absorb water through the root system to satisfy plant photosynthesis and respiration. In addition, as water is an important driving factor for enhancing the fertility of plant growth substrates, soil water content directly affects the growth and survival of plants (Lohier et al. 2014). The untreated copper tailings have extremely low water content, resulting in soil compaction and poor air permeability, thereby affecting plant root growth. The composite amendments can effectively adjust the water content of copper tailings, which in the experimental area is 15.57-16.16% after repairing with ryegrass, vetiver grass, and tall fescue. The main reason is that the water retaining agent in the conditioner can effectively inhibit water evaporation, improve the saturated water content of copper tailings, weaken the saturated water conductivity of copper tailings, slow down the speed of water release from copper tailings, and thus reduce the infiltration and loss of copper tailings water . As well as calcite and medical stone in the conditioner, the effective porosity of copper tailing was improved, which makes it more conducive to the growth and development of plant roots (Lai et al. 2017). In addition, the root system of plants and straws can reduce water and soil loss caused by surface runoff or wind erosion in an effective way, improve soil aggregate structure, and thus enhance the water retention capacity of copper tailings (Meisrimler et al. 2015).
Soil nutrients play the key role in plant growth and development (Jabborova et al. 2021). Mineral nutrients in the soil, including nitrogen, phosphorus, and potassium and other elements can be absorbed by plant roots directly or after transformation (He et al. 2019). The macronutrients such as organic matter, available potassium, and total nitrogen, and available phosphorus of copper tailings repaired by the composite amendment were all enhanced to a certain extent, which may be because the peat and organic fertilizer in the conditioner are rich in organic matter, nitrogen, phosphorus, potassium, and other nutrient elements, and straw decomposition will also increase the nutrients in copper tailings to a certain extent (Zheng et al. 2021). Compared with the CK group, the content of available potassium in the planting area where the compound amendment was applied showed an upward trend, which was mainly due to the fact that the potassium in the soil mostly came from the potassium-containing minerals in the soil parent texture (Zafarul-Hye et al. 2021). However, when the soil pH decreased, the potassium fixation ability of the soil was weakened, and the available potassium content in copper tailings increased as the slow available potassium in copper tailings was converted to available potassium. Meanwhile, H + in soil, the amount of non-specific adsorbed potassium in the soil colloid under displacement, and the content of water-soluble and exchangeable potassium (available potassium) available to plants increased (Adekiya et al. 2020). Besides, the organic matter of the copper tailings was more than that of the control group after planting the three plants without any measures, mainly because the nutritive sterility of copper tailings tended to make the plants wither and wither, and the microorganisms decomposed the litter layer or copper tailing surface to form humus, hence increasing the organic matter content in the soil (Hashimoto et al. 2020). The organic matter content of the treated herbaceous plants decreased compared with that of the composite amendments group, and ryegrass consumed the most soil organic matter, maybe because of the short growth time of herbaceous plants, and the consumption of organic matter is greater than the production, resulting in the decrease of soil organic matter content (Jung et al. 2021). Beyond that, soil total nitrogen content of different herb planting areas is different, maybe due to different plant residues and root distribution of herbaceous plants, which make plant rhizosphere soil properties, soil nitrogen cycle processes, soil microbial quantity, and life activities change, thus further leading to changes in soil nitrogen content for different herbaceous plants (Ju et al. 2020). The total nitrogen content of copper tailings in the three herb planting areas decreased to a certain extent after the application of the composite amendments, which may be due to the fact that all the plants were in the growth and development stage during the measurement period, and all were fast-growing plants featured with higher consumption of nitrogen content (Razaq et al. 2017). When the nitrogen consumption of plants was greater than the amount of nitrogen returned, the total nitrogen content in the soil showed a downward trend. Furthermore, the available phosphorus content in each herb planting area was lower than that in the control group and CS test area, respectively, maybe because the phosphorus content in the soil is affected by the change of soil pH value (Nur Aainaa et al. 2018). Water-soluble phosphorus is gradually released and absorbed by the plant, thereby reducing the available phosphorus content of copper tailings. In addition, phosphorus is easily fixed by calcium in alkaline soils, and phosphates mostly react with calcium salts in the soil to form various calcium phosphate salts with low solubility, hence reducing the availability of phosphorus and resulting in a lower content of available phosphorus in the soil (Pérez-Rodriguez et al. 2020;Naik et al. 2008).

Conclusions
In conclusion, straw is mainly used as a crop base fertilizer at present. Different from previous studies; in this study, straw and conditioner were utilized as composite amendments for copper tailing remediation, and copper tailings replaced guest soil as the plant growth substrate for mine ecological remediation. The results show that the application of composite amendments can significantly improve the physical and chemical properties of copper tailings and promote plant growth. Besides, the gray relational analysis demonstrates that planting vetiver grass after applying composite amendments had the best effect on improving the comprehensive physical and chemical properties of copper tailings. Moreover, based on this study, the effect of composite amendments on the remediation of heavy metals in copper tailings by plant and the improvement of microbial activity can be focused on in the future, for the purpose of analyzing the mechanism of composite amendments on the remediation of copper tailings by plants.
Author contributions Weiwei Wang performed the data analyses and wrote the manuscript.
Jinchun Xue contributed to the conception of the study. Jiajia You contributed significantly to the analysis. Huaqin Han and Hui Qi performed the experiment. Xiaojuan Wang helped perform the analysis with constructive discussions.