Comparison of Phytoremediation Potential of Nerium indicum with Inorganic Modifier Calcium Carbonate and Organic Modifier Mushroom Residue to Lead–Zinc Tailings

At present, the application of phytoremediation technology in the ecological remediation of heavy metal tailings is receiving more and more attention. In this study, the physiological and biochemical response and tolerance mechanism of woody plant Nerium indicum to Pb and Zn under different proportions of inorganic modifier calcium carbonate (C1: 5%, C2: 10%, C3: 15%) and organic modifier mushroom residue (M1: 10%, M2: 20%, M3: 30%) was compared. The results showed that the pH value has a trend of C group > M group > CK group and organic matter has a trend of M group > CK group > C group. Phosphatase activity and catalase activity has a trend of M group > C group > CK group, but catalase was more vulnerable to the calcium carbonate concentration. Both modifiers can promote the transformation of Pb, Zn, Cu, and Cd in tailings to more stable organic bound and residual states. However, the stabilization effect of mushroom residue is better, and its stability is Pb, Zn > Cd, Cu. Both modifiers can increase the biomass of Nerium indicum and the modification effect of mushroom residue is better than calcium carbonate. Pb/Zn content and accumulation in Nerium indicum organs showed root > stem > leaf in all groups. Compared with the CK group, the enrichment coefficient of Pb/Zn in C1 and M1 groups decreased, while the translocation factor of Pb/Zn in C1 and M1 groups increased. With the increase in modifier concentration, the enrichment coefficient increases about 1.75~52.94%, but the translocation factor decreases rapidly (20.01~64.46%). Clearly, both the calcium carbonate and mushroom residue amendment could promote the growth ability of Nerium indicum in lead–zinc tailings and strengthen the phytoremediation potential.


Introduction
The prevention and control of heavy metal pollution is one of the research hotspots in the environmental field [1,2]. Lead-zinc tailings are solid wastes discharged from the beneficiation process after the mining of non-ferrous metal mines. After being discharged, they are generally stacked in the tailings pond, which forms a tailings wasteland after being closed [3,4]. Lead-zinc tailings have the characteristics of high metal content, poor nutrition and poor permeability. They not only occupy a lot of land, but also have multiple metal toxicity, which makes it difficult for plants to grow [5,6]. At the same time, tailings and their wastes enter the water environment and soil environment through surface runoff, leaching and seepage [7,8]. Heavy metals in the soil will destroy plant root tissue and functional

. Physical and Chemical Properties of Tailing
The pH value of tailings was measured using the method below. Weigh 10.0 g of tailing that has passed through a 2 mm sieve into a 50 mL-tall beaker, add 25 mL of carbon dioxide-free water or calcium chloride, stir vigorously with a glass rod for 1-2 min, let it stand for 30 min, and measure with a pH meter. Organic matter content in tailings was determined by hydrated hot potassium dichromate oxidation-colorimetric method [32]. The improved BCR three-step extraction method was used to determine the content of Pb and Zn in various forms of tailings [33]. The catalase activity was determined by potassium permanganate titration, and the phosphatase activity was determined by benzyl disodium phosphate [34].

Biomass Increment, Enrichment, and Transport of Heavy Metals of Plants
The biomass of plants is determined by subtracting the dry weight at harvest from the dry weight before planting. The content of Pb, Zn, Cu, and Cd in plants and substrates was determined by flame atomic absorption spectrophotometer. The plant samples were digested by the "nitric acid-perchloric acid" electrothermal plate digestion method. Root soil samples were digested by the "aqua regia-perchloric acid" electrothermal plate digestion method [35].

Data Analysis
The data are all mean ± standard deviation. Excel was used for basic data processing. All results were tested by one-way ANOVA using the SPSS 19.0 (OriginLab Corporation, Northampton, MA, USA) statistical package. All figures were drawn using the Origin 2018 statistical package. The Duncan test at 5% probabilities was performed for later comparison to test for treatment differences. Different lowercase letters in the following figure and table indicate significant differences among different treatment groups. Bioconcentration factor (BCF), translocation factor (TLF), and transfer factor (TSF) of heavy metals were calculated as follows [36]: As shown in Figure 1, the application of both calcium carbonate and mushroom residue can improve the pH value of tailing, and the improvement effect is proportional to the proportion of the modifier. The improvement effect of calcium carbonate is better than that of mushroom residue. After adding different modifiers, the pH value of tailings showed a trend of calcium carbonate improvement groups > mushroom residue improvement groups > the control group (p < 0.01), the pH values of the improvement groups were significantly higher than those of the control group. With the increase in the proportion of the modifier, there were significant differences in the increase in pH among the three improved gradients in the improvement groups. The pH value of 15% calcium carbonate treatment increased the most. improvement groups > the control group (p < 0.01), the pH values of the improv groups were significantly higher than those of the control group. With the increase proportion of the modifier, there were significant differences in the increase in pH a the three improved gradients in the improvement groups. The pH value of 15% ca carbonate treatment increased the most.

Effects of different modifiers and their gradients on the organic matter of tail
The addition of two modifiers can effectively improve the organic matter con tailing, and the effect of mushroom residue is better than that of calcium carbon Figure 2. Under different modifier gradients, the organic matter content in t showed a trend of mushroom residue improvement groups > calcium carbonate im ment groups > the control group (p < 0.01), and the organic matter content of mush residue improvement groups was significantly higher than that of the control group 1.82 times). The effect of increasing the proportion of the two modifiers on the con organic matter in tailings is different. With the increase in proportion, there was a icant difference between 10% and 20% improvement gradients of mushroom resid provement groups, but there was no significant difference between calcium carbona provement groups. The highest content of organic matter was treated with 30% room residue. Organic matter content mg/kg

Effects of Different Modifiers and Their Gradients on the Organic Matter of Tailing
The addition of two modifiers can effectively improve the organic matter content in tailing, and the effect of mushroom residue is better than that of calcium carbonate in Figure 2. Under different modifier gradients, the organic matter content in tailings showed a trend of mushroom residue improvement groups > calcium carbonate improvement groups > the control group (p < 0.01), and the organic matter content of mushroom residue improvement groups was significantly higher than that of the control group (1.42-1.82 times). The effect of increasing the proportion of the two modifiers on the content of organic matter in tailings is different. With the increase in proportion, there was a significant difference between 10% and 20% improvement gradients of mushroom residue improvement groups, but there was no significant difference between calcium carbonate improvement groups. The highest content of organic matter was treated with 30% mushroom residue. improvement groups > the control group (p < 0.01), the pH values of the improvement groups were significantly higher than those of the control group. With the increase in the proportion of the modifier, there were significant differences in the increase in pH among the three improved gradients in the improvement groups. The pH value of 15% calcium carbonate treatment increased the most.

Effects of different modifiers and their gradients on the organic matter of tailing
The addition of two modifiers can effectively improve the organic matter content in tailing, and the effect of mushroom residue is better than that of calcium carbonate in Figure 2. Under different modifier gradients, the organic matter content in tailings showed a trend of mushroom residue improvement groups > calcium carbonate improvement groups > the control group (p < 0.01), and the organic matter content of mushroom residue improvement groups was significantly higher than that of the control group (1.42-1.82 times). The effect of increasing the proportion of the two modifiers on the content of organic matter in tailings is different. With the increase in proportion, there was a significant difference between 10% and 20% improvement gradients of mushroom residue improvement groups, but there was no significant difference between calcium carbonate improvement groups. The highest content of organic matter was treated with 30% mushroom residue. Organic matter content mg/kg

Effects of Different Modifiers and Their Gradients on Soil Enzyme Activity
It can be seen from Figure 3 that both modifiers can improve the activities of phosphatase and catalase in tailing, but the effect of mushroom residue is more significant. Under different improvement gradients of the two modifiers, the phosphatase activ-ity and catalase activity in the tailings showed a trend of mushroom residue improvement groups > calcium carbonate improvement groups > the control group (p < 0.01) in Figure 3a,b. The activity of phosphatase and catalase in mushroom residue improvement groups was significantly increased (phosphatase, 22.00-54.00 times and catalase, 1.50-1.56 times), while only the activity of catalase was significantly increased in calcium carbonate improvement groups (catalase, 1.17-1.42 times).

Effects of Different Modifiers and Their Gradients on Soil Enzyme Activity
It can be seen from Figure 3 that both modifiers can improve the activities of phosphatase and catalase in tailing, but the effect of mushroom residue is more significant. Under different improvement gradients of the two modifiers, the phosphatase activity and catalase activity in the tailings showed a trend of mushroom residue improvement groups > calcium carbonate improvement groups > the control group (p < 0.01) in Figure  3a,b. The activity of phosphatase and catalase in mushroom residue improvement groups was significantly increased (phosphatase, 22.00-54.00 times and catalase, 1.50-1.56 times), while only the activity of catalase was significantly increased in calcium carbonate improvement groups (catalase, 1.17-1.42 times). In terms of the influence on phosphatase activity, with the increase in the improved proportion, there was no significant difference among the three groups of improvement gradients of the calcium carbonate group (p > 0.05). However, there was a significant difference among the three groups of improvement gradients of the mushroom residue group (p > 0.05). This indicated that the increase in calcium carbonate proportion could not significantly improve the phosphatase activity, while the increase in mushroom residue proportion could significantly improve the phosphatase activity.
In terms of the influence on catalase activity, with the increase in the improved proportion, there were significant differences among the three improvement gradients of the calcium carbonate improvement groups. This indicated that the increase in calcium carbonate proportion could significantly improve catalase activity. There was no significant difference between the treatment with 10% and 20% of mushroom residue, but there was a significant difference between the treatment with 30% of mushroom residue and the first two groups. This indicated that the increase in mushroom residue proportion had little influence on the improvement in catalase activity.

Effects of Different Modifiers and Their Gradients on the Morphology of Heavy Metals in Tailings
The morphological content of heavy metals in the tailings under improved treatment is shown in Figure 4. In the control group, Pb mainly exists in the acid-extractable state and Fe-Mn binding state in Figure 4a. Zn and Cd mainly exist in the acid-extractable state, and Cu mainly exists in the acid-extractable state and organic binding state in Figure 4bd. After modified treatment of the two kinds of modifier, the content of Pb, Zn, Cu, and Cd showed a phenomenon of a significant decrease in the acid-extractable state and a In terms of the influence on phosphatase activity, with the increase in the improved proportion, there was no significant difference among the three groups of improvement gradients of the calcium carbonate group (p > 0.05). However, there was a significant difference among the three groups of improvement gradients of the mushroom residue group (p > 0.05). This indicated that the increase in calcium carbonate proportion could not significantly improve the phosphatase activity, while the increase in mushroom residue proportion could significantly improve the phosphatase activity.
In terms of the influence on catalase activity, with the increase in the improved proportion, there were significant differences among the three improvement gradients of the calcium carbonate improvement groups. This indicated that the increase in calcium carbonate proportion could significantly improve catalase activity. There was no significant difference between the treatment with 10% and 20% of mushroom residue, but there was a significant difference between the treatment with 30% of mushroom residue and the first two groups. This indicated that the increase in mushroom residue proportion had little influence on the improvement in catalase activity.

Effects of Different Modifiers and Their Gradients on the Morphology of Heavy Metals in Tailings
The morphological content of heavy metals in the tailings under improved treatment is shown in Figure 4. In the control group, Pb mainly exists in the acid-extractable state and Fe-Mn binding state in Figure 4a. Zn and Cd mainly exist in the acid-extractable state, and Cu mainly exists in the acid-extractable state and organic binding state in Figure 4b-d. After modified treatment of the two kinds of modifier, the content of Pb, Zn, Cu, and Cd showed a phenomenon of a significant decrease in the acid-extractable state and a significant increase in the residue state, and a small decrease in the Fe-Mn binding state and a small increase in the organic binding state. The decreased range of acid-extractable state content and the increased range of organic binding state and residue state content of four metals in mushroom residue improvement groups were higher than those in calcium carbonate improvement groups. This indicated that both of the modifiers could promote the transformation of Pb, Zn, Cu, and Cd in tailings from the unstable acidextractable and Fe-Mn binding state to the more stable organic binding state and residue state. The stabilization effect of mushroom residue on the four metals was better than that of calcium carbonate. state content and the increased range of organic binding state and residue state content of four metals in mushroom residue improvement groups were higher than those in calcium carbonate improvement groups. This indicated that both of the modifiers could promote the transformation of Pb, Zn, Cu, and Cd in tailings from the unstable acid-extractable and Fe-Mn binding state to the more stable organic binding state and residue state. The stabilization effect of mushroom residue on the four metals was better than that of calcium carbonate.  It can be seen from Figure 4 that the increase in modifier proportion can improve the stabilization degree of Pb, Zn, Cu, and Cd. The decreasing range of acid-extractable and Fe-Mn binding state content of Pb, Zn, Cu, and Cd and the increasing of organic binding and residue state content increased with the increase in modifier proportion. In the calcium carbonate treatment groups, the decreasing range of acid-extractable state content and the increasing range of residue state content of Pb, Zn, Cu, and Cd were Pb, Zn > Cu, Cd. The decreasing range of Fe-Mn binding state content and the increasing of organic binding state content were Pb, Cu > Zn, Cd. That indicated that the stabilization of calcium carbonate on Pb, Zn, Cu and Cd was Pb > Zn > Cu > Cd. In the mushroom residue treatment groups, the decreasing range of acid-extractable state content of Pb, Zn, Cu, and Cd was Pb > Zn > Cd > Cu. The decreasing range of Fe-Mn binding state content was Zn, Cd > Cu > Pb. The increasing range of organic binding state content was Cu > Pb > Cd > Zn, and the increasing range of residue state content was Zn > Cd > Pb > Cu. That indicated that the stabilization of mushroom residue on Pb, Zn, Cu, and Cd had the trend of Pb, Zn > Cd > Cu. It can be seen from Figure 4 that the increase in modifier proportion can improve the stabilization degree of Pb, Zn, Cu, and Cd. The decreasing range of acid-extractable and Fe-Mn binding state content of Pb, Zn, Cu, and Cd and the increasing of organic binding and residue state content increased with the increase in modifier proportion. In the calcium carbonate treatment groups, the decreasing range of acid-extractable state content and the increasing range of residue state content of Pb, Zn, Cu, and Cd were Pb, Zn > Cu, Cd. The decreasing range of Fe-Mn binding state content and the increasing of organic binding state content were Pb, Cu > Zn, Cd. That indicated that the stabilization of calcium carbonate on Pb, Zn, Cu and Cd was Pb > Zn > Cu > Cd. In the mushroom residue treatment groups, the decreasing range of acid-extractable state content of Pb, Zn, Cu, and Cd was Pb > Zn > Cd > Cu. The decreasing range of Fe-Mn binding state content was Zn, Cd > Cu > Pb. The increasing range of organic binding state content was Cu > Pb > Cd > Zn, and the increasing range of residue state content was Zn > Cd > Pb > Cu. That indicated that the stabilization of mushroom residue on Pb, Zn, Cu, and Cd had the trend of Pb, Zn > Cd > Cu.

Effects of Different Modifiers and Their Gradients on Growth and Tolerance of Nerium indicum 3.3.1. Effects of Different Modifiers and Their Gradients on Nerium indicum Biological Increment
Under different improvement gradients, the biomass increments of each organ and whole plant of Nerium indicum showed a trend of M3 > M2 > M1 > C3 > C2 > C1 > CK in Table 3. The biomass increment of mushroom residue improvement groups was 2.3~4.4 times as much as the control group (the whole plant biomass 108.82 ± 3.76 g/pot), while that of calcium carbonate improvement groups was 1.2~1.4 times as much as the control group. The results showed that the two modifiers could increase the biomass of Nerium indicum, and the effect of mushroom residue was better than calcium carbonate. In the modified calcium carbonate groups, with the increase in the proportion of the modifier, there was no significant difference in biomass increment among the three treatment groups (p > 0.05). In the mushroom residue treatment groups, there was a significant difference when the additive amount was 30% (p < 0.05). This indicated that the biomass of Nerium indicum could not be significantly increased by increasing the proportion of calcium carbonate, while it could be significantly increased by increasing the proportion of the mushroom residue to 30%.

Effects on Heavy Metal Content and Accumulation in Each Part of Nerium indicum
The contents of Pb and Zn in different organs of Nerium indicum treated with modifiers are shown in Table 4 group. The concentration of Pb/Zn in root is inversely proportional to the proportion of modifier, while it is directly proportional in stems and leaves. The results showed that the addition of calcium carbonate could reduce the Zn content in all organs of Nerium indicum, reduce the Pb content in root, and increase the Pb content in stem and leaf. As shown in Table 4, the addition of mushroom residue could reduce the Pb/Zn content in all organs of Nerium indicum. From this, the addition of calcium carbonate and mushroom residue modifiers can reduce the pressure of Pb/Zn on Nerium indicum. In all improvement gradients of the two kinds of improver, the accumulation of Pb and Zn in each organ of Nerium indicum showed the tendency of root > stem > leaf in Table 5. According to the control group, Pb and Zn in Nerium indicum were mainly concentrated in the root (Pb 71.85%, Zn 70.55%), and the accumulation of Zn (6.69 mg/pot) in Nerium indicum was significantly higher than that of Pb (4.44 mg/pot). In the calcium carbonate improved groups, the accumulation of Pb and Zn in the root of Nerium indicum revealed a phenomenon of a downward trend first and then an upward trend with the proportion (Pb: CK 3.19 mg/pot, C1 1.98 mg/pot, C2 2.81 mg/pot, C3 3.61, mg/pot and Zn: CK 4.72 mg/pot, C1 3.30 mg/pot, C2 4.16 mg/pot, C3 5.06 mg/pot). However, the accumulation of Pb in the stem and leaf is higher than the CK group, while the accumulation of Zn is lower than the CK group. It was consistent with the regularity of the content of Pb/Zn in each organ of Nerium indicum. The Pb/Zn content of all Nerium indicum organs in the mushroom residue improvement groups was higher than that in the control group (increase rate: 5.55-396.23%), which indicated that the addition of mushroom residue could improve the accumulation ability of Pb and Zn in Nerium indicum. Except Pb in stem, the accumulation ability is positively correlated with the proportion of mushroom residue. The biological concentration factor (BCF) of Pb and Zn in Nerium indicum under the two modifiers and their different improvement gradients are shown in Figure 5. The enrichment ability of Nerium indicum to Pb and Zn is weak, and the enrichment ability of Pb is weaker than Zn. The BCF of Pb and Zn in Nerium indicum showed a trend of CK > improvement groups (p < 0.01), indicating that the two modifiers can reduce the enrichment of Pb and Zn in Nerium indicum. Compared with the CK group, the BCF of Pb in C1 and M1 groups decreased by 42.29% and 35.49%, respectively. Compared with the CK group, the BCF of Zn in C1 and M1 groups decreased by 49.73% and 26.48%, respectively. However, the BCF of Pb and Zn increased with the modified proportion.
According to the translocation factor (TLF) of Pb and Zn in the Nerium indicum under two kinds of improvers and different improvement gradients in Figure 6, compared with the CK group, the BLF of Pb/Zn in C1, C2, and C3 groups increased by 313.51%, 230.77%, 169.49%, and 109.83%, 25.65%, −4.55%, respectively. The ability of Nerium indicum to transport Pb and Zn from underground to aboveground was enhanced after being treated with calcium carbonate, and the migration ability of Pb was stronger than that of Zn. Compared with the CK group, the BLF of Pb/Zn in M1, M2, and M3 groups increased by 10.04%, −30.81%, −60.89%, and 25.39%, −9.81%, −22.93%, respectively. On the contrary, the ability of Nerium indicum to transport Pb and Zn from underground to overground was weakened, and the migration ability of Nerium indicum to Zn was stronger than that of Pb in the mushroom residue improvement groups.  According to the translocation factor (TLF) of Pb and Zn in the Nerium indicum under two kinds of improvers and different improvement gradients in Figure 6, compared with the CK group, the BLF of Pb/Zn in C1, C2, and C3 groups increased by 313.51%, 230.77%, 169.49%, and 109.83%, 25.65%, −4.55%, respectively. The ability of Nerium indicum to transport Pb and Zn from underground to aboveground was enhanced after being treated with calcium carbonate, and the migration ability of Pb was stronger than that of Zn. Compared with the CK group, the BLF of Pb/Zn in M1, M2, and M3 groups increased by 10.04%, −30.81%, −60.89%, and 25.39%, −9.81%, −22.93%, respectively. On the contrary, the ability of Nerium indicum to transport Pb and Zn from underground to overground was weakened, and the migration ability of Nerium indicum to Zn was stronger than that of Pb in the mushroom residue improvement groups.
Compared with the control group, the TLF of Nerium indicum to Pb and Zn in the calcium carbonate improvement groups increased. The translocation factor of Pb was basically between 0.6 and 1.0, and that of Zn was basically between 0.2 and 0.6. This indicated that the ability of calcium carbonate to promote the migration of lead and zinc from underground to aboveground is enhanced. Compared with the control group, the TLF of Pb and Zn of Nerium indicum in the mushroom residue improvement groups was lower. The TLF of Zn was higher than that of Pb, but the TLF of Pb and Zn was still lower than 0.4. This indicated that the transport capacity of Pb and Zn in Nerium indicum improved by mushroom residue was very weak. However, the increase in two kinds of improver proportion can reduce the transportability of Nerium indicum to Pb and Zn. With the increase in modifier proportion, the TLF of Pb and Zn decreased, and there was no significant difference among the treatment groups. The transfer factor (TSF) of heavy metals is based on the heavy metal accumulation, which is most accurately defined as the percentage of the cumulative amount per unit time (year/season) per unit area (mu/ha) to the total absorption. It is usually used to measure the remediation capacity of plants for heavy metals in soil [37]. The transfer factor (TSF) of Pb and Zn in Nerium indicum is shown in Figure 7. Compared with the control group, the TLF of Nerium indicum to Pb and Zn in the calcium carbonate improvement groups increased. The translocation factor of Pb was basically between 0.6 and 1.0, and that of Zn was basically between 0.2 and 0.6. This indicated that the ability of calcium carbonate to promote the migration of lead and zinc from underground to aboveground is enhanced. Compared with the control group, the TLF of Pb and Zn of Nerium indicum in the mushroom residue improvement groups was lower. The TLF of Zn was higher than that of Pb, but the TLF of Pb and Zn was still lower than 0.4. This indicated that the transport capacity of Pb and Zn in Nerium indicum improved by mushroom residue was very weak. However, the increase in two kinds of improver proportion can reduce the transportability of Nerium indicum to Pb and Zn. With the increase in modifier proportion, the TLF of Pb and Zn decreased, and there was no significant difference among the treatment groups.
The transfer factor (TSF) of heavy metals is based on the heavy metal accumulation, which is most accurately defined as the percentage of the cumulative amount per unit time (year/season) per unit area (mu/ha) to the total absorption. It is usually used to measure the remediation capacity of plants for heavy metals in soil [37]. The transfer factor (TSF) of Pb and Zn in Nerium indicum is shown in Figure 7.
The transfer factor (TSF) of heavy metals is based on the heavy metal accumulation, which is most accurately defined as the percentage of the cumulative amount per unit time (year/season) per unit area (mu/ha) to the total absorption. It is usually used to measure the remediation capacity of plants for heavy metals in soil [37]. The transfer factor (TSF) of Pb and Zn in Nerium indicum is shown in Figure 7. It can be seen that the Pb transfer factor of Nerium indicum showed a trend of calcium carbonate treatment groups (C1 0.48, C2 0.44, C3 0.41) > control group (CK 0.29). Pb content was low in all organs of Nerium indicum, but its aboveground biomass was significantly higher than the control group, which led to the accumulation of Pb in the aboveground part of Nerium indicum in the improved calcium carbonate groups higher than that in the control group. This increased the TSF of Pb in Nerium indicum. Except for the C1 group (C1 0.34), the Zn content in all organs of Nerium indicum in the C2 and C3 groups (C2 0.24, C3 0.21) was much lower than that in the control group (CK 0.30). Since zinc is mainly concentrated in the root, the accumulation of Zn in the aboveground part of Nerium indicum is lower than that in the control group. Pb/Zn transfer factor is inversely proportional to calcium carbonate concentration in the calcium carbonate treatment groups. It can be seen that the Pb transfer factor of Nerium indicum showed a trend of calcium carbonate treatment groups (C1 0.48, C2 0.44, C3 0.41) > control group (CK 0.29). Pb content was low in all organs of Nerium indicum, but its aboveground biomass was significantly higher than the control group, which led to the accumulation of Pb in the aboveground part of Nerium indicum in the improved calcium carbonate groups higher than that in the control group. This increased the TSF of Pb in Nerium indicum. Except for the C1 group (C1 0.34), the Zn content in all organs of Nerium indicum in the C2 and C3 groups (C2 0.24, C3 0.21) was much lower than that in the control group (CK 0.30). Since zinc is mainly concentrated in the root, the accumulation of Zn in the aboveground part of Nerium indicum is lower than that in the control group. Pb/Zn transfer factor is inversely proportional to calcium carbonate concentration in the calcium carbonate treatment groups.
In the mushroom residue improvement groups, except for the 10% proportion treatment group (M1 0.35), the Pb transfer factor of Nerium indicum showed a trend of control group (CK 0.29) > mushroom residue treatment groups (M2 0.27, M3 0.16). The aboveground biomass of the improvement groups was significantly higher than that of the control group. Since Pb was mainly concentrated in the root, and the accumulation of Pb in the shoot was small, this led to the Pb accumulation of the control group being higher than the improvement groups. Except for the 10% proportion treatment group (M3 0.16), the Zn transfer factor of Nerium indicum showed a trend of mushroom residue treatment groups (M1 0.40, M2 0.34) > control group (CK 0.30). However, the biomass of the improvement groups was significantly higher than the control group, which led to the Zn accumulation in the aerial part of the improvement groups being higher than that of the control group. The Pb/Zn transfer factor is inversely proportional to mushroom residue concentration in the mushroom residue treatment groups.
In general, both calcium carbonate and mushroom residue can reduce the stress of Pb/Zn on Nerium indicum, promote the Nerium indicum biomass, and improve tolerance of Nerium indicum to Pb/Zn. These findings indicate that the phytoremediation technology of Nerium indicum combined with calcium carbonate or mushroom residue modifier has a good application potential in the ecological remediation of lead-zinc tailings [38].

Discussion
The abandoned land of lead-zinc tailings has a high content of heavy metals and low content of organic matter, which is not conducive to plant growth (Zhang et al. 2014). It is an effective way to restore the tailings field by planting tolerant plants combined with modifiers on it. In this study, the pH of the tailings increased after adding calcium carbonate. As a weak alkaline modifier, calcium carbonate can promote the hydrolysis of Al 3+ in soil and neutralize H + produced by hydrolysis; thus, effectively increasing the soil pH. This is similar to the experimental research results of Guo [39]. The report of Chien showed that the higher the pH value of the soil within a certain range, the higher the adsorption capacity of the soil for heavy metals [40]. This may be because after calcium carbonate enters the soil, OH − and CO 2− 3 will form precipitation with heavy metal ions [41,42], which will stabilize the heavy metal ions in the soil; thus, reducing the contents of acid-extractable and Fe-Mn binding state of Pb and Zn with high activity, and increasing the contents of organic binding state and residue state with low activity. Soil enzyme activity is highly sensitive to heavy metals, which is an important index for evaluating soil health status and determining soil heavy metal content [43]. In this study, both of the modifiers can improve the activity of phosphatase and catalase in tailing, which is consistent with the increase in organic matter content and the decrease in unstable heavy metal content in tailing.
In this study, it was found that after calcium carbonate was applied, the content of Zn in organs of Nerium indicum substantially decreased, while the content of Pb in stems and leaves increased. This may be because the morphology of heavy metals in the soil directly affects the absorption of heavy metal ions by plants [44]; thus, affecting the transport of elements from plant roots to the aboveground, and the effects on different heavy metal elements vary to different degrees [45]. Moreover, the absorption of Pb and Zn in Nerium indicum was inhibited. This may be related to Ca 2+ competing adsorption sites and ion channels with heavy metal ions after calcium carbonate is applied to the soil [46]. It may also be that the increase in soil pH after the application of calcium carbonate increases the negative charge on the surface of soil particles. That increases the adsorption strength of Pb and Zn, and forms a carbonate bound state and hydroxide precipitation; thus, reducing the migration of Pb and Zn ions in the soil and the absorption of heavy metals by crops [47]. The same results were also shown in the study of Zhao [48]. The decrease in Pb and Zn content in Nerium indicum reduced the stress degree of Nerium indicum under Pb and Zn stress and promoted the growth of Nerium indicum, so as its biomass increased [49]. At the same time, the accumulation of Pb and Zn in Nerium indicum increased. Therefore, the phenomenon of low concentration and high accumulation of Pb and Zn appeared in Nerium indicum [38].
As an improved material with high organic matter content, the mushroom residue can not only improve the physical and chemical properties of tailings, improve the water holding rate of the soil, chelate with heavy metal ions, but also reconstruct the microbial community while releasing nutrients [50][51][52]. The mushroom residue is rich in organic acids such as Fulvic acid and amino acid. These groups in the organic acids such as −COOH, −OH, and −CHO can react with heavy metal ions to form heavy metal-organic chelate [53] or generate precipitation, co-precipitation [54], etc. They may also make heavy metal ions directly adsorb on their surface [50] and reduce the content of forms with high activity of heavy metals, with active low residue in the soil. It was found that the addition of mushroom residue significantly increased the pH value of tailing, organic matter content, soil phosphatase, and catalase activities, and decreased the Pb and Zn contents in Nerium indicum, which was similar to the results of Zhang [21]. The study of Peng found that three kinds of organic industrial wastes decreased the heavy metal content in plants while increasing the nutrient element content in the substrate and increasing the soil enzyme activity, and the former and the latter showed a very significant negative correlation [55]. The effect of organic modifiers on vegetation restoration is obvious, which may be because the addition of organic modifiers can reduce the accumulation of heavy metals in plants and increase the biomass of plants after reducing the effective state content of heavy metals in the substrate. That reduces the concentration of heavy metals in plants and accumulation of heavy metals under the condition of ensuring the normal growth of plants [56,57].

Conclusions
Separate application of mushroom residue and calcium carbonate can effectively increase the pH value, organic matter content, and phosphatase and catalase activities of the tailing. It can also improve the stability of lead, zinc, copper, and cadmium in the tailing. The effect of mushroom tailings is better than that of calcium carbonate. Both the enzyme activity and metal stability in the tailings are positively correlated with the addition ratio of the modifier. Adding calcium carbonate can reduce the zinc content in all organs of Nerium indicum. The addition of mushroom residue can reduce the content of Pb and Zn in the organs of Nerium indicum, thereby reducing the stress of Pb and Zn on Nerium indicum, promoting the increase in Nerium indicum biomass, and improving the phytoremediation ability of Nerium indicum. In general, modifiers can effectively improve the adaptability and phytoremediation ability of Nerium indicum to heavy metal, and the effect of organic modifier mushroom residue is better than that of inorganic calcium carbonate.

Data Availability Statement:
The authors confirm that the data supporting the findings of this study are available within the article.