Coal resources have always been an important energy source in China, accounting for about 70% of the country’s primary energy production and consumption [1
]. This country is the largest producer and consumer of coal in the world, and the coal output of China reached 3.52 billion tons in 2017, accounting for 46% of the world’s total output and 51% of the world’s coal consumption [2
]. The regions of Shanxi, Shaanxi and Inner Mongolia account for 66.82% of China’s coal output. Open-pit coal mining has a greater efficiency, higher recovery rate, and lower cost than coal mining via underground mining. The percentage of mining in China that is done by open-pit mining has increased from 4% to 15% in recent years [3
]. The open-pit coal mining areas in China are mainly distributed in desert, hilly, and grassland areas, including those in Shanxi, Shaanxi, and Inner Mongolia, which are ecologically sensitive [4
]. While the large-scale exploitation of coal resources meets the needs of China’s economic construction, it also leads to a series of ecological and environmental problems [5
], as well as social problems [6
]. Compared with underground mining, the land damage caused by open-pit mining is more serious. In the process of open-pit coal mining, it is necessary to strip away all the of rock and soil layers above the coal seam, which will inevitably have large ecological and environmental impacts [8
]. According to incomplete statistics, the area of land damaged by open-pit coal mining in China reaches 45,000 hm2
every year [9
]. Therefore, it is imperative that the reclamation of damaged land in open-pit coal mining areas is vigorously promoted.
Soil reconstruction is the core component of land reclamation [11
], and thus the quality of reconstructed soil directly determines the quality of land reclamation that is possible. The Quality Control Standard for Land Reclamation (TD/T 1036-2013) promulgated by the former Ministry of Land and Resources in 2013 stipulated that, in the northern grassland areas of China, the effective soil layer thickness should be greater than 30 cm when the purpose of reclamation is to recreate grasslands. In the process of open-pit mining, the platform-slope form of accumulation is often used in the construction dumping areas, which causes an increase in surface area that results in there being an insufficient amount of topsoil available to be used as soil cover. Soil is a key component in many ecosystem processes, such as nutrient circulation, water balance, and litter decomposition [12
]. Soil structure and nutrient status are among the key indicators that are used to assess the restoration and maintenance of ecological functions in degraded ecosystems [13
]. And Burley et al. [14
] did several seminal works and important advances crossing top soil adaptation for vegetation growth. Therefore, the selection of topsoil substitutes has become a key part of the process of soil reconstruction in zones of topsoil scarcity.
The preferred substitutes for topsoil are generally industrial solid wastes from mining areas. Many laws and regulations concerning land reclamation in Western countries clearly stipulate that it is necessary to analyze the physicochemical properties and heavy metals content of the matrix of the soil and the overlying strata, and then select suitable substitutes for top soil before mining [17
]. For example, in German reclamation, there are fewer topsoil sources, some of which come from the topsoil stripped from mining areas, and some from artificial soil [19
]. In the United States of America (USA), when the amount of stripped topsoil is insufficient, it is necessary to find and screen suitable soil substitutes from the overburdened strata of coal seams. Baker et al. [20
] obtained a kind of stable soil by mixing a fly ash series of combustion products and sludge compost into coal gangue (pH < 2), which could be directly applied in the field. Wilson-Kokes and Skousen [21
] used weathered brown sandstone and un-weathered grey sandstone for soil reconstruction in an open-pit mine in West Virginia, USA. Through monitoring soil physical and chemical properties and measuring tree growth for eight consecutive years, that study found that the weathered brown sandstone soil had better physical and chemical properties, making it an ideal substitute material for topsoil. Inoue [22
] introduced the application of fly ash as a topsoil substitute material in the land reclamation of an open pit mining area in Indonesia. Paradela et al. [23
] tested and analyzed the basic characteristics of the slate powder produced in the open-pit mining process, and found that it could be used as a topsoil substitute material in the process of reclaiming open-pit mining. Domestic research on topsoil substitute materials in China started relatively late. Through pot and plot experiments, Ma et al. [24
] found that the application of 20% fly ash in base soil was the best composition of substitutes for topsoil among those they tested to improve the yield and growth of soybean and maize. Working in an open pit mine in Inner Mongolia, Hu et al. [26
] selected the sub-clay III layer of sub-clay as a topsoil substitute material through analyses of its physical and chemical properties seedling growth rate. On this basis, his team added peat [27
], modified straw [28
], vermiculite [29
], and humic acid [30
], and improved and optimized the formula of the substitute material to apply through assessing alfalfa growth and resisting performance. Because of the different ore-forming geological conditions that occur in different regions, the physical and chemical properties of mining solid wastes are different, so the proportion of solid wastes has strong regional characteristics. Thus, with the aim of solving regional problems, the best way to solve the problem of topsoil scarcity in mining areas is to select typical mining areas in which to carry out topsoil substitution material ratio tests, assess the impacts of the use of different reconstructed soil materials on the biomass of Melilotus officinalis
, and determine the optimal ratio of different components to include in the topsoil substitute material [31
Inner Mongolia is an important open-pit coal mining area in China, and it is also an ecologically sensitive area. The Shengli Mining Area in Inner Mongolia is located in the grassland region of eastern China, where the primary topsoil is relatively barren, which has become a serious bottleneck in the process of land reclamation in this area. This restriction causes a series of problems in land reclamation, such as insufficient soil cover thickness and poor vegetation growth. Therefore, the Shengli Mining area in Inner Mongolia was selected as a typical mining area for examination in the present study. This study was done with the aim of overcoming such obstacles in the process of land reclamation in the eastern grassland open-pit mining areas in Inner Mongolia; to do so, a method of reconstructing soil was put forward, starting from making full use of the common raw materials in the mining area; then combining the topsoil, coal gangue, fly ash, rock and soil stripping materials, and ash soil into different formulations of reconstructed soil; and then applying these in pot experiments. This was also done to reduce the cost of ecological land restoration and solve the environmental problems caused by the stacking of solid wastes, such as coal gangue and fly ash, as well as to provide a theoretical basis for soil reconstruction and the rational utilization of coal gangue and fly ash in the studied mining area.
2. Material and Methods
2.1. The Phase of Pot Experiment Design
In the pre-sampling process (Figure 1
), we found that the thickness of the topsoil is about 20 cm in the undisturbed area, the following is the calcic horizon. The sampling of the reclaimed dumping site in the study area found that the thickness of the covering soil in most areas is about 10–15 cm. Owing to the randomness of construction, some areas will have a thickness of 5–10 cm. Therefore, there are two reconstruction modes, the layered mode and mixed mode. Four groups of schemes are set up in the layering mode. The purpose of determining the suitable plant growth with different soil cover thickness is to explore whether the plant can grow normally under this highly random construction condition. The comparison between topsoil and reconstructed soil is to explore whether the reclamation mode of reconstructed soil + coal gangue will have adverse effects on plant growth. The topsoil scheme is set as the control scheme, the topsoil scheme refers to the scheme in which only topsoil is used and no other substitute materials. The scheme is used as a control scheme. The topsoil used in the experiment is more than 20 cm soil collected in the field.
The following explains why Melilotus officinalis is suitable as a test species. First of all, this species is a legume herb with the effect of fertilizing the soil. Secondly, the characteristics of grass-tolerant, barren-resistant, and alkali-tolerant soils are highly adaptable to the region. Finally, in the reclamation work before, the grass raft was also planted and grown well in the study area. On the basis of the above three points, this species is suitable as a test species. Each group was repeated three times.
2.2. Proportion Determination of Mixing Method
Before conducting the experiments’ design (Figure 1
), we consulted the relevant literature, combined with the test results of different materials, and had a preliminary understanding of different topsoil substitutes materials. Compared with topsoil, the rock and soil stripping is mainly attributable to poor nutrient status and large chunks of gravel; however, the stripping material is the material with the most similar physical properties to the surface soil. Therefore, in the reconstruction of soil, the content of stripping material should be controlled within a certain range. In the experiment, the content of stripping material is controlled below 60%. Coal gangue has a large particle size; it will cause a large loss of water, while the content of coal gangue is excessive, but coal gangue can improve the soil nutrient status. Therefore, the amount of gangue should not be excessive in the proportion. Fly ash is poor in nutrient status and is often used as a modifier. However, in order to fully demonstrate that the fly ash will cause poor plant growth, 10%, 30% and 60% are selected. It was also found that, under the condition that the content of fly ash was 60%, the leaves of the hibiscus were yellow and a large number of deaths occurred. This is also a prerequisite for our experimental design. Finally, on this basis, according to the texture, the control is in the range of loam after proportioning, according to different combinations and different proportioning tests. The experimental materials used in this study included topsoil (sandy loam soil), fly ash, coal gangue, and rock and soil stripping materials (a mixture of parent material and soil) (Table 1
). The above experimental materials were obtained from the Shengli Mining Area of Inner Mongolia. A certain amount of ash soil was also used as fertilizer.
The experiment was carried out in the greenhouse of the China University of Geosciences (Beijing), China. The diameter and height of the flowerpots used were 20 cm and 22 cm, respectively, and the thickness of the reconstructed soil applied was 20 cm. On the basis of different reconstruction methods, the experimental scheme was divided into two groups, the layered variants schemes (Table 2
) and the mixed schemes (Table 3
), while an additional scheme, the total topsoil scheme (D1), was used as a control. Each scheme was repeated three times. A gradient test was set up to examine different mixtures of topsoil, rock and soil stripping materials, fly ash, and coal gangue, in different proportions, to form different reconstructed soils. Ash soil was used as a base fertilizer before the pot experiment, and was applied to a thickness of 2 cm per pot. To ensure uniform mixing, the required materials were poured onto a piece of canvas in turn according to the mixing ratio, and then the materials were turned from the bottom to the top by hand to mix them; this procedure was repeated at least five times until all of the materials were evenly mixed.
2.3. Monitoring of Indicators
Indicators are divided into two categories (Figure 1
): biomass represents the overall growth status, and leaf width, leaf length, and root length represent the monomer growth status. The aboveground biomass of Melilotus officinalis
grown in each pot was measured by harvesting. The aboveground parts of the potted vegetation were harvested and numbered in a ready-sealed bag. After that, the samples were dried until they reached a constant weight in an indoor oven at 65 °C, and then their weights were recorded.
The leaf width and total plant height of each Melilotus officinalis plant was measured with a ruler. Three Melilotus officinalis plants in each pot were selected for the measurement of these dimensions.
After harvesting the aboveground parts of Melilotus officinalis plants, the flower pot was cut longitudinally and the root system was collected. Six roots of different lengths were selected from each pot, and their lengths were then measured with a ruler.
The experimental instruments used included a PL303 electronic balance (METTLER TOLEDO Instrument (Shanghai) Co., Ltd., Shanghai, China), a DHG-9245A electric heating blast dryer (Shanghai Yiheng Technology Co., Ltd., Shanghai, China), a ruler. The CP114 electronic balance is used to measure the biomass; the 101-2AB electric heating blast dryer is used to dry grass samples; the ruler is used to measure plant height, leaf width, and leaf length.
2.4. Data Analysis
All schemes are divided into two categories (Figure 1
): the difference analysis in layered mode and the difference analysis in mixed mode. In the mixed mode, all schemes are divided into three groups, namely, fly ash group, coal gangue group, and rock soil stripping group. Take the fly ash group as an example, select the same or similar scheme of fly ash content, form a group together with the control scheme, and analyze the difference within the group. The difference analysis is divided into two parts: one is to analyze the general trend of the influence of some substitute materials on the growth difference of Melilotus officinalis
under different content; the other is to analyze the difference of the proportion of other substitute materials on the growth of Melilotus officinalis
under the same content of some substitute materials. The data for the growth status of Melilotus officinalis
were analyzed by one-way analysis of variance (ANOVA) in SPSS 20.0 (IBM SPSS Statistics, Chicago, IL, USA). Through the results of the difference analysis, the range value of the content of each topsoil substitute material in the reconstructed soil was determined, and the best formula of the reconstructed soil was obtained in the ratio test.
4.1. Reasons for the Differences in the Physicochemical Properties of Reconstructed Soils and the Growth Status of Melilotus officinalis
The theory of soil-forming factors posits that soil is the product of multiple natural factors, such as biology, climate, parent material, topography, and time, as well as human activities. In this study, on the basis of the theory of soil-forming factors and the reconstruction of soil parent material, different soil profiles were reconstructed using solid wastes (coal gangue, rock and soil stripping materials, and fly ash) generated during mining as surface soil substitutes. Dong et al. [32
] have shown that the physicochemical properties of soils that developed from different parent materials are quite different. For instance, coal gangue has a large particle size and high organic matter content [33
]; fly ash has a large particle size [35
], is hydrophilic [36
], and has poor nutrient status [37
]; and the physical properties of rock and soil exfoliates from stripping are similar to those of the original topsoil, but with a lower organic matter content, as the organic matter content, total nitrogen content, available phosphorus, and available potassium content of the rock and soil stripping are more than double that of the topsoil. Therefore, the physicochemical properties of reconstructed soils vary greatly if different combinations of materials are used, for example, the organic matter content of different schemes vary greatly (Table 4
The effects of including materials in different ratios on the production of plant biomass were greater than those of these differences on leaf width, plant height, and root length. During the experiment, it was found that the mixture schemes resulting in lower biomass tended to have fewer plants and lower coverage, but the growth of a single plant in these cases was sometimes better; further, the schemes resulting in lower biomass but more plants tended to have poor performance in terms of leaf width or plant height. The reason for this result is that there were differences in the proportions of different components included in the soil substitute in these different schemes, resulting in differences in their physicochemical properties. Different endowments of soil resources result in different productivity of the replacement soil. Therefore, in the actual process of vegetation reconstruction, the use of a reasonable planting density and topdressing will also play important roles in the successful growth of Melilotus officinalis.
The number of plants in the potted plants is not indicated in the text. This result is derived from the data collected by the camera. The number of plants can be seen from the photographs, so this conclusion is reached. However, this result is more because of our subjective knowledge.
4.2. Reasons for the Better Growth of Melilotus officinalis under Specific Substitute Material Ratios
When the coal gangue content of the mixture used was below 30% or 10%, the growth of Melilotus officinalis
performed better in terms of all four indicators assessed. According to the physical and chemical properties of solid waste, the average organic matter content of coal gangue is 43.90 g/kg, which is about 25% higher than the organic matter content of topsoil, and the available K content of coal gangue is about 10% higher than that of topsoil. Coal gangue has a large particle size and large pores; its particle size is 2–5 cm. When stacked, there are many and large pores between coal gangue particles. This characteristic means that, when coal gangue is piled up, moisture can easily seep downward through it [38
], so the content of coal gangue in the soil replacement material should not be too high. However, including coal gangue can improve the soil nutrient status [39
]. At the same time, as the degree of weathering increases, the sizes of the internal weathering cracks in coal gangue increase, which makes coal gangue have more water-holding capacity [40
], and thus be more conducive to the growth of vegetation. However, in the actual production process, coal gangue is generally excessively produced, whereas the availability of the surface soil is limited, so the optimal amount of coal gangue used should be about 30%. When layered and overlapped coal gangue and reconstructed soil or topsoil is used, the roots of Melilotus officinalis
can penetrate into the coal gangue layer; the root length of Melilotus officinalis
is about 11–12 cm. The development of crevices in the coal gangue layer can provide the necessary water and nutrients for Melilotus officinalis
, helping this plant to grow better.
Fly ash is mainly used to improve the physical structure of clay because of its large particle size [41
], but clay did not exist in the sandy loam soil used in this study’s experiments. The hydrophilicity of fly ash has little effect on plant growth under conditions in which an adequate supply of the water required for the growth of vegetation is available or provided. Fly ash can reduce soil bulk density, increase porosity, adjust the three-phase ratio, and increase ground temperature. If used to improve sandy soil, it can increase water holding capacity and hydraulic conductivity, which is helpful to prevent crust. However, owing to the fact that there is almost no nitrogen in fly ash, it was also found that the leaves of Melilotus officinalis
began yellowing during the experiment when this material was included. According to Liebig’s law of the minimum, this characteristic thus became a limiting factor for the growth of Melilotus officinalis
. That is to say, under the conditions of an indoor pot experiment, as the fly ash content of the mixture increases, the soil nutrient condition becomes worse, resulting in lower production of Melilotus officinalis
biomass. In 2008, Chen et al. [42
] also showed that, with an increase in fly ash consumption, the growth of plants deteriorated.
The use of weathered fly ash can increase the accumulation of Se in crops [43
]. Some short-term indoor incubation experiments have found that adding un-weathered fly ash to sandy soil can inhibit microbial respiration, enzyme activity, and soil N cycling [44
]. Fly ash contains 5–30% toxic elements; especially Cd, Cu and Pb can be filtered out, which may cause soil, water, and biological pollution, especially the high content of soluble salt in weathered fly ash, which is more likely to cause groundwater pollution [46
However, according to the pot experiment results of this study, when the content of fly ash was below 10%, it had little effect on the growth of Melilotus officinalis.
4.3. Benefits and Limitations of Using Solid Waste from the Mining Industry as a Soil Reconstruction Material
The selection of a reasonable compositional scheme based on experiments using the different solid waste produced in mining processes as soil substitute materials is of great significance for land reclamation in mining areas with scarce topsoil. Firstly, this allows the problem of poor vegetation growth caused by the insufficient thickness of the overlying soil in such areas to be solved. Secondly, the cost to mining enterprises of purchasing topsoil is greatly reduced, as reconstructing the soil using substitute materials can save more than 50% of the available topsoil; for example, assuming a soil cover thickness of 30 cm, the total depth of the surface soil needed is thus less than 1500 m3 per hectare, and if the local price of surface soil is about 30 yuan/m3, then in this case, the reclamation investment cost is reduced by 45,000 yuan per hectare. Finally, the problem of the disposal of solid waste generated in the mining process is solved. It can thus be seen that using solid waste as a substitute material for topsoil has certain economic and ecological benefits. However, at the same time, the current study’s results are based on laboratory tests, so their applicability in the field is not yet known. The next step is to carry out plot experiments in the field to determine the best application scheme. The pot experiment can only simulate the conditions of field crop growth to the maximum extent, it cannot achieve complete consistency, so it will cause differences in crop growth. In the pot experiment, the mixing of different materials is more uniform, but in field work, this effect can not be achieved. In the field experiment, we found that there may be only one material in some areas, but no mixing. In the actual construction process, the compaction of reconstructed soil by large-scale machinery will result in the increase of bulk density and the decrease of porosity, which will inevitably affect the growth of plants.
(1) On the basis of the present study’s results, the use of mining solid wastes as a substitute for topsoil to sustain plant growth appears to be feasible according to analyses of the four selected indices of the growth status of Melilotus officinalis tested. This conclusion can help the owner of the mining area solve the problem of resource utilization of solid waste and scarcity of topsoil in similar areas.
(2) When the thickness of the reconstructed upper soil was greater than 10 cm, the biomass of Melilotus officinalis was higher than that obtained with pure topsoil, and when the reconstructed upper soil was placed above the natural soil, the biomass of Melilotus officinalis obtained was higher than that obtained on topsoil alone. When the amount of coal gangue added was controlled to be 30% of the mixture, the biomass of Melilotus officinalis obtained was the best, and the values of other growth indicators were also better. The overall biomass obtained with the mixture schemes containing fly ash was lower, and was obviously different from that obtained in the control scheme, but the growth of individual plants was better; therefore, the growth of Melilotus officinalis was better when the amount of fly ash added was controlled to be 10% or less. When the content of rock and soil stripping materials from mining was controlled to be 40% or less, Melilotus officinalis showed good growth in terms of all four indicators. This scheme was thus more suitable for sustaining the growth of Melilotus officinalis than all of the others tested. Also, when the ratio of topsoil, coal gangue, rock and soil stripping materials was 3:3:4, respectively, the biomass of Melilotus officinalis was the highest, and the increases in leaf width, plant height, and root length were also better. However, this conclusion comes from laboratory tests, and it is unknown whether it is applicable to the local area. This result only shows that these materials can be used as substitute materials for topsoil. In different mining areas, the specific proportion needs to be tested before it can be obtained.
(3) The biomass of Melilotus officinalis significantly differed among the schemes tested, but the other three indicators did not significantly differ among schemes in many cases. In treatments in which plants had small biomass and also tended to produce fewer plants, the growth statuses of individual plants were better. Treatments that produced average amounts of plant biomass, but more plants tended to have poor performance in terms of leaf width or plant height. Therefore, in the actual reclamation process, planting density should be reasonably arranged according to the physical and chemical properties of the soil. Of course, the best way is to reclaim land strictly according to the best proportion obtained from the experiment, but the economic and ecological conditions of different mining areas are different, so it is likely that the best proportion cannot be chosen. Therefore, the combination of substitute materials proportion and planting method can achieve the goal of reclamation better.