# Reclaiming Subsidized Land: An Evaluation of Coal Gangue Interlayers

Authors: Yuling Gong; Zhenqi Hu; Kevin McSweeney
Journal: Advances in Materials Science and Engineering
 (2020)

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## Abstract

Rehabilitation of areas impacted by subsidence following coal mining is a pressing need in eastern China, especially where availability of suitable soil material is limited. A field experiment was established to evaluate performance of varying layered combinations of soil and coal gangue materials as measured by maize (Zea mays L.) growth and yield. Two control treatments and eight experimental treatments were constructed. All treatments had a 30 cm surface layer of topsoil. CK1 consisted of native soil material. CK2 consisted of 50 cm gangue covered by topsoil and 40 cm subsoil. Group 1 treatments (T1–T3) had a 15 cm layer of subsoil immediately below the topsoil, underlain, respectively, by progressively thicker gangue layers (20, 30, and 40 cm) overlying another 15 cm subsoil layer, in turn underlain by gangue. Group 2 treatments (T4–T7) consisted of 40 cm subsoil. T4-T5 followed the same pattern as Group 1 except that the lower subsoil layer was 25 cm thick. T6-T7 differed from T4-T5 by having 25 cm upper and 15 cm lower subsoil layers between different thicknesses of gangue. Group3 (T8) consisted of three 10 cm layer of subsoil separating 2 gangue layers and gangue layer below. Key plant performance indicators, biomass and yield, were significantly better under CK1-2 than in the other treatments (T1–T8). Below the topsoil Eh, pH and total salt content were higher and water content was less in T1–T8 than in CK1-2, reflecting the influence of gangue in 30–70 cm zone in these treatments. The nutrient content (TN, AN, TP, OP, TK, AK, and OM) of topsoil in experimental treatments was lower than or similar to that of the control treatments. Results indicate that minimizing the adverse impacts of gangue requires a combined top-subsoil cover of at least 70 cm.


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## Body

## 1. Introduction

Fossil fuels have been the basis of energy production all over the world and 85% of the energy consumption is met by fossil fuels nowadays [1, 2]. Coal, as one of the most important fossil fuels, accounts for the majority of electricity generation in many countries [3]. Even in the year 2018, coal was responsible for 38% of total global electricity generation [4]. In China, more than 90% of coal resources come from underground coal mining and these methods can cause surface collapse and ecological and environmental damage [5]. Subsided land in China occupies about 1.35 million ha and is increasing at an annual rate of 70,000 hm2 [6]. The subsidence area will reach 2.19 million ha by 2030 in China according to the current projections for coal production and anticipated subsidence. The destruction or degradation has caused serious shortage of cultivated land in the mining areas, and the socioeconomic and ecological environments of the mining areas have also been adversely affected. The rate of mining subsidence reclamation is only about 35% at present [7]. Therefore, reclamation of subsidence caused by coal mining has become an urgent problem to be solved in China.Meanwhile, a lot of gangue is produced during coal mining and accounts for about 10–15% of total coal production [8]. Gangue is composed of carbon-bearing rocks and rocks discharged from coal mines in the process of well construction, excavation, coal mining, and coal washing [9]. It refers to the aggregation of solid waste discharged during coal mine construction and production [10]. Accumulation of gangue in China has reached 3.8 billion tons and is increasing at an annual rate of 200 million tons [11]. Gangue stockpiles occupy large land areas and also pollute the atmosphere, water, and soil and even endanger human health [12, 13]. A large amount of technology development has been invested in the utilization of gangue, but the utilization rate is less than 15% [11]. Due to the shortage of soil resources in coal mining subsidence areas, gangue is usually used as substrate for soil profile reconstruction and for ecological and environmental restoration in coal mine subsidence areas [14, 15]. Gangue is inherently infertile and has very limited water holding capacity due to its large porosity and lack of plant available nutrients [16].Up to now, soil construction on land reclaimed with gangue consists of placing local soil over gangue [17–19]. All these studies focused on the effects of different thicknesses of soil cover over gangue substrate on crop growth. These results showed that the crop yield was close to that of local farmland when the soil cover thickness was greater than 70 cm. Some studies showed that the interlayered profiles can improve the water and fertilizer retention of soil profiles [20–23]. In order to solve the problems of the poor water and fertilizer conservation of coal gangue and the scarcity of soil resources in coal mining area, we proposed evaluating interlayered profiles, in which the thickness of overlying soil in some sections is less than that of 70 cm. We hypothesised that plant growth in interlayered profiles is better than that of the traditional placement of local soils over gangue.The objective of this study is to compare plant performance of the traditional method using local soil placed over gangue with the varying interlayered combinations of soil and gangue materials as measured by maize (Zea mays L.) growth and yield. Finally, a new soil profile is determined, which is suitable for crop growth and can also save soils.


## 2. Materials and Methods

### 2.1. Experimental Field Design

The experimental site was located in Tianji town, Panji district (32°49′ and 116°48′), Huainan City, and Anhui Province (Figure1). This area is located in the transitional zone of the subtropical and warm temperate zone. Annual rainfall is 937.2 mm and yearly average temperature is 15.3°C per year. The average frost-free period is 223.7 days per year [24]. Due to abundant sunshine, abundant heat, moderate precipitation, and long frost-free period, it is suitable for rice, wheat, corn, and other grain crops [23].Figure 1
The location of experiment site.The experiment plots were constructed using soil and gangue from the adjacent coal mining subsided area in January 2018 under conditions when the groundwater level was high. The gangue used in this experiment was selected from the coal seam. The gangue is dense, large in size (10 mm–50 mm), and largely devoid of plant available nutrients. The soil is lime concretion black soil. It belongs to the vertosols according to the China Soil Taxonomy [25], US Soil Taxonomy [26], and World Reference Base for Soil Resources [27]. The lime concretion black soil is a semiaqueous soil formed under swamp vegetation on fluvial and lacustrine sediments under warm temperate and semihumid climate conditions [28]. The dominant clay mineral is montmorillonite, which is characterized by heavy texture, low organic matter content, and a large shrink-swell capacity, and is prone to desiccation and waterlogging [29]. Selected soil and gangue properties are shown in Table 1. Gangue trace element content is shown in Table 2. The experiment plots consisted of various combinations of interlayered gangue and soil. Ten different interlayer soil profiles were designed (Figure 2). There are two control treatments (CK1 and CK2) and 8 more experimental treatments (T1, T2, T3, T4, T5, T6, T7, and T8). Each treatment was replicated three times. All treatments had a 30 cm surface layer of topsoil. CK1 was filled with native soil (120 cm) and CK2 consisted of gangue (50 cm) covered by 70 cm soil (30 cm topsoil and 40 cm subsoil), which represents a traditional reclaimed soil profile in this region. Treatments T1–T8 had various combinations of soil and gangue layers as shown in Figure 1. According to the subsoils’ thickness, position, and number of the interlayers, T1–T8 can be divided into 3 Groups. Group 1 composed of T1, T2, and T3. In Group 1, thickness of the subsoil is 30 cm and each interlayer is 15 cm thick, located in the upper, middle, and lower layers, respectively. Group 2 consisted of T4, T5, T6, and T7. In Group 2, the subsoil thickness is 40 cm and the thickness of interlayers is 25 cm, 25 cm, 15 cm, and 15 cm, respectively. The interlayers are located in the middle layer, lower layer, lower layer, and middle layer, respectively. In Group 3 (T8), total subsoil thickness is 30 cm. There are two gangue interlayers separating three 10 cm layers of subsoil; they are, respectively, located in the upper layer and lower layer. These combinations were based on the orthogonal experimental design and chosen from all the schemes. The experiment consisted of 30 rectangular plots (each plot with 6 m × 7 m = 42 m2) and all plots were randomly arranged (Figure 3). A 0.5 m wide buffer area was set between plots. The experimental field was surrounded by 1 m wide soil buffer area after it was constructed. Excavating machinery was used to level soil and gangue and to compact gangue during plot construction (Figure 4).Table 1
Selected chemical and physical properties of topsoil, subsoil, and coal gangue.
Properties
Texture
Total potassium (g/kg)
Total nitrogen (g/kg)
Total phosphorus (g/kg)
Organic matter (g/kg)
Available phosphorus (mg/kg)
Alkali- hydrolysable nitrogen (mg/kg)
Available potassium (mg/kg)
pH
Sand (%)
Silk (%)
Clay (%)
Topsoil
8.35
73.98
17.67
12.56
0.71
0.16
6.99
0.88
27.93
241.1
7.62
Subsoil
9.11
74.14
16.75
13.21
0.60
0.15
5.08
1.46
46.31
247.28
7.67
Coal gangue
Gravel
8.47
1.25
0.04
82.71
0.42
13.23
352.32
10.04Table 2
Coal gangue trace elements.
S (%)
Cr (mg/kg)
Cu (mg/kg)
Zn (mg/kg)
As (mg/kg)
Pb (mg/kg)
Hg (mg/kg)
Cd (mg/kg)
Ni (mg/kg)
Trace elements content in coal gangue
0.34
40
60
90
<2
70
0.19
0.02
40
The average background of soil in Huainan [30]
—
64.93
24.6
80.81
10.45
30.47
0.02
0.06
25.74
Soil environment quality standard value in China [31]
—
350
100
300
20
350
1.0
1.0
60Figure 2
Diagram of different multilayered soil-gangue profiles for reclaiming subsided land with gangue.Figure 3
Layout of experiment plots of different interlayer soil-coal gangue profiles.Figure 4
Experiment site construction.
(a)
(b)Maize (Zea mays L.) was sown on 4 April 2018, with plant spacing of 30 cm and row spacing of 50 cm, and planting density was about 68400 plants per ha. Maize was harvested on 24 June 2018. Experiment field management followed practices used by local farmers.


### 2.2. Soil, Gangue, Plant Sampling, and Measurements

Soil and gangue samples were collected by sampling the profile at 0–10 cm, 20–30 cm, 40–50 cm, 60–70 cm, 80–90 cm, and 100–110 cm depth intervals following harvest. Three samples were randomly removed by a ring sampler (5 cmI.D × 5 cm height) at the same increments used previously for analyzing the soil water content. The method of Chen et al. [32] was used to determine field moisture content of gangue. This involved extracting gangue samples from soil pits at designated locations and weighing them in the field (M1). Subsequently, the samples were oven-dried at 100°C in the laboratory until the quality of the samples does not change and then were weighed again (M2) [33]. The gangue water content was estimated by computing (M1 − M2)/M2. Meanwhile, the soil and gangue incremental samples were mixed and then stored in insulated and tied plastic bags to prevent moisture loss and brought back to the laboratory promptly and stored at 4°C until chemical analyses were conducted [34]. Soil and gangue were air-dried for 15 d at room temperature, sieved through a 1 mm screen, and mixed, and soil and gangue were tested for pH, available phosphorus, alkali-hydrolysable nitrogen, available potassium, and electrical conductivity. The remaining air-dried soil and gangue were ground to pass through a 0.25 mm sieve to determine total nitrogen, total phosphorous, total potassium, and organic matter. The methods used to measure these element contents were the protocols of US EPA methods 9045D [35], LY/T 1233-1999 [36], DB13/T 843-2007 [37], LY/T 1236-1999 [38], HJ 802-2016 [39], NY/T1121.6-2006 [40], LY/T 1228-1999 [41], LY/T 1232-1999 [42], and LY/T 1234-1999 [43].Plant height, stem diameter, chlorophyll, and leaf areas of maize were measured at seedling, jointing, and tasseling stages (on the 30th, 51st, and 73rd day, respectively). Dry biomass and yield were measured after maize matured. At maize seeding stage, 3 plants with similar growth were selected in each plot and labeled as samples for later monitoring of maize growth. Plant height was measured as the distance from the surface of soil to the upper most extended lead tip of the plant [44]. Plant stem diameter was measured by using a vernier caliper at the first node above ground level [45]. Leaf chlorophyll was measured using a SPAD-502 chlorophyll meter [46]. Leaf area was measured following the method of Maddonni et al., which is based on multiplying 0.75 times leaf length by leaf width [47]. Dry shoot biomass samples were oven-dried for 24 h at 80°C and then weighed. Yield was measured according to Circular of the General Office of the Ministry of Agriculture, China, on issuing the Measures for the Examination and Acceptance of the Establishment of the National Grain Yield for High Yield [48].


### 2.3. Statistical Analysis

The data were analyzed by one-way analysis of variance (ANOVA) to detect differences between treatments. Treatment differences were shown using the SNK (S) test at the significance level ofP<0.05 by the SPSS 19.0 system. The standard deviation (SD) of each treatment was analyzed by SPSS19.0 program.


## 2.1. Experimental Field Design

The experimental site was located in Tianji town, Panji district (32°49′ and 116°48′), Huainan City, and Anhui Province (Figure1). This area is located in the transitional zone of the subtropical and warm temperate zone. Annual rainfall is 937.2 mm and yearly average temperature is 15.3°C per year. The average frost-free period is 223.7 days per year [24]. Due to abundant sunshine, abundant heat, moderate precipitation, and long frost-free period, it is suitable for rice, wheat, corn, and other grain crops [23].Figure 1
The location of experiment site.The experiment plots were constructed using soil and gangue from the adjacent coal mining subsided area in January 2018 under conditions when the groundwater level was high. The gangue used in this experiment was selected from the coal seam. The gangue is dense, large in size (10 mm–50 mm), and largely devoid of plant available nutrients. The soil is lime concretion black soil. It belongs to the vertosols according to the China Soil Taxonomy [25], US Soil Taxonomy [26], and World Reference Base for Soil Resources [27]. The lime concretion black soil is a semiaqueous soil formed under swamp vegetation on fluvial and lacustrine sediments under warm temperate and semihumid climate conditions [28]. The dominant clay mineral is montmorillonite, which is characterized by heavy texture, low organic matter content, and a large shrink-swell capacity, and is prone to desiccation and waterlogging [29]. Selected soil and gangue properties are shown in Table 1. Gangue trace element content is shown in Table 2. The experiment plots consisted of various combinations of interlayered gangue and soil. Ten different interlayer soil profiles were designed (Figure 2). There are two control treatments (CK1 and CK2) and 8 more experimental treatments (T1, T2, T3, T4, T5, T6, T7, and T8). Each treatment was replicated three times. All treatments had a 30 cm surface layer of topsoil. CK1 was filled with native soil (120 cm) and CK2 consisted of gangue (50 cm) covered by 70 cm soil (30 cm topsoil and 40 cm subsoil), which represents a traditional reclaimed soil profile in this region. Treatments T1–T8 had various combinations of soil and gangue layers as shown in Figure 1. According to the subsoils’ thickness, position, and number of the interlayers, T1–T8 can be divided into 3 Groups. Group 1 composed of T1, T2, and T3. In Group 1, thickness of the subsoil is 30 cm and each interlayer is 15 cm thick, located in the upper, middle, and lower layers, respectively. Group 2 consisted of T4, T5, T6, and T7. In Group 2, the subsoil thickness is 40 cm and the thickness of interlayers is 25 cm, 25 cm, 15 cm, and 15 cm, respectively. The interlayers are located in the middle layer, lower layer, lower layer, and middle layer, respectively. In Group 3 (T8), total subsoil thickness is 30 cm. There are two gangue interlayers separating three 10 cm layers of subsoil; they are, respectively, located in the upper layer and lower layer. These combinations were based on the orthogonal experimental design and chosen from all the schemes. The experiment consisted of 30 rectangular plots (each plot with 6 m × 7 m = 42 m2) and all plots were randomly arranged (Figure 3). A 0.5 m wide buffer area was set between plots. The experimental field was surrounded by 1 m wide soil buffer area after it was constructed. Excavating machinery was used to level soil and gangue and to compact gangue during plot construction (Figure 4).Table 1
Selected chemical and physical properties of topsoil, subsoil, and coal gangue.
Properties
Texture
Total potassium (g/kg)
Total nitrogen (g/kg)
Total phosphorus (g/kg)
Organic matter (g/kg)
Available phosphorus (mg/kg)
Alkali- hydrolysable nitrogen (mg/kg)
Available potassium (mg/kg)
pH
Sand (%)
Silk (%)
Clay (%)
Topsoil
8.35
73.98
17.67
12.56
0.71
0.16
6.99
0.88
27.93
241.1
7.62
Subsoil
9.11
74.14
16.75
13.21
0.60
0.15
5.08
1.46
46.31
247.28
7.67
Coal gangue
Gravel
8.47
1.25
0.04
82.71
0.42
13.23
352.32
10.04Table 2
Coal gangue trace elements.
S (%)
Cr (mg/kg)
Cu (mg/kg)
Zn (mg/kg)
As (mg/kg)
Pb (mg/kg)
Hg (mg/kg)
Cd (mg/kg)
Ni (mg/kg)
Trace elements content in coal gangue
0.34
40
60
90
<2
70
0.19
0.02
40
The average background of soil in Huainan [30]
—
64.93
24.6
80.81
10.45
30.47
0.02
0.06
25.74
Soil environment quality standard value in China [31]
—
350
100
300
20
350
1.0
1.0
60Figure 2
Diagram of different multilayered soil-gangue profiles for reclaiming subsided land with gangue.Figure 3
Layout of experiment plots of different interlayer soil-coal gangue profiles.Figure 4
Experiment site construction.
(a)
(b)Maize (Zea mays L.) was sown on 4 April 2018, with plant spacing of 30 cm and row spacing of 50 cm, and planting density was about 68400 plants per ha. Maize was harvested on 24 June 2018. Experiment field management followed practices used by local farmers.


## 2.2. Soil, Gangue, Plant Sampling, and Measurements

Soil and gangue samples were collected by sampling the profile at 0–10 cm, 20–30 cm, 40–50 cm, 60–70 cm, 80–90 cm, and 100–110 cm depth intervals following harvest. Three samples were randomly removed by a ring sampler (5 cmI.D × 5 cm height) at the same increments used previously for analyzing the soil water content. The method of Chen et al. [32] was used to determine field moisture content of gangue. This involved extracting gangue samples from soil pits at designated locations and weighing them in the field (M1). Subsequently, the samples were oven-dried at 100°C in the laboratory until the quality of the samples does not change and then were weighed again (M2) [33]. The gangue water content was estimated by computing (M1 − M2)/M2. Meanwhile, the soil and gangue incremental samples were mixed and then stored in insulated and tied plastic bags to prevent moisture loss and brought back to the laboratory promptly and stored at 4°C until chemical analyses were conducted [34]. Soil and gangue were air-dried for 15 d at room temperature, sieved through a 1 mm screen, and mixed, and soil and gangue were tested for pH, available phosphorus, alkali-hydrolysable nitrogen, available potassium, and electrical conductivity. The remaining air-dried soil and gangue were ground to pass through a 0.25 mm sieve to determine total nitrogen, total phosphorous, total potassium, and organic matter. The methods used to measure these element contents were the protocols of US EPA methods 9045D [35], LY/T 1233-1999 [36], DB13/T 843-2007 [37], LY/T 1236-1999 [38], HJ 802-2016 [39], NY/T1121.6-2006 [40], LY/T 1228-1999 [41], LY/T 1232-1999 [42], and LY/T 1234-1999 [43].Plant height, stem diameter, chlorophyll, and leaf areas of maize were measured at seedling, jointing, and tasseling stages (on the 30th, 51st, and 73rd day, respectively). Dry biomass and yield were measured after maize matured. At maize seeding stage, 3 plants with similar growth were selected in each plot and labeled as samples for later monitoring of maize growth. Plant height was measured as the distance from the surface of soil to the upper most extended lead tip of the plant [44]. Plant stem diameter was measured by using a vernier caliper at the first node above ground level [45]. Leaf chlorophyll was measured using a SPAD-502 chlorophyll meter [46]. Leaf area was measured following the method of Maddonni et al., which is based on multiplying 0.75 times leaf length by leaf width [47]. Dry shoot biomass samples were oven-dried for 24 h at 80°C and then weighed. Yield was measured according to Circular of the General Office of the Ministry of Agriculture, China, on issuing the Measures for the Examination and Acceptance of the Establishment of the National Grain Yield for High Yield [48].


## 2.3. Statistical Analysis

The data were analyzed by one-way analysis of variance (ANOVA) to detect differences between treatments. Treatment differences were shown using the SNK (S) test at the significance level ofP<0.05 by the SPSS 19.0 system. The standard deviation (SD) of each treatment was analyzed by SPSS19.0 program.


## 3. Results

In order to analyze the effect of different soil profiles on the growth of maize, maize growth (plant height, stem diameter, chlorophyll content, and leaf areas) at different stages (seedling, jointing, and tasseling stage) and biomass, yield, soil water content, and nutrients after harvest were analyzed. The following results from the study were obtained.


### 3.1. Plant Height, Stem Diameter, Chlorophyll Content, and Leaf Areas

The planting height, steam diameter, chlorophyll, and leaf areas of different treatments at seedling stage, jointing stage, and tasseling stage are shown in Figure5(a). It can be seen that the differences of each treatment at jointing and tasseling stages were significant (P<0.05) but there was a nonsignificant difference of maize growth among the different treatments at seedling stage. Maize growth in T8 was the poorest during the experiment period. Notably, at jointing and tasseling stages, there was significantly better maize growth between other treatments and T8. Maize growth in CK1 was similar to that in CK2. This indicated that a 70 cm cover of soil above a coal gangue layer was superior for maize growth compared with other treatments.Figure 5
Planting height, stem diameter, chlorophyll, and leaf areas of different treatments at seedling, jointing, tasseling, and mature stages.
(a)
(b)
(c)
(d)


### 3.2. Dry Biomass and Corn Yield

The characteristics of above-ground dry biomass and yield in different experimental treatments were similar (Figure6). There were significant differences of biomass or yield between control treatments (CK1 and CK2) and experimental treatments (T1–T8). The biomass and yield of the control treatments were significantly higher than those of the experimental treatments. The average biomass and yield of the control treatments were 1.30 times and 1.43 times higher than those of the experimental treatments, respectively. The biomass and yield values of CK1 were largest and were 220.70 g and 8739.15 kg/ha, respectively, followed by CK2. The biomass and yield values of CK2 were 207.27 g and 8239.60 kg/ha, respectively. There was no significant difference of biomass or yield between CK1 and CK2. The biomass and yield of CK2 were 1.25 times and 1.45 times higher than those of Group 1, respectively. The biomass and yield of CK2 were 1.23 times and 1.31 times higher than those of Group 2, respectively. The biomass and yield of CK2 were 1.38 times and 1.58 times higher than those of Group 3, respectively. The difference of biomass and yield between Group 2 and CK2 was the smallest, followed by Group 1, and the difference between Group 3 and CK2 was the largest. Due to the fact that biomass and yield of CK2 were obviously higher than those of the experimental treatments, CK2 can be adopted when lime concretion black soil and gangue were used as filling materials.Figure 6
Dry biomass and corn yield of different treatments at mature stages.
(a)
(b)


### 3.3. Soil and Coal Gangue Attributes

#### 3.3.1. Water Content

Soil and coal gangue water content in different interlayered soil profiles are shown in Figure7. With depth increase, water content of soil or gangue increased. There was no significant difference of topsoil (0–30 cm) water content between control experiments and Group 1 and Group 2. However, the difference of topsoil water content between control experiments and Group 3 was significant. At the same depth, the soil water content was significantly higher than that of gangue. This indicated coal gangue has poor water holding capacity creating a hydrological discontinuity. The water content of interlayered soil was higher than or similar to that of CK1 or CK2. This indicated that gangue has poor water holding capacity and the interlayer has larger water holding capacity.Figure 7
Soil water content of interlayered profiles.
(a)
(b)
(c)


#### 3.3.2. Acidity Alkalinity (pH), Electrical Conductivity (Eh), and Total Salt Content

There was a significant difference of pH value at topsoil (0–10 cm) among T8 (Group 3) and control treatments (Table3). The gangue layer of T8 was closer to the soil surface than in other treatments. This resulted in slower water infiltration and limited base cation leaching, causing a relatively high degree of base saturation and higher pH value [49]. Meanwhile, differences in pH values in all treatments at 20–30 cm were not significant. Moreover, there were significant differences in Eh values or total soil salt content in the topsoil (0–10 cm) among T8 and control treatments. There were no significant differences of these three indices topsoils among all experimental treatments (T1–T8). At the same depth, the pH value, Eh value, and total soil salt content at the interlayer position were higher than or similar to those of CK1 and CK2. Values of pH, Eh, and total salt content of gangue were significantly higher than those of soil, which were 1.15 times, 1.40 times, and 1.44 times those in soil, respectively. So measures should be taken to reduce the values of pH, Eh, and total soil salt content of gangue and interlayered soil.Table 3
Acidity alkalinity, electrical conductivity (Eh), and total soil salt content.
Treatment
0–10 cm
20–30 cm
40–50 cm
60–70 cm
80–90 cm
100–110 cm
pH
CK1
7.40 ± 0.09c
7.32 ± 0.11a
7.60 ± 0.33c
7.59 ± 0.26b
7.57 ± 0.12b
7.55 ± 0.14d
CK2
7.42 ± 0.01abc
7.37 ± 0.16a
7.58 ± 0.25c
7.68 ± 0.00b
8.92 ± 0.26a
8.94 ± 0.08ab
T1
7.53 ± 0.19abc
7.61 ± 0.18a
9.00 ± 0.44a
7.70 ± 0.18b
8.87 ± 0.14a
9.14 ± 0.13a
T2
7.58 ± 0.05abc
7.61 ± 0.22a
8.02 ± 0.29bc
8.44 ± 0.26a
7.50 ± 0.10b
8.00 ± 0.10c
T3
7.67 ± 0.05ab
7.68 ± 0.13a
8.25 ± 0.28abc
8.61 ± 0.37a
7.48 ± 0.06b
8.84 ± 0.31ab
T4
7.47 ± 0.03abc
7.56 ± 0.10a
8.19 ± 0.35abc
7.60 ± 0.04b
7.69 ± 0.10b
8.69 ± 0.13ab
T5
7.44 ± 0.02abc
7.45 ± 0.15a
8.57 ± 0.28ab
8.80 ± 0.10a
7.71 ± 0.20b
8.77 ± 0.14ab
T6
7.43 ± 0.05abc
7.45 ± 0.11a
7.41 ± 0.11c
8.90 ± 0.27a
7.56 ± 0.06b
8.61 ± 0.12b
T7
7.46 ± 0.01abc
7.39 ± 0.16a
7.49 ± 0.16c
8.53 ± 0.19a
7.86 ± 0.04b
8.93 ± 0.07ab
T8
7.68 ± 0.06a
7.71 ± 0.08a
8.95 ± 0.04a
8.77 ± 0.15a
7.62 ± 0.12b
9.11 ± 0.10a
EC (us/cm)
CK1
63.00 ± 2.45d
75.33 ± 0.47c
94.00 ± 3.74c
95.33 ± 5.73b
98.67 ± 8.05c
109.33 ± 13.42a
CK2
73.33 ± 4.64cd
83.00 ± 3.74bc
100.33 ± 2.36c
107.33 ± 7.72b
159.00 ± 4.32a
152.00 ± 6.16b
T1
96.00 ± 5.89ab
101.00 ± 4.08abc
138.67 ± 2.05a
112.33 ± 4.50b
148.33 ± 4.03a
151.33 ± 12.36b
T2
101.00 ± 4.32a
109.33 ± 4.19ab
135.33 ± 4.99a
153.67 ± 10.50a
108.00 ± 9.09bc
157.33 ± 19.95b
T3
102.00 ± 2.94a
112.33 ± 8.73ab
134.67 ± 4.78a
151.33 ± 8.99a
112.67 ± 9.39bc
169.67 ± 9.67b
T4
93.67 ± 8.73abc
99.67 ± 6.02abc
139.33 ± 4.19a
114.00 ± 11.43b
112.00 ± 6.48bc
163.00 ± 1.63b
T5
89.00 ± 8.49abc
91.67 ± 17.33abc
133.67 ± 4.50a
162.33 ± 7.04a
111.33 ± 5.44bc
166.00 ± 8.04b
T6
86.50 ± 0.41abc
94.67 ± 1.25abc
112.00 ± 7.35b
158.00 ± 5.72a
114.67 ± 4.64bc
153.67 ± 11.44b
T7
78 ± 14.90bcd
84.67 ± 9.00bc
115.00 ± 6.68b
160.67 ± 7.59a
121.00 ± 2.16b
159.33 ± 21.82b
T8
104.67 ± 5.73a
118.67 ± 20.55a
146.00 ± 3.56a
152.33 ± 4.11a
124.17 ± 9.85b
160.67 ± 9.74b
Total salt content (g/kg)
CK1
0.20 ± 0.01e
0.24 ± 0.00b
0.32 ± 0.05b
0.35 ± 0.04b
0.31 ± 0.03b
0.31 ± 0.02a
CK2
0.27 ± 0.02c
0.25 ± 0.05b
0.32 ± 0.01b
0.36 ± 0.04b
0.51 ± 0.04a
0.51 ± 0.03b
T1
0.30 ± 0.01ab
0.32 ± 0.02ab
0.53 ± 0.06a
0.43 ± 0.02ab
0.54 ± 0.03a
0.55 ± 0.04b
T2
0.30 ± 0.01ab
0.32 ± 0.01ab
0.52 ± 0.04a
0.51 ± 0.02a
0.44 ± 0.02ab
0.51 ± 0.03b
T3
0.32 ± 0.01a
0.35 ± 0.01ab
0.52 ± 0.04a
0.52 ± 0.04a
0.44 ± 0.02ab
0.54 ± 0.03b
T4
0.28 ± 0.01bc
0.30 ± 0.00ab
0.51 ± 0.04a
0.43 ± 0.02ab
0.43 ± 0.02ab
0.52 ± 0.01b
T5
0.28 ± 0.00bc
0.27 ± 0.03ab
0.51 ± 0.14a
0.52 ± 0.02a
0.44 ± 0.03ab
0.52 ± 0.02b
T6
0.23 ± 0.00d
0.29 ± 0.06ab
0.33 ± 0.02b
0.51 ± 0.04a
0.44 ± 0.03ab
0.52 ± 0.04b
T7
0.23 ± 0.02d
0.26 ± 0.03b
0.33 ± 0.02b
0.54 ± 0.03a
0.43 ± 0.02ab
0.51 ± 0.03b
T8
0.33 ± 0.01a
0.38 ± 0.07a
0.51 ± 0.04a
0.51 ± 0.05a
0.44 ± 0.09ab
0.54 ± 0.02b


#### 3.3.3. Total and Available Nutrients

The content of total nitrogen, available nitrogen, total phosphorus, available phosphorus, total potassium, available potassium, and organic matter of topsoil in all experimental treatments was lower than or similar to the control treatments (Tables4–7). But the content of these total and available nutrients of the interlayers in all experimental treatments was higher than or the same as the control treatments. The available nitrogen, total phosphorus, available phosphorus, and total potassium contents of soil were larger than those of gangue, except that the total nitrogen, available potassium, and organic matter contents of soil were lower than those of gangue. Due to the high carbon content in gangue, the nitrogen and organic matter content in gangue were very high. However, nitrogen and organic matter in gangue are not readily available to plants.Table 4
Total and available nitrogen of interlayered soil profiles.
Treatment
0–10 cm
20–30 cm
40–50 cm
60–70 cm
80–90 cm
100–110 cm
TN (g/kg)
CK1
0.56 ± 0.03a
0.50 ± 0.06a
0.43 ± 0.02b
0.43 ± 0.02b
0.38 ± 0.05d
0.44 ± 0.37e
CK2
0.53 ± 0.03a
0.49 ± 0.05a
0.41 ± 0.02b
0.45 ± 0.04b
1.26 ± 0.03a
1.28 ± 0.03bc
T1
0.44 ± 0.03b
0.34 ± 0.02b
1.25 ± 0.09a
0.47 ± 0.01b
1.18 ± 0.01b
1.26 ± 0.02bc
T2
0.43 ± 0.02b
0.33 ± 0.04b
1.13 ± 0.08a
1.13 ± 0.08a
0.40 ± 0.03d
1.20 ± 0.05bcd
T3
0.41 ± 0.01b
0.34 ± 0.05b
1.33 ± 0.13a
1.41 ± 0.27a
0.43 ± 0.02d
1.32 ± 0.08ab
T4
0.44 ± 0.02b
0.35 ± 0.05b
1.24 ± 0.05a
0.47 ± 0.6b
0.58 ± 0.04c
1.14 ± 0.05cd
T5
0.45 ± 0.01b
0.35 ± 0.05b
1.32 ± 0.09a
1.26 ± 0.07a
0.54 ± 0.02c
1.40 ± 0.02a
T6
0.45 ± 0.04b
0.36 ± 0.04b
0.41 ± 0.02b
1.22 ± 0.01a
0.55 ± 0.06c
1.15 ± 0.07cd
T7
0.46 ± 0.01b
0.37 ± 0.04b
0.38 ± 0.02b
1.36 ± 0.01a
0.55 ± 0.05c
1.03 ± 0.05d
T8
0.40 ± 0.03b
0.32 ± 0.04b
1.36 ± 0.19a
1.26 ± 0.03a
0.40 ± 0.03d
1.18 ± 0.01bcd
AN (mg/kg)
CK1
25.19 ± 0.98a
21.85 ± 1.38a
18.89 ± 0.44a
15.16 ± 0.38b
14.52 ± 0.12a
13.55 ± 1.41a
CK2
24.96 ± 1.31a
21.27 ± 1.55a
18.29 ± 0.77a
14.09 ± 0.25b
9.94 ± 0.38b
10.30 ± 0.06b
T1
21.24 ± 1.50b
17.61 ± 0.24bc
10.63 ± 0.95c
14.73 ± 0.69b
9.96 ± 0.40b
8.69 ± 0.11bc
T2
19.96 ± 1.56b
15.74 ± 0.16bc
10.40 ± 0.60c
9.97 ± 0.20c
15.53 ± 1.32a
9.53 ± 0.27bc
T3
19.33 ± 0.25b
15.79 ± 0.02bc
10.48 ± 0.50c
9.81 ± 0.80c
14.68 ± 0.78a
10.25 ± 0.57b
T4
20.06 ± 0.41b
18.09 ± 0.72b
10.35 ± 0.50c
16.35 ± 0.28a
15.52 ± 0.54a
8.14 ± 0.33c
T5
21.94 ± 0.55b
15.92 ± 0.60bc
10.63 ± 0.59c
9.78 ± 0.69c
15.91 ± 0.60a
9.40 ± 0.39bc
T6
21.32 ± 0.91b
17.51 ± 0.35bc
15.53 ± 1.58b
9.71 ± 0.42c
15.27 ± 0.20a
10.25 ± 0.67b
T7
21.94 ± 1.16b
17.64 ± 0.87bc
15.43 ± 0.35b
9.79 ± 0.54c
15.22 ± 0.81a
10.09 ± 0.28b
T8
19.06 ± 0.13b
15.12 ± 0.48c
10.16 ± 0.42c
9.80 ± 0.39c
14.66 ± 0.94a
9.35 ± 0.24bcTable 5
Total and available phosphorus of interlayered soil profiles.
Treatment
0–10 cm
20–30 cm
40–50 cm
60–70 cm
80–90 cm
100–110 cm
TP (g/kg)
CK1
0.40 ± 0.04a
0.38 ± 0.05a
0.34 ± 0.03a
0.32 ± 0.02a
0.32 ± 0.04a
0.33 ± 0.02a
CK2
0.38 ± 0.05a
0.37 ± 0.05a
0.32 ± 0.02a
0.31 ± 0.02a
0.23 ± 0.03b
0.26 ± 0.01b
T1
0.34 ± 0.03a
0.33 ± 0.02a
0.21 ± 0.02b
0.33 ± 0.01a
0.20 ± 0.02b
0.24 ± 0.01b
T2
0.35 ± 0.03a
0.31 ± 0.02a
0.21 ± 0.03b
0.21 ± 0.02b
0.33 ± 0.02a
0.22 ± 0.02b
T3
0.33 ± 0.02a
0.32 ± 0.06a
0.21 ± 0.03b
0.21 ± 0.01b
0.32 ± 0.03a
0.24 ± 0.01b
T4
0.36 ± 0.04a
0.32 ± 0.03a
0.21 ± 0.01b
0.32 ± 0.01a
0.32 ± 0.03a
0.23 ± 0.03b
T5
0.34 ± 0.03a
0.33 ± 0.04a
0.21 ± 0.02b
0.22 ± 0.03b
0.33 ± 0.03a
0.26 ± 0.02b
T6
0.34 ± 0.02a
0.33 ± 0.02a
0.34 ± 0.01a
0.23 ± 0.02b
0.33 ± 0.01a
0.25 ± 0.01b
T7
0.35 ± 0.02a
0.34 ± 0.04a
0.33 ± 0.03a
0.22 ± 0.03b
0.34 ± 0.01a
0.24 ± 0.02b
T8
0.33 ± 0.02a
0.32 ± 0.04a
0.19 ± 0.02b
0.21 ± 0.01b
0.32 ± 0.04a
0.23 ± 0.01b
OP (mg/kg)
CK1
8.78 ± 0.22a
8.35 ± 0.26a
7.53 ± 0.77a
6.26 ± 0.13a
6.14 ± 0.35a
6.22 ± 0.11a
CK2
8.61 ± 0.12ab
7.94 ± 0.78a
7.23 ± 0.16a
6.44 ± 0.11a
4.48 ± 0.33b
4.90 ± 0.16b
T1
7.67 ± 0.59bc
5.27 ± 0.43b
4.72 ± 0.62a
6.30 ± 0.07a
4.25 ± 0.16b
4.44 ± 0.25b
T2
6.75 ± 0.37c
5.86 ± 0.41b
4.68 ± 0.31c
4.50 ± 0.21b
6.24 ± 0.15a
4.34 ± 0.31b
T3
6.70 ± 0.33c
5.90 ± 0.29b
4.74 ± 0.28c
4.60 ± 0.33b
6.23 ± 0.27a
4.33 ± 0.39b
T4
7.64 ± 0.27bc
5.76 ± 0.67b
4.80 ± 0.25c
6.70 ± 0.36a
6.71 ± 0.41a
4.31 ± 0.25b
T5
6.95 ± 0.84c
5.65 ± 0.17b
4.81 ± 0.41c
4.79 ± 0.52b
6.74 ± 0.40a
4.21 ± 0.20b
T6
7.08 ± 0.12c
6.19 ± 0.60b
5.74 ± 0.31bc
4.76 ± 0.42b
6.98 ± 0.35a
4.47 ± 0.23b
T7
7.18 ± 0.14c
6.30 ± 0.20b
6.16 ± 0.28b
4.88 ± 0.44b
6.95 ± 0.73a
4.67 ± 0.28b
T8
6.69 ± 0.32c
5.15 ± 0.16b
4.67 ± 0.22c
4.25 ± 0.18b
6.18 ± 0.27a
4.27 ± 0.49bTable 6
Total and available potassium of interlayered soil profiles.
Treatment
0–10 cm
20–30 cm
40–50 cm
60–70 cm
80–90 cm
100–110 cm
TK (g/kg)
CK1
18.80 ± 0.10a
18.78 ± 0.38a
18.09 ± 0.42a
17.57 ± 0.21a
17.10 ± 0.24a
17.41 ± 0.29a
CK2
19.05 ± 0.59a
18.85 ± 0.75a
17.85 ± 0.59ab
15.36 ± 0.12b
11.50 ± 0.11c
11.42 ± 0.28b
T1
18.78 ± 0.08ab
18.18 ± 0.17ab
11.55 ± 0.08c
17.68 ± 0.31a
12.14 ± 0.27b
11.29 ± 0.21bc
T2
17.90 ± 0.13abc
17.33 ± 0.31b
11.56 ± 0.31c
11.42 ± 0.35c
17.45 ± 0.35a
11.35 ± 0.12bc
T3
18.75 ± 0.79ab
17.21 ± 0.17b
10.76 ± 0.33 cd
10.68 ± 0.13 cd
17.18 ± 0.10a
11.30 ± 0.12bc
T4
17.57 ± 0.21bc
17.80 ± 0.23ab
10.05 ± 0.10d
17.79 ± 0.33a
17.40 ± 0.17a
11.08 ± 0.10bc
T5
18.56 ± 0.07ab
17.91 ± 0.37ab
10.76 ± 0.19 cd
10.25 ± 0.17d
17.17 ± 0.22a
10.39 ± 0.12d
T6
17.69 ± 0.51bc
18.05 ± 0.54ab
17.45 ± 0.39ab
10.35 ± 0.24d
17.21 ± 0.24a
10.84 ± 0.17c
T7
18.56 ± 0.27ab
18.01 ± 0.41ab
17.19 ± 0.29b
10.90 ± 0.39 cd
17.73 ± 0.11a
10.80 ± 0.09c
T8
17.36 ± 0.22c
17.16 ± 0.30b
11.18 ± 0.14c
11.14 ± 0.51 cd
17.34 ± 0.15a
11.10 ± 0.07bc
AK (mg/kg)
CK1
215.80 ± 3.69a
208.87 ± 8.04a
203.67 ± 12.44ab
200.20 ± 12.91ab
194.13 ± 12.15a
197.60 ± 10.29a
CK2
218.40 ± 2.14a
210.30 ± 12.85a
190.27 ± 19.27ab
188.50 ± 15.07b
227.93 ± 14.59a
230.53 ± 10.36a
T1
198.47 ± 5.87b
187.60 ± 11.03ab
225.07 ± 13.58ab
204.00 ± 13.64ab
232.07 ± 13.36a
229.67 ± 12.97a
T2
197.6 ± 4.25b
181.53 ± 6.13ab
233.87.87 ± 17.48a
232.27 ± 13.92ab
201.20 ± 12.51a
237.47 ± 14.30a
T3
196.73 ± 1.52b
185.00 ± 3.68ab
220.80 ± 17.76ab
240.93 ± 13.93a
204.10 ± 13.96a
227.40 ± 13.41a
T4
204.53 ± 2.47b
189.40 ± 4.90ab
235.60 ± 10.00a
206.97 ± 12.20ab
201.07 ± 14.97a
225.80 ± 14.16a
T5
199.33 ± 4.42b
191.27 ± 7.44ab
227.40 ± 16.05ab
223.73 ± 10.80ab
206.40 ± 14.79a
229.53 ± 14.27a
T6
201.07 ± 6.49b
190.20 ± 14.86ab
182.80 ± 10.99b
228.80 ± 12.42ab
205.07 ± 13.90a
226.27 ± 13.15a
T7
207.20 ± 4.61b
192.93 ± 8.49ab
180.60 ± 16.33b
230.33 ± 10.62ab
207.53 ± 17.98a
220.60 ± 13.87a
T8
196.73 ± 2.45b
175.53 ± 5.03b
222.07 ± 12.49ab
225.60 ± 13.75ab
209.17 ± 12.43a
222.73 ± 10.82aTable 7
Organic matter of soil and coal gangue of interlayered soil profiles.
Treatment
0–10 cm
20–30 cm
40–50 cm
60–70 cm
80–90 cm
100–110 cm
OM (g/kg)
CK1
6.26 ± 0.05a
5.96 ± 0.30a
5.53 ± 0.43f
3.36 ± 0.43e
3.41 ± 0.29b
3.40 ± 0.28f
CK2
6.13 ± 0.23a
5.85 ± 0.40a
5.51 ± 0.02f
4.63 ± 0.38e
96.99 ± 1.76a
70.38 ± 4.64de
T1
5.07 ± 0.07b
4.26 ± 0.18b
77.34 ± 3.40d
3.77 ± 0.17e
97.81 ± 4.78a
83.42 ± 2.88c
T2
4.83 ± 0.28b
4.20 ± 0.21b
71.29 ± 0.38e
79.36 ± 3.55d
3.72 ± 0.08b
67.72 ± 3.47e
T3
4.76 ± 0.54b
4.08 ± 0.26b
80.05 ± 1.85d
117.31 ± 3.24a
3.66 ± 0.05b
91.21 ± 1.24b
T4
4.99 ± 0.13b
4.29 ± 0.12b
107.45 ± 1.33c
5.09 ± 0.58e
4.38 ± 0.23b
74.85 ± 1.37de
T5
5.18 ± 0.46b
4.36 ± 0.62b
138.23 ± 1.24a
107.34 ± 5.24b
3.92 ± 0.41b
106.95 ± 2.11a
T6
5.28 ± 0.17b
4.32 ± 0.13b
4.45 ± 0.26f
93.11 ± 2.41c
4.12 ± 0.64b
102.17 ± 2.04a
T7
5.32 ± 0.58b
4.37 ± 0.21b
4.12 ± 0.08f
106.44 ± 3.42b
4.01 ± 0.47b
76.74 ± 3.27d
T8
4.65 ± 0.27b
3.94 ± 0.66b
120.20 ± 3.26b
106.41 ± 2.87b
3.56 ± 0.33b
71.73 ± 0.94de


## 3.1. Plant Height, Stem Diameter, Chlorophyll Content, and Leaf Areas

The planting height, steam diameter, chlorophyll, and leaf areas of different treatments at seedling stage, jointing stage, and tasseling stage are shown in Figure5(a). It can be seen that the differences of each treatment at jointing and tasseling stages were significant (P<0.05) but there was a nonsignificant difference of maize growth among the different treatments at seedling stage. Maize growth in T8 was the poorest during the experiment period. Notably, at jointing and tasseling stages, there was significantly better maize growth between other treatments and T8. Maize growth in CK1 was similar to that in CK2. This indicated that a 70 cm cover of soil above a coal gangue layer was superior for maize growth compared with other treatments.Figure 5
Planting height, stem diameter, chlorophyll, and leaf areas of different treatments at seedling, jointing, tasseling, and mature stages.
(a)
(b)
(c)
(d)


## 3.2. Dry Biomass and Corn Yield

The characteristics of above-ground dry biomass and yield in different experimental treatments were similar (Figure6). There were significant differences of biomass or yield between control treatments (CK1 and CK2) and experimental treatments (T1–T8). The biomass and yield of the control treatments were significantly higher than those of the experimental treatments. The average biomass and yield of the control treatments were 1.30 times and 1.43 times higher than those of the experimental treatments, respectively. The biomass and yield values of CK1 were largest and were 220.70 g and 8739.15 kg/ha, respectively, followed by CK2. The biomass and yield values of CK2 were 207.27 g and 8239.60 kg/ha, respectively. There was no significant difference of biomass or yield between CK1 and CK2. The biomass and yield of CK2 were 1.25 times and 1.45 times higher than those of Group 1, respectively. The biomass and yield of CK2 were 1.23 times and 1.31 times higher than those of Group 2, respectively. The biomass and yield of CK2 were 1.38 times and 1.58 times higher than those of Group 3, respectively. The difference of biomass and yield between Group 2 and CK2 was the smallest, followed by Group 1, and the difference between Group 3 and CK2 was the largest. Due to the fact that biomass and yield of CK2 were obviously higher than those of the experimental treatments, CK2 can be adopted when lime concretion black soil and gangue were used as filling materials.Figure 6
Dry biomass and corn yield of different treatments at mature stages.
(a)
(b)


## 3.3. Soil and Coal Gangue Attributes

### 3.3.1. Water Content

Soil and coal gangue water content in different interlayered soil profiles are shown in Figure7. With depth increase, water content of soil or gangue increased. There was no significant difference of topsoil (0–30 cm) water content between control experiments and Group 1 and Group 2. However, the difference of topsoil water content between control experiments and Group 3 was significant. At the same depth, the soil water content was significantly higher than that of gangue. This indicated coal gangue has poor water holding capacity creating a hydrological discontinuity. The water content of interlayered soil was higher than or similar to that of CK1 or CK2. This indicated that gangue has poor water holding capacity and the interlayer has larger water holding capacity.Figure 7
Soil water content of interlayered profiles.
(a)
(b)
(c)


### 3.3.2. Acidity Alkalinity (pH), Electrical Conductivity (Eh), and Total Salt Content

There was a significant difference of pH value at topsoil (0–10 cm) among T8 (Group 3) and control treatments (Table3). The gangue layer of T8 was closer to the soil surface than in other treatments. This resulted in slower water infiltration and limited base cation leaching, causing a relatively high degree of base saturation and higher pH value [49]. Meanwhile, differences in pH values in all treatments at 20–30 cm were not significant. Moreover, there were significant differences in Eh values or total soil salt content in the topsoil (0–10 cm) among T8 and control treatments. There were no significant differences of these three indices topsoils among all experimental treatments (T1–T8). At the same depth, the pH value, Eh value, and total soil salt content at the interlayer position were higher than or similar to those of CK1 and CK2. Values of pH, Eh, and total salt content of gangue were significantly higher than those of soil, which were 1.15 times, 1.40 times, and 1.44 times those in soil, respectively. So measures should be taken to reduce the values of pH, Eh, and total soil salt content of gangue and interlayered soil.Table 3
Acidity alkalinity, electrical conductivity (Eh), and total soil salt content.
Treatment
0–10 cm
20–30 cm
40–50 cm
60–70 cm
80–90 cm
100–110 cm
pH
CK1
7.40 ± 0.09c
7.32 ± 0.11a
7.60 ± 0.33c
7.59 ± 0.26b
7.57 ± 0.12b
7.55 ± 0.14d
CK2
7.42 ± 0.01abc
7.37 ± 0.16a
7.58 ± 0.25c
7.68 ± 0.00b
8.92 ± 0.26a
8.94 ± 0.08ab
T1
7.53 ± 0.19abc
7.61 ± 0.18a
9.00 ± 0.44a
7.70 ± 0.18b
8.87 ± 0.14a
9.14 ± 0.13a
T2
7.58 ± 0.05abc
7.61 ± 0.22a
8.02 ± 0.29bc
8.44 ± 0.26a
7.50 ± 0.10b
8.00 ± 0.10c
T3
7.67 ± 0.05ab
7.68 ± 0.13a
8.25 ± 0.28abc
8.61 ± 0.37a
7.48 ± 0.06b
8.84 ± 0.31ab
T4
7.47 ± 0.03abc
7.56 ± 0.10a
8.19 ± 0.35abc
7.60 ± 0.04b
7.69 ± 0.10b
8.69 ± 0.13ab
T5
7.44 ± 0.02abc
7.45 ± 0.15a
8.57 ± 0.28ab
8.80 ± 0.10a
7.71 ± 0.20b
8.77 ± 0.14ab
T6
7.43 ± 0.05abc
7.45 ± 0.11a
7.41 ± 0.11c
8.90 ± 0.27a
7.56 ± 0.06b
8.61 ± 0.12b
T7
7.46 ± 0.01abc
7.39 ± 0.16a
7.49 ± 0.16c
8.53 ± 0.19a
7.86 ± 0.04b
8.93 ± 0.07ab
T8
7.68 ± 0.06a
7.71 ± 0.08a
8.95 ± 0.04a
8.77 ± 0.15a
7.62 ± 0.12b
9.11 ± 0.10a
EC (us/cm)
CK1
63.00 ± 2.45d
75.33 ± 0.47c
94.00 ± 3.74c
95.33 ± 5.73b
98.67 ± 8.05c
109.33 ± 13.42a
CK2
73.33 ± 4.64cd
83.00 ± 3.74bc
100.33 ± 2.36c
107.33 ± 7.72b
159.00 ± 4.32a
152.00 ± 6.16b
T1
96.00 ± 5.89ab
101.00 ± 4.08abc
138.67 ± 2.05a
112.33 ± 4.50b
148.33 ± 4.03a
151.33 ± 12.36b
T2
101.00 ± 4.32a
109.33 ± 4.19ab
135.33 ± 4.99a
153.67 ± 10.50a
108.00 ± 9.09bc
157.33 ± 19.95b
T3
102.00 ± 2.94a
112.33 ± 8.73ab
134.67 ± 4.78a
151.33 ± 8.99a
112.67 ± 9.39bc
169.67 ± 9.67b
T4
93.67 ± 8.73abc
99.67 ± 6.02abc
139.33 ± 4.19a
114.00 ± 11.43b
112.00 ± 6.48bc
163.00 ± 1.63b
T5
89.00 ± 8.49abc
91.67 ± 17.33abc
133.67 ± 4.50a
162.33 ± 7.04a
111.33 ± 5.44bc
166.00 ± 8.04b
T6
86.50 ± 0.41abc
94.67 ± 1.25abc
112.00 ± 7.35b
158.00 ± 5.72a
114.67 ± 4.64bc
153.67 ± 11.44b
T7
78 ± 14.90bcd
84.67 ± 9.00bc
115.00 ± 6.68b
160.67 ± 7.59a
121.00 ± 2.16b
159.33 ± 21.82b
T8
104.67 ± 5.73a
118.67 ± 20.55a
146.00 ± 3.56a
152.33 ± 4.11a
124.17 ± 9.85b
160.67 ± 9.74b
Total salt content (g/kg)
CK1
0.20 ± 0.01e
0.24 ± 0.00b
0.32 ± 0.05b
0.35 ± 0.04b
0.31 ± 0.03b
0.31 ± 0.02a
CK2
0.27 ± 0.02c
0.25 ± 0.05b
0.32 ± 0.01b
0.36 ± 0.04b
0.51 ± 0.04a
0.51 ± 0.03b
T1
0.30 ± 0.01ab
0.32 ± 0.02ab
0.53 ± 0.06a
0.43 ± 0.02ab
0.54 ± 0.03a
0.55 ± 0.04b
T2
0.30 ± 0.01ab
0.32 ± 0.01ab
0.52 ± 0.04a
0.51 ± 0.02a
0.44 ± 0.02ab
0.51 ± 0.03b
T3
0.32 ± 0.01a
0.35 ± 0.01ab
0.52 ± 0.04a
0.52 ± 0.04a
0.44 ± 0.02ab
0.54 ± 0.03b
T4
0.28 ± 0.01bc
0.30 ± 0.00ab
0.51 ± 0.04a
0.43 ± 0.02ab
0.43 ± 0.02ab
0.52 ± 0.01b
T5
0.28 ± 0.00bc
0.27 ± 0.03ab
0.51 ± 0.14a
0.52 ± 0.02a
0.44 ± 0.03ab
0.52 ± 0.02b
T6
0.23 ± 0.00d
0.29 ± 0.06ab
0.33 ± 0.02b
0.51 ± 0.04a
0.44 ± 0.03ab
0.52 ± 0.04b
T7
0.23 ± 0.02d
0.26 ± 0.03b
0.33 ± 0.02b
0.54 ± 0.03a
0.43 ± 0.02ab
0.51 ± 0.03b
T8
0.33 ± 0.01a
0.38 ± 0.07a
0.51 ± 0.04a
0.51 ± 0.05a
0.44 ± 0.09ab
0.54 ± 0.02b


### 3.3.3. Total and Available Nutrients

The content of total nitrogen, available nitrogen, total phosphorus, available phosphorus, total potassium, available potassium, and organic matter of topsoil in all experimental treatments was lower than or similar to the control treatments (Tables4–7). But the content of these total and available nutrients of the interlayers in all experimental treatments was higher than or the same as the control treatments. The available nitrogen, total phosphorus, available phosphorus, and total potassium contents of soil were larger than those of gangue, except that the total nitrogen, available potassium, and organic matter contents of soil were lower than those of gangue. Due to the high carbon content in gangue, the nitrogen and organic matter content in gangue were very high. However, nitrogen and organic matter in gangue are not readily available to plants.Table 4
Total and available nitrogen of interlayered soil profiles.
Treatment
0–10 cm
20–30 cm
40–50 cm
60–70 cm
80–90 cm
100–110 cm
TN (g/kg)
CK1
0.56 ± 0.03a
0.50 ± 0.06a
0.43 ± 0.02b
0.43 ± 0.02b
0.38 ± 0.05d
0.44 ± 0.37e
CK2
0.53 ± 0.03a
0.49 ± 0.05a
0.41 ± 0.02b
0.45 ± 0.04b
1.26 ± 0.03a
1.28 ± 0.03bc
T1
0.44 ± 0.03b
0.34 ± 0.02b
1.25 ± 0.09a
0.47 ± 0.01b
1.18 ± 0.01b
1.26 ± 0.02bc
T2
0.43 ± 0.02b
0.33 ± 0.04b
1.13 ± 0.08a
1.13 ± 0.08a
0.40 ± 0.03d
1.20 ± 0.05bcd
T3
0.41 ± 0.01b
0.34 ± 0.05b
1.33 ± 0.13a
1.41 ± 0.27a
0.43 ± 0.02d
1.32 ± 0.08ab
T4
0.44 ± 0.02b
0.35 ± 0.05b
1.24 ± 0.05a
0.47 ± 0.6b
0.58 ± 0.04c
1.14 ± 0.05cd
T5
0.45 ± 0.01b
0.35 ± 0.05b
1.32 ± 0.09a
1.26 ± 0.07a
0.54 ± 0.02c
1.40 ± 0.02a
T6
0.45 ± 0.04b
0.36 ± 0.04b
0.41 ± 0.02b
1.22 ± 0.01a
0.55 ± 0.06c
1.15 ± 0.07cd
T7
0.46 ± 0.01b
0.37 ± 0.04b
0.38 ± 0.02b
1.36 ± 0.01a
0.55 ± 0.05c
1.03 ± 0.05d
T8
0.40 ± 0.03b
0.32 ± 0.04b
1.36 ± 0.19a
1.26 ± 0.03a
0.40 ± 0.03d
1.18 ± 0.01bcd
AN (mg/kg)
CK1
25.19 ± 0.98a
21.85 ± 1.38a
18.89 ± 0.44a
15.16 ± 0.38b
14.52 ± 0.12a
13.55 ± 1.41a
CK2
24.96 ± 1.31a
21.27 ± 1.55a
18.29 ± 0.77a
14.09 ± 0.25b
9.94 ± 0.38b
10.30 ± 0.06b
T1
21.24 ± 1.50b
17.61 ± 0.24bc
10.63 ± 0.95c
14.73 ± 0.69b
9.96 ± 0.40b
8.69 ± 0.11bc
T2
19.96 ± 1.56b
15.74 ± 0.16bc
10.40 ± 0.60c
9.97 ± 0.20c
15.53 ± 1.32a
9.53 ± 0.27bc
T3
19.33 ± 0.25b
15.79 ± 0.02bc
10.48 ± 0.50c
9.81 ± 0.80c
14.68 ± 0.78a
10.25 ± 0.57b
T4
20.06 ± 0.41b
18.09 ± 0.72b
10.35 ± 0.50c
16.35 ± 0.28a
15.52 ± 0.54a
8.14 ± 0.33c
T5
21.94 ± 0.55b
15.92 ± 0.60bc
10.63 ± 0.59c
9.78 ± 0.69c
15.91 ± 0.60a
9.40 ± 0.39bc
T6
21.32 ± 0.91b
17.51 ± 0.35bc
15.53 ± 1.58b
9.71 ± 0.42c
15.27 ± 0.20a
10.25 ± 0.67b
T7
21.94 ± 1.16b
17.64 ± 0.87bc
15.43 ± 0.35b
9.79 ± 0.54c
15.22 ± 0.81a
10.09 ± 0.28b
T8
19.06 ± 0.13b
15.12 ± 0.48c
10.16 ± 0.42c
9.80 ± 0.39c
14.66 ± 0.94a
9.35 ± 0.24bcTable 5
Total and available phosphorus of interlayered soil profiles.
Treatment
0–10 cm
20–30 cm
40–50 cm
60–70 cm
80–90 cm
100–110 cm
TP (g/kg)
CK1
0.40 ± 0.04a
0.38 ± 0.05a
0.34 ± 0.03a
0.32 ± 0.02a
0.32 ± 0.04a
0.33 ± 0.02a
CK2
0.38 ± 0.05a
0.37 ± 0.05a
0.32 ± 0.02a
0.31 ± 0.02a
0.23 ± 0.03b
0.26 ± 0.01b
T1
0.34 ± 0.03a
0.33 ± 0.02a
0.21 ± 0.02b
0.33 ± 0.01a
0.20 ± 0.02b
0.24 ± 0.01b
T2
0.35 ± 0.03a
0.31 ± 0.02a
0.21 ± 0.03b
0.21 ± 0.02b
0.33 ± 0.02a
0.22 ± 0.02b
T3
0.33 ± 0.02a
0.32 ± 0.06a
0.21 ± 0.03b
0.21 ± 0.01b
0.32 ± 0.03a
0.24 ± 0.01b
T4
0.36 ± 0.04a
0.32 ± 0.03a
0.21 ± 0.01b
0.32 ± 0.01a
0.32 ± 0.03a
0.23 ± 0.03b
T5
0.34 ± 0.03a
0.33 ± 0.04a
0.21 ± 0.02b
0.22 ± 0.03b
0.33 ± 0.03a
0.26 ± 0.02b
T6
0.34 ± 0.02a
0.33 ± 0.02a
0.34 ± 0.01a
0.23 ± 0.02b
0.33 ± 0.01a
0.25 ± 0.01b
T7
0.35 ± 0.02a
0.34 ± 0.04a
0.33 ± 0.03a
0.22 ± 0.03b
0.34 ± 0.01a
0.24 ± 0.02b
T8
0.33 ± 0.02a
0.32 ± 0.04a
0.19 ± 0.02b
0.21 ± 0.01b
0.32 ± 0.04a
0.23 ± 0.01b
OP (mg/kg)
CK1
8.78 ± 0.22a
8.35 ± 0.26a
7.53 ± 0.77a
6.26 ± 0.13a
6.14 ± 0.35a
6.22 ± 0.11a
CK2
8.61 ± 0.12ab
7.94 ± 0.78a
7.23 ± 0.16a
6.44 ± 0.11a
4.48 ± 0.33b
4.90 ± 0.16b
T1
7.67 ± 0.59bc
5.27 ± 0.43b
4.72 ± 0.62a
6.30 ± 0.07a
4.25 ± 0.16b
4.44 ± 0.25b
T2
6.75 ± 0.37c
5.86 ± 0.41b
4.68 ± 0.31c
4.50 ± 0.21b
6.24 ± 0.15a
4.34 ± 0.31b
T3
6.70 ± 0.33c
5.90 ± 0.29b
4.74 ± 0.28c
4.60 ± 0.33b
6.23 ± 0.27a
4.33 ± 0.39b
T4
7.64 ± 0.27bc
5.76 ± 0.67b
4.80 ± 0.25c
6.70 ± 0.36a
6.71 ± 0.41a
4.31 ± 0.25b
T5
6.95 ± 0.84c
5.65 ± 0.17b
4.81 ± 0.41c
4.79 ± 0.52b
6.74 ± 0.40a
4.21 ± 0.20b
T6
7.08 ± 0.12c
6.19 ± 0.60b
5.74 ± 0.31bc
4.76 ± 0.42b
6.98 ± 0.35a
4.47 ± 0.23b
T7
7.18 ± 0.14c
6.30 ± 0.20b
6.16 ± 0.28b
4.88 ± 0.44b
6.95 ± 0.73a
4.67 ± 0.28b
T8
6.69 ± 0.32c
5.15 ± 0.16b
4.67 ± 0.22c
4.25 ± 0.18b
6.18 ± 0.27a
4.27 ± 0.49bTable 6
Total and available potassium of interlayered soil profiles.
Treatment
0–10 cm
20–30 cm
40–50 cm
60–70 cm
80–90 cm
100–110 cm
TK (g/kg)
CK1
18.80 ± 0.10a
18.78 ± 0.38a
18.09 ± 0.42a
17.57 ± 0.21a
17.10 ± 0.24a
17.41 ± 0.29a
CK2
19.05 ± 0.59a
18.85 ± 0.75a
17.85 ± 0.59ab
15.36 ± 0.12b
11.50 ± 0.11c
11.42 ± 0.28b
T1
18.78 ± 0.08ab
18.18 ± 0.17ab
11.55 ± 0.08c
17.68 ± 0.31a
12.14 ± 0.27b
11.29 ± 0.21bc
T2
17.90 ± 0.13abc
17.33 ± 0.31b
11.56 ± 0.31c
11.42 ± 0.35c
17.45 ± 0.35a
11.35 ± 0.12bc
T3
18.75 ± 0.79ab
17.21 ± 0.17b
10.76 ± 0.33 cd
10.68 ± 0.13 cd
17.18 ± 0.10a
11.30 ± 0.12bc
T4
17.57 ± 0.21bc
17.80 ± 0.23ab
10.05 ± 0.10d
17.79 ± 0.33a
17.40 ± 0.17a
11.08 ± 0.10bc
T5
18.56 ± 0.07ab
17.91 ± 0.37ab
10.76 ± 0.19 cd
10.25 ± 0.17d
17.17 ± 0.22a
10.39 ± 0.12d
T6
17.69 ± 0.51bc
18.05 ± 0.54ab
17.45 ± 0.39ab
10.35 ± 0.24d
17.21 ± 0.24a
10.84 ± 0.17c
T7
18.56 ± 0.27ab
18.01 ± 0.41ab
17.19 ± 0.29b
10.90 ± 0.39 cd
17.73 ± 0.11a
10.80 ± 0.09c
T8
17.36 ± 0.22c
17.16 ± 0.30b
11.18 ± 0.14c
11.14 ± 0.51 cd
17.34 ± 0.15a
11.10 ± 0.07bc
AK (mg/kg)
CK1
215.80 ± 3.69a
208.87 ± 8.04a
203.67 ± 12.44ab
200.20 ± 12.91ab
194.13 ± 12.15a
197.60 ± 10.29a
CK2
218.40 ± 2.14a
210.30 ± 12.85a
190.27 ± 19.27ab
188.50 ± 15.07b
227.93 ± 14.59a
230.53 ± 10.36a
T1
198.47 ± 5.87b
187.60 ± 11.03ab
225.07 ± 13.58ab
204.00 ± 13.64ab
232.07 ± 13.36a
229.67 ± 12.97a
T2
197.6 ± 4.25b
181.53 ± 6.13ab
233.87.87 ± 17.48a
232.27 ± 13.92ab
201.20 ± 12.51a
237.47 ± 14.30a
T3
196.73 ± 1.52b
185.00 ± 3.68ab
220.80 ± 17.76ab
240.93 ± 13.93a
204.10 ± 13.96a
227.40 ± 13.41a
T4
204.53 ± 2.47b
189.40 ± 4.90ab
235.60 ± 10.00a
206.97 ± 12.20ab
201.07 ± 14.97a
225.80 ± 14.16a
T5
199.33 ± 4.42b
191.27 ± 7.44ab
227.40 ± 16.05ab
223.73 ± 10.80ab
206.40 ± 14.79a
229.53 ± 14.27a
T6
201.07 ± 6.49b
190.20 ± 14.86ab
182.80 ± 10.99b
228.80 ± 12.42ab
205.07 ± 13.90a
226.27 ± 13.15a
T7
207.20 ± 4.61b
192.93 ± 8.49ab
180.60 ± 16.33b
230.33 ± 10.62ab
207.53 ± 17.98a
220.60 ± 13.87a
T8
196.73 ± 2.45b
175.53 ± 5.03b
222.07 ± 12.49ab
225.60 ± 13.75ab
209.17 ± 12.43a
222.73 ± 10.82aTable 7
Organic matter of soil and coal gangue of interlayered soil profiles.
Treatment
0–10 cm
20–30 cm
40–50 cm
60–70 cm
80–90 cm
100–110 cm
OM (g/kg)
CK1
6.26 ± 0.05a
5.96 ± 0.30a
5.53 ± 0.43f
3.36 ± 0.43e
3.41 ± 0.29b
3.40 ± 0.28f
CK2
6.13 ± 0.23a
5.85 ± 0.40a
5.51 ± 0.02f
4.63 ± 0.38e
96.99 ± 1.76a
70.38 ± 4.64de
T1
5.07 ± 0.07b
4.26 ± 0.18b
77.34 ± 3.40d
3.77 ± 0.17e
97.81 ± 4.78a
83.42 ± 2.88c
T2
4.83 ± 0.28b
4.20 ± 0.21b
71.29 ± 0.38e
79.36 ± 3.55d
3.72 ± 0.08b
67.72 ± 3.47e
T3
4.76 ± 0.54b
4.08 ± 0.26b
80.05 ± 1.85d
117.31 ± 3.24a
3.66 ± 0.05b
91.21 ± 1.24b
T4
4.99 ± 0.13b
4.29 ± 0.12b
107.45 ± 1.33c
5.09 ± 0.58e
4.38 ± 0.23b
74.85 ± 1.37de
T5
5.18 ± 0.46b
4.36 ± 0.62b
138.23 ± 1.24a
107.34 ± 5.24b
3.92 ± 0.41b
106.95 ± 2.11a
T6
5.28 ± 0.17b
4.32 ± 0.13b
4.45 ± 0.26f
93.11 ± 2.41c
4.12 ± 0.64b
102.17 ± 2.04a
T7
5.32 ± 0.58b
4.37 ± 0.21b
4.12 ± 0.08f
106.44 ± 3.42b
4.01 ± 0.47b
76.74 ± 3.27d
T8
4.65 ± 0.27b
3.94 ± 0.66b
120.20 ± 3.26b
106.41 ± 2.87b
3.56 ± 0.33b
71.73 ± 0.94de


## 3.3.1. Water Content

Soil and coal gangue water content in different interlayered soil profiles are shown in Figure7. With depth increase, water content of soil or gangue increased. There was no significant difference of topsoil (0–30 cm) water content between control experiments and Group 1 and Group 2. However, the difference of topsoil water content between control experiments and Group 3 was significant. At the same depth, the soil water content was significantly higher than that of gangue. This indicated coal gangue has poor water holding capacity creating a hydrological discontinuity. The water content of interlayered soil was higher than or similar to that of CK1 or CK2. This indicated that gangue has poor water holding capacity and the interlayer has larger water holding capacity.Figure 7
Soil water content of interlayered profiles.
(a)
(b)
(c)


## 3.3.2. Acidity Alkalinity (pH), Electrical Conductivity (Eh), and Total Salt Content

There was a significant difference of pH value at topsoil (0–10 cm) among T8 (Group 3) and control treatments (Table3). The gangue layer of T8 was closer to the soil surface than in other treatments. This resulted in slower water infiltration and limited base cation leaching, causing a relatively high degree of base saturation and higher pH value [49]. Meanwhile, differences in pH values in all treatments at 20–30 cm were not significant. Moreover, there were significant differences in Eh values or total soil salt content in the topsoil (0–10 cm) among T8 and control treatments. There were no significant differences of these three indices topsoils among all experimental treatments (T1–T8). At the same depth, the pH value, Eh value, and total soil salt content at the interlayer position were higher than or similar to those of CK1 and CK2. Values of pH, Eh, and total salt content of gangue were significantly higher than those of soil, which were 1.15 times, 1.40 times, and 1.44 times those in soil, respectively. So measures should be taken to reduce the values of pH, Eh, and total soil salt content of gangue and interlayered soil.Table 3
Acidity alkalinity, electrical conductivity (Eh), and total soil salt content.
Treatment
0–10 cm
20–30 cm
40–50 cm
60–70 cm
80–90 cm
100–110 cm
pH
CK1
7.40 ± 0.09c
7.32 ± 0.11a
7.60 ± 0.33c
7.59 ± 0.26b
7.57 ± 0.12b
7.55 ± 0.14d
CK2
7.42 ± 0.01abc
7.37 ± 0.16a
7.58 ± 0.25c
7.68 ± 0.00b
8.92 ± 0.26a
8.94 ± 0.08ab
T1
7.53 ± 0.19abc
7.61 ± 0.18a
9.00 ± 0.44a
7.70 ± 0.18b
8.87 ± 0.14a
9.14 ± 0.13a
T2
7.58 ± 0.05abc
7.61 ± 0.22a
8.02 ± 0.29bc
8.44 ± 0.26a
7.50 ± 0.10b
8.00 ± 0.10c
T3
7.67 ± 0.05ab
7.68 ± 0.13a
8.25 ± 0.28abc
8.61 ± 0.37a
7.48 ± 0.06b
8.84 ± 0.31ab
T4
7.47 ± 0.03abc
7.56 ± 0.10a
8.19 ± 0.35abc
7.60 ± 0.04b
7.69 ± 0.10b
8.69 ± 0.13ab
T5
7.44 ± 0.02abc
7.45 ± 0.15a
8.57 ± 0.28ab
8.80 ± 0.10a
7.71 ± 0.20b
8.77 ± 0.14ab
T6
7.43 ± 0.05abc
7.45 ± 0.11a
7.41 ± 0.11c
8.90 ± 0.27a
7.56 ± 0.06b
8.61 ± 0.12b
T7
7.46 ± 0.01abc
7.39 ± 0.16a
7.49 ± 0.16c
8.53 ± 0.19a
7.86 ± 0.04b
8.93 ± 0.07ab
T8
7.68 ± 0.06a
7.71 ± 0.08a
8.95 ± 0.04a
8.77 ± 0.15a
7.62 ± 0.12b
9.11 ± 0.10a
EC (us/cm)
CK1
63.00 ± 2.45d
75.33 ± 0.47c
94.00 ± 3.74c
95.33 ± 5.73b
98.67 ± 8.05c
109.33 ± 13.42a
CK2
73.33 ± 4.64cd
83.00 ± 3.74bc
100.33 ± 2.36c
107.33 ± 7.72b
159.00 ± 4.32a
152.00 ± 6.16b
T1
96.00 ± 5.89ab
101.00 ± 4.08abc
138.67 ± 2.05a
112.33 ± 4.50b
148.33 ± 4.03a
151.33 ± 12.36b
T2
101.00 ± 4.32a
109.33 ± 4.19ab
135.33 ± 4.99a
153.67 ± 10.50a
108.00 ± 9.09bc
157.33 ± 19.95b
T3
102.00 ± 2.94a
112.33 ± 8.73ab
134.67 ± 4.78a
151.33 ± 8.99a
112.67 ± 9.39bc
169.67 ± 9.67b
T4
93.67 ± 8.73abc
99.67 ± 6.02abc
139.33 ± 4.19a
114.00 ± 11.43b
112.00 ± 6.48bc
163.00 ± 1.63b
T5
89.00 ± 8.49abc
91.67 ± 17.33abc
133.67 ± 4.50a
162.33 ± 7.04a
111.33 ± 5.44bc
166.00 ± 8.04b
T6
86.50 ± 0.41abc
94.67 ± 1.25abc
112.00 ± 7.35b
158.00 ± 5.72a
114.67 ± 4.64bc
153.67 ± 11.44b
T7
78 ± 14.90bcd
84.67 ± 9.00bc
115.00 ± 6.68b
160.67 ± 7.59a
121.00 ± 2.16b
159.33 ± 21.82b
T8
104.67 ± 5.73a
118.67 ± 20.55a
146.00 ± 3.56a
152.33 ± 4.11a
124.17 ± 9.85b
160.67 ± 9.74b
Total salt content (g/kg)
CK1
0.20 ± 0.01e
0.24 ± 0.00b
0.32 ± 0.05b
0.35 ± 0.04b
0.31 ± 0.03b
0.31 ± 0.02a
CK2
0.27 ± 0.02c
0.25 ± 0.05b
0.32 ± 0.01b
0.36 ± 0.04b
0.51 ± 0.04a
0.51 ± 0.03b
T1
0.30 ± 0.01ab
0.32 ± 0.02ab
0.53 ± 0.06a
0.43 ± 0.02ab
0.54 ± 0.03a
0.55 ± 0.04b
T2
0.30 ± 0.01ab
0.32 ± 0.01ab
0.52 ± 0.04a
0.51 ± 0.02a
0.44 ± 0.02ab
0.51 ± 0.03b
T3
0.32 ± 0.01a
0.35 ± 0.01ab
0.52 ± 0.04a
0.52 ± 0.04a
0.44 ± 0.02ab
0.54 ± 0.03b
T4
0.28 ± 0.01bc
0.30 ± 0.00ab
0.51 ± 0.04a
0.43 ± 0.02ab
0.43 ± 0.02ab
0.52 ± 0.01b
T5
0.28 ± 0.00bc
0.27 ± 0.03ab
0.51 ± 0.14a
0.52 ± 0.02a
0.44 ± 0.03ab
0.52 ± 0.02b
T6
0.23 ± 0.00d
0.29 ± 0.06ab
0.33 ± 0.02b
0.51 ± 0.04a
0.44 ± 0.03ab
0.52 ± 0.04b
T7
0.23 ± 0.02d
0.26 ± 0.03b
0.33 ± 0.02b
0.54 ± 0.03a
0.43 ± 0.02ab
0.51 ± 0.03b
T8
0.33 ± 0.01a
0.38 ± 0.07a
0.51 ± 0.04a
0.51 ± 0.05a
0.44 ± 0.09ab
0.54 ± 0.02b


## 3.3.3. Total and Available Nutrients

The content of total nitrogen, available nitrogen, total phosphorus, available phosphorus, total potassium, available potassium, and organic matter of topsoil in all experimental treatments was lower than or similar to the control treatments (Tables4–7). But the content of these total and available nutrients of the interlayers in all experimental treatments was higher than or the same as the control treatments. The available nitrogen, total phosphorus, available phosphorus, and total potassium contents of soil were larger than those of gangue, except that the total nitrogen, available potassium, and organic matter contents of soil were lower than those of gangue. Due to the high carbon content in gangue, the nitrogen and organic matter content in gangue were very high. However, nitrogen and organic matter in gangue are not readily available to plants.Table 4
Total and available nitrogen of interlayered soil profiles.
Treatment
0–10 cm
20–30 cm
40–50 cm
60–70 cm
80–90 cm
100–110 cm
TN (g/kg)
CK1
0.56 ± 0.03a
0.50 ± 0.06a
0.43 ± 0.02b
0.43 ± 0.02b
0.38 ± 0.05d
0.44 ± 0.37e
CK2
0.53 ± 0.03a
0.49 ± 0.05a
0.41 ± 0.02b
0.45 ± 0.04b
1.26 ± 0.03a
1.28 ± 0.03bc
T1
0.44 ± 0.03b
0.34 ± 0.02b
1.25 ± 0.09a
0.47 ± 0.01b
1.18 ± 0.01b
1.26 ± 0.02bc
T2
0.43 ± 0.02b
0.33 ± 0.04b
1.13 ± 0.08a
1.13 ± 0.08a
0.40 ± 0.03d
1.20 ± 0.05bcd
T3
0.41 ± 0.01b
0.34 ± 0.05b
1.33 ± 0.13a
1.41 ± 0.27a
0.43 ± 0.02d
1.32 ± 0.08ab
T4
0.44 ± 0.02b
0.35 ± 0.05b
1.24 ± 0.05a
0.47 ± 0.6b
0.58 ± 0.04c
1.14 ± 0.05cd
T5
0.45 ± 0.01b
0.35 ± 0.05b
1.32 ± 0.09a
1.26 ± 0.07a
0.54 ± 0.02c
1.40 ± 0.02a
T6
0.45 ± 0.04b
0.36 ± 0.04b
0.41 ± 0.02b
1.22 ± 0.01a
0.55 ± 0.06c
1.15 ± 0.07cd
T7
0.46 ± 0.01b
0.37 ± 0.04b
0.38 ± 0.02b
1.36 ± 0.01a
0.55 ± 0.05c
1.03 ± 0.05d
T8
0.40 ± 0.03b
0.32 ± 0.04b
1.36 ± 0.19a
1.26 ± 0.03a
0.40 ± 0.03d
1.18 ± 0.01bcd
AN (mg/kg)
CK1
25.19 ± 0.98a
21.85 ± 1.38a
18.89 ± 0.44a
15.16 ± 0.38b
14.52 ± 0.12a
13.55 ± 1.41a
CK2
24.96 ± 1.31a
21.27 ± 1.55a
18.29 ± 0.77a
14.09 ± 0.25b
9.94 ± 0.38b
10.30 ± 0.06b
T1
21.24 ± 1.50b
17.61 ± 0.24bc
10.63 ± 0.95c
14.73 ± 0.69b
9.96 ± 0.40b
8.69 ± 0.11bc
T2
19.96 ± 1.56b
15.74 ± 0.16bc
10.40 ± 0.60c
9.97 ± 0.20c
15.53 ± 1.32a
9.53 ± 0.27bc
T3
19.33 ± 0.25b
15.79 ± 0.02bc
10.48 ± 0.50c
9.81 ± 0.80c
14.68 ± 0.78a
10.25 ± 0.57b
T4
20.06 ± 0.41b
18.09 ± 0.72b
10.35 ± 0.50c
16.35 ± 0.28a
15.52 ± 0.54a
8.14 ± 0.33c
T5
21.94 ± 0.55b
15.92 ± 0.60bc
10.63 ± 0.59c
9.78 ± 0.69c
15.91 ± 0.60a
9.40 ± 0.39bc
T6
21.32 ± 0.91b
17.51 ± 0.35bc
15.53 ± 1.58b
9.71 ± 0.42c
15.27 ± 0.20a
10.25 ± 0.67b
T7
21.94 ± 1.16b
17.64 ± 0.87bc
15.43 ± 0.35b
9.79 ± 0.54c
15.22 ± 0.81a
10.09 ± 0.28b
T8
19.06 ± 0.13b
15.12 ± 0.48c
10.16 ± 0.42c
9.80 ± 0.39c
14.66 ± 0.94a
9.35 ± 0.24bcTable 5
Total and available phosphorus of interlayered soil profiles.
Treatment
0–10 cm
20–30 cm
40–50 cm
60–70 cm
80–90 cm
100–110 cm
TP (g/kg)
CK1
0.40 ± 0.04a
0.38 ± 0.05a
0.34 ± 0.03a
0.32 ± 0.02a
0.32 ± 0.04a
0.33 ± 0.02a
CK2
0.38 ± 0.05a
0.37 ± 0.05a
0.32 ± 0.02a
0.31 ± 0.02a
0.23 ± 0.03b
0.26 ± 0.01b
T1
0.34 ± 0.03a
0.33 ± 0.02a
0.21 ± 0.02b
0.33 ± 0.01a
0.20 ± 0.02b
0.24 ± 0.01b
T2
0.35 ± 0.03a
0.31 ± 0.02a
0.21 ± 0.03b
0.21 ± 0.02b
0.33 ± 0.02a
0.22 ± 0.02b
T3
0.33 ± 0.02a
0.32 ± 0.06a
0.21 ± 0.03b
0.21 ± 0.01b
0.32 ± 0.03a
0.24 ± 0.01b
T4
0.36 ± 0.04a
0.32 ± 0.03a
0.21 ± 0.01b
0.32 ± 0.01a
0.32 ± 0.03a
0.23 ± 0.03b
T5
0.34 ± 0.03a
0.33 ± 0.04a
0.21 ± 0.02b
0.22 ± 0.03b
0.33 ± 0.03a
0.26 ± 0.02b
T6
0.34 ± 0.02a
0.33 ± 0.02a
0.34 ± 0.01a
0.23 ± 0.02b
0.33 ± 0.01a
0.25 ± 0.01b
T7
0.35 ± 0.02a
0.34 ± 0.04a
0.33 ± 0.03a
0.22 ± 0.03b
0.34 ± 0.01a
0.24 ± 0.02b
T8
0.33 ± 0.02a
0.32 ± 0.04a
0.19 ± 0.02b
0.21 ± 0.01b
0.32 ± 0.04a
0.23 ± 0.01b
OP (mg/kg)
CK1
8.78 ± 0.22a
8.35 ± 0.26a
7.53 ± 0.77a
6.26 ± 0.13a
6.14 ± 0.35a
6.22 ± 0.11a
CK2
8.61 ± 0.12ab
7.94 ± 0.78a
7.23 ± 0.16a
6.44 ± 0.11a
4.48 ± 0.33b
4.90 ± 0.16b
T1
7.67 ± 0.59bc
5.27 ± 0.43b
4.72 ± 0.62a
6.30 ± 0.07a
4.25 ± 0.16b
4.44 ± 0.25b
T2
6.75 ± 0.37c
5.86 ± 0.41b
4.68 ± 0.31c
4.50 ± 0.21b
6.24 ± 0.15a
4.34 ± 0.31b
T3
6.70 ± 0.33c
5.90 ± 0.29b
4.74 ± 0.28c
4.60 ± 0.33b
6.23 ± 0.27a
4.33 ± 0.39b
T4
7.64 ± 0.27bc
5.76 ± 0.67b
4.80 ± 0.25c
6.70 ± 0.36a
6.71 ± 0.41a
4.31 ± 0.25b
T5
6.95 ± 0.84c
5.65 ± 0.17b
4.81 ± 0.41c
4.79 ± 0.52b
6.74 ± 0.40a
4.21 ± 0.20b
T6
7.08 ± 0.12c
6.19 ± 0.60b
5.74 ± 0.31bc
4.76 ± 0.42b
6.98 ± 0.35a
4.47 ± 0.23b
T7
7.18 ± 0.14c
6.30 ± 0.20b
6.16 ± 0.28b
4.88 ± 0.44b
6.95 ± 0.73a
4.67 ± 0.28b
T8
6.69 ± 0.32c
5.15 ± 0.16b
4.67 ± 0.22c
4.25 ± 0.18b
6.18 ± 0.27a
4.27 ± 0.49bTable 6
Total and available potassium of interlayered soil profiles.
Treatment
0–10 cm
20–30 cm
40–50 cm
60–70 cm
80–90 cm
100–110 cm
TK (g/kg)
CK1
18.80 ± 0.10a
18.78 ± 0.38a
18.09 ± 0.42a
17.57 ± 0.21a
17.10 ± 0.24a
17.41 ± 0.29a
CK2
19.05 ± 0.59a
18.85 ± 0.75a
17.85 ± 0.59ab
15.36 ± 0.12b
11.50 ± 0.11c
11.42 ± 0.28b
T1
18.78 ± 0.08ab
18.18 ± 0.17ab
11.55 ± 0.08c
17.68 ± 0.31a
12.14 ± 0.27b
11.29 ± 0.21bc
T2
17.90 ± 0.13abc
17.33 ± 0.31b
11.56 ± 0.31c
11.42 ± 0.35c
17.45 ± 0.35a
11.35 ± 0.12bc
T3
18.75 ± 0.79ab
17.21 ± 0.17b
10.76 ± 0.33 cd
10.68 ± 0.13 cd
17.18 ± 0.10a
11.30 ± 0.12bc
T4
17.57 ± 0.21bc
17.80 ± 0.23ab
10.05 ± 0.10d
17.79 ± 0.33a
17.40 ± 0.17a
11.08 ± 0.10bc
T5
18.56 ± 0.07ab
17.91 ± 0.37ab
10.76 ± 0.19 cd
10.25 ± 0.17d
17.17 ± 0.22a
10.39 ± 0.12d
T6
17.69 ± 0.51bc
18.05 ± 0.54ab
17.45 ± 0.39ab
10.35 ± 0.24d
17.21 ± 0.24a
10.84 ± 0.17c
T7
18.56 ± 0.27ab
18.01 ± 0.41ab
17.19 ± 0.29b
10.90 ± 0.39 cd
17.73 ± 0.11a
10.80 ± 0.09c
T8
17.36 ± 0.22c
17.16 ± 0.30b
11.18 ± 0.14c
11.14 ± 0.51 cd
17.34 ± 0.15a
11.10 ± 0.07bc
AK (mg/kg)
CK1
215.80 ± 3.69a
208.87 ± 8.04a
203.67 ± 12.44ab
200.20 ± 12.91ab
194.13 ± 12.15a
197.60 ± 10.29a
CK2
218.40 ± 2.14a
210.30 ± 12.85a
190.27 ± 19.27ab
188.50 ± 15.07b
227.93 ± 14.59a
230.53 ± 10.36a
T1
198.47 ± 5.87b
187.60 ± 11.03ab
225.07 ± 13.58ab
204.00 ± 13.64ab
232.07 ± 13.36a
229.67 ± 12.97a
T2
197.6 ± 4.25b
181.53 ± 6.13ab
233.87.87 ± 17.48a
232.27 ± 13.92ab
201.20 ± 12.51a
237.47 ± 14.30a
T3
196.73 ± 1.52b
185.00 ± 3.68ab
220.80 ± 17.76ab
240.93 ± 13.93a
204.10 ± 13.96a
227.40 ± 13.41a
T4
204.53 ± 2.47b
189.40 ± 4.90ab
235.60 ± 10.00a
206.97 ± 12.20ab
201.07 ± 14.97a
225.80 ± 14.16a
T5
199.33 ± 4.42b
191.27 ± 7.44ab
227.40 ± 16.05ab
223.73 ± 10.80ab
206.40 ± 14.79a
229.53 ± 14.27a
T6
201.07 ± 6.49b
190.20 ± 14.86ab
182.80 ± 10.99b
228.80 ± 12.42ab
205.07 ± 13.90a
226.27 ± 13.15a
T7
207.20 ± 4.61b
192.93 ± 8.49ab
180.60 ± 16.33b
230.33 ± 10.62ab
207.53 ± 17.98a
220.60 ± 13.87a
T8
196.73 ± 2.45b
175.53 ± 5.03b
222.07 ± 12.49ab
225.60 ± 13.75ab
209.17 ± 12.43a
222.73 ± 10.82aTable 7
Organic matter of soil and coal gangue of interlayered soil profiles.
Treatment
0–10 cm
20–30 cm
40–50 cm
60–70 cm
80–90 cm
100–110 cm
OM (g/kg)
CK1
6.26 ± 0.05a
5.96 ± 0.30a
5.53 ± 0.43f
3.36 ± 0.43e
3.41 ± 0.29b
3.40 ± 0.28f
CK2
6.13 ± 0.23a
5.85 ± 0.40a
5.51 ± 0.02f
4.63 ± 0.38e
96.99 ± 1.76a
70.38 ± 4.64de
T1
5.07 ± 0.07b
4.26 ± 0.18b
77.34 ± 3.40d
3.77 ± 0.17e
97.81 ± 4.78a
83.42 ± 2.88c
T2
4.83 ± 0.28b
4.20 ± 0.21b
71.29 ± 0.38e
79.36 ± 3.55d
3.72 ± 0.08b
67.72 ± 3.47e
T3
4.76 ± 0.54b
4.08 ± 0.26b
80.05 ± 1.85d
117.31 ± 3.24a
3.66 ± 0.05b
91.21 ± 1.24b
T4
4.99 ± 0.13b
4.29 ± 0.12b
107.45 ± 1.33c
5.09 ± 0.58e
4.38 ± 0.23b
74.85 ± 1.37de
T5
5.18 ± 0.46b
4.36 ± 0.62b
138.23 ± 1.24a
107.34 ± 5.24b
3.92 ± 0.41b
106.95 ± 2.11a
T6
5.28 ± 0.17b
4.32 ± 0.13b
4.45 ± 0.26f
93.11 ± 2.41c
4.12 ± 0.64b
102.17 ± 2.04a
T7
5.32 ± 0.58b
4.37 ± 0.21b
4.12 ± 0.08f
106.44 ± 3.42b
4.01 ± 0.47b
76.74 ± 3.27d
T8
4.65 ± 0.27b
3.94 ± 0.66b
120.20 ± 3.26b
106.41 ± 2.87b
3.56 ± 0.33b
71.73 ± 0.94de


## 4. Discussion

### 4.1. Distribution Characteristics of Filling Material Water Content and Nutrients in Layered Soil Profile

The texture, bulk density, and organic matter content of the filling material will affect the hydraulic properties [34, 50]. When the wetting front passes through the fine soil-coarse soil interface, due to the large porosity and small suction of the coarse soil, the wetting front stops at the interface causing a hydrological discontinuity [51], and the water content of the upper soil increased. When the gangue is located in the middle layer of soil profile, its presence inhibits the process of soil water infiltration and correspondingly increases the moisture of the upper soil [52]. However, the soil in this study area is the lime concretion black soil that has a large silt content and limited water infiltration [53], making it difficult for water to pass through the 40–70 cm soil layer. Therefore, there was no significant difference of the surface soil water content among the control group, Group 1, and Group 2. There were significant differences in topsoil water content between T8 and the other soil profiles. This can be explained by the proximity of the gangue (40 cm) to the soil surface in T8, which impedes downward movement of water at the interface of the hydrological discontinuity between the upper soil layer and the gangue. The soil water content and nutrients in the interlayer were higher than those in the control group at the same depth. This is due to the fine texture and suction of lime concretion black soil and the coarse texture and poor suction of the lower gangue, which leads to water and nutrients being ponded in the upper soils [53, 54]. The gangue is coarse-textured, low in plant available nutrients, and high in pH value, which is not conducive to the growth and development of plant root. The root system mainly absorbed water from the upper soil, which may explain the lower subsoil water content of T8. Gangue has the characteristics of large particle size, small suction, and poor water holding capacity [54]. So the water content of gangue was obviously lower than that of soil layer. Water movement and solute transport in soil are interconnected and affected. Therefore, the distribution characteristics of nutrients content in different soil profiles were similar to those of soil water content.


### 4.2. The Effects of Filling Material Properties on the Corn Root Distribution

The growth of maize roots directly affects the growth of shoots. Only when the root system is well developed can the needs of above-ground growth be fully satisfied [55]. In addition to the water and nutrient conditions, due to the different physical and chemical properties of different soils, the changes of soil mechanical strength, bulk density, temperature heat, air permeability, fertilizer and water retention, and other factors are also different, which will affect the growth of maize roots [56]. Water and fertilizer in soil are more abundant than in gangue, so it can be expected that corn roots will favor soil over gangue as a growth medium. In the early stage of maize growth, 85%–90% of roots were distributed in the soil layer above 40 cm [57]. So there was no significant difference among the control groups and treatments during the seedling stage. However, with the prolongation of growth period, the roots of quantity of deep soil continued to increase. It has been demonstrated that 93.85% of maize roots are distributed above 60 cm in lime concretion black soil during maize maturation [58]. Because the thickness of surface soil in the control group was more than 70 cm, the biomass and yield were significantly higher than those in the treatments. Guo et al. [59] carried out a field experiment using 30 cm topsoil and a subsoil composed of a 1 : 1 mix local subsoil and saprophytic topsoil overlying gangue. The combined thickness of soil in the treatments overlying gangue was 30 cm, 50 cm, 70 cm, and 100 cm, respectively. The experiment was conducted in the coal gangue filling and reclamation subsidence areas in Kailuan Coal Mine in Tangshan City, Hebei Province. Tangshan City is located in the east part of the North China Plain. The climate of Tangshan City is warm, semihumid, monsoon, and continental with four distinct seasons. The average annual temperature in Tangshan is 10°C–11.3°C. The average annual rainfall in Tangshan is 620 mm–750 mm. Rainfall is mainly concentrated from July to August [60]. The results showed that the maize yield in the soil profile with 70 cm soil thickness was not significantly different from that of the control treatment. Liu et al. [61] investigated land reclamation area in the coal mining subsidence area of Huainan City in Anhui Province. Treatments consisted of the following soil thicknesses over gangue: 40 cm, 60 cm, 75 cm, and 90 cm. The results indicated that the soil productivity of the 75 cm soil thickness and 90 cm soil thickness treatments was similar to that of the control treatments. But the soil productivity of the 40 cm and 60 cm soil thickness treatments was significantly lower than that of the control treatments. The results of this study also showed that there was no significant difference in corn yield between a soil thickness of 70 cm (CK2) and the control treatment (CK1), but the maize yield of CK2 was significantly higher than that of T1–T8. This result is consistent with the other studies referenced above and suggests that, under these climatic conditions, a soil profile with a thickness of 70 cm (CK2) is optimal for use as a cover over gangue for filling subsided areas.


### 4.3. The Potential Pollution from the Application of Gangue

Gangue can be used as filling and reclaiming materials if the trace elements in gangue will not pollute the soil, groundwater, and surrounding environment. Studies show that trace elements (Cr, Pb, Hg, As, Cu, Se, etc.) in gangue vary greatly from several times to dozens of times in different areas of China, and not all trace elements in gangue exceed the national or local standards [62–64]. In order to ensure the safety of gangue for agricultural use, the standards for pollution control on the storage and disposal site for general industrial solid wastes and environmental quality standard for soils should be observed when reclaiming subsidence areas with gangue [65, 66].


## 4.1. Distribution Characteristics of Filling Material Water Content and Nutrients in Layered Soil Profile

The texture, bulk density, and organic matter content of the filling material will affect the hydraulic properties [34, 50]. When the wetting front passes through the fine soil-coarse soil interface, due to the large porosity and small suction of the coarse soil, the wetting front stops at the interface causing a hydrological discontinuity [51], and the water content of the upper soil increased. When the gangue is located in the middle layer of soil profile, its presence inhibits the process of soil water infiltration and correspondingly increases the moisture of the upper soil [52]. However, the soil in this study area is the lime concretion black soil that has a large silt content and limited water infiltration [53], making it difficult for water to pass through the 40–70 cm soil layer. Therefore, there was no significant difference of the surface soil water content among the control group, Group 1, and Group 2. There were significant differences in topsoil water content between T8 and the other soil profiles. This can be explained by the proximity of the gangue (40 cm) to the soil surface in T8, which impedes downward movement of water at the interface of the hydrological discontinuity between the upper soil layer and the gangue. The soil water content and nutrients in the interlayer were higher than those in the control group at the same depth. This is due to the fine texture and suction of lime concretion black soil and the coarse texture and poor suction of the lower gangue, which leads to water and nutrients being ponded in the upper soils [53, 54]. The gangue is coarse-textured, low in plant available nutrients, and high in pH value, which is not conducive to the growth and development of plant root. The root system mainly absorbed water from the upper soil, which may explain the lower subsoil water content of T8. Gangue has the characteristics of large particle size, small suction, and poor water holding capacity [54]. So the water content of gangue was obviously lower than that of soil layer. Water movement and solute transport in soil are interconnected and affected. Therefore, the distribution characteristics of nutrients content in different soil profiles were similar to those of soil water content.


## 4.2. The Effects of Filling Material Properties on the Corn Root Distribution

The growth of maize roots directly affects the growth of shoots. Only when the root system is well developed can the needs of above-ground growth be fully satisfied [55]. In addition to the water and nutrient conditions, due to the different physical and chemical properties of different soils, the changes of soil mechanical strength, bulk density, temperature heat, air permeability, fertilizer and water retention, and other factors are also different, which will affect the growth of maize roots [56]. Water and fertilizer in soil are more abundant than in gangue, so it can be expected that corn roots will favor soil over gangue as a growth medium. In the early stage of maize growth, 85%–90% of roots were distributed in the soil layer above 40 cm [57]. So there was no significant difference among the control groups and treatments during the seedling stage. However, with the prolongation of growth period, the roots of quantity of deep soil continued to increase. It has been demonstrated that 93.85% of maize roots are distributed above 60 cm in lime concretion black soil during maize maturation [58]. Because the thickness of surface soil in the control group was more than 70 cm, the biomass and yield were significantly higher than those in the treatments. Guo et al. [59] carried out a field experiment using 30 cm topsoil and a subsoil composed of a 1 : 1 mix local subsoil and saprophytic topsoil overlying gangue. The combined thickness of soil in the treatments overlying gangue was 30 cm, 50 cm, 70 cm, and 100 cm, respectively. The experiment was conducted in the coal gangue filling and reclamation subsidence areas in Kailuan Coal Mine in Tangshan City, Hebei Province. Tangshan City is located in the east part of the North China Plain. The climate of Tangshan City is warm, semihumid, monsoon, and continental with four distinct seasons. The average annual temperature in Tangshan is 10°C–11.3°C. The average annual rainfall in Tangshan is 620 mm–750 mm. Rainfall is mainly concentrated from July to August [60]. The results showed that the maize yield in the soil profile with 70 cm soil thickness was not significantly different from that of the control treatment. Liu et al. [61] investigated land reclamation area in the coal mining subsidence area of Huainan City in Anhui Province. Treatments consisted of the following soil thicknesses over gangue: 40 cm, 60 cm, 75 cm, and 90 cm. The results indicated that the soil productivity of the 75 cm soil thickness and 90 cm soil thickness treatments was similar to that of the control treatments. But the soil productivity of the 40 cm and 60 cm soil thickness treatments was significantly lower than that of the control treatments. The results of this study also showed that there was no significant difference in corn yield between a soil thickness of 70 cm (CK2) and the control treatment (CK1), but the maize yield of CK2 was significantly higher than that of T1–T8. This result is consistent with the other studies referenced above and suggests that, under these climatic conditions, a soil profile with a thickness of 70 cm (CK2) is optimal for use as a cover over gangue for filling subsided areas.


## 4.3. The Potential Pollution from the Application of Gangue

Gangue can be used as filling and reclaiming materials if the trace elements in gangue will not pollute the soil, groundwater, and surrounding environment. Studies show that trace elements (Cr, Pb, Hg, As, Cu, Se, etc.) in gangue vary greatly from several times to dozens of times in different areas of China, and not all trace elements in gangue exceed the national or local standards [62–64]. In order to ensure the safety of gangue for agricultural use, the standards for pollution control on the storage and disposal site for general industrial solid wastes and environmental quality standard for soils should be observed when reclaiming subsidence areas with gangue [65, 66].


## 5. Conclusions

The differences in plant height, stem diameter, chlorophyll, and leaf areas of different treatments increased in significance as the growth period extended, especially in the tasseling stage. Maize growth of the control groups was significantly better than T8. But the differences of maize growth among the treatments groups are not significant. The biomass and yield of the control groups were significantly better than the experimental treatments. The biomass of CK1 and CK2 was 220.70 g and 207.27 g, respectively. The yields of CK1 and CK2 were 8739.15 kg/ha and 8239.60 kg/ha, respectively. The water and nutrient contents of topsoil in the control group were higher or similar to those of the experiment treatments. But there were no significant differences of biomass, yield, subsoil water content, and nutrients between CK1 and CK2. Results indicated minimizing the adverse impacts of gangue requires a combined top-subsoil cover of at least 70 cm. This result is consistent with other studies conducted in the same area of China.In the future, repeated measurements of maize growth and yield are recommended to gauge how the various treatments respond to weather variation. For example, significant changes in soil hydrology could impact transport of soluble salts within profiles that might hinder plant growth.



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*Source: 5740659-2020-01-20.xml*