4.1. Comparative Structures of Soil Microbial Communities in Reclaimed and Control Lands
The distinct soil microbial composition may represent a core to maintain ecosystem functions during harsh conditions [
28]. After severe disturbances, such as coal mining, land subsidence, and coal gangue filling and reclamation, soil ecology was seriously disturbed, and soil microbial diversity tended to decrease. Our study supported findings reported in other pyrosequencing studies in mining areas after phytoremediation, where increased microbial diversity followed improvements in soil conditions [
29,
30].
The bacterial community abundance of reclaimed soil was lower than that of control soil, which supports the findings of other researchers [
21,
31,
32]. The bacterial community diversity in reclaimed land was also inferior to that of natural soils. Previous investigations of reclamation soil reported increased microbial diversity following the addition of compost, lime, and vegetation [
33,
34,
35,
36].
Comparison of the bacterial community compositions of the control and reclaimed lands indicated that while the dominant bacteria in soil bacterial communities were consistent, the structural proportions of the bacterial populations differed. Even after 15 years of reclamation, soil bacterial community structures in reclaimed land had not recovered completely to the levels found in control land, which is consistent with the results of previous studies [
32,
37,
38]. Lewis (2012) reported that in the composition of microbial communities persists even after more than 20 years of recovery, revealing the significant negative effects of mining on the soil [
38].
4.2. Variation in Microbial Community Structure after Different Reclamation Periods
The diversity of soil microorganisms represents the stability of microbial communities and reflects the effects of soil ecological mechanisms on communities [
39]. We compared the degree of restoration of diversity and richness of the bacterial community following different reclamation period and found that samples with a 15-year reclamation periods were extremely close to those of normal farmland. These results supported the findings of a previous investigation of the effects of different reclamation methods and periods on soil microorganisms [
13]. Variations in bacterial diversity in the 0–20-cm soil layer were more marked than those in the 20–40-cm layer, which may be because the soil environment in the 0–20-cm soil layer was more easily altered by artificial disturbance than that in the 20–40-cm soil layer.
Microbial community succession was found in reclaimed soils, and the ratios of dominant bacterial communities in the soil ecological system from reclaimed land successively changed. The role of the dominant bacterial community is crucial in recovering soil quality. Most Firmicutes have unique physiological structures, allowing them to adapt to the lack of water and extreme environment of mining areas; therefore, Firmicutes were more abundant in reclaimed land than in control land. Bacterial genera from Firmicutes, such as
Bacillus, which are characterized by resistance to extreme environmental conditions and their ability to eliminate heavy metals from soil, and
Enterococcus, which are characterized by rapid growth, became the dominant bacterial community members in reclamation soil. These findings indicate that Firmicutes made a major contribution to ecological remediation and re-establishment of soil ecology in coal mining subsidence areas, which supports the findings of Chen (2012) and Poncelet (2014) [
30,
40]. In addition, some bacteria such as
Bacillus and
Enterococcus, can grow and propagate in nutrient-depleted environments, playing an essential role in the recovery of soil fertility, and contributing to soil remediation and ecological improvement of coal mines after reclamation.
We speculate that samples of reclaimed land with unstable soil ecology lack some beneficial bacteria; namely, Proteobacteria. Previous studies have indicated that the resilience of a microbial community will be promoted by increasing the abundance of bacteria that can be classified as copiotrophs, such as members of Proteobacteria [
41,
42], as well as decreasing abundance of oligotrophs, such as many members of Acidobacteria [
43]. This may be because higher proportions of Proteobacteria in soil communities have been associated with facilitated plant growth during remediation of contaminated soils, suggesting a positive feedback relationship between available carbon from root growth and members of this phylum [
44].
Therefore, the predominant bacteria found in degraded soils often can adapt to ecological changes, improve the soil environment, and replenish soil that is relatively nutrient poor. He et al., (2017) [
45] reported similar findings, stating that the dominant bacterial communities in reclaimed open-pit coal mining land were mostly chemoautotrophic or chemoheterotrophic, or that they were functional bacterial genera participating in the nitrogen cycle or in degradation of polycyclic aromatic hydrocarbon organic matter, making them beneficial for ecological remediation and recovery of fertility in polluted soil. Therefore, bio-organic fertilizers or specific microbial fertilizers can be applied to adjust the soil micro-ecological environment, and promote the succession of bacterial communities and the stability of soil ecosystems. This will facilitate the practice of soil remediation.
At present, numerous studies have analyzed the effect of compound microbial agents on the quality of reclaimed soil. For example, when continuously applying ripening biofertilizer to reclaimed soil for three years, total phosphorus, available phosphorus, and organic matter content in the soil increased by 7.1%, 27.9% and 30.2%, respectively, comparing to the control group. Moreover, the number of bacteria, fungi, and actinomycetes increased by 28.2%, 32.2% and 22.8% respectively [
46]. Further, different microbial agents have been shown to have a great influence on the quantity and structure of microorganisms in the soil [
47]. Hence, it is important to identify suitable and efficient bacterial strains to improve soil quality, which can promote the rapid restoration of soil function for the reclamation soil. Moreover, when applying bacterial fertilizer to reclaimed soil, managers should pay special attention to the influence of soil heavy metals on bacterial activity [
48].
4.3. Comparative Analysis of Bacterial Communities and Dehydrogenase Activity of Soil during Different Reclamation Periods
Soil enzyme activities play a pivotal role in soil biochemical processes and have often been used as indicators in the evaluation of soil recovery conditions in different ecosystems [
40,
49,
50]. Previous research has shown that interactions between soil enzymes and bacteria can promote the mineralization and decomposition of organic matter and the cycling and transformation of nutrients in soil [
51]. The results of our study demonstrate a relationship between the structure of the microbial community (both by PLFA and genomic approaches) and soil enzyme activities [
42].
Based on the correlation analysis conducted in the current study (
Table 5), a strong correlation was found between soil dehydrogenase and Firmicutes, Proteobacteria, Chloroflexi, Acidobacteria, as well as
Bacillus,
Enterococcus,
Lactococcus,
Paenibacillus,
Cronobacter, and
Alkaliphilus. This is because soil dehydrogenases are redox enzymes and are mainly derived from soil microorganisms, plant root exudates, and animal and plant residues [
4]. The activity of dehydrogenases changes with changes in soil biota, thus, soil dehydrogenase activity can be used as an index of changes in bacterial community structure and soil fertility, as well as to provide a theoretical basis for improving the soil quality of reclaimed land. These findings are similar to those of Meng et al. [
16], who reported that the soil enzyme activities of the main Proteobacteria in reclaimed land were highly correlated with the Olsen phosphorous value and alkaline phosphatase and could be used to measure the effectiveness of soil phosphorus by evaluating relationships between bacterial colonies and enzyme activity.
4.4. Comparative Analysis of Physicochemical Properties and Microbial Communities of Soil with Different Reclamation Periods
The soil microbial community composition and soil environment are inseparable. Soil properties affect the activity and diversity of the soil microbial community, while soil microorganisms gradually improve soil quality through decomposition of organic matter [
52]. Interactions between the soil and its microorganisms drive the normal function of this ecosystem, and any modification in this relationship may influence microbial structures, which in turn affect soil ecological processes [
53].
At the phylum level, different relationships between bacterial communities and the soil quality of reclaimed land were observed after different reclamation periods. As the reclamation period increased, the reclaimed land showed increasing P and K with increasing Firmicutes and Proteobacteria, indicating a direct correlation between increased availability of these minerals and better growth of Firmicutes and some Proteobacteria. The quantities of other bacterial phyla have been shown to be increased by increasing the TOC, BD, pH, and EC, which supports the findings of Nkongolo et al. (2016) [
54].
Our research revealed that the distribution of soil bacterial communities was significantly influenced by the MioC, TOC, EC, K, and P, but that pH had no effect on the bacterial community. This may have occurred because the pH of our samples did not differ significantly. However, other studies have found a strong influence of pH on the composition of soil microbial communities [
22,
55,
56,
57].
Carter (1986) [
58] reported that increasing soil organic matter content could significantly influence soil microbial diversity. Moreover, He et al. (2017) [
45] found that the C:N of soil was the main factor affecting variations in soil bacterial colonies. The easy-available organic matter in the soil was beneficial for the potential development and activity of copiotrophs [
42]. Other authors have observed a negative impact of salinity on bacterial diversity [
59].
Gemmatimonadetes are suitable for survival under low soil moisture conditions [
60]. Acidocbacillus is weakly positively correlated with pH. However, some studies have reported that the relative abundance of Acidocbacillus in soil was significantly negatively correlated with soil pH [
61,
62]. In addition, changes in land use patterns also have a significant impact on the distribution of Acidocbacillus [
63].