4.1 Reasons for ecological disturbance in natural and resource development
Natural ecological disturbance is mainly affected by the average annual temperature and annual precipitation. Precipitation indirectly affects vegetation cover and soil physical and chemical properties in the study area (Grothmann et al. 2017), whereas temperature can affect soil physical and chemical properties in the study area by changing the growth pattern of plants in the study area (Hao et al. 2020). Less precipitation and unsuitable temperatures can exacerbate natural ecological disturbance. In this study, the ecological disturbance of all natural subsystems was classified as Grade IV, the annual average temperature to Grade IV, and the average annual precipitation to Grade III (Fig. 6). The mining area is small, and the overall climate is uniform. There were no significant climatic differences among the regions. While the annual precipitation in the area is 194.7-531.6 mm (average precipitation of 322.5 mm), the annual evaporation of 2297.4–2833 mm.This severe natural climate causes relatively strong disturbances in the natural ecological subsystem.
In the ecological subsystem of resource development, mining thickness is the most important ecological disturbance index. It is the driving factor of ecological disturbance caused by the resource development subsystem. The thickness of coal mines increases the surface displacement and deformation value (Wu et al. 2021), thereby aggravating the ecological disturbance of resource development. In addition to the independent effects of different ecological indicators, the resource development subsystem is also affected by the synergistic disturbance of mining intensity, mining thickness, burial depth, and mining thickness ratio. The three ecological indicators were significantly correlated (Fig. 9). This indicates that coal mining is affected by the actual occurrence of coal in the study area. The ecological indicators in the ecological subsystem of resource development are easily influenced and restricted by each other, forming a disturbance chain of "mining intensity–mining thickness–burial depth and mining thickness ratio” and leading to more complex ecological disturbances than any single indicator would cause. Measures such as layered driving, height-limited mining, and infill mining to interrupt the connection between these ecological indicators may reduce the ecological disturbance of the resource development subsystem.
4.2 Reasons for groundwater and surface ecological disturbance
The evaluation of groundwater and surface ecological disturbance (Fig. 6b) identified the chemical factor as the ecological factor with the largest weight, followed by physical and ecological component factors. Chemical factors are the disturbance center in the region, with available potassium and available phosphorus being the top two ecological indicators. Potassium and phosphorus are essential mineral nutrients for crop growth and development. Available potassium and available phosphorus reflect the ecological quality of surface soil to a certain extent—the less potassium and phosphorus, the lower the ecological diversity in the region, and the higher the ecological disturbance of groundwater and surface. The prominence of potassium and phosphorus in an assessment portrays these two elements as the driving indicators affecting the ecological disturbance of chemical ecological factors in the region. However, given that the ecological indicators of soil chemical factors were correlated before and after mining (Fig. 9), the soil chemical indicators in the study area can be regarded as the driving factors affecting the ecological disturbance of chemical ecological factors in the region. Consequently, Grade I was assigned to the non-mining and two-year mining areas and Grade II to the one-year mining area (Fig. 7). The reasons for such classification are multiple and complex: (1) Soil chemical factors may be positively regulated by the mining depth. The mining depth in the study area was Grade I, and the one-year and two-year mining areas were Grade II (Fig. 5). In the one-year mining area, the mining depth had a strong positive correlation with chemical ecological indicators except for pH (Fig. 9), which is contrary to the traditional view that coal mining will directly cause soil nutrient loss (Ma et al. 2019). This has been attributed to surface deformation, which arises with increasing mining depth, and the available fertility of soil surface, which is enriched in low stand areas with changing terrain (Fig. 10). However, this correlation disappears with the change in mining stop (Fig. 9). Therefore, the increase in mining depth reduces the disturbance to soil chemical ecological factors, strongly correlates the soil physical factors and chemical factors, and produces a series of indirect ecological disturbances under the interaction of different ecological subsystems. (2) The soil physical and chemical factors are correlated (Fig. 9). The mining process indirectly disrupts the chemical factors by disturbing the soil physical factors, thereby affecting the natural ecosystem. (3) Coal mining leads to surface subsidence and ground fissures (Liu et al. 2019), which leads to soil nutrient loss along ground fissures. However, ground fissures produced by coal mining can be self-repaired and closed, thus recovering available nutrients (Fu et al. 2021) (Fig. 10). In conclusion, the ecological disturbance caused by chemical ecological factors in the study area is determined by three ecological disturbance chains, namely "coal mining–soil chemical factor," "natural climate–soil physical factor–soil chemical factor," and "coal mining–surface subsidence–soil chemical factor." The assessment results of soil chemical factor disturbance are consistent with the generation and restoration process of ground fissures, indicating the leading role of "coal mining–surface subsidence–soil chemical factor" in the disturbance chain group.
Physical factors may be indirectly affected by soil chemical factors while being affected by natural ecological subsystems. This impact may be triggered by mining. Soil bulk density is the most important ecological index, followed by soil porosity. Soil bulk density and soil porosity are indicators of soil permeability and water retention capacity (Dang et al. 2021) and have a significant impact on the overall quality of soil. The lower the soil bulk density and porosity, the greater the possibility of intensified ecological disturbance of groundwater and surface. In the past, it was believed that coal mining caused surface deformation, which led to local soil compaction or looseness, as well as changes in soil bulk density and porosity in the subsidence area (Wang et al. 2015). This is the main reason for the increased disturbance of groundwater and surface ecosystem. However, the assessment result of surface settlement in the non-mining area was Grade I (Fig. 7), and the assessment of soil physical indicators assigned Grade IV to soil bulk density and Grade III to soil porosity (Fig. 7). Our study could not establish a correlation between soil physical factors and surface subsidence. The causes of soil physical factors disturbance are the following: (1) During mining, soil physical and chemical factors are correlated (Fig. 9), generating a disturbance chain of "coal mining–soil chemical factors–soil physical factors," increasing the ecological disturbance of soil physical factors in the study area, and establishing the coexistence between positive and negative ecological disturbance. (2) The average annual temperature and average annual precipitation in the region were assigned Grade IV (Fig. 6), consistent with the grade classification of soil physical factors. The presence of the disturbance chain "natural climate–soil physical factors,” which plays a leading role in the region, is thus confirmed. The ecological component factor is mainly disturbed by mining, but it remains unverified whether this factor is affected by other ecological factors or subsystems. Groundwater loss and surface subsidence rate are the two most important ecological weight indicators. Groundwater is water located in the formation voids below the vadose zone, which is sensitive to arid and semi-arid environmental changes and is an important environmental energy parameter (Zhao et al. 2018). Surface subsidence is a geological hazard phenomenon that occurs when the upper rock layer is caving, fractured, squeezed, bent, moved, and otherwise deformed under the action of gravity after the mineral is mined and spreads to the ground. Surface subsidence directly destroys the natural state of surface land and reduces its use value (Cui et al. 2020). Both groundwater loss and the surface subsidence rate are important indicators for measuring the contribution of coal mining to geological damage. The greater the groundwater loss, the greater the surface subsidence rate and the greater the possibility of increased groundwater and surface ecological disturbance. In this study, the unmined area was assigned Grade I, whereas the one-year and two-year mining areas were both reduced to Grade III (Fig. 7). The resource development subsystem in the unmined area was downgraded to level III in the one-year and two-year mining areas (Fig. 6). This is because coal seam mining leads to changes in the surface and topography, especially when the thick coal seam and mining face in the study area are large; severe deformation of the overlying strata will cause ground fissures and deformation on the surface, and the subsidence rate will also increase (Cui et al. 2020). Coal mining leads to drying of coal-measure aquifers, which may have varying degrees of impact on bedrock and phreatic aquifers (Wu et al. 2021). As a result, the loss of groundwater increases, and the ecological components are seriously disturbed. Because most of the ecological component factors are with regional influences, the sites in the same area are subject to the same disturbance in the evaluation; therefore, it cannot be confirmed whether they will be affected by ecological subsystems or ecological factors other than resource development. However, the evaluation results suggest that the ecological component factor is the main barrier to the reduced ecological disturbance in the coal mining subsidence area.
4.4 Reasons for increased ecological disturbance in coal mining subsidence areas
The ecological disturbance in the coal mining subsidence area is negatively correlated with the degree of mining; that is, the ecological disturbance in the coal mining subsidence area decreased in the order two-year mining area > one-year mining area > non-mining area (Fig. 8). The number of ecological disturbance chains decreased as follows: one-year mining area > two-year mining area > non-mining area (Fig. 9). This reveals that, in the first year of mining, the generation of ecological disturbance chains was the main reason for the increase in regional ecological disturbance. After two years of mining, although the number of disturbance chains decreased (Fig. 9), the disturbance of the ecological component factors did not end (Fig. 7), which further increased the ecological disturbance in the study area. In conclusion, coal mining deepens the connection between different ecological subsystems and factors. Multiple ecological disturbance chains are formed in the region, resulting in ecological disturbances that are difficult to resolve.