Relationship between GWT and soil characteristics
Due to the limited size of the study area (760 km2) and the consistent climate change. According to the topographic and geomorphological characteristics of the watershed, in the natural state without human intervention, it can be postulated that the soil physicochemical indexes and the species and growth of vegetation communities on the northern and southern sides of the lake should have a certain symmetry, with the Tao-Ahaizi lake as the centre. However, the northern side of the lake was affected by coal mine development, resulting in a significant change in the groundwater-soil-vegetation system compared with the southern side.
After statistical analysis of soil and vegetation characteristics, it is found that the limit depth of groundwater level in the Bojianghaizi basin is about 7 m, so the vegetation system dependent on groundwater in the study area can be distinguished from the vegetation system driven by precipitation. SWC, SEC, SOM, SAK, SAN, pH and vegetation coverage decreased significantly with the increase of GWT when the groundwater level was within the limited depth of groundwater. On the contrary, when the limit depth of groundwater is exceeded, the change of each detection index is not obvious with the increase of GWT.
It has been demonstrated that the moisture and salt content of surface soils in the vadose zone are primarily influenced by both the GWT and capillary rise height (Dong et al. 2022). Firstly, within the capillary rise height, groundwater and its dissolved salts can directly enter the soil (Yeh et al., 1998). Secondly, the entry of water and salts alters the soil's geochemical environment, modifying the original biogeochemical reactions, which in turn affects the structural and functional attributes of the surface vegetation (Zhang et al., 2018). Furthermore, as the groundwater depth increases, the capillary rise height becomes insufficient to support the migration of water and its dissolved salts to the surface. Consequently, at Y8 and Y9, the closest sampling points to the lake on the north and south sides, groundwater can transport water and salts to the surface soil through capillary action due to the shallowest GWT within the capillary rise height range. This results in the highest SWC and SEC values observed in the study area.
The extensive groundwater extraction associated with coal mining activities in the study area has led to the formation of a pronounced groundwater depression funnel at the center of the mining operations. Consequently, this has resulted in a significant reduction in SWC at the sampling points Y4 and Y5 within the mine site. Furthermore, coal mine development activities have transformed the previously confined subsurface environment into an exposed state. This exposure facilitates the oxidation of sulfurous iron ore and sulfur-containing organics within the coal bed in the presence of oxygen, producing acid (Eqs. 1-4). This process also enhances the solubility of minerals, resulting in a relatively low soil pH in the mine area (pH 6.78 and 7.53 at Y4 and Y5, respectively) and an increase in the SEC.
Water table-soil-vegetation interaction relationship
SOM, SAN, and SAK are essential indicators for assessing soil fertility, closely tied to the physical structure, chemical properties, and biological activity of the soil (Balesdent et al., 2000; Tian et al., 2019; Zörb et al., 2014). Although soil fertility indicators can vary by region and scale, research shows that under natural conditions, the concentrations of SOM, SAN, and SAK are predominantly influenced by the intensity of plant and microbial fixation and consumption (Hassanzadeh Bashtian et al., 2024; Yu et al., 2020; L. Zhang et al., 2021). Farmland distribution in the Bojianghaizi basin is sparse, and the sample sites chosen for this study are all located on wasteland (or sandy land), where vegetation communities are in a natural state, thereby minimizing potential interference from artificial nutrient sources. Correlation analysis (Fig. 5) in the study area reveals a significant negative correlation between the content of SOM, SAK, SAN and, GWT (correlation coefficients of −0.848, −0.775, and −0.758, respectively), and a significant positive correlation with vegetation cover (correlation coefficients of 0.861, 0.765, and 0.835, respectively).
In regions with a shallow groundwater table, high plant coverage facilitates the movement of SAK and SAN into the soil via capillary action. This process also supplies the soil with SOM, SAN, and SAK. In humid conditions, the soil microbial community is more functionally diverse, enabling a wider range of metabolic activities. This diversity enhances the decomposition of surface plant residues and organic matter, thereby increasing the levels of organic matter, available nitrogen, and available potassium in the soil (Zhao et al., 2023). The underground water level in the mining area (Y4, Y5) decreased significantly from 4-7m to 20-24m due to the development of the coal mine. Compared with the undeveloped area on the south side of the lake, the soil fertility in the mining area is relatively low due to the lack of water and nutrients recharged by capillary action. Furthermore, coal mining operations will result in the destruction of the land structure and vegetation resources, which will not only lead to a reduction in the soil and vegetation's water storage capacity, but also cause the loss and degradation of organic matter in the soil. Consequently, accumulating soil organic matter becomes increasingly difficult. Moreover, industrial pollution from coal mining adversely affects environmental factors such as soil, vegetation, and microorganisms, hindering the accumulation of SOM, SAN, and SAK in the soil (Mirlean et al., 2007; Semhi et al., 2013; H. Zhang et al., 2023).
In arid and semiarid regions, where precipitation is limited and water evaporation rates are high, groundwater resources frequently serve as the primary source of water for vegetation growth (Mata-González et al., 2021). In the hierarchical structure of surface, vadose, and saturated water zones, the vegetation root systems are predominantly found within the vadose zone. The lithological and hydrogeochemical properties of the vadose zone significantly influence vegetation growth conditions (Grimaldi et al., 2015).
A significant negative correlation (correlation coefficient of −0.923) is observed between GWT and vegetation cover (Fig. 5). In the lakeshore zone where the GWT is <0.5 m, the high SWC and strong evaporation result in the continuous loss of water and the accumulation of salts in the surface soil. This ultimately leads to the formation of a high-salt soil environment. The vegetation in this area primarily relies on groundwater for its water needs and has evolved to thrive in the high-salt and high-alkali environments. Consequently, the vegetation species are relatively sparse, comprising primarily hydrophilic and halophytic species, such as Leymus secalinus (Georgi) Tzvelev, Neotrinia splendens (Trin.) M. Nobis, P. D. Gudkova & A. Nowak, Medicago falcata L., etc. (Fig. 6).
When the GWT is between 0.5 to 7 m, the soil fertility level is high in this area due to the relatively high content of SOM, SAK, and SAN at this location, and it is within the range of the height of the capillary water rise, which makes the soil fertility level high in this area. There is a relatively rich variety of vegetation growing in these areas, most of which is recharged by groundwater in addition to moisture from atmospheric precipitation, and thus has a certain degree of drought tolerance. These areas are dominated by semi-dependent groundwater vegetation, such as Stipa tianschanica Roshev. var. gobica (Roshev.) P. C. Kuo, Artemisia annua L., Setaria viridis (L.) P. Beauv., Kali collinum (Pall.) Akhani & Roalson, etc.
When the GWT is> 7 m, it is difficult for groundwater to rise to the root of vegetation by capillary force. The primary factor influencing the ability of a vegetation community to obtain water is atmospheric precipitation. This process results in a shift from a semi-dependent groundwater type to a xerophilous type of vegetation community. The main plant species are Stipa tianschanica Roshev. var. gobica (Roshev.) P. C. Kuo, Leymus chinensis (Trin. ex Bunge) Tzvelev, Artemisia annua L., Setaria viridis (L.) P. Beauv., Thymus mongolicus (Ronniger) Ronniger, Artemisia desertorum Spreng., etc.
The vegetation succession caused by the change in groundwater level is particularly significant in the mining area (Y4, Y5). Before mining activities, the GWT of Y4 and Y5 in the mining area is 4-7 m, and semi-dependent groundwater plant species should be grown, such as Artemisia annua L., Leymus chinensis (Trin. ex Bunge) Tzvelev, and Chenopodium acuminatum Willd.. Due to the large amount of groundwater pumped out by coal mining activities, the groundwater level in the mining area has dropped sharply and a groundwater depression funnel has been formed, which has destroyed the stability of the original groundwater-soil-plant system and promoted the evolution of the vegetation community in the mining area. Now it is mostly grown as xerophilous vegetation such as Leymus chinensis (Trin. ex Bunge) Tzvelev, Stipa tianschanica Roshev. var. gobica (Roshev.) P. C. Kuo, Artemisia annua L. and Artemisia desertorum Spreng..