Open-pit mining is globally the main coal mining method. Many countries have realized the importance of land reclamation and have successively promulgated laws and regulations with respect to land reclamation of abandoned mining sites. Soil reconstruction is the core of land reclamation, and the quality of soil reconstruction directly determines the land reclamation status (Wang et al., 2021). The original topsoil is the best choice to reconstruct the soil. However, the poor soil development in mining areas due to mining activities causes serious problems of surface soil scarcity (Robson et al., 2018). When topsoil is scarce, suitable soil substitute materials are often screened from the geotechnical layer above the coal seam (Nicolini and Topp, 2005; Wilson-Kokes et al., 2013). Open-pit coal mines in China are mostly located in arid and semi-arid areas. Numerous domestic and foreign research results have shown that the native soil in open-pit mines is deficient in nutrients, and the soil fertility and plant diversity are poor, which is insufficient to realize the functions of soil nutrient cycling and microbial regulation (Wang et al., 2022; Feng, 2019; Hamid et al., 2022). The types of green vegetation available for ecological restoration are limited (Wu et al., 2023; Zhao et al., 2023). Thus, it is necessary to find an amendment for surface reconstruction soil that can balance the requirements of soil microorganism reproduction and the growth needs of locally adapted plants.

Root exudates are the general term for organic compounds released from the roots to the growth medium during the growth and development of plants, which can generally account for more than 10% of the photosynthetic carbon sequestration of plants (Haichar et al., 2016). Root exudates include low molecular weight organic substances, high molecular weight mucilage substances, root cell abscission, and its decomposition products, as well as gases, protons, and nutrient ions (Jiang et al., 2022). They are key factors in regulating the micro-ecological functions of the rhizosphere and also the main medium for communication between plants and soil (Chen et al., 2014; Pausch and Kuzyakov, 2018). They are of great significance for the elemental cycling of soil in the reclamation of mining areas, the absorption of plant nutrients, and the improvement of microbial functions (Ulbrich et al., 2022; Damin et al., 2008). A large number of previous empirical studies have shown that the growth of plants under the influence of root exudates is relatively good (Carla et al., 2020; Viviane et al., 2021). To adapt to the soil environment, plants will adjust the microbial community’s absorption and transformation of nutrient elements by changing the secretion of their root systems (Qu et al., 2020; Sun et al., 2021; Wang et al., 2023; Rahman et al., 2021). Plant root secretion is influenced by biological factors (plant species genotype) and abiotic factors (soil properties, climatic conditions) (Huang et al., 2019; Hu et al., 2021). The components of root exudates secreted by different kinds of plants grown under specific soil conditions are variable (Alberto et al., 2019; Sun et al., 2018). In the rhizosphere soil, bacteria have an absolute preponderant number of rhizosphere microorganisms, which plays an important role in maintaining plant growth and reducing the influence of environmental stress on plants (Jones et al., 2017; Meihong et al., 2019). Plant roots affect the population structure and diversity of rhizosphere bacteria by regulating the type and quantity of root exudates (Cordovez et al., 2024). Rhizosphere bacteria regulate the secretion of plant roots by regulating soil nutrients, thus affecting the growth and development of plants and nutrient absorption (Lyu et al., 2024). The nutrient content, transformation, and utilization efficiency of thin reconstructed soil in mining areas are often low, and the biological adaptability of the reconstructed soil is poor (Ju et al., 2021; Demyan and Smeck, 2022). According to the above, a way to overcome some of these problems may be to use root exudates to recover soil fertility and microbial diversity. It has been shown that low soil microbial biomass can restrict the recovery of soil functioning during the restoration of post-mining lands in semi-arid regions. Thus, especially for mining areas in areas with naturally nutrient-poor soils in arid and semiarid regions, this could be helpful in plant nutrients’ absorption, transformation, and utilization, as an organic modifier and assist microorganism breeding and ecological function recovering in the reconstruction soil land (Lu et al., 2018; Li et al., 2019). The research team selected the typical open-pit mine in China - Zahanoer open-pit mine, Inner Mongolia as the research object. Based on the investigation results, sweet clover (Melilotus officinalis L. Pall) was selected as the root exudate extraction body because it is a common reclamation species in mines and has high economic efficiency. Based on the research findings regarding the growth characteristics of common reclamation plants in open-pit mines and in accordance with experimental requirements, it is determined that Medicago sativa (Medicago Sativa Linn) exhibits significant salt-alkali tolerance and drought resistance. Its genetic mechanism provides a biological foundation for soil improvement in open-pit mines. Medicago sativa demonstrates strong adaptability to the reclaimed areas of open-pit mines. As the “king of forage grass, “its cultivation techniques are well-established, and extensive large-scale application experience has been accumulated in arid and semi-arid regions such as Inner Mongolia. Moreover, the molecular mechanisms and field monitoring technologies related to Medicago sativa have developed into a relatively comprehensive research system, facilitating experimental design and data collection. Consequently, this study ultimately selects Medicago sativa, a pioneer herbaceous reclamation plant, as the experimental subject to investigate the functions of root secretion improvement and microbial regulation. The results can be used to provide basic technical support for ideas and inspiration in the ecological restoration of opencast coal mine areas.

A main ecological problem from open coal mine use is restoring the soil surface environment. This is due to the shortage of topsoil and low soil fertility in many arid and semi-arid regions. Soil improvement is a critical component of ecological restoration in open-pit mining areas (Cantó et al., 2020; Malik and Jawad, 2022). Traditional fertilizers and root exudates, as two primary soil improvement methods, exhibit significant differences in their mechanisms and effects. Traditional fertilizers include chemical and organic fertilizers. The mechanism of chemical fertilizer is to improve soil fertility by directly supplementing nitrogen, phosphorus, potassium, and other nutrients in the soil (Tong et al., 2024). Studies have shown that chemical fertilizers can significantly increase the content of available nutrients in soil; for example, nitrogen fertilizers can raise soil alkali-hydrolyzable nitrogen content by 30–50% (Dakora and Phillips, 2002; Badri and Vivanco, 2009). However, the long-term application of chemical fertilizers may lead to adverse effects, such as soil acidification, compaction, and reduced microbial diversity (Wan et al., 2018).

The mechanism of action of organic fertilizer is to convert organic matter into humus through microbial decomposition, thereby improving soil structure and enhancing its water-holding and nutrient retention capacity (Wen et al., 2020). Research shows that the application of organic fertilizer can significantly increase soil organic matter content. For instance, applying cow manure or biogas slurry can increase the soil organic matter content by more than 30% (Hammad et al., 2020). Moreover, organic fertilizer can promote the formation of soil aggregates, improve soil aeration, and enhance the water–air ratio (Wang et al., 2024). However, the improvement effect of organic fertilizer is relatively slow, and its contribution to enhancing soil microbial diversity is limited (Tittonell et al., 2008). Root exudates indirectly improve soil structure and quality by activating insoluble nutrients and promoting microbial activity. Organic acids (e.g., oxalic acid and citric acid) in root exudates facilitate the dissolution of insoluble minerals through acidolysis and chelation (Bais et al., 2006). For instance, root exudates from cypress can increase soil available phosphorus content by 20–30%. Additionally, root exudates provide abundant microbial nutrients and carbon sources, enhancing rhizosphere microbial metabolic activity and accelerating nutrient mineralization and transformation (Li et al., 2012; Deru et al., 2023). Polysaccharides in root exudates also promote the formation of soil aggregates, thereby improving soil structure (Zeng et al., 2008; Yang et al., 2024). Through literature review and data comparison (Hao et al., 2022; Nannipieri et al., 2008; Shen et al., 2019), the soil improvement effect indicators of traditional fertilization and applied root secretion were quantitatively analyzed and compared, as shown in Table 2.

In summary, although traditional fertilization has relatively better immediate effects on improving soil nutrients, considering the flexibility and sustainability of the improvement effect, the improved root secretion method is more suitable for open-pit soil with poor nutrients, high soil strength, and low microbial activity (Table 2) (Deonalli et al., 2017; Zhao et al., 2025; Keiluweit et al., 2015). In this study, root exudates were used as soil amendments to improve the soil content of C and N in an abandoned open-pit coal mine area. Soil OM is the limiting factor to the growth of both plants and microbes (Lal, 2020). The root exudates contain vitamins, peptides, organic acids, lipids, carbohydrates, and other important plant organic nutrient substances. These substances promote root growth, absorb more water from the soil, and support the whole plant growing stage (Han et al., 2018). Furthermore, secondary metabolites in root exudates can help plants adapt to poor fertility and the biotic stress of impoverished soils (Vives-Peris et al., 2020). The results of this study also showed that the higher the amount of root exudates, the higher the soil fertility and root-shoot ratio.

Microorganisms play an important role in the nutrient transformation between amended soil and plants. However, the microorganism density is low with small diversity, especially in the mine ecosystem (Zhou et al., 2018). Previous studies have shown that root exudates can stimulate the growth of microorganisms and improve soil bacterial abundance (Zhou et al., 2012). When root exudates are sufficient, they can provide more energy for microbial metabolism and improve the abundance of soil bacteria, which is consistent with the conclusion of this study. On the one hand, the improved fertility of amended soil provides a substrate that is easily decomposed by microorganisms (Clocchiatti et al., 2021). The soil bacteria density increased with the continuous supply of nutrients (Figure 7). This was shown by the positive correlation between soil bacterial community distribution and soil fertility from OM and TN (Figure 9). Furthermore, increased plant growth by adding root exudates improved the ecological function of amended soil (Parray et al., 2016). Plant growth promotes rhizobacteria interactions with plants, such as element fixation and solubilization of N and P (Rosier et al., 2018), as well as improving plant drought resistance and the antiviral ability in the rhizosphere (Morales et al., 2022). Thus, root exudates improve soil bacteria by promoting plant growth, especially root growth (Ge et al., 2017).

In our experiments, the local dominating bacteria community was driven by root exudates and plant growth, as also shown in previous studies (Freedman et al., 2015; Lanoue et al., 2010; Maurer et al., 2021). The bacteria were sensitive to nutrients and organic matter in poor soils (Badri and Vivanco, 2009). The results of this study indicate that the bacterial diversity was highest for a root exudate concentration of 40–70%. If the added root exudates cause an imbalance between C and N in terms of bacterial use, the diversity of bacteria could be reduced (Jiang et al., 2021). Consequently, bacterial abundance decreased for a root exudate concentration of 85%.

A high concentration of root exudates was most obvious in promoting Actinobacteriota, Proteobacteria, and Chloroflexi. Actinobacteriota are an important bacterium in soil. Its slender and complex mycelia contribute to the formation of soil aggregates and the accumulation of organic carbon (Xie et al., 2022), which is of great significance to the improvement of basic soil fertility (Hernández-Álvarez et al., 2022). Proteobacteria are the most widely distributed and complex functional flora in farmland and natural soil. Proteobacteria help the fixation and transformation, especially mineralization, in the soil N cycle (Li et al., 2022). Alphaproteobacteria and Gammaproteobacteria are closely related to plant N-uptake and utilization from soil (Bugg et al., 2011). Chloroflexi is a facultative anaerobic bacteria that can generate energy through photosynthesis (Wang et al., 2019). It does not produce oxygen or fix nitrogen in photosynthesis, but it is helpful for the circulation of soil nutrients (Speirs et al., 2019). Firmicutes can produce spores with strong resistance to drought and extreme environments. All abundant bacteria in our experiments can adapt well to harsh soil conditions (Hernández-Álvarez et al., 2022). They, as well, can improve the efficiency of amended soil to reconstruct and enhance plant growth.

Activating soil nutrients through root exudates and promoting the absorption of soil nutrients by crops is of great significance for improving the productivity of poor soil. The soil in open-pit mining areas is nutrient-poor, making it essential to select locally adapted reclamation plants that are most suitable for growth in the planting system. Additionally, the impact of root exudates from these reclamation plants on soil properties and plant growth should be considered. Direct or indirect effects of root secretion from reclaimed plant species can affect the growth and development of their own or neighboring crops, improving the properties of soil under nutrient stress, and enhancing the effect of plant reclamation. The findings regarding the interaction between plant root exudates and rhizosphere microorganisms in this study hold significant theoretical and practical value for elucidating soil ecosystem functions and guiding vegetation restoration and ecological management in mined areas. In the future, the appropriate ecological planting model and adaptation mechanism of plants mediated by plant root exudates can be systematically studied to guide the ecological planting work in mines.

Applying root exudates in impoverished soil ecosystems of open-pit coal mines could increase soil fertility and improve the plant nutrient status. In the amended soil, the root exudates can simultaneously promote plant growth and bacterial proliferation. The growing plant roots will gradually increase bacterial abundance and diversity. The local dominant bacterial species of soil nutrient cycling increased and became more abundant. In a certain range, increasing the amount of root exudates can significantly increase the bacterial abundance.

This study demonstrates the practical application feasibility of root exudate-mediated soil amendment technology in the ecological restoration of open-pit coal mines. However, prior to large-scale implementation, region-specific analyses integrating pedological characteristics, reclamation objectives, and monitoring technologies must be conducted. A three-phase implementation strategy should be rigorously followed. In the initial restoration stage, concentrated root exudate solutions should be sprayed preferentially to activate the local microbial community and create a micro-environment for subsequent plant colonization. In the mid-term collaborative stage, synchronized with the sowing of stress-tolerant plants, continuous input of low-concentration exudates from the drip irrigation system should be adopted to promote root development and colonization of microorganisms. In the later maintenance stage, soil microbial diversity should be regularly monitored, and the amount of exudate application should be adjusted according to the abundance of dominant bacteria to avoid excessive stimulation that leads to community imbalance.