1. Introduction
Changes in land use are key factors promoting global climate change, and the side effects of mining activity that destroy the soil, vegetation, and biodiversity lead to imbalanced carbon cycling in terrestrial ecosystems. The United Nations General Assembly has emphasized the necessity of large-scale restoration of degraded land and has declared 2021–2030 the Global Decade for Ecosystem Restoration
[1],
[2]. Major mining companies worldwide have been strongly advised to minimize their carbon emissions within the next 10–15 years and to achieve net-zero emissions by 2050
[3],
[4]. The Chinese government has reduced its annual coal production capacity by 320 million metric tons
[4]. Furthermore, the Chinese government has ordered the closure of over 20 000 mines, and these abandoned mines urgently require ecological restoration. Although mining land reclamation and ecological governance have been continuing for over 40 years in China, the main focus has been on vegetation restoration and eliminating geological disasters, with little research being carried out on carbon sequestration in degraded mining land
[5].
In this paper, we briefly introduce the spatial distribution of mineral resources, the mining damaged area, and the ecological governance of mines in China. We explore the carbon-sequestration mechanism involved in ecological restoration in mining areas and propose key technical approaches for enhancing carbon sequestration in mining areas. Thus, this study provides a scientific basis for global decision-makers and practitioners.
2. Land damaged by mining in China and its ecological governance
There are approximately 110 000 non-oil-and-gas mines in China, with a total land area of 104 000 km
2, accounting for 1.08% of the territorial area
[6]. There are 77 700 active mines, over 11 300 mines under construction, and 22 200 closed mines, accounting for 69.8%, 10.2%, and 20.0% of the total area of non-oil-and-gas mines in China, respectively. By 2019, the mining industry had damaged 36 100 km
2 of various land types, with 81.5% of the land damaged by mining being distributed in the north, northeast, and northwest regions of China. Six regions—Shanxi, Inner Mongolia, Hebei, Shandong, Xinjiang, and Gansu—have damaged land areas exceeding 200 000 hectares (ha; 1 ha = 10
4 m
2). The total area of damaged land is 1 459 300 ha, the area of subsidence land is 844 500 ha, and the land area occupied by solid waste tips is 1 306 700 ha. Coal mining is responsible for the largest area of damage, at approximately 135 260 ha, accounting for 37.5% of the total area of mining damage in China. Nonmetal mines for building materials and chemical raw materials follow closely, accounting for 32.4% and 14.7% of the total area of mining damage in China
[7], respectively (
Fig. 1).
In the 1990s, the Chinese government implemented the ecological restoration of mines on a large scale, aiming to eliminate geological hazards and restore vegetation. In 2006, the Ministry of Finance of the People’s Republic of China, Ministry of Natural Resources of the People’s Republic of China, and Ministry of Ecology and Environment of the People’s Republic of China jointly issued
Guiding Opinions on Gradually Establishing a Responsibility Mechanism for Mine Environmental Governance and Ecological Restoration, proposing the establishment of a special fund specifically for the governance of the mine geological environment
[7]. However, the mining economy is greatly affected by fluctuations in the international market. When the market is sluggish, some mining enterprises would rather give up their mining rights than pay enormous costs for ecological restoration, which greatly interferes with the implementation of geological environmental management in mines. Moreover, historically untreated mines are very common. Compared with a mining land reclamation rate of 50%–70% in some other countries, the mining land reclamation rate in China is 30% lower
[3]. In 2019, the cumulative ecological management area of mines in China was only 930 800 ha, with 2 679 700 ha of damaged land still awaiting treatment. However, there are some successful examples of completed ecological restoration projects near mines. For example, Xuzhou City in Jiangsu Province (China) has combined the management of coal mining subsidence with urban planning, new rural construction, and ecological agriculture to form the nationally well-known Pan’an Lake urban wetland ecosystem. In 2019, the Ministry of Natural Resources of the People’s Republic of China issued
Opinions on Exploring and Utilizing Marketization to Promote Ecological Restoration of Mines, stating that the mining damage control area in China was 65 000 ha, which for the first time exceeded the newly added mining damage land area (48 000 ha)
[8]. Nevertheless, under the current conditions of mine restoration technology and capital investment, the treatment of the 2 679 700 ha of abandoned mining land that has yet to be treated and of the mining-damaged land that will be newly added in the future is a massive and complex undertaking.
3. The carbon-sequestration mechanism in mining ecological restoration
Approximately 75% of the carbon pool in terrestrial ecosystems is stored in the soil and is mainly derived from the transfer of plant photosynthetic carbon
[9]. The main reasons for the loss of carbon stored in soil in mining areas are the vegetation damage and soil erosion caused by mining. The stability of the soil carbon pool and surface vegetation cover are low in damaged mines, leading to an increase in soil temperatures that then increases the mineralization rate of organic carbon
[10]. Therefore, the most important steps in the ecological restoration of mining areas are restoring the vegetation cover and increasing the soil fertility. Land reclamation near mines usually involves filling the mines with solid wastes such as coal gangue and fly ash, backfilling and mechanically compacting with topsoil, and finally reestablishing the vegetation cover. However, reclaimed soils have a poor texture, loose structure, low fertility, weak permeability, and different capacities to support new vegetation. Some mining areas have very harsh natural conditions, making it difficult for vegetation to survive and contribute to carbon sequestration
[11].
The mechanism of carbon sequestration in mining areas includes physical, chemical, and biological processes and their interactions, as described in more detail below.
3.1. Soil aggregation
Soil restoration and vegetation reconstruction can promote the development of aggregates and increase organic carbon accumulation. Soil aggregates are very important in long-term carbon sequestration because they protect the carbon from decomposition and prolong the retention time of organic carbon. The components of active organic carbon and soil properties, such as the nutrients, pH value, and salt content, directly influence the distribution and storage of organic carbon
[12].
3.2. Clay mineral interactions
The presence of high-valent iron and aluminum oxides and clay minerals increases the stability of soil organic carbon through ligand substitution, high-valent ion bonding bridges, van der Waals forces, and complexation
[13]. Due to the ability of minerals to preserve soluble organic compounds produced from animal and plant residues or by means of microbial metabolism, organic carbon accumulation in soil largely depends on the capacity of large-surface-area iron and aluminum oxides to adsorb soluble organic compounds. The interactions between microorganisms, minerals, and organic carbon play an important regulatory role in preserving carbon in mining areas
[14]. The type and amount of metal oxides, clay mineral composition, clay content, and surface properties strongly affect the stability of soil organic carbon. Clay minerals have a high surface-adsorption capacity and can adsorb hydrophobic organic carbon with poor biodegradability. In addition to directly affecting the stability of organic carbon, clay minerals and iron and aluminum oxides indirectly affect stability through soil aggregation processes
[15].
3.3. Litter decomposition
The reconstructed vegetation is an important factor affecting the concentration of soil organic matter in mining areas because it directly affects the quality and quantity of litter input. For example, grassland soils have higher organic carbon storage capacity than forest soils owing to the higher organic matter productivity of the former
[16]. Other studies have shown that the increase in the organic carbon content in reclaimed topsoil may be because of the high vegetation cover and litter inputs
[17],
[18]. In contrast, in grasslands, the decomposition of underground roots (especially fine roots) is the main source of organic carbon
[19].
3.4. Microbial regulation
The storage capacity and stability of soil carbon pools are significantly correlated with the microbial activity, and the contributions of different groups of microorganisms to organic matter decomposition differ. For example, microorganisms and their enzymes play important roles in the mineralization of organic matter, and their selective decomposition of organic matter can enrich more degraded complex chemical substances, having a significant effect on soil organic matter
[20]. In addition, some microorganisms (e.g., chlorophyll-containing microorganisms such as the bacterial genus
Pseudomonas) have special carbon-sequestration mechanisms that can fix organic carbon or degrade aromatic compounds for respiration (e.g.,
β-Proteobacteria)
[21]. Research has shown that higher growth rates and efficiency of microbial communities promote carbon accumulation in the soil
[22]. The chemical structure and complexity of organic carbon depend on the relative contributions of the two pathways of microbial
in vivo turnover and
ex vivo modification. Through the continuous “
in vivo turnover” of microorganisms, the input of exogenous organic carbon from different chemical components tends to be the same, while “
ex vivo modification” makes the carbon components of plant sources tend to differ
[23]. In addition, certain microorganisms and their metabolites (e.g., the recalcitrant cell walls and complex polymers produced by microorganisms) are important components of recalcitrant organic carbon (
Fig. 2).
3.5. Mediation by biotic processes
Unlike naturally formed soils, the reclaimed soils in mining areas are constructed from the cover layer excavated during mining. These soils typically have unfavorable physical, chemical, and biological conditions such as low nutrient levels, poor soil structure, and contamination by leaching of metals and organic matter, resulting in high levels of disturbance
[24]. Moreover, the ecological conditions at mines in China have complex, multidimensional, and regional differences. For example, the components of soil organic carbon with the properties of humic and non-humic substances are considered the main forms of carbon present in the soils of humid and semi-humid mining areas. In arid and semi-arid mining areas, soil inorganic carbon composed of carbonates and bicarbonate is the main form of carbon
[25],
[26].
Factors such as climate change, land use, and management practices further influence the process and stability of carbon sequestration. These factors gradually reduce the rate of formation and the stability of organic carbon during the land restoration process, which is the result of the transformation of the mine restoration ecosystem from secondary succession to near-natural succession. In the early stages of mine restoration, vegetation growth, soil nutrients, soil microorganisms, and so forth are extremely sensitive to climate and human management, so these can affect the rate of formation and stability of organic carbon
[27].
4. Sustainable improvement strategies for mining ecological restoration in China
On a global scale, mining and related land use are important contributors to soil carbon pool losses and greenhouse gas emissions. Therefore, sustainable restoration practices and appropriate postmining ecosystem management are necessary to mitigate the global climate crisis. Hence, we propose the following four research directions and key strategies to support sustainable carbon sequestration in degraded mining areas in the future (
Fig. 3).
4.1. Accurately estimating ecological carbon sequestration for mining restoration
Accurate estimation of the contribution of ecological restoration in mining areas to global carbon sequestration remains an enormous challenge, mainly due to the high spatial heterogeneity of mining-area carbon sequestration under different reclamation models and the errors in methods for mining-area carbon sequestration estimation. Although many studies have quantified carbon storage in vegetation and soils at the site scale, there is great uncertainty when the research results are extrapolated to regional scales. The wind lateral transfer of carbon is rarely estimated, whether through inventory studies or vortex covariance measurements
[28]. In addition, most carbon flux monitoring data are derived from locations with low human interference, and there is a lack of long-term monitoring in degraded mining areas. Therefore, the integration of various methods is necessary to improve the efficiency of measuring organic and inorganic carbon dynamics and lateral carbon transfer in degraded mining areas. This can be achieved by combining ground inventory, magnetic flux monitoring, and simulation. In addition, modern remote-sensing technologies such as hyperspectral imaging, digital photogrammetry, and laser scanning can monitor vegetation dynamics at a large regional scale and effectively estimate the size and distribution of soil carbon pools. By accurately quantifying changes in the carbon balance in mining areas at different temporal and spatial scales, the optimal location for priority restoration or the implementation of carbon sequestration projects can also be determined
[29].
4.2. Reversing the impact of non-biological processes
Non-biological processes drive soil carbon cycling, possibly leading to the loss of organic and inorganic carbon in soil. Recent studies have shown that non-biological processes and the plant and microbial activities driven by them can greatly alter soil carbon sequestration and nutrient cycling
[30]. Moreover, the response of non-biological processes to global climate change is not clearly understood. In contrast to natural abiotic processes, abiotic processes caused by mining and their mediated carbon effects have not yet attracted much attention from researchers. For example, coal mining disrupts the original stress balance of the underground surrounding rock, greatly increasing the frequency of ground cracks and collapses, causing soil erosion, and leading to rapid carbon loss. In addition, the enrichment of trace metals hinders the regulation of exogenous carbon by microorganisms, and an increase in acid deposition leads to the loss of inorganic carbon and the activation of metal toxicity, which is not conducive to the carbon storage of the entire ecosystem
[31]. In addition, other non-biological processes such as changes in rainfall, temperature, or degradation may significantly alter the carbon balance in mining areas, but these processes have received limited attention. Therefore, further research is needed to determine whether changes in non-biological processes may affect the carbon balance in mining areas that will be subjected to ecological restoration and to clarify the relationship of non-biological processes with each other and with biological processes. A better understanding of the relative contribution of non-biological processes to the carbon cycle in mining areas will provide vital scientific evidence for protecting or increasing carbon reserves.
4.3. Developing multifunctional green technologies
Appropriate reclamation techniques and post-mining management strategies are important in the ecological restoration of mines. Research has shown that using mining and other organic wastes to construct soil for mine reclamation can offset up to 60% (1.00 Gt CO
2 equivalence (CO
2e)) of soil CO
2 emissions
[32]. Redesigning the terrain and water flow can minimize soil erosion and provide a medium for vegetation restoration. In addition, increasing evidence suggests that carbon-based technologies have enormous potential for improving soil quality and remediating heavy metal pollution in soils. Biomass-derived carbon materials have become popular materials in remediation projects. For example, if all agricultural biomass can be carbonized and returned to agricultural land in Guangdong Province, China, approximately nine million tons of carbon can be sequestered in the soil annually
[33]. However, the prospects of the persistent use of biochar to alleviate greenhouse gases and stabilize soil carbon in mining restoration have yet to be quantified. As an exogenous carbon input, the assessment of biochar’s sustainability in remediation and its interaction with abiotic factors still need further exploration.
The restoration of vegetation to a near-natural state is the key to mining area reclamation. However, the quantity and quality of vegetation litter and the carbon input of mycorrhizal fungi differ considerably. Different vegetation types have different adaptability to the environment, and future research needs to screen plant species suitable for local mining area reclamation. In addition, the land after mining must be restored by developing layered vegetation cover consisting of ground vegetation (e.g., grass and leguminous herbs), shrub layer, subtree layer, canopy layer, and emergent layer, but the timing of the introduction of each vegetation type and carbon footprint analyses have not been fully considered
[34].
Currently, microbial technology is a research priority in various fields. Autotrophic microorganisms (e.g., cyanobacteria and photoautotrophic bacteria) that fix CO2 via the Calvin–Benson–Bassham cycle play a central role in the soil carbon balance. The rapid development of modern molecular biology and the popularization of high-throughput and sequencing technologies have greatly increased the possibility of the gene-technology-targeted screening and synthesis of microorganisms with specific functions. In short, green technology has become a promising new avenue under the Chinese vision of carbon neutrality. To maximize the carbon-sequestration effect of mining area restoration in the shortest possible time, coordination and an increase in the efficiency of these technologies must be pursued.
4.4. Improving adaptive management strategies
The mining ecosystem is a multistable system with dynamic characteristics. Therefore, after the implementation of a reclamation project, a comprehensive, adaptive management framework and action guidelines for measuring the degree of mining area restoration and responding to emergency plans need to be established. For example, ecological restoration measures combining biotechnology and engineering technology are being adopted in the Loess Plateau region. Pioneer plants are introduced first to promote the growth of organic matter; then, suitable vegetation is introduced
[35]. In the Inner Mongolia grassland area, the aim is to restore grassland and meadow vegetation, with biotechnology as the main focus and engineering technology as the auxiliary
[36].
In addition, based on changes in the ecosystem, the policies and practical measures for ecological protection and restoration in mining areas need to be revised and improved. Restoration of the mining ecosystem is necessary, and a stable self-sustaining and self-recovery state must be attained.
As mentioned above, a recovering mining ecosystem is dynamic, so assessing the degree of ecological restoration in mining areas must be based on a comprehensive adaptive management framework, which we must emphasize. In mining area ecological restoration, the carbon sink advances from a sensitive state in the early stage of restoration to a self-sustaining stage. Therefore, it is necessary to determine the main driving factors limiting the improvement of carbon sinks in an ecological restoration stage, and the technical means of ecological restoration should be reasonably configured according to the reclamation outcomes in mining areas. This will be conducive to the long-term stability and improvement of carbon sinks.
Currently, China’s economic development cannot be separated from its energy consumption, which is closely related to carbon emissions. Approaches that both ensure energy security and reduce carbon emissions are important in changing the current ineffective carbon-reduction practices of some industries and local governments. Therefore, as per the economic structure in China, a mere reduction in coal usage is not advisable at present, and the coal industry must become “greener.” In the past 30 years, Western countries have shifted most of their manufacturing to China; this has resulted in pollution of China’s land and atmosphere, and—while the Western countries retain their own “post-industrial” territories—China has been forced to over-industrialize. In addition, Western governments have repeatedly refused to fulfill corresponding emission-reduction obligations while urging China to reduce its carbon emissions. For example, on June 1, 2017, the US government announced plans to reject the 2015 Paris Climate Agreement.
The Chinese authorities have planned for the next few decades and have expressed concerns about economic growth. In July of 2021, the Chinese Communist Party explicitly corrected China’s “carbon-reduction” campaign and required the establishment of a systematic plan in which the gradual withdrawal of traditional fossil fuels is based on the safety and reliability of new energy. For survival, China’s coal industry may achieve a “greener” model with almost zero emissions. Therefore, mining carbon offsets action is top-down and robust in China and has nothing to do with the twists and turns of carbon-emission-control policies of the United States or globalists
[37].
Acknowledgments
This work was supported by the National Natural Science Foundation of China (52374170 and 51974313) and the National Key Research and Development Plan Project (2022YFF1303300). We thank LetPub (www.letpub.com.cn) for its linguistic assistance during the preparation of this manuscript.
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