In the Anthropocene, human activities, such as mining, have caused large-scale ecosystem disruptions and changes to the earth’s surface ^[1][2][1,2]. Human activities have also altered global element cycles, including carbon, while the gradual increase in carbon dioxide content in the atmosphere continues to be a driving factor for climate change.

The removal of vegetation cover and topsoil causes a severe decrease in soil organic carbon (SOC) content, making mining one of the most significant human impacts on the environment ^[3][4][3,4]. Shrestha and Lal [5] found that SOC in reclaimed mine soils decreased by up to 83% when compared with undisturbed sites. Land productivity is also diminished as a result of soil profile disturbance during mining [6]. The exposed or deposited post-mining substrates become the parent rock of the developing soils called reclaimed mine soils (RMS) or Technosols ^[7][8][9][7,8,9]. These substrates typically display a lack of SOM, nutrient deficiency (mainly of nitrogen and phosphorus) and disturbed nutrient ratios, low pH-values, and unfavorable air–water properties ^[2][9][10][2,9,10].

When it comes to restoring the ecological function of mining sites after exploitation, two basic questions should be considered: succession or reclamation [11]? Succession does not always allow for the restoration of soil function and plant communities in a timely manner. As a result, anthropological intervention through reclamation stimulates the long-term process of succession in post-mining sites ^[10][12][10,12]. The reclamation of mining sites improves the productivity of the sites as well as the sequestration of lost carbon (C) and the reduction in CO₂ emissions [7]. For example, Pietrzykowski and Socha [13] revealed that Scots pine (Pinus sylvestris L.) productivity in post-mining ecosystems was comparable with that of a managed forest on a natural pine site. As a result, mine heap rehabilitation is critical for the restoration of ecosystem services ^[14][15][14,15]. The goal of reclamation (how it will be used later); natural conditions, such as climate, the availability of soil substrates, and water balance; technical (mining) management, including management of rock overburden; and the relief of the site designed for reclamation all influence the design and detailed planning of reclamation treatments ^[9][11][13][9,11,13]. In areas with a well-formed landscape, favorable hydrological conditions, and potentially productive soils, reclamation may entail using biological methods such as agricultural practices, mineral fertilization, and the introduction of appropriate humus-forming vegetation to target and skillfully accelerate soil formation processes. In places with barren or phytotoxic soils (e.g., the acidic and sulfurous Miocene sands, etc.), the basic restoration technique, in addition to appropriate landscape formation and hydrological condition regulation, consists of sealing and neutralization, followed by biological reclamation [16].

Planting vegetation is the most popular biological restoration approach for mining sites ^[10][17][10,17]. Mining constrains the establishment of vegetation by affecting essential soil components for plant growth, such as organic matter, microbial activity, and water, as well as by increasing bulk density and heavy metals ^{[17][18][19][20]}[17,18,19,20]. Reclaiming a rehabilitation site with various methods such as topsoiling (i.e., respreading of salvaged topsoil or spreading of uppermost soil layer from surrounding arable fields), green manure such as lupine and alfalfa, biochar, inclusion of plant litters, fertilizer treatment, and planting of grass and leguminous shrubs aids in the growth of vegetation ^[21][22][23][21,22,23]. Planting vegetation at a mining site plays a crucial role in the production of organic matter, which supports soil biota and releases carbon and essential nutrients into the soil ^{[15][24][25][26][27][28]}[15,24,25,26,27,28]. Improved soil organic matter may also improve the soil moisture-holding capacity and soil cation exchange, which are vital in the restoration of degraded mining soils [29]. Evaluating a mine site after restoration is vital to ensuring successful restoration. Among the reclamation quality indicators, SOC is very reliable, and evaluating its’ status over time shows the quality of the reclaimed land ^{[4][28][30][31][32]}[4,28,30,31,32].

The accumulation rate of SOC in a reclaimed site is affected by different driving factors, such as the age of the stand, the type of vegetation, treatments applied before restoration, the approach of rehabilitation, and agro-climate ^[13][28][33][13,28,33]. Litterfall from the planted vegetation is one of the major sources of SOC in reclaimed sites, and its accumulation over time has a direct relationship with SOC. The findings of Barliza et al. [34] in spoil heap reclaimed from an opencast coal mine in Colombia indicates that the highest fine litterfall recorded in the 21-year-old site (2.3 Mg ha⁻¹ year⁻¹) was more than double that recorded in the 7-year-old site (1.1 Mg ha⁻¹ year⁻¹) and concluded the enriching role of litterfall on soil organic matter content and nutrient status with the increasing age of the stand. An increase in the age of the stand also increases the microbial community [35], which results in the decomposition of organic matter and increases the amount of carbon in the soil with increasing age. Mukhopadhyay et al. [36] also concluded that age of reclamation has a significant effect on the nutritional and microbial properties of the mine soils.

Since the beginning of the history of mining, many studies have been conducted on reclamation methods for degraded mining sites to minimize its environmental impact and to enhance ecosystem services in the changing climate. Several studies have evaluated the effects of mining site reclamation on carbon sequestration ^{[33][37][38][39][40][41]}[33,37,38,39,40,41].

2. Factors Affecting Carbon Accumulation in RMS

The restoration of mining sites often contributes to SOC accumulation, but its rate may be affected by different factors such as the reclamation method, the age of restoration, the climate, soil properties, soil moisture, and vegetation ^{[17][28][33][37][42][43]}[17,28,33,37,42,43] (Table 1).

Table 1. The effect of age on soil organic carbon accumulation.

Tree Species	Age of the Plantation	SOC Total Stock (Mg ha	−1	)	SOC Accumulation Rate (Mg ha	−1	Year	−1	)	Soil Depth (cm)	MAP(mm)	MAT (°C)	Reclaimed Mine Soil Substrate Type	General Reclamation Techniques	References
Mixed Forest	5	9.11	1.82	0–20	1000	26	Coal	Top soil with mixed forest	[28]
	10	19.89	1.99
	25	41.37	1.65
Quercus liaotungensis	11	32.59	1.59	0–30	431.1	10.0	Coal	Leveling and top soiling	[44]
Pinus tabuliformis		16.04	0.37
Mixed	Acacia auriculiformis, Sennasiamea, Acacia catechu	and	Dalbergia sissoo	3	1.83	0.61	0–15	1375	25.7	Coal	Regrading of spoil materials and plantation of tree species	[8]
				7	3.65	0.52
				10	5.82	0.58
				15	7.60	0.51
Prosopis juliflora	2	8.1	4.05	0–60	975	27.5	Coal	Regrading and top soiling	[33]
	3	12.6	4.20
	4	17	4.25
	5	19.2	3.84
	6	27.5	4.58
	7	32.8	4.69
	8	45.4	5.68
Robinia pseudoacacia	L.	2	11.7	4	0–30	569	9.4	Lignite	NK fertilization, and spread of a mixture of rye and alfalfa	[45]
		14	59.8
Dalbergia sissoo	2	1.1	0.55	0–15	1308	27	Coal	Top soiling, farm yard manure, and NPK fertilizers	[46]
	16	8.91	0.56
Mixed Forest	5	7.02	1.40	0–25	1230	16.2	Coal	Loose-graded, hydroseeded, and NPK	[47]
	11	13.52	1.23
	21	21.35	1.02
Alder (	Alnus glutinosa	)	28	33.49	1.20	0–20	650	6.8	Coal	n/a *	[48][49]	[48,49]
Lime (	Tilia cordata	)	31	34.51	1.12
Oak (	Quercus robur	)	28	15.01	0.54
Spruce (	Picea	sp.)	27	8.46	0.32
Pine (	Pinus	sp.)	22	8.80	0.40
Casuarina equisetifolia	6	3.19	0.53	0–30	1228	21.7	Heavy mineral	Topsoil with a seed mixture of short-lived annual species	[50]
	9	3.75	0.42
	12	9.35	0.78
	15	11.55	0.77
Mixed Forest	2	5.4	2.70	0–30	975	23	Coal	Only backfilled dumps	[51]
	8	16.4	2.05
	14	26.4	1.89
Scots pines and giant miscanthus plants.	n/a	33	n/a	0–20	n/a	n/a	Lignite	Sewage sludge	[52]
		45	n/a					Compost
Scots pines	25	27.2	1.1	O to C2 horizon	n/a	n/a	Lignite	Liming and NPK fertilizers	[53]
		37.4	1.50					NPK fertilizers
Scots pines	12	63.1	5.20	0–110	580	7.6	Lignite	Liming, NPK fertilization and sowing a mixture of grasses and leguminous plants	[54]
	17	45.9	2.70
	21	22.6	1.08	0–110	700	8	Sand	Top soiling, NPK fertilization, and lupine as green manure
	23	16.8	0.73
	30	65.0	2.17	0–110	650	7	Sulfur	Leguminous and grass crop with NPK fertilization
	30	34.4	1.15
Pasture land	25	36.7	1.47	0–30	n/a	n/a	Coal	Grading and application of stored top and subsoils	[55]
Forest land		37.1	1.48

* n/a = data is not available.

2.1. The Age of Restoration and Dynamics of SOC Accumulation in Restored Novel Ecosystems

A chronosequence study of reclaimed mining sites found that carbon stock increases with age ^{[8][28][47][50][51]}[8,28,47,50,51] (Table 1), but this depends on the vegetation type. For example, the increase in SOC with age for deciduous vegetation reaches a maximum at 10 years and then increases at a slower rate, whereas for coniferous vegetation, SOC slowly increases with age up to 40 years [49]. Furthermore, Sperow [56] found that the annual rate of SOC accumulation increases at an increasing rate for the first 11 years for pasture and 13 years for forest and then decreases for the remaining 20 years. The increase in restoration age promotes the growth of various trees, shrubs, and grasses, which contribute to the addition of soil organic matter via litterfall, twigs, and roots and which also serves as a starting point for soil development in post-mining sites [57].

Carbon is added to the soil each year; therefore, the age of restoration increases the SOC. Adeli et al. [58] showed the strong and positive correlation of SOC accumulation with reclamation age for both forest and pasture ecosystems. The higher SOC at the oldest site (17 years old) was due to the accumulation of leaf litter and its decomposition, which forms humus [36]. Additionally, Adeli et al. [58] mentioned the manure addition from grazing wildlife animals as an additional source of SOC under pasture. Ahirwal and Maiti [59] revealed that afforestation with tree species showed a rate of SOC accumulation in the range of 1.2–2.8 Mg SOC ha⁻¹ year⁻¹. As indicated in Table 1, the annual SOC ranged from approximately 0.32 Mg ha^{−1 [48][49]}[48,49] to 5.0 Mg ha^{−1 [33][54]}[33,54]; the calculated average annual increment was 1.84 Mg ha⁻¹; and the variation in the annual increment may be due to tree species, climate, and reclamation methods ^[33][37][42][33,37,42]. Similarly, Pietrzykowski and Krzaklewski [60] also reported SOC accumulation at a rate of 1.5 Mg C ha⁻¹ year⁻¹ in mine soil in Poland. The findings of Stahl et al. [61] revealed increased SOC content with age and the 22-year-old and 32-year-old reclaimed soils had higher mean SOC contents than the undisturbed soil. This indicates that the age of restoration has a significant impact on the accumulation of SOC in RMS.

2.2. The Effect of Vegetation Types on SOC Accumulation

In the case of restoration by afforestation, the success of reclamation mainly depends on the type of plant species selected for revegetation for the particular site and climate [28]. The most common vegetations types used for restoration are mixed forest, deciduous, ever green, legumes, and grass ^[4][62][63][4,62,63]. Different tree species play different roles in amending mine soil properties even in the same climate ^{[38][64][65][66]}[38,64,65,66]. Field experiment studies indicated significant differences in the SOC stocks under different afforested trees on the same soil substrate, which may be due to the amount of litter, the decomposition of dead roots, and the elemental composition of the individual biomass ^[37][67][37,67]. The decomposition of leaf litter releases the bound nutrients into the soil, which increases the SOC concentration of mine soils over time [62]. Ahirwal and Maiti [59] stated that the increase in SOC concentration is due to the accumulation of leaf litter and its subsequent decomposition to humus. Yan et al. [44] recommended Quercus liaotungensis compared with Rhus typhina and Pinus tabuliformis is associated with its significantly higher organic carbon sequestration rate (1.59 t ha⁻¹ yr⁻¹) for reclamation management of degraded mining lands in China. Frouz et al. [48] also observed significantly different SOC accumulation rates among different tree species, ranging from 0.15 to 1.28 t ha⁻¹ yr⁻¹ in post-mining sites in the Czech Republic.