Sampling Site

The sampling sites are located in the southwest Fujian, China, where is one of the REE supplying centers [16]. Changting County (25°18’-26°02’LN, 116°01’-116°39’LE) that covers a district area of 3099.59 km², and Liancheng County (25°13’-25°56’LN, 116°32’-117°10’LE) where the overall area is around 2578.59 km² (Fig.1).

Fig. 1.

Location of the sampling site.

Changting County and Liancheng County are located in the mid-subtropical humid monsoon climate zone. The landforms in both counties are hilly areas with a well-developed water system and streams. Soil types in Changting County are mainly red soil and eroded red soil, which developed by the weathering of biotite granite with coarse mineral grains under a long-term of humid and hot climate. The soil is about 10-20 m with poor corrosion resistance, strong acidity, and low water and fertilizer retention capacity [14]. Red soil developed from biotite shells and gneissic porphyritic biotite granite dominates in Liancheng County. Both sampling sites are rich in REE deposits. The zonal vegetation in Changting County and Liancheng County both dominated with subtropical evergreen broad-leaved forest, however anthropogenic activities deconstruct the original forest. Nowadays, mono-structural secondary pine forests replaced most surface vegetation in Changting County, whereas in Liancheng County, secondary monsoon pines forest (Pinus massoniana) and fir (Cunninghamia lanceolata) forest occupies 82.8% of the total vegetation.

Methods

Physical and Chemical Properties of Soils with Different Restoration Years

The Soil pH and Mechanical Properties

The soil pH in areas processed with heap leaching varied between 4.31-5.22 (Table 1). All of the samples’ pH were lower than 5, except 10-20 cm soil at S1, where had the longest restoration time. The order of the pH was S3>S1>S2>CK>S4 and S1>CK>S3>S2>S4 in the soil of 0-10 cm and 10-20 cm respectively. The difference in sand, silt, and clay content between soil layers in the same plot was not significant, except for the sand content in S2 and clay content in the CK. The sand and silt content in mining areas were both significantly higher than that in the CK, while the clay content was opposite. The silt content in restoration plots was significantly higher in CK and lower in S4. The maximum soil clay content from 0-10 cm was found in S4, which was significantly higher than the CK. While in 10-20 cm soil shown in contrast. Both CK and S4 that without restoration were significantly higher than restoration plots [18].

Table 2 showed that the soil pH after the in-situ mining process varied between 4.31-4.71. In mined areas, the pH showed an increasing trend downwards with all values lower than 4.5, while the trend was contrasting in the control site and all higher than 4.5. The soil sand content increased vertically downward in both the mining area and the unmined area. In the mined area. the lower slope was significantly higher than the middle and upper slopes. At the same slope, the upper slope of the mine area was significantly higher than that of the unmined area, while the differences in the middle and lower slope were not significant. The silt content of soil increased first and then decreased in the mining area, whereas the control site shows a decreasing trend from the top to the bottom. On the upper slope, the unmined area was significantly higher than the mine area, but this does not show at the middle and lower slopes. The changing trend of soil clay content was opposite to that of sand content, which decreased downward in both sites. The two plots have significant differences at each slope. At the same slope point, the upper slope and lower slope in the control site were significantly higher than those in the mining area, but not in the middle slope.

Table 1.

Soil pH and mechanical composition by heap leaching.

Sampling site	Restoration time /year	Sampling depth /cm	pH	Mechanical Composition /%
				Sand particle	Silt particle	Clay particle
S1	8	0-10	4.64±0.15	51.44±5.19aA	39.13±3.43aB	9.43±1.99aC
		10-20	5.22±0.17	51.10±12.02aA	39.15±8.31aB	9.75±3.73aB
S2	6	0-10	4.35±0.12	44.97±10.59bB	44.89±8.72aB	10.13±2.03aC
		10-20	4.44±0.32	52.83±3.83aA	38.64±3.59aB	8.53±0.26aB
S3	3	0-10	4.72±0.03	53.61±4.40aA	38.42±3.52aB	8.05±1.24aC
		10-20	4.58±0.15	54.42±5.52aA	37.36±4.92aB	8.23±0.74aB
S4	0 (no restoration)	0-10	4.31±0.01	53.63±2.60aA	27.35±3.74aC	19.02±1.14aA
		10-20	4.42±0.01	54.54±1.29aA	26.78±2.81aC	18.58±4.11aA
CK	0 (unmined)	0-10	4.31±0.14	23.93±7.25aC	61.07±7.45aA	15.01±2.04bB
		10-20	4.60±0.15	20.56±6.42aB	60.11±3.86aA	19.63±2.68aA

[i] Note: S1: Niushitang, S2: Xiakeng, S3: Santongbazhou, S4: Huangnikeng, CK: Longjing. Low case letters indicate significant difference between sampling depths in the same sampling plot, capital letters indicate significant difference between sampling plots at the same depth (p<0.05).

Table 2.

Soil pH and mechanical composition by in-situ leaching

Sampling site	Slope position	pH	Mechanical Composition /%
			Sand particle	Silt particle	Clay particle
Mined area	upper slope	4.31±0.01	26.11±1.05bA	51.54±0.28bB	22.35±0.77aB
	middle slope	4.37±0.02	28.77±0.76bA	54.73±0.62aA	16.49±0.14bA
	lower slope	4.40±0.01	38.87±4.87aA	49.29±3.34bA	11.84±1.52aA
Control	upper slope	4.71±0.01	15.06±0.37cB	60.60±0.35aA	24.33±0.22aA
	middle slope	4.58±0.02	27.36±1.59bA	54.19±2.10bA	18.45±0.50bA
	lower slope	4.52±0.02	36.06±1.90aA	49.22±1.23bA	14.72±0.67cA

[i] Note: Different lowercase letters mean significant difference in the different sampling position of the same sample plot, different capital letters indicate significant difference in the same sampling position of the different sample plot (p<0.05).

Fig. 2.

The soil nutrient contents after heap leaching (p<0.05).

Soil Physical and Chemical Properties of Different Mining Techniques

The soil pH is one of the most decisive properties, which impacts the availability of soil nutrients and indicates soil environmental characteristics [18]. The main factors that affect soil pH are the natural soil-forming conditions, followed by farming or resource extraction. For all soil samples from sampling sites after the heap leaching process, the pH were less than 5, besides the the 10-20 cm soil of S1. S1 was under the longest vegetation restoration time of eight years. While at S4 that without restoration, the pH values were less than 4.5 at each soil layer. The soil samples from mined areas were all showed strong acidity and visible piles. The heap leaching process significantly increased soil acidity, which was consistent with the results of most studies [8]. The 0-10 cm soil pH of CK was lower than 4.5, which may be because of the acidity of the subtropical red soil itself and the growth of Dicranopteris pedata. As the root system of Dicranopteris pedata concentrated in the surface layer and produced organic acid caused the soil pH decreases [19]. After in-situ leaching, the soil pH (4.31-4.71) in the mining area were lower than the unmined area, which demonstrated the in-situ leaching process also leads to soil acidity. Furthermore, liquid ammonia accelerates soil acidification [20]. Therefore, mining activities by heap leaching and in-situ leaching techniques both coursed soil acidity and nutrients decreasing.

Fig. 3.

The soil carbon, nitrogen and phosphorus contents after heap leaching (p<0.05).

The soil mechanical composition is important to either the soil structure characterization or as an index to measure soil quality [21]. The soil structure components varied significantly in the sampling sites after the heap leaching compared to the CK. The biggest difference between sampling sites and the CK was the sand content significantly increased, while silt and clay contents were significantly reduced. With the increase of management years, silt content raised and the clay content decreased. The significantly increased sand content demonstrates that REE mining has resulted in soil destruction, which has not improved after years of restoration. The increase in silt contents mainly because of the growth of plants that make fine particles more effective. Compared to heap leaching, the impacts of in-situ leaching technology on the mechanical composition of the topsoil was lighter because this technique did not excavate the mountain, vegetation was removed only for deploying the injection holes on the slopes. Thus, the proper operation in the in-situ leaching process can avoid dramatic damage to the mountain structure, and the impact on the soil’s mechanical composition can be small.

The soil organic carbon determines various soil functions and plays an important role in the formation of soil structure, nutrient release, and microbial activities. At the 0-10cm soil of the CK, the organic carbon content was similar to the background value of Fujian Province red soil (22.1 g/kg^-1) [22]. In the sampling sites, the organic carbon contents were significantly lower than CK, although they were improved after restoration. The maximum content in the mining area was only 3.76 g/kg^-1 at the S1 where after 8 years of restoration, this demonstrates that the increase in soil organic carbon content in the mining areas is a slow process. Correspondingly, Zhang’s study [23] in these areas also showed that the organic carbon content of 10-20 cm soil was about 5 g/kg^-1 after 30 years of restoration. The soil organic carbon content fluctuated with the changing of restoration time. At the early restoration stage, in the mining area the organic carbon content increased because fertilizer was applied to improve planted vegetation growth. However, the plantation dying out under the harsh growth environment of the mining area and soil erosion by rain, the organic carbon content began to decrease. Then the soil organic carbon content increased with the gradual invasion of some native fern species, such as Dicranopteris pedata. Compared with the unmined area, the change in soil organic carbon in the in-situ leaching mined area showed an opposite trend on different slope positions. The trend in unmined areas was agreed with studies of Seibert [24], the soil organic content at the bottom of the slope was higher than that at the upper slope. We also found that the growth of plants on the upper slope in the mining area was better than the middle and lower slope. It is because of the steep slope at the middle and lower slopes caused stronger nutrients loss by cutting off the herbaceous plants.

Soil nitrogen, phosphorus, and potassium were the most important nutrient elements for plant growth. Under the heap leaching mining process, the total nitrogen in the soil was relatively high at the initial stage of restoration, and a large amount of (NH₄)₂SO₄ remained after the mining area was abandoned. After 3 years of restoration (S3), the total nitrogen content of 10-20 cm soil reached 1.21 g/kg^-1, while in the unmined red soil site was only about 0.6 g/kg^-1, which demonstrated the residual leachate (NH₄)₂SO₄ in the early restoration stage increased soil nitrogen content. However, the nitrogen content was similar in S1, S2, and S4 indicates that plants are difficult to effectively uptake residual nitrogen from the soil. A further decrease of nitrogen in those sampling plots was because the residual (NH₄)₂SO₄ in the soil was easily washed away by precipitation. The nitrogen content after 6 years (S2) of restoration was about 0.3 g/kg^-1in both soil layers. The scouring and leaching effect of rainwater resulted in the nitrogen content at 10-20 cm soil layer was higher than that of the 0-10 cm soil layer after 8years of restoration. The soil total nitrogen after in-situ leaching process on the slope position was similar to the change of organic carbon. The residual nitrogen remained at the top of the slope because of the platform shape at the top of the ore. The leaching effect was the main cause of nitrogen loss in the middle and lower slopes. This means that in-situ leaching results in less nitrogen residue in the surface soil, and more nitrogen will stay in the mountain interior or migrate downstream with slope runoff. A dramatically larger amount of leachate is needed in the in-situ leaching, however, the nitrogen content in all slopes of the mining area was lower than the S3 after heap leaching, this further proof that the anthropogenic nitrogen did not be used by plants, they translocate and migrate to lower basins by leaching effects or wash off by precipitation. Therefore, the high potential risk of NH₄⁺ and SO₄^2- pollution after in-situ leaching should be emphasized.

By further compared correlation of organic carbon content and total nitrogen after two mining methods, the in-situ leaching mined area was consistent with any type of soil, while the correlation in the heap leaching mining area was low, which indicated that the soil destruction led to nutrient imbalance strongly. This consist to the strong correlation between soil organic carbon and total nitrogen, it can reach 0.95 at the significant level of 0.01. Fig. 4 showed the equating relationship between soil organic carbon and total nitrogen, which suggested the lack of organic carbon was a critical factor imbalanced carbon and nitrogen nutrients in heap leaching areas.

The soil phosphorus mainly comes from the parent rock. In southern China, subtropical soils are deficient in phosphorus, and the total phosphorus content of 0-20 cm in the unmined red soil mountain is only 0.15 g/kg^-1 [22], CK of the heap leaching the total phosphorus content of 0-10 cm soil was similar to that of uneroded red soil, but at 10-20 cm were also low. The total phosphorus content in the mining area was even lower. The lack of phosphorus restricted the growth of soil microorganisms, which made it difficult to improve the soil environment in the mining area and restricted plant growth [25]. The changing trend of the total phosphorus content in the four mining areas increased first and then decreased with the extension of the restoration time. Because plants absorbed a certain of phosphorus at the beginning of the restoration, the total phosphorus content in the soil after 3 years of restoration (S3) was relatively low. With the death of plants, phosphorus released into the soil, therefore the total phosphorus content increased after 6 years of restoration (S2). Afterward, new plants invaded and absorbed phosphorus, and the total phosphorus content showed a downward trend. However, the total phosphorus content of the soil in the in-situ leaching mined area was higher in the mining area than in the unmined area. The largest difference reflected at the top of the slope, it may due to the absorption and usage of residual ammonia nitrogen in the vegetation at the upper slope after the mining area was abandoned. The growth status was good, and plants absorbed and transformed more phosphorus in the underlying soil and returns it to the topsoil. The phosphorus content difference between the middle and lower slopes was less obvious. It was because less mobility of phosphorus in the soil as there was no erosion happened during in-situ mining [25].

Fig. 4.

Relationship between soil organic carbon and total nitrogen: a) heap leaching area, b) in-situ leaching area.

The total potassium content in the heap leaching mining area was less than 10 g/kg^-1, showing an increasing trend with the increase of management period, and all are higher than the unmined Control area. The lowest content was in the S4, indicated the heap leaching mining method caused potassium leaching. Although the restoration of mining areas used a certain amount of potassium fertilizer, however, potassium was easy to leache downward at the initial stage. Only with the increase of time, the vegetation growth improved the use of potassium fertilizer. So that the potassium content in the top soil rose, but overall the potassium content was low. The total potassium in the in-situ leaching area was higher than the unmined area, which can be affected by plant growth, brought potassium from the deep soil to the top soil.

Different Impacts between Heap Leaching and in-situ Leaching Techniques

The heap leaching has been applied since the 1960s for IAT-Res. The heap leaching mining caused distinct and long-term ecological hazards, such as deforestation, and soil deconstruction. The rare earth mining in Ganzhou region has left 302 abandoned mines and 191 million tons of tailings and the area of destroyed forests increased from 23 km² in 2000 to 153 km² in 2010, and in June 2011, the Chinese central government enforced a ban on surface mining, such as tank/heap leaching, in-situ leaching was implemented for IAT-Res. The in-situ leaching is considered less harmful to the ecosystem. The in-situ leaching process does not require stripping large-area of the topsoil and less damage to vegetation. However, to excavate injection wells and catchment ditches within the ore still generate waste tailings and damages the surface flora [26]. Research on soil erosion in mining areas of Longnan County, Ganzhou, Jiangxi Province showed that although the in-situ leaching technology reduced soil erosion, it posed a potential threat of landslides [9]. The in-situ leaching technique caused surface soil erosion was lower compared to the heap leaching method. However, the injection wells on hills may collapse as the time gone by can trigger landslides. The study of Sun [27] showed landslides happened in the Longnan mine area after in-situ leaching mining. The causes of landslides mainly because of changes in slope stability and the destruction of the ore body by leaching. The problem of landslides and overloaded leachate pollution consequences by in-situ leaching has attracted the attention of researchers. Moreover, as the leachates residue generate NH⁴⁺ and SO₄^2- after mining. Those compounds can into the underground water by percolation [28], under the rain and surface runoff contaminate the down string and soil [17]. Ammonium nitrogen promotes vegetative growth and inhibits the reproductive growth of downstream crops, resulting in a substantial reduction in the yield or no harvest. The presence of ammonium nitrogen can also easily cause soil mineral depletion affects the fixation and release of metal ions by changing the ion composition and strength of the soil solution. During the acidification process, ammonia nitrogen invades as an activated ion, which intensifies the acidification process [29]. Ammonia nitrogen is also harmful to aquatic organisms, reducing the number of fish and other aquatic animal species, poisoning, and even death. Therefore, the uncertainty of the time, the location of the landslide, and governance goal are unclear after in-situ leaching makes it difficult to conduct targeted governance.