# Effect of Ecologically Restored Vegetation Roots on the Stability of Shallow Aggregates in Ionic Rare Earth Tailings Piles

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## Abstract

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## 1. Introduction

_{4})

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_{4}leaching fluid leads to particle movement and reconstruction, reducing the number of pore sizes and pore quantity. In addition, it was found that the seepage effects are caused by a chemical action of ion exchange rather than the physical action caused [15]. Lingbo Zhou et al. explored the evolution of pore structure in the leaching process through indoor experiments. They found that the number of small and medium pores increased significantly, and the number of medium–large pores decreased sharply during the ion exchange process. The porous structure evolution showed the opposite trend with the completion of the ion exchange process [16]. YunZhang Rao et al., through indoor column leaching tests, direct shear tests and the application of fractal theory, found that the ion exchange during the leaching process destroyed the soil skeleton. Moreover, the overall shear strength of the soil is declining. The fractal theory has a good effect on the characterization of particle gradation and shear strength parameters [17]. GuanShi Wang et al. analyzed the shear expansion characteristics of ionic rare earth ore bodies through plastic work increments, put forward the shear expansion equation of ionic rare earth ore bodies and constructed an elastic–plastic constitutive model suitable for ionic rare earth ore bodies. The elastic–plastic stiffness matrix of this model, under a general stress state, proves that it has a good fitting effect on the indoor three-axis CD test results of ionic rare earth ore bodies [18].

## 2. Materials and Methods

#### 2.1. Study Area and Grass Selection

#### 2.2. Sample Collection and Parameters

#### 2.3. Root System Characteristic Parameters

^{−3}; ${L}_{A}$ is the sum of root lengths per unit volume, in mm; ${V}_{h}$ is the unit volume, which is taken as 1000 cm

^{3}in this paper, ${\rho}_{RD}$ is the root density in root·cm

^{−3}; $M$ is the sum of the number of roots per unit volume, in roots; ${\rho}_{RV}$ is the volume of the root system in cm

^{3}·cm

^{−3}; ${V}_{A}$ is the sum of the root volume per unit volume in cm

^{3}. Vegetation root system characteristics parameters are provided in Table 4.

#### 2.4. Sieve Test for Aggregates Content

#### 2.5. Correlation Analysis Method Selection

#### 2.6. Introduction of Stability Evaluation Index

## 3. Results

#### 3.1. Effect of Root System Parameters on the Physical Properties of Rare Earth Tailings

#### 3.2. Effect of Root System on the Stability of Mechanical Aggregates

#### 3.2.1. Distribution Characteristics of Aggregates at the Same Depth

- Horizon 0–10 cm:

- Horizon 10–20 cm:

- Horizon 20–30 cm:

#### 3.2.2. Effect of Root System of Different Species on Aggregate Characteristics and Distribution

- Paspalum notatum Flugge:

- Setaria viridis:

- Cynodon dactylon (L.):

## 4. Discussion

#### 4.1. Analysis of the Effect of Root System Action on the Stability of Rare Earth Tailings

#### 4.2. Mechanisms of the Influence of Root Characteristic Parameters on the Stability of Tailings’ Aggregates

## 5. Conclusions

- The vegetation roots effectively improved shallow aggregates’ content and spatial distribution in the rare earth-tailing pile. The vegetation root system is not limited to transforming small aggregates and sticking to large aggregates but changes the distribution of soil aggregates according to its own growth needs. By changing the content and distribution of aggregates, the root system changes the soil of rare earth tailings from disorderly to orderly, thus relieving soil erosion and improving the overall stability of shallow soil. This shows that the rare earth tailings pile can improve the overall stability of the soil through afforestation during ecological restoration.
- An analysis of stability indicators of rare earth tailings’ aggregates under the influence of root systems found that the vegetation root system effectively improved the stability of rare earth tailing pile aggregates, enhanced their ability to resist external forces, hydraulic dispersion or changes in external hydrological conditions while maintaining their original form, increased the corrosion resistance of their aggregates, optimized spatial distribution, improved physical properties and enhanced structural stability. The stability index of rootless tailings’ aggregates varies haphazardly. The stability of root-containing tailings’ aggregates shows a continuous weakening with increasing depth until it tends to be similar to rootless tailings, indicating that the vegetation root system has a specific improvement effect on the aggregates at their depths and gradually weakens with depth downward, and does not significantly modify the aggregates below their root distribution areas.
- Statistical analysis of root system characteristic parameters and aggregates stability was performed. It was found that the root system of Paspalum notatum Flugge is superior to other root systems in maintaining the stability of rare earth tailings, because all of its root parameters are greater than those of other root systems. Different root parameters played different roles in the stability index of the aggregates, and the root length density, RL, is the critical factor affecting the stability of the aggregates. Therefore, when we carry out ecological restoration of rare earth tailings piles, we can prioritize Paspalum notatum Flugge with long roots for ecological restoration.

## Supplementary Materials

## Author Contributions

## Funding

## Data Availability Statement

## Acknowledgments

## Conflicts of Interest

## References

- Xiao, Z.J.; Liu, Z.W.; Zhang, N. Environmental impact analysis and control technology of ion rare earth mining in south of Jiangxi Province. Chin. Rare Earths
**2014**, 35, 56–61. [Google Scholar] - Zou, G.L.; Yi-Ding, W.U.; Cai, S.J. Impacts of ion-adsorption rare earth’s leaching process on resources and environment. Nonferrous Met. Sci. Eng.
**2014**, 5, 100–106. [Google Scholar] - Li, S.Y.; Li, H.K.; Xu, F. Comparison of Remote Sensing Monitoring Methods for Land Desertification in Ion-adsorption Rare Earth Mining Areas. Chin. Rare Earths
**2021**, 12, 9–20. [Google Scholar] - Shi, X.Y.; Chen, H.W. Contamination and Restoration of Abandoned Pool and Heap Leaching Sites of Rare Earth Mine. J. Chin. Soc. Rare Earths
**2019**, 37, 409–417. [Google Scholar] - Zhang, C.Y.; Li, J.; Lei, S.G.; Yang, J.Z.; Yang, N. Progress and Prospect of the Quantitative Remote Sensing for Monitoring the Eco-environment in Mining Area. J. Met. Mater. Min.
**2022**, 3, 1–27. [Google Scholar] - Guo, Z.; Zhang, L.; Yang, W.; Hua, L.; Cai, C. Aggregate Stability under Long-Term Fertilization Practices: The Case of Eroded Ultisols of South-Central China. Sustainability
**2019**, 11, 1169. [Google Scholar] [CrossRef] [Green Version] - Saha, J.K.; Selladurai, R.; Coumar, M.V.; Dotaniya, M.L.; Kundu, S.; Patra, A.K. Assessment of Heavy Metals Contamination in Soil. Soil Pollut. Emerg. Threat Agric.
**2017**, 10, 155–191. [Google Scholar] - Tiller, K.G. Heavy Metals in Soils and Their Environmental Significance. Adv. Soil Sci.
**1989**, 9, 113–142. [Google Scholar] - Zhang, P.; Yang, F.L.; Lan, M.M.; Liu, W.S.; Yang, W.J.; Teng, Y.T.; Qiu, R.L. Phytostabilization with tolerant plants and soil amendments of the tailings of the Dabaoshan polymetallic mine in Guangdong Province. Huanjing Kexue Xuebao
**2019**, 39, 545–552. [Google Scholar] - Li, H.X.; Wang, B.; Wang, Y.J.; Wang, Y.Q. Impact of different forest types on stability and organic carbon of soil aggregates. J. Beijing For. Univ.
**2016**, 5, 84–91. [Google Scholar] - Cui, H.B.; Li, H.T.; Zhang, S.W.; Yi, Q.T.; Zhou, J.; Fang, G.D.; Zhou, J. Bioavailability and mobility of copper and cadmium in polluted soil after phytostabilization using different plants aided by limestone. Chemosphere
**2020**, 242, 1252521–1252528. [Google Scholar] [CrossRef] [PubMed] - Hao, H.X.; Di, H.Y.; Jiao, X.; Wang, J.G.; Shi, Z.H. Fine roots benefit soil physical properties key to mitigate soil detachment capacity following the restoration of eroded land. Plant Soil
**2020**, 446, 487–501. [Google Scholar] [CrossRef] - Le Bissonnais, Y.; Prieto, I.; Roumet, C.; Nespoulous, J.; Metayer, J.; Huon, S.; Villatoro, M.; Stokes, A. Soil aggregate stability in Mediterranean and tropical agro-ecosystems: Effect of plant roots and soil characteristics. Plant Soil
**2018**, 424, 303–317. [Google Scholar] [CrossRef] - Greinwald, K.; Gebauer, T.; Treuter, L.; Kolodziej, V.; Musso, A.; Maier, F.; Lustenberger, F.; Scherer-Lorenzen, M. Root density drives aggregate stability of soils of different moraine ages in the Swiss Alps. Plant Soil
**2021**, 468, 439–457. [Google Scholar] [CrossRef] - Wang, X.J.; Zhuo, Y.L.; Deng, S.Q.; Li, Y.X.; Zhong, W.; Zhao, K. Experimental Research on the Impact of Ion Exchange and Infiltration on the Microstructure of Rare Earth Orebody. Adv. Mater. Sci. Eng.
**2017**, 1, 1–8. [Google Scholar] [CrossRef] [Green Version] - Zhou, L.B.; Wang, X.J.; Zhuo, Y.L.; Hu, K.J.; Zhong, W.; Huang, G.L. Dynamic pore structure evolution of the ion adsorbed rare earth ore during the ion exchange process. R. Soc. Open Sci.
**2019**, 6, 213–224. [Google Scholar] [CrossRef] [PubMed] - Rao, Y.Z.; Jiang, F.C.; Chen, J.L.; Yu, B. Research on Fractal Characteristics of Shear Strength for Ion-absorbed Rare Earth Deposits in Column Leaching Test. Min. Res. Dev.
**2018**, 38, 35–39. [Google Scholar] - Hong, B.G.; Hu, S.L.; Luo, S.H.; Wang, Y.L.; Wang, G.S. Dilatancy behaviors and construction of elastoplastic constitutive model of ion-absorbed rare earth orebody. Chin. J. Nonferrous Met.
**2020**, 30, 1957–1966. [Google Scholar] - Cao, X.Z.; Li, X.Q.; Chi, M.R.; Zhang, G.N.; Chen, C.C. Comparison of planting suitability of four herbaceeous plants on rare earth tailings in south china. Acta Agric. Univ. Jiangxiensis
**2012**, 34, 35–41. [Google Scholar] - Jiao, P.P. Study of Plant Roots Affecting Soil Strength and Creep Properties. Ph.D. Thesis, China University of Mining and Technology, Beijing, China, 2019. [Google Scholar]
- Guan, X.; Yang, P.T.; Lu, Y. Relationships between soil particle size distribution and soil physical properties based on multifractal. Trans. Chin. Soc. Agric. Mach.
**2011**, 42, 44–50. [Google Scholar] - Hao, Y.Z.; Zhao, J.Y.; Lu, M.; Wang, Q.; Peng, W.Q.; Chen, Z. Effect of Plant Roots on River Bank Stabilization after Composite Vegetation Planting. J. Hydroecol.
**2020**, 41, 42–50. [Google Scholar] - Black, C.K.; Masters, M.D.; Lebauer, D.S.; Anderson-Teixeira, K.J.; Delucia, E.H. Root volume distribution of maturing perennial grasses revealed by correcting for minirhizotron surface effects. Plant Soil
**2017**, 419, 391–404. [Google Scholar] [CrossRef] - Saha, R.; Ginwal, H.S.; Chandra, G.; Barthwal, S. Root distribution, orientation and root length density modelling in Eucalyptus and evaluation of associated water use efficiency. New For.
**2020**, 51, 1023–1037. [Google Scholar] [CrossRef] - Peng, X.; Hu, D.; Zeng, W.Z.; Wu, J.w.; Huang, J.S. Estimating soil moisture from hyperspectra in saline soil based on EPO-PLS regression. Trans. Chin. Soc. Agric. Eng.
**2016**, 32, 167–173. [Google Scholar] - Dong, H.H.; ShangGuan, D.H. Spatial Distribution of Precipitation in Shiyang River Basin Based on PLS Regression Model. Mt. Res. Dev.
**2016**, 34, 591–598. [Google Scholar] - Chaplot, V.; Cooper, M. Soil aggregate stability to predict organic carbon outputs from soils. Geoderma
**2015**, 243, 205–213. [Google Scholar] [CrossRef] - Liu, Y.; Zha, T.-G.; Wang, Y.-K.; Wang, G.-M. Soil aggregate stability and soil organic carbon characteristics in Quercus variabilis and Pinus tabulaeformis plantations in Beijing area. Yingyong Shengtai Xuebao
**2013**, 24, 607–613. [Google Scholar] - He, Y.J.; Lv, D.Y. Fractal expression of soil particle-size distribution at the basin scale. Open Geosci.
**2022**, 14, 70–78. [Google Scholar] [CrossRef] - Yu, J.; Miao, S.J.; Qiao, Y.F. The stabilization mechanism of different types of soil aggregates. Chin. Agric. Sci. Bull.
**2022**, 38, 89–95. [Google Scholar] - Pinheiro, E.F.M.; Pereira, M.G.; Anjos, L.H.C. Aggregate distribution and soil organic matter under different tillage systems for vegetable crops in a Red Latosol from Brazil. Soil Tillage Res.
**2003**, 77, 79–84. [Google Scholar] [CrossRef] - Guo, Z.L.; Chang, C.P.; Zou, X.Y.; Wang, R.D.; Li, J.F.; Li, Q. A model for characterizing dry soil aggregate size distribution. Catena
**2021**, 198, 105018. [Google Scholar] [CrossRef]

**Figure 5.**Distribution characteristics of aggregates. (

**a**) Effect of the root system on aggregates’ content; (

**b**) particle size distribution of aggregates.

**Figure 6.**Distribution characteristics of aggregates. (

**a**) Effect of the root system on aggregates’ content; (

**b**) particle size distribution of aggregates.

**Figure 7.**Distribution characteristics of aggregates. (

**a**) Effect of root system on aggregates’ content; (

**b**) particle size distribution of aggregates.

**Figure 8.**Distribution characteristics of aggregates. (

**a**) Effect of depth on aggregates’ content; (

**b**) effect of depth on the particle size of aggregates.

**Figure 9.**Distribution characteristics of aggregates. (

**a**) Effect of depth on aggregates’ content; (

**b**) effect of depth on the particle size of aggregates.

**Figure 10.**Distribution characteristics of aggregates. (

**a**) Effect of depth on aggregates’ content; (

**b**) effect of depth on the particle size of aggregates.

**Figure 14.**Standardized regression coefficient graph. (

**a**) Paspalum notatum Flugge; (

**b**) Setaria viridis; (

**c**) Cynodon dactylon (L.).

Vegetation Sample | Feature Description |
---|---|

Paspalum notatum Flugge | The vegetation has a well-developed root system, suitable for tropical and subtropical growth, drought-resistant and erosion-resistant root system and a high foliage survival rate. |

Setaria viridis | This annual herbaceous vegetation has well-developed fibrous roots, wide rhizomes, a warm and temperate climate, strong water absorption and good survival ability. |

Cynodon dactylon (L.) | Growing in warm areas and wasteland slopes, its rhizome has a robust spreading ability, strong resistance, high coverage and a good function of fixing and retaining soil. |

Sample Type | Depth /cm | Length of Horizontal Extension /cm | Number of Root Systems /Root | Average Diameter /cm | Root Depth /cm |
---|---|---|---|---|---|

Root of Setaria viridis | 0–10 | 16.0 | 103 | 0.371 | 29 |

10–20 | 9.3 | 49 | 0.283 | ||

20–30 | 8.3 | 28 | 0.226 | ||

Root of Cynodon dactylon (L.) | 0–10 | 18.0 | 116 | 0.419 | 26 |

10–20 | 11.9 | 52 | 0.297 | ||

20–30 | 9.6 | 28 | 0.187 | ||

Root of Paspalum notatum Flugge | 0–10 | 18.5 | 122 | 0.436 | 33 |

10–20 | 14.6 | 75 | 0.302 | ||

20–30 | 9.8 | 42 | 0.251 |

Sample Type | Depth /cm | Weight Capacity /g·cm ^{−3} | Water Content /% | Porosity /% |
---|---|---|---|---|

Tailings with Paspalum notatum Flugge root system | 0–10 | 1.31 | 11.0 | 37.4 |

10–20 | 1.46 | 15.2 | 35.3 | |

20–30 | 1.54 | 18.1 | 34.1 | |

Tailings with Setaria viridis root system | 0–10 | 1.34 | 10.5 | 36.5 |

10–20 | 1.58 | 12.5 | 34.5 | |

20–30 | 1.62 | 17.1 | 33.4 | |

Tailings with Cynodon dactylon (L.) root system | 0–10 | 1.32 | 9.7 | 37.2 |

10–20 | 1.48 | 10.3 | 34.9 | |

20–30 | 1.58 | 11.8 | 33.8 | |

Rootless tailings | 0–10 | 1.62 | 4.5 | 33.4 |

10–20 | 1.77 | 5.4 | 33.2 | |

20–30 | 1.73 | 7.8 | 33.3 |

Root Samples | Depth /cm | RL /cm·cm ^{−3} | RD /Root·cm ^{−3} | RV /cm ^{3}·cm^{−3} |
---|---|---|---|---|

Root of Paspalum notatum Flugge | 0–10 | 2.26 | 0.120 | 0.334 |

10–20 | 1.10 | 0.075 | 0.078 | |

20–30 | 0.41 | 0.042 | 0.020 | |

Root of Setaria viridis | 0–10 | 1.65 | 0.103 | 0.178 |

10–20 | 0.46 | 0.063 | 0.029 | |

20–30 | 0.23 | 0.053 | 0.010 | |

Root of Cynodon dactylon (L.) | 0–10 | 2.09 | 0.116 | 0.288 |

10–20 | 0.42 | 0.052 | 0.043 | |

20–30 | 0.27 | 0.028 | 0.007 |

Parameter Type | Relevance | RL | RD | RV | Bulk Density | Water Content | Porosity |
---|---|---|---|---|---|---|---|

RL | Correlation coefficient | 1.000 ** | 0.998 ** | 0.979 * | −0.961 * | −0.999 ** | 0.998 ** |

value of p | 0.000 | 0.002 | 0.021 | 0.037 | 0.001 | 0.002 | |

RD | Correlation coefficient | 0.998 ** | 1.000 ** | 0.966 * | −0.918 | −0.902 | 0.998 ** |

value of p | 0.002 | 0.000 | 0.034 | 0.082 | 0.082 | 0.002 | |

RV | Correlation coefficient | 0.979 * | 0.966 * | 1.000 ** | −0.851 | −0.969 * | 0.971* |

value of p | 0.021 | 0.034 | 0.000 | 0.149 | 0.031 | 0.029 | |

Bulk density | Correlation coefficient | −0.961 * | −0.918 | −0.851 | 1.000 ** | 0.920 | −0.935 |

value of p | 0.037 | 0.082 | 0.149 | 0.000 | 0.080 | 0.065 | |

Water content | Correlation coefficient | −0.999 ** | −0.902 | −0.969 * | 0.920 | 1.000 ** | −0.998 ** |

value of p | 0.001 | 0.082 | 0.031 | 0.080 | 0.000 | 0.002 | |

Porosity | Correlation coefficient | 0.998 ** | 0.998 ** | 0.971 * | −0.935 | −0.998 ** | 1.000 ** |

value of p | 0.002 | 0.002 | 0.029 | 0.065 | 0.002 | 0.000 |

**Table 6.**Biased correlation analysis of tailings containing Paspalum notatum Flugge (* p < 0.05 ** p < 0.01).

Parameter Type | Relevance | RLRD | RLRV | RDRL | RDRV | RVRL | RVRD |
---|---|---|---|---|---|---|---|

Bulk density | Correlation coefficient | 0.898 * | −0.418 | 0.653 | −0.409 | 0.893 | 0.865 |

value of p | 0.038 | 0.484 | 0.232 | 0.495 | 0.052 | 0.078 | |

Water content | Correlation coefficient | −0.656 | 0.332 | 0.873 | 0.982 ** | −0.870 * | 0.830 |

value of p | 0.229 | 0.586 | 0.053 | 0.003 | 0.049 | 0.082 | |

Porosity | Correlation coefficient | 0.976 ** | −0.654 | −0.651 | −0.936 * | 0.965 * | 0.945 * |

value of p | 0.003 | 0.231 | 0.234 | 0.042 | 0.035 | 0.038 |

**Table 7.**Correlation equation for the variable tailings containing the Paspalum notatum Flugge root system.

Root | Dependent Variable | Relational Equation of the Independent Variable |
---|---|---|

Paspalum notatum Flugge | MWD | MWD = −45 × RL + 37 × RD + 8 × RV (R2 = 0.973 SEE = 21) |

GMD | GMD = −66 × RL + 54 × RD + 12 × RV (R2 = 0.965 SEE = 26) | |

D | D = −20 × RL + 16 × RD + 3 × RV (R2 = 0.975 SEE = 20) | |

Setaria viridis | MWD | MWD = 6.161 × RL − 0.267 × RD − 4.980 × RV (R2 = 0.962 SEE = 30) |

GMD | GMD = −6.439 × RL + 4.606 × RD + 2.802 × RV (R2 = 0.981 SEE = 15) | |

D | D = −4.556 × RL + 0.343 × RD + 3.250 × RV (R2 = 0.959 SEE = 35) | |

Cynodon dactylon (L.) | MWD | MWD = 14.802 × RL − 5.698 × RD − 8.186 × RV (R2 = 0.963 SEE = 28) |

GMD | GMD = 29.903 × RL − 12.271 × RD − 16.839 × RV (R2 = 0.978 SEE = 19) | |

D | D = −87.427 × RL + 41.237 × RD + 45.778 × RV (R2 = 0.983 SEE = 13) |

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**MDPI and ACS Style**

Zhong, W.; Shuai, Q.; Zeng, P.; Guo, Z.; Hu, K.; Wang, X.; Zeng, F.; Zhu, J.; Feng, X.; Lin, S.;
et al. Effect of Ecologically Restored Vegetation Roots on the Stability of Shallow Aggregates in Ionic Rare Earth Tailings Piles. *Agronomy* **2023**, *13*, 993.
https://doi.org/10.3390/agronomy13040993

**AMA Style**

Zhong W, Shuai Q, Zeng P, Guo Z, Hu K, Wang X, Zeng F, Zhu J, Feng X, Lin S,
et al. Effect of Ecologically Restored Vegetation Roots on the Stability of Shallow Aggregates in Ionic Rare Earth Tailings Piles. *Agronomy*. 2023; 13(4):993.
https://doi.org/10.3390/agronomy13040993

**Chicago/Turabian Style**

Zhong, Wen, Qi Shuai, Peng Zeng, Zhongqun Guo, Kaijian Hu, Xiaojun Wang, Fangjin Zeng, Jianxin Zhu, Xiao Feng, Shengjie Lin,
and et al. 2023. "Effect of Ecologically Restored Vegetation Roots on the Stability of Shallow Aggregates in Ionic Rare Earth Tailings Piles" *Agronomy* 13, no. 4: 993.
https://doi.org/10.3390/agronomy13040993