# 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

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**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