## 3 Results and Discussion
### Discussion  
In this paper, laboratory work to improve the geotechnical performances of acid- and alkali-contaminated clayey soil with the XG-oat combination is presented. Both the plant cultivation tests and direct shear tests were conducted to identify the effect of biopolymer in promoting plant growth in acidic and alkaline soil environments and explore the efficiency of the XG-oat combination in improving the shear strength of the acid- and alkali-contaminated clayey soil. The main observations are as follows:  
1. The exposure to acidic and alkaline pollutants changed the Afterberg limits and shear strength of the clayey soil (Figures 5 and 6). The acid-contaminated soil underwent more severe degradation in geotechnical properties compared with the alkali-contaminated soil (Figures 5 and 6).
2. An appropriate amount of XG (e.g., 0.25% and 0.50%) promoted the plant growth and increased the seed germination ratio (Figure 12). Excessive XG content (e.g., 0.75% and 1.00%) may lead to the formation of a hard XG-soil matrix, preventing seed germination and root growth (Figure 12).
3. The shear strength of the vegetated soil was positively related to the seed germination ratio (Figure 16). Both 0.25% XG and 0.50% XG that mostly promoted the plant growth, also led to the higher soil shear strength (Figure 16). In addition, a higher XG content (i.e., 0.50% XG) was demanded in the acid-contaminated soil, while a lower XG content (i.e., 0.25% XG) was demanded in the alkali-contaminated soil (Figures 14 and 16).
4. Apart from enhancing soil strength, the XG-oat combination reduced the types and contents of heavy metal elements in the soil as well (Figure 17).  
biopolymer-plant combinations in reinforcing clayey soils with different pore fluid pH values. Biopolymers with an appropriate blending content are effective in promoting plant growth, leading to the enhanced shear strength of the vegetated soil. The plant roots, due to their heavy metal elements sorption capacity, also contribute to the reduction in the types and contents of the heavy metal elements in the soil. Hence, it is suggested to use biopolymers in combination with plants to improve the geotechnical performances of the acid- and alkali-contaminated soils.  
Conceptualization, J.N. and G.H.; investigation, G.H., J.C. and S.L.; writing--original draft preparation, J.C.; writing--review and editing, J.N., X.G. and J.C.; funding acquisition, J.N. and X.G. All authors have read and agreed to the published version of the manuscript.  
This research was funded by National Natural Science Foundation of China, grant number 51608323, and 51978533; European Union's Horizon 2020 Framework program Marie Sklodowska-Curie Individual Fellowships, grant number 897701; Marie Sklodowska-Curie Actions Research and Innovation Staff Exchange (RISE), grant number 778360.  
Not applicable.  
The work described in this paper was supported by the National Natural Science Foundation of China (51608323, 51978533). In addition, this project has received funding from the European Union's Horizon 2020 Framework programme Marie Sklodowska-Curie Individual Fellowships under grant agreement No. 897701 and Marie Sklodowska-Curie Actions Research and Innovation Staff Exchange (RISE) under grant agreement No. 778360.  
The authors declare no conflict of interest.