## 1 Introduction  
In the last decade, a new type of soil reinforcement material biopolymer, which is environmentally friendly and quick-acting, has been gaining an increasing opportunity to be used in the field of geotechnical engineering [1; 2; 3; 4; 5; 6]. Biopolymer-based soil treatment technology is used to improve the geotechnical properties of soils by adding plant or microbial secretions, as well as some artificially synthesized organic polymers to the soil [7]. The viscous biopolymer hydrogels can improve soil cohesion immediately after being mixed with soils through cementing [8]. When the biopolymer hydrogels are dehydrated and converted to rigid fibers, the compression and shear strength of the treated soils can be further improved [9; 10; 11]. Since biopolymers have high specific surface area with surface charges, they are able to bind directly to clay particles through ionic and hydrogen bonds, resulting in an efficient soil enhancement [8; 12]. Soil reinforcement with vegetation is also deemed to be a sustainable and cost-effective method for stabilizing shallow soil slopes. While plant roots are able to provide direct mechanical reinforcement through the aggregation of rhizosphere soil in close proximity to the roots [13; 14; 15; 16], the plant transpiration that discharges water from the soil through transportation pathways (i.e., from roots to leaves) also increases the matrix suction, thus enhancing the soil stability [17; 18; 19; 20]. However, soil stabilization using vegetation is always faced with the problem of insufficient water supply [21; 22]. According to [PERSON] et al. [23], soils treated with biopolymers (e.g., \(\beta\)-glucan and xanthan gum) lead to promoted germination and growth of oats, probably owing to the enhanced water retention capacity of the biopolymer-treated soils. [PERSON] et al. [24] later reported that \(\beta\)-glucan is able to increase the vegetation's survivability in severe drought environments (i.e., arid and semiarid regions) because \(\beta\)-glucan not only helps to reserve water in the soil, but serves as a nutrient for plant metabolism as well, thereby increasing the resilience of vegetation to the extreme environmental conditions. As biopolymers contribute to both soil strength enhancement and vegetation growth promotion, their usage is expected to increase the efficiency of soil slope reinforcement with vegetation, especially at the initial stage of vegetation growth when the plant roots are not well developed and are more vulnerable to the variation in water supply. In this context, [PERSON] et al. [25] carried out a study to test the feasibility of using a biopolymer-plant soil reinforcing method for improving a clayey soil, by examining both the vegetation growth in the biopolymer-treated soil and the shear strength of the vegetated soil (with biopolymer). The results indicated that the biopolymer (i.e., xanthan gum) can improve the water-holding capacity and fertility of the soil, and therefore increase the root content per unit of soil. In addition, in either the early stage of seed germination or later stage of root growth, the shear strength of the soil reinforced with the biopolymer-plant combination is superior to that treated with the single method (i.e., biopolymer treatment or planting vegetation). Therefore, the usage of combined biopolymer and plants seems to be a desirable alternative for soil slope stabilization. Hitherto, the performance of the biopolymer-plant soil reinforcing method has not been examined in acid- and alkali-contaminated soils. When the soil is eroded by acidic or alkaline pollutants (e.g., acid rain, industrial wastewaters, etc. ), its physicochemical and mechanical properties change accordingly, in terms of Atterberg limits [26; 27; 28], compressibility [29; 30], and mechanical strength [26; 27; 31; 32; 33; 34; 35]. This study aims to expand the application of the biopolymer-plant combination to clayey soil exposed to acidic and alkaline contaminations. Xanthan gum (XG) and oat are used in combination for soil improvement.