**6. General discussion**

**Species Root pattern Type of root system Root tensile strength (MPa)**

104

61

54


29

**VH type VH type**

**R type R type**

**H type H type**

**M type M type**

**Table 4.** Potential slope plants and their mechanical characteristics [13,16,35].

**Heart**

*Leucaena leucocephala*

538 Environmental Risk Assessment of Soil Contamination

*Peltophorum pterocarpum*

*Acacia mangium*

*Dillenia suffruticosa*

*Melastoma malabathricum* The observation of this study provides the key findings and contribution of tropical plants to alleviate soil acidity and soil erosion. The bioengineering characteristics of selected tropical plants have been intensively assessed to identify their potentiality towards slope stabilization. This observation also revealed the contribution of pioneer species to enhance the process of natural succession on slope. Amongst the tropical plants, few were suitable for reinforcing slope and rehabilitation of acidic slope, exhibiting tolerance mechanism in soil acidic condition. Based on the field studies, significant morphological and physiological changes were observed in *M. malabathricum* in response to severe acidic (< 3) condition. These changes include the improved photosynthetic rate, transpiration rate, LAI and root system. In addition, the highest concentration of Al (> 1200 ppm) in the *M. malabathricum* leaves, either Al-treated or not, has made this species the most suitable plant for severe acidic slope. This study also reveals an alternative approach to alleviate the acidity. Besides liming, legume trees were recommended for acidic slope rehabilitation due to their nitrogen-fixing abilities and extensive root systems that can penetrate a deeper soil depth. The deeper the soil depth, the higher the soil pH (less acidic), which makes this condition more conducive for the root establishment as well as the plant growth as a whole [18].

In terms of the alleviation of soil erosion, the soil reinforcement of slope stability is mainly depending on the properties of root systems of plant species. Root biomass, root architecture and tensile strength were remarked as prominent engineering properties of plants to assess its potentiality for soil reinforcement [13]. The extensive root growth and development of the taproot (VH-type) of *L. leucocephala* has resulted in a tremendous enhancement of mechanical effects on soil. As inferred from these findings, M-type root was suitable in protecting ground cover from surficial erosion. In relations to this mechanical aspect, plant density also showed an important contribution for controlling soil erosion. Research findings exhibited that higher plant density would reduce the soil erosion rate, indicating a high soil-root interaction and canopy interception. Apart from that, carbon sink potentiality of plants was also measured *via* photosynthetic components of plants grown on slope. It can be envisaged that the outstanding carbon sink potential of the slope plant community would confer numerous advantages, not only to improve the global warming and sustain the ecosystem but also to produce more stable and safe slopes.
