**3. Impacts of vegetation on soil erosion and carbon sequestration**

#### **3.1. Soil erosion**

The use of vegetation to control soil erosion has been practiced for many centuries, firstly introduced in China in 16th century to stabilize dam [19]. Nowadays, this practice of vegetation has been successfully applied to stabilize slope throughout the world. The vegetation and erosion process are interrelated by the ability of the plant life growing on soil and the inter‐ action of root and soil [20]. But the interaction of vegetation and soil are complex as it involved with, inter alia, the combination of soil types, plant coverage and the steepness of slope. There are many factors also responsible for controlling soil erosion such as soil elements, soil density, slope length, existing plant species and plant position on slope, plant age, plant coverage and plant root distribution. Moreover, the revegetation process also influenced by the plant-soil interaction such as soil acidity, nutrient content, and drought conditions. Toriman and Shukor [21] found that in a forest area of Malaysia, interception reduces 23.9% of the total rainfall and it is varying subjected to plant canopy, density and types of plants.

In our research findings, the plant density treatment (i.e. low, medium and high densities) of the potential slope plant species, *Melastoma malabathricum* provided the significant findings on the interception process at the sloping areas. A higher plant density increased the leaf area index (LAI) (Figure 5) as well as contributed to a higher plant growth. In addition, the highest plant density in the studied plots recorded the lowest erosion rate, indicating that soil erosion was lower at the area with a higher vegetation density by intercepting rainfall by plant canopy (LAI) (Figure 6).

**Figure 5.** The Leaf Area Index (LAI) of *M. malabathricum* of three different density treatments.

**Figure 4.** Topographic map of surveyed slopes in Peninsular Malaysia; a) Faculty of Science, University of Malaya; lati‐ tude 030 07' 28.5'' N, longitude 1010 39' 14.6'' E and b) Batu 38, Pusat Pengajian Luar, University of Malaya, Ulu Gom‐

The use of vegetation to control soil erosion has been practiced for many centuries, firstly introduced in China in 16th century to stabilize dam [19]. Nowadays, this practice of vegetation has been successfully applied to stabilize slope throughout the world. The vegetation and erosion process are interrelated by the ability of the plant life growing on soil and the inter‐ action of root and soil [20]. But the interaction of vegetation and soil are complex as it involved with, inter alia, the combination of soil types, plant coverage and the steepness of slope. There are many factors also responsible for controlling soil erosion such as soil elements, soil density, slope length, existing plant species and plant position on slope, plant age, plant coverage and plant root distribution. Moreover, the revegetation process also influenced by the plant-soil interaction such as soil acidity, nutrient content, and drought conditions. Toriman and Shukor

**3. Impacts of vegetation on soil erosion and carbon sequestration**

bak; latitude 030 20' 45.27'' N, longitude 1010 46' 26.52'' E.

524 Environmental Risk Assessment of Soil Contamination

**3.1. Soil erosion**

Furthermore, the higher the plant density, the higher the soil carbon content is. This result also indicates that the increased of species density influenced in carbon cycle via storing the large amount of carbon in soil through photosynthesis and respiration. It can be explained by the large amount of litter fall on the soil surface, thus, enhanced the decomposition process which in turns, increased the organic matter and mineral content at the top layer of soil. Apart from that, the amount of soil carbon was directly related to the root length density (Figure 7). Higher soil carbon content was observed at the greater root length density (RLD) area, indicating the distribution of soil carbon was induced by the root distribution. The root system supplies decomposable organic matter in soil and supports a large microbial community in the rhizosphere [22,23], thus, help in distribution of soil carbon. In addition, a higher plant density produced more litter on the top layer of the soil surface, which in turns increased the decom‐ position process via carbon and nitrogen cycles [24], hence, increased the soil pH value as well. In aftermath, the soil pH was enhanced.

**Figure 6.** The erosion rate on slope at different plant density of *M. malabathricum*.

**Figure 7.** Soil carbon content and root length density (RLD) with depth of the species studied at different plant density.

#### **3.2. Carbon sequestration and carbon sink potentiality**

Carbon sequestration, a natural processes in ecosystems where CO2 is absorbed from the atmosphere and stored it in plants and soil. During the photosynthesis process, plants absorb CO2 and converted into carbohydrate or starch [25]. In this way, atmospheric carbon is stored in the leaves, stems, and roots for a long period of time (Figure 8). When a tree is utilized for wood, its ability to sequester carbon is extended, and the carbon is not released until the product burns or decomposes. Vegetation plays an important role in sequestrating carbon, as one way to alleviate global warming, a global issue discussed nowadays. Whereas, carbon sink potential is defined as a natural entity, process, activity or mechanism such as plants which can alleviate greenhouse gases from the atmosphere. It has been reported that forest, for example the tropical rainforest, is one of the largest carbon sinks in the world [26]. The higher the potential of the plant to absorb CO2, the greater is the capacity of the plant to be a carbon sink potential. The quantitative meas‐ ure on the CO2 absorption by individual plant will assist to assess the carbon sink potentiality of plants. The plants which exhibited high photosynthetic components i.e. Amax, A400, light and CO2 saturation levels, are the good carbon sink plants.

Amax is an indicator of acclimatisation towards elevated CO2 and can be used to determine the plant growth capacity in a future climatic situation. In our study, *Leucaena leucocephala, Peltophorum pterocarpum* and *Justicia betonica* exhibited higher Maximum Assimilation Rate (Amax) and Quantum Efficiency (QE) than *Lantana camara* and *Thunbergia erecta* (Table 3). From the simulated CO2 experiments, *L. leucocephala, P. pterocarpum* and *J. betonica* seem to show the ability to utilize high concentrations of CO2 in order to enhance photosynthetic rate. Further‐ more, in the simulated light experiments, no photo-oxidation occurs. It has been reported that plants which can maintain the use of captured light energy for NADPH and ATP synthesis, may provide more sink capacity. This ability, then, would diminish the accumulation of excitation energy in the photosynthetic pigments, which is a major cause of photo-oxidative damage*.* Thus, the capacity to resist photo-oxidation is an indication that *L. leucocephala, P. pterocarpum* and *J. betonica* are the good potential carbon sink species. In addition, *L. leucoce‐ phala* and *P. pterocarpum* remain photosynthetically comparatively active at lower CO2 concentrations, indicating low CO2 is required to initiate the photosynthesis process of *L. leucocephala* and *P. pterocarpum*. Changes in CO2 concentrations affected the photosynthesis of both species similarly; *L. leucocephala* and *P. pterocarpum* seemed to show higher ability to utilize high concentration of CO2 in order to enhance photosynthetic rate as compared to other species studied. As inferred from these findings, *L. leucocephala* is a good carbon sink plant. Other species also showed considerably higher carbon sequestration capacity in which they can be regarded as supportive plants enhancing the carbon sink source when combine-grown on slope.
