**4.1. Liming**

Lime is a material that contains calcium (Ca) or magnesium (Mg) and will neutralize soil acidity. Carbonates are the most available and widely used liming materials. Lime decreases the acidity level of soil (increases pH) by changing the hydrogen ions of soil into water and carbon dioxide (CO2) molecule. One calcium ion (Ca++) from the lime replaces two hydrogen ions (H+ ) of soil complex. In addition, one carbonate ion (CO3 2-) reacts with water molecule (H2O) to form bicarbonate ion (HCO3 - ). These react with hydrogen ion (H+ ) to form H2O and CO2. Thus, the pH of soil increases due to the concentration of hydrogen ions (H+ ) has been reduced (Figure 9).

**Figure 9.** Reduction of soil acidity (or H+ ions) by lime [28].

Regarding the mechanism of acid soil rehabilitation, liming changes the biological, structural and chemical components of soils. Organic matter decay is slow in acid soils due to the low activity levels of soil organisms. However, excessive liming rate would pollute the soil and cause environmental hazards.

#### **4.2. Planting legumes species**

Legumes foster production of a greater total biomass in the soil by providing additional nitrogen. Soil microbes use the increased nitrogen to break down carbon-rich residues of crops like wheat or corn. Normaniza and Barakbah [1] introduced a planting technique and docu‐ mented that legumes plants showed high tolerance to acidic condition when planted with lime, rock phosphate and sphagnum moss (Figure 10). This planting technique known as "Micro‐ climate Plant Propagation Technique" and the supplied chemicals (CaCO3, K2SO4 and MgSO4, and rock phosphate) assist as a suitable plant supplement to enhance the plant growth.

Another advantage of tree legumes is their deep root systems, a characteristic which confers persistence even on infertile soils [18]. Several legumes have aggressive taproots reaching six to eight feet deep and half inch in diameter that open pathways deep into the soil. Legumes contribute to an increased diversity of soil flora and fauna lending a greater stability to the total life of the soil [18]. In conclusion, nitrogen-fixing abilities of legumes plants are important for alleviating soil acidity, maintaining ecosystem fertility and long-term slope stabilization.

#### **4.3. Planting Al-tolerant plants**

Acid soil rehabilitation is an essential process of minimizing the acidity level of the soil and providing a better environment for plant growth. This process also will help offset greenhouse gas emissions, guarantee more food for an increasing population and contribute to the economic progress of future generations. There are several methods used in rehabilitating acidic soil. However, only three methods, which are liming, planting legumes and acid tolerant

Maximum Assimilation Rate (Amax), Light Compensation point (Г1), Quantum Efficiency (QE), Photosynthesis at ambient CO2 concentration (A400), Mesophyll conductance (gm), CO2 compensation point (ГCO2), LL: *Leucaena leucocephala,* PP;

plants will be discussed in this chapter.

**Figure 8.** The process of carbon sequestration [27].

528 Environmental Risk Assessment of Soil Contamination

Г1 (µE m−2 s−1)

**Light response curves Carbon response curves**

*Peltophorum pterocarpum*, JB: *Justicia betonica,* LC: *Lantana camara,* TE: *Thunbergia erecta*

**Table 3.** Photosynthetic components of plants grown on slope [18]

Amax (µmol m−2 s−1)

LL 62 27 0.13 124 58 3 2 PP 36 85.5 0.1 80 34 2 9 JB 12.6 42.8 0.06 37.6 16.2 0.11 70 LC 9.3 55.9 0.06 16.7 10 0.08 48.5 TE 4.3 31.0 0.04 17.7 2.4 0.08 96.5

A400 (µmol m−2 s−1) gm

(µmol m−2 s−1)

ГCO2 (ppm)

QE (µE m−2 s−1)

**Plant species**

Amax (µmol m−2 s−1)

> Aluminium was the major factor for slope acidity and the presence of Aluminium was unavoidable because it was a part of most clay particles. The mechanism of Al accumulator

plant to alleviate soil acidity has attracted the interest of plant ecologist and physiologist as well as evolutionary biologist. In the presence of Al, the tolerant cultivars have efficiently uptake and utilized Ca and P. The susceptible (Al-sensitive) and intermediate cultivars exhibited less Ca and P uptake and utilization. The effect of Al on roots indicated that the nutrient solution having Al at a concentration below 40 mM has stimulated root growth, increasing the size and number of central cap cells. Beyond 60 mM, root growth was strongly inhibited with cellular damage in peripheral root cap cells.

**Figure 10.** Plantation of legume seedling on slope [1].

Revegetation with Al-tolerant plants can be a valuable rehabilitation tool. Al-tolerant plants can tolerate and accumulate high concentrations of Al in the shoot whereby the growth of the plants was not affected by Al toxicity. Plants can deal with Al toxicity by setting up several aluminium tolerance mechanisms. Therefore, on such Al-contaminated soil, planting Altolerant plants plays increasingly important phytoremediation role. Proper management of these kind of plants in acidic soil may significantly contribute to restoring the natural envi‐ ronment. On the other hand, most of Al-tolerant plants are shrubs for example, *M. malabathri‐ cum*. These kind of shrubs have woody root systems (M type) that give mechanical support to slopes stability. Moreover, when *M. malabathricum* are planted with grass, they can help to prevent sloughing of the shallow sod layer. The woody top growth also helps to stabilize rehabilitated areas by reducing surface wind velocity. These shrubs also improve soil and forest floors by drying them out, adding organic matter, and fix some nitrogen.

#### **4.4. Tolerance mechanism of plants in acidic slope**

plant to alleviate soil acidity has attracted the interest of plant ecologist and physiologist as well as evolutionary biologist. In the presence of Al, the tolerant cultivars have efficiently uptake and utilized Ca and P. The susceptible (Al-sensitive) and intermediate cultivars exhibited less Ca and P uptake and utilization. The effect of Al on roots indicated that the nutrient solution having Al at a concentration below 40 mM has stimulated root growth, increasing the size and number of central cap cells. Beyond 60 mM, root growth was strongly

Revegetation with Al-tolerant plants can be a valuable rehabilitation tool. Al-tolerant plants can tolerate and accumulate high concentrations of Al in the shoot whereby the growth of the plants was not affected by Al toxicity. Plants can deal with Al toxicity by setting up several aluminium tolerance mechanisms. Therefore, on such Al-contaminated soil, planting Altolerant plants plays increasingly important phytoremediation role. Proper management of these kind of plants in acidic soil may significantly contribute to restoring the natural envi‐ ronment. On the other hand, most of Al-tolerant plants are shrubs for example, *M. malabathri‐ cum*. These kind of shrubs have woody root systems (M type) that give mechanical support to slopes stability. Moreover, when *M. malabathricum* are planted with grass, they can help to prevent sloughing of the shallow sod layer. The woody top growth also helps to stabilize rehabilitated areas by reducing surface wind velocity. These shrubs also improve soil and

forest floors by drying them out, adding organic matter, and fix some nitrogen.

inhibited with cellular damage in peripheral root cap cells.

530 Environmental Risk Assessment of Soil Contamination

**Figure 10.** Plantation of legume seedling on slope [1].

Some tolerance mechanisms to ensure its survival and growth vary amongst the species. In some cases, the plant produces small leaves as a response to acidic condition of the soil, for example smaller and thinner leaves in *L. leucocephala* as observed in our research (Figure 11).

**Figure 11.** Leaf necrosis to reduce leaf area - a visual symptom of Aluminium toxicity, a mechanism to resist acidic condition [18].

Additionally, this visual observation indicated that plants reduced leaf number by leaf wilting and necrosis. This necrosis symptom is the sign of acid tolerance mechanism of plants, possibly as a mechanism to reduce leaf area [18]. However, high leaf chlorophyll content was observed in adapted plants in acidic soil and the plant seems to have recovered from the toxicity effect by increasing the nodulation activity. In addition, leaf analysis of acidic and non-acidic treated *L. leucocephala* showed that Aluminium concentration was higher by 36% in acidic treated than those in non-acidic treated *L. leucocephala*, implying a high Al uptake of *L. leucocephala* in acidic slope. The excessive accumulation of Al in leaf may indicate the mechanism of acidity tolerance of this species. Moreover, the plant which exhibits Al concentration in leaf more than 1000 ppm is reportedly called an Al accumulator, a mechanism of Al adaptation of a plant to acidity (Figure 12). *M. malabathricum* accumulated the highest concentration of Aluminium, which was almost 1850 ppm, which makes both species an Al accumulator (more than 1000 ppm) and *A. mangium* is non-accumulator.

As similarly reported by Watanabe and Osaki [29], most of the plant samples (*Evodia latifolia* and *Justicia betonica*) exhibited general symptoms of Al toxicity which includes curly young leaves, reduced leaf number and necrosis, as mechanisms to reduce leaf area.

In relation to the Al concentration of the leaf, the soil pH grown with *M. malabathricum* increased up to 6.0, meanwhile 5.5 and 5.1 for *L. leucocephala* and *A. mangium*, respectively (Figure 13). The results imply a positive relationship between the concentration of Al in the leaf and soil pH; as the Aluminium accumulation in leaf increased, the value of soil pH increased as well. The experiment showed the importance role of plant as an Aluminium accumulator in rehabilitating the acidic slope.

**Figure 12.** Al concentration of leaves of three species studied

Other possible tolerance mechanisms that could be identified in this project were increasing root length, stomatal conductance and LAI (Figure 14-17). The interaction and compilation of all tolerance mechanism contribute to the rehabilitation of the acidic soil. The photosynthetic rate and chlorophyll content of Al-tolerant plants increased with the increasing of Al concen‐ tration. Therefore, as the Al concentration increased, the tolerance mechanism has also enhanced by increasing the transpiration rate of plant.

**Figure 13.** Soil pH changes grown by treated plants of the three species studied.

**Figure 14.** Root length in sandy loam and acidic soil of three species studied

**Figure 12.** Al concentration of leaves of three species studied

532 Environmental Risk Assessment of Soil Contamination

enhanced by increasing the transpiration rate of plant.

**Figure 13.** Soil pH changes grown by treated plants of the three species studied.

Other possible tolerance mechanisms that could be identified in this project were increasing root length, stomatal conductance and LAI (Figure 14-17). The interaction and compilation of all tolerance mechanism contribute to the rehabilitation of the acidic soil. The photosynthetic rate and chlorophyll content of Al-tolerant plants increased with the increasing of Al concen‐ tration. Therefore, as the Al concentration increased, the tolerance mechanism has also

**Figure 15.** Leaf Area Index in sandy loam and acidic soil of three species studied

**Figure 16.** Stomatal conductance in (a) sandy loam and (b) acidic soil of three species studied




**Figure 17.** Tolerance mechanism of plants [30]
