soil remediation and plants || soil pollution status and its remediation in nepal

313Soil Remediation and Plants. http://dx.doi.org/10.1016/B978-0-12-799937-1.00011-5Copyright © 2015 Elsevier Inc. All rights reserved.

Soil Pollution Status and Its Remediation in Nepal

Anup K.C.* and Subin Kalu†

*Department of Environmental Science, Amrit Campus, Tribhuvan University, Thamel, Kathmandu, Nepal, †Central Department of Environmental Science, Tribhuvan University, Kirtipur, Kathmandu, Nepal

INTRODUCTION

Nepal is located in Southeast Asia between 80°04′–88°12′E longitude and 26°22′–30°27′N latitude, having borders with China in the north and India in the east, west and south. The total area is 147,181 km2, extending 800 km from east to west and 144 km to 240 km north to south. The country is blessed with tremendous geographical diversity ranging from an altitude of 60 m to 8848 m from south to north. In this roughly rectangular outlined country, 83% of the land is mountainous and 17% is formed by the alluvial plains of the Gangetic basin (Paudyal, 2002).

Physiographically, Nepal can be divided into eight distinct divisions from south to north (Hagen, 1969):

1. The Terai, 2. The Siwalik (Churia) Range, 3. The Dun Valleys, 4. The Mahabharat Range, 5. The Midlands, 6. The Fore Himalayas, 7. The Higher Himalayas, and 8. The Inner and Trans Himalayan Valleys.

The provinces run from east to west and are therefore incorporated into the Indian Himalayan belt. Each of these divisons has distinct altitu-dinal, topographical, climatic and vegetational characteristics. A detailed description of physiographic provinces is presented in Table 11.1 and Figures 11.1 and 11.2.

Chapter 11

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TABLE 11.1 Physiographical Division of the Nepal Himalaya (Upreti, 1999)

SN Geomorphic unit Width (km) Altitudes (m) Main rock typesMain processes for Landform development

1 Terai (Northern edge of the Gangetic Plain)

20–50 100–200 Alluvium: coarse gravels in the north near the foot of the mountains, gradually becoming finer southward

River deposition, erosion and tectonic upliftment

2 Churia Range (Siwaliks)

10–50 200–1300 Sandstone, mudstone, shale and conglomerate.

Tectonic upliftment, erosion and slope failure

3 Dun Valleys 5–30 200–300 Valleys within the Churia Hills filled up by coarse to fine alluvial sediments

River deposition, erosion and tectonic upliftment

4 Mahabharat Range 10–35 1000–3000 Schist, phyllite, gneiss, quartzite, granite and limestone belonging to the Lesser Himalayan Zone

Tectonic upliftment, weathering, erosion and slope failure

5 Midlands 40–60 300–2000 Schist, phyllite, gneiss, quartzite, granite, limestone geologically belonging to the Lesser Himalayan Zone


6 Fore Himalaya 20–70 2000–5000 Gneisses, schists, phyllites and marbles mostly belonging to the northern edge of the Lesser Himalayan Zone


7 Higher Himalaya 10–60 > 5000 Gneisses, schists, migmatites and marbles belonging to the Higher Himalayan Zone

Tectonic upliftment, weathering, erosion (rivers and glaciers) and slope failure

8 Inner and Trans Himalaya

5–50 2500–4500 Gneisses, schists and marbles of the Higher Himalayan Zone and Tethyan sediments (limestones, shale, sandstone, etc.) belonging to the Tibetan-Tethys Zone

Tectonic upliftment, wind and glacial erosion and slope degradation by rock disintegrations

315Chapter | 11 Soil Pollution Status and Its Remediation in Nepal

FIGURE 11.1 Physiography of the Nepal Himalaya. For a colour version of this figure, please see the section at the end of this book. Source: Dahal, 2006.

FIGURE 11.2 Generalized geological cross section of the Nepal Himalaya. Source: Dahal, 2006.

SOIL CHARACTERISTICS

Soil, except for open water surfaces and rock outcrops, is a thin layer covering the entire surface of the earth and is a complex mixture of mineral nutrients, organic matter, water, air and living organisms (Kang and Tripathi, 1992). There is a large number of different kinds of soils whose characteristics are deter-mined by various environmental parameters such as climate, parent material,

316 Soil Remediation and Plants

relief, organisms and time factor, reflecting different kinds and degrees of soil formation factors and their combinations (Kang and Tripathi, 1992).

The major factor related to the agricultural output is soil fertility – one of the key factors in determining soil productivity. The most important problem currently is the land degradation which is a primary constraint to improve food security in industrializing countries (Drechsel et al., 2004). Physiography and the water resources help to make the soil fertile to some extent, but the total nutrient content varies from soil to soil depending on the nature of parent material and other soil-forming processes (Anup et al., 2013). Only the plant- available form of the nutrients in the soil is relevant for the crops and is chemically determined through appropriate testing methods (Reddy et al., 2012). The primary nutrients for plant growth are nitrogen, phosphorus and potassium (known collectively as NPK). When they are insufficient, they will be responsible for limiting crop growth (Gruhn et al., 2000). Poor soil fertility, low levels of mineral nutrients in soil, improper nutrient management and lack of plant genotypes are major con-straints contributing to food insecurity, malnutrition and ecosystem degradation in developing countries (Cakmak, 2002).

SOILS OF NEPAL

The soils of Nepal are highly variable and are derived mainly from young parent material (Manandhar, 1989). Various factors such as geology, climate and vegetation types have resulted in the variations in soil characteristics. The classification given for the soils in Nepal on the basis of soil texture, mode of transportation, and color, can broadly be outlined as: alluvial, sandy and alluvial, gravelly, residual and glacial soils (Figure 11.3).

Alluvial Soil

Alluvial soil is found in the valleys of the Terai region and in the middle hill valleys around Kathmandu and Pokhara. The valleys lie between the Siwalik and Mahabharat hills which widen out in places to form flat fertile valleys called Dun valleys. New alluvial soil with more sand and silt than clay is being deposited in the flood plain areas along the river courses. Alluvial soil is also found in the higher areas above the flood plain covering a greater part of the Terai. The nutrient content of new alluvial soil is fair to medium depending on how long it has been cultivated. Conversely, the nutrient content of old alluvial soils is very low.

Sandy and Alluvial Soil

Valleys in the mid-hills of Kathmandu and Pokhara are composed of sandy and silty alluvial soils, which are fairly fertile. In the Kathmandu valley, some deposits of peat mare (Kumero) have been found. This is diatomaceous clay which is used for painting house walls during festivals in rural areas.


In addition, the Kathmandu valley is a source of dark clay or silty clay (Kali-mati) soil which is obtained from deep underground pits and is used as manure for potato and other vegetable crops. This soil is rich in humus, potash and calcium.

Gravelly Soil

The foot of the Churia hills has soils of mixed gravel and pebbles. This soil is not useful for agriculture as it has a very coarse texture and cannot hold suffi-cient moisture for plant growth. Such soils were deposited by rivers originating in the Churia hills and have high lime content. Some soils in high mountain areas are also coarse-textured gravels.

Residual Soil

Residual soil is found mostly on the ridges and slopes of the mountains. Soils of the Churia hills are very young and coarse, and are dry for most of the year. Soils on the slopes of the mid-hills are medium to low in plant nutrients but less productive due to moisture and climatic limitations.

Glacial Soil

Glacial soil is found in high Himalayan regions having rocky terrain with ice blocks. They are covered with snow for most of the year. The soil is much less exposed to the air due to snow cover.

LRMP (1986) reported that 14 dominant soil groups covering four soil orders are encountered in Nepal. Major soil orders of Nepal according to

FIGURE 11.3 Soils of Nepal. For a colour version of this figure, please see the section at the end of this book. Source: Pariyar, 2008.


US Department of Agriculture (USDA) taxonomy are Entisols, Inceptisols, Mollisols and Alfisols. Soil orders like Spodosols, Histosols, Utisols and Aridi-sols are occasionally found in Nepal. The country resource profile of Nepal pre-pared by Food and Agriculture Organization (FAO) has also described these soil groups and orders (Pariyar, 2008).

Entisols

These are the youngest and least-developed soils, generally found on hillsides and adjacent to river courses. They are formed through deposition of colluvium and alluvium and are present throughout the country.

Three great groups of this order are recorded, namely, ustifluvents, ustorth-ents and fluvaquents.

Ustifluvents

These are commonly found in the depositional stage of rivers. Horizons of deposi-tion are identifiable but soil does not show any pedogenetic development. They are mostly coarse textured, highly permeable and well drained. Depending upon the type of materials transported by rivers, they can be calcareous or non-calcareous.

Ustorthents

These develop through colluvial deposition and are found in landslide scars and on slopes of more than 35 degrees. As the soil develops it is constantly removed by erosion. They are shallow, near the bedrock, coarse-textured and poorly vegetated, but used for grazing, fodder and firewood collection.

Fluvaquents

These entisols are also found adjacent to rivers but are poorly to imperfectly drained, vary in texture and occasionally flooded. If they are suited to cultivation, rice can be grown.

Inceptisols

These cover the largest area in Nepal and are the most important soils. They occur on more stable slopes and show distinct weathering in the subsoil. The vast difference in landscape, climate, geology and parent materials have helped to develop a variety of Inceptisols.

Haplaquents

This soil is dominant in the lower piedmont plain of the Terai where drainage is restricted. It is also found in duns (broad flat valleys) valleys and limited areas of


the Middle hills. The B-horizon is well developed. As water remains for more than 3 months, the subsoil shows gleying and mottling. Due to the moisture regime rice grows well on the soils, whereas crops requiring aeration do not thrive. These soils are common in the low-relief areas adjacent to major river systems.

Dystrochrepts

These are the common soils in the Terai as well as in the Middle hills, mostly below 1500 m. They have developed on the acidic or neutral bedrock including lacustrine deposits, with a well-developed B-horizon and base saturation below 60%. They develop under forest and are found on steeper slopes, can be stony, well drained and deeper with ample stones and gravel. The pH is below 5.5 and they have low base saturation. Organic matter plays an important role in retain-ing soil plant nutrients suppressing the possibility of aluminium toxicity. These soils are used cautiously by maintaining high organic matter content. Prolonged use of nitrogenous fertilizers alone may increase acidity of these soils and need to be amended with high rates of lime. Erosion control on the hill slopes is a must to maintain the productivity of Dystrochrepts.

Ustochrepts

These are commonly found on alluvial plains of the Terai and Siwalik regions and develop on phyllite, schists, quartzite and limestone on the Middle and High hills. They are common on the Western and Middle hills and are diagnosed by a well-developed B-horizon, pale surface soils, high base saturation, variable soil texture and structure. The soils developing on colluvial deposits are stonier, those on calcareous parent materials are non-calcareous at the surface. As depth increases, calcium carbonate increases due to the leaching and precipitation of the calcium carbonate in the lower horizons. On hilly areas these soils are prone to heavy erosion. Ustochrepts in the Terai are deep, well drained, with loamy texture, non-stony and non-calcareous with high base saturation. When they are irrigated they have wide productive potential. Ustochrepts in the Siwalik, Mid-dle and Mountain regions are deep to shallow, stony, coarse to loamy texture, well-drained calcareous or non-calcareous forms but have high base saturation.

Cryumbrepts

These are the soils of the High Himalayan and High Hill regions, generally found above 3000 m. Annual mean temperature is below 8°C. Soils of this great group have dark A-horizon, high organic matter with wide C / N ratio, low base saturation and contain no free carbonate. They are rubbly and silty in texture. Near the settlements trees are cleared for fodder and firewood so bare areas are prone to soil erosion. Pathways of gullies caused by melting snow are common. Areas under these soils are extensively used for seasonal grazing.


Haplumbrepts

These are the soils of the High and Middle hill regions developed in cool temperatures on the acidic bedrocks in mixed forest. They have low base saturation and moisture regimes. Soils under forest and on steep slopes are shallow and stony but the cultivated ones are fertile due to a high organic matter content, which inactivates the toxic effect of aluminium by its chelating action. Frequency of stones on the surface hinders cultivation. Soil fertility is regularly maintained by grazing animals, leaving fallow for 2–3 year periods. Barley, millet and potato are the main crops grown on these soils.

Cryochrepts

These soils are similar to Ustochrepts and are found above 3000 m. They are of no importance for agricultural production.

Eutrochrepts

These soils are similar to Ustochrepts but develop on calcium-rich parent materials under a definite moisture regime.

Spodosols

These are soils with high organic matter. The active amorphous materials contain Al with or without Fe. They develop between 3000- and 4000-m altitudes in a humid, cool climate, mainly occurring in the higher parts of the High Hills and the lower parts of the high Himalayan region, but occupy a very small area. Agriculturally they are of very little importance, have low pH, which restricts growth of agricultural crops.

Mollisols

Soils with high organic matter content, usually under thick grass or forest, dark colour and high base saturation. They develop on basic parent materials at higher elevations.

Haplustolls

These are common in the sub-tropical mixed forest of the Terai and inner valleys. They develop on alluvial materials and are distinguished by a soft and dark-coloured mollic A horizon with high base saturation and a well-developed B-horizon under a moisture regime. Haplustolls develop under forest but not under grassland. Land with old alluvial deposition and forest litter which, on decomposition, contributes high base saturation helps in the development of mol-lisols. They are usually very fertile and produce high crop yields for the first few years after clearing. The yields decrease as organic matter content decreases.


Cryoborolls

These differ from Haplustolls mainly in their development on base-rich parent materials under thick grassland of the high mountains in high Himalayan regions. They are found in cooler climates and moisture regimes.

Alfisols

These soils are found on the higher river terraces with accumulation of a leached layer of lattice of silicate clays in their B horizon and high base saturation. They are available on stable slopes of the Middle and High hill regions where cli-mate helps the development of mature pedogenetic argillic horizons. The great groups of Alfisols found in Nepal are as follows:

RhodustalfsThese soils are found in Upper River terraces especially in the Siwaliks and Middle hills and mostly develop on green phyllite. They are not present in the Terai and High hills. Base saturation is more than 35%. Fertility is maintained with the application of ample organic matter. Decrease in the organic matter con-tent from loss of fertile soil decreases crop productivity. These occur on ancient river terraces (tars) and the upper alluvial terraces where water for irrigation is scarce. Rainfed cultivation is practiced with maize / millet being the major crops.

EutroboralfsThese Alfisols develop on calcium-rich material under cold temperatures in the high Himalayan region.

HaplustalfsThese are Alfisols similar to the Rhodustalf but do not meet the criteria of the Rhodustalfs.

Ultisols

Ultisols are not very common in Nepal. Only one great group, Rhodudults, is found in small pockets of upper terraces formed by rivers. They are similar to the Rhodustalfs but soil pH is low in these soils. Phosphorous management is a problem to maintain productivity.

Aridosols

These soils are rare in Nepal and occur in the north of Jhomsom in Mustang district where rainfall is less than 250 mm a year. Soils have calcium and other salts accumulated on the surface. Depending on the local microclimate, these soils can be fertile and produce good crops.


Soils originating from weathered soft rocks (Phyllite, Quartzite, Sandstone, Granites, Gneiss and Schists) are characterized by a high degree of porosity, poor slope stability, shallow soil depth, course texture and acidic reaction.

Nepal has complex topography and diversified agricultural practices. Soil fertility loss is the main problem in the hills due to uncontrollable soil erosion and improper soil management. The problem of soil fertility deterioration is increasing with the increase in cropping intensity, use of high-yielding crop varieties, low and unbalanced application of chemical fertilizers and decreasing use of organic manure are major problems (Mandal, 2002). Agricultural expan-sion has converted steep shrub and grasslands to rainfed agriculture, leading to soil erosion. The forests expanded during 1980s due to an intensive afforestation program and these are now playing a key role in sustaining agriculture. Both expansion of agricultural land into marginal grass and shrub land and agricul-tural intensification are taking place simultaneously, and nutrient inputs appear to be insufficient to sustain long-term productivity. Nutrients present in the for-est biomass are continuously removed which is expected to lead to a remarkable decline on a long-term basis in the forest soil fertility. There is clear evidence of soil acidification, leading to phosphorus deficiencies and impairing decomposi-tion processes. Organic matter and associated nitrogen, soil pH, exchangeable Ca, and available phosphorus are all key indicators of soil fertility degradation. The forest soils have the worst fertility status followed by grassland and rainfed agricultural land (Schreier et al., 2007).

Subsistence agriculture and lack of technical knowledge and inputs lead to improper soil nutrient management in Nepal. Cropping intensity has resulted in depletion of nutrients, while socioeconomic and cultural factors are lead-ing to poor nutrient concentration. Pedologically, most of the soil in the hilly areas is derived from phyllite and schist (leading to modestly inherent soil fertility) or sandstones, quartzite and granite (leading to infertile sandy soil (Andersen, 2007)).

NUTRIENT AND HEAVY METAL STATUS IN THE SOILS OF NEPAL

High concentration of heavy metals in the soils has negative impacts on both the environment and human health. Population growth, industrialization and unsustainable urbanization in developing countries like Nepal leads to an accu-mulation of heavy metals in the soil system. These heavy metals can be bio-accumulated and bio-magnified to have serious ecological and health problems. Heavy metal pollution is threatening atmosphere, water and soil systems in the country. The indigenous technology of Nepal in activities like gold-plating tech-niques heavily depends on the use of mercury and the use of leaded gasoline adds to it. The latter is used in greater amounts in vehicles which release more and more lead in the form of exhaust gas into the atmosphere (Shrestha, 2003). It gets deposited in the soil due to gravity polluting, the soil. The soil in the river


bank of Nepal is polluted by cadmium,salts of lead and ferrous etc. The efflu-ents of battery industries, leather factories and dye factories are directly dumped into the river system of urban areas in Nepal (Shrestha, 2003).

Several soil scientists have studied the nutrient status of the soils throughout the country. Some of the outcomes of these studies are presented below.

A report by Sippola and Lindstedt (1994) in central Mid Hills of Nepal was based on 150 soil samples collected from cultivated fields. Paddy crop rotations had the lowest concentrations of all nutrients, especially phosphate (P), potas-sium (K), sulphur (S), manganese (Mn) and zinc (Zn). Heavy metals such as cadmium (Cd) and lead (Pb) were not high at the sites investigated. Sulphur (S), calcium (Ca) and magnesium (Mg) content was generally low with acidic pH (average 5.8, minimum 4.4). The main deficiency was of boron (B), where 58% of the samples had very low, and 36% had low boron content. Zinc was very low in 14% of the samples, and low in 42% of the samples. Only a few samples were very low in Mo, but half the samples were in the lower category.

Gupta et al. (1989) analyzed the soils of citrus orchards in Dhankuta District in the Eastern Hills of Nepal for their N, P, K, Mg, Mn, Cu, Zn and B contents. The soil samples were tested for total element concentration. Leaf samples were analyzed and cross-correlated with soil values. The results show widespread deficiency of Zn, B, N, Mg and Cu.

Reports from the Terai plains show similar situations. One report from the Chitwan District in central Terai by Khatri-Chhetri and Ghimire (1992) men-tions that 100% of 70 soil samples were ‘very low to low’ in B, 83% were ‘very low to low’ in Zn, and 23% were ‘very low to low’ in Mn. The trends were confirmed by plant tissue analysis and yield responses in trials.

The study of Turton et al. (1997) shows that at low altitudes, the majority of the farmers reported unchanged or increasing soil fertility. However, soil analy-sis in these villages highlighted critically low levels of organic matter, as well as nitrogen and acidification problems. Management of soil fertility increasingly relies on chemical fertilizers. The soil fertility is declining at higher altitudes. The measured soil nutrient levels are higher than for villages. From group dis-cussions, several key factors were identified that may account for the perceived decline in soil fertility. These include the deterioration in forest resources and a decline in livestock numbers. These villages do not have access to alterna-tive nutrient sources. In the midlands, there was a pattern of low nutrient sta-tus and declining soil fertility. The soil samples analyzed in the laboratory of Department of Agriculture and in NARC indicate that 48% of soils are high in available P and 39% in available K but low in nitrogen. Available B, Mo, Zn and N content in the samples is low. Soil erosion is one of the major causes that threatens soil sustainability in Nepal. Due to mountainous physiography, poorly managed slopy terraces and degraded rangelands, erosion on these lands is highest ( Turton et al., 1997). In the Western Hills of Nepal, Tripathi (1999) found that 87% of samples are deficient (< 1 mg kg−1) in B, and 10–20% of sam-ples are low in Zn (< 0.5 mg kg−1), Mn (< 10 mg kg−1kg) and Cu (< 0.5 mg kg−1).


Micronutrients often vary in relation to soil types. Zinc is less available in sandy and alkaline soils, whereas Mo deficiency is only a problem in acidic soils.

Andersen and Sandvold (2000) studied 102 samples from altitudes of 300–2200 m in the Arun Valley in Eastern Nepal. Boron was deficient in 86 samples and zinc was deficient in 34 samples, with deficiency limits of 0.5 and 0.6 mg kg−1, respectively. Zinc values were larger under maize–potato and horti-culture at high altitudes, whereas B was deficient everywhere. The Zn values were largely deficient in khet fields. Tripathi (2003) presented data from the Western hills by correlating nutrient content, altitude and land type. The main division of land types in Nepal is between khet (paddy fields) and bari (dryland terraces). On altitudes ranging from 600 to 2200 m the mean values of available nutrients are Zn 0.92 mg kg−1, Fe 180.6 mg kg−1, Mn 58 mg kg−1, Cu 1.50 mg kg−1 and B 0.59 mg kg−1. In this study, altitude did not affect micronutrient concentrations except for B, which increased with altitude. Comparing khet with bari, there were larger Zn values on khet, whereas all other micronutrients were lower in khet soil than in bari. Bhatta et al. (2005) gave a map of the areas in Nepal affected by wheat sterility in Terai and in the districts surrounding the Kathmandu Valley. Trials with and without boron supply at a rate of 2 kg ha−1 proved that sterility was caused by boron, as reported by Ozturk et al. (2010). But the susceptibility of wheat variet-ies varied greatly from 0 to almost 100% sterility. A paper by Karki et al. (2005) included maps of the district-wide distribution of B, Zn, Cu, Fe and Mn shown in classes of low, medium, and high concentrations. Of the 75 Nepalese districts, 21 are represented in the survey. It was concluded that Fe and Mn are sufficient in most soils, but B, Zn and Mo are commonly deficient in the soils. Rai et al. (2005) has presented a statistical analysis of Zn in the soils of Rupandehi district in Terai with a mean value of 0.29 mg kg−1 (0.002–1.641 mg kg−1). Sapkota and Andersen (2005) presented a case study of intensive horticulture from the Kathmandu Val-ley. About 75 soil samples had sufficient macro- and micronutrients, except for B.

Geology and soil type affect supply of micronutrients such as selenium (Se) and zinc (Zn). Zinc, Mo and B deficiencies affect the yields of pulse crops and reduce the availability of protein, iron, folate and other nutrients. Most micro-nutrient problems in the Nepal region are due to soil deficiencies, but excess can also be a problem. Calcium (Ca), boron (B), zinc (Zn) and magnesium (Mg) con-tent in soils are low. In addition, Mo deficiency is likely to occur in Nepal, most probably in the soils of the Bagmati area. Among the other elements studied, the status of copper (Cu), iron (Fe) and manganese (Mn) are not considered to be a problem (Andersen, 2007). Soil acidification, associated with high inputs of acid-causing fertilizers (urea and ammonium based fertilizers) and acid bedrock geology, is becoming a major problem in the double and triple crop rotation systems in the Jhikhu Khola area. This acidification has serious implications as low soil pH (< 5.0) slows down the rate of organic matter decomposition, and leads to the leaching of base cations (calcium and magnesium) and the fixing of available phosphorous in the soil, making it unavailable to plants and causing aluminium toxicity and micronutrient deficiencies.


REMEDIATION OF TOXICITY FROM SOIL

Remediation deals with the removal of contaminants of pollutants from the resource. In case of soils, once polluted, it needs to be cleaned and purified for betterment of its quality and fertility. The process of purifying and revitalizing the soil is known as soil remediation. Remedy technologies can be categorized as ex situ and in situ methods. Ex situ methods involve excavation of affected soils and subsequent treatment at the surface, whereas in situ methods seek to treat the contamination without removing the soils. Among the processes used in soil remediation, excavation and dredging are the most common. This pro-cess involves extracting soil that is contaminated and deemed to be unrecover-able using current technology, and transporting it to a landfill set aside for this purpose. Often, purified soil is used to fill in the area where the extraction took place. Soil remediation is also accomplished by using a process known as pump and treat. Essentially, this approach involves the removal of contaminated ground water, using various methods to purify the extracted liquid. While the water is purified, the soil is extracted and filtered to remove various contaminants and returned back to its original position. The purified water is pumped back into the purified soil, effectively restoring the ecological balance of the area. As technol-ogy advances, newer methods of reclaiming contaminated soil are also in thede-velopment phase. This will make it possible to purify land and use the area for growing food. It will also help in creating wildlife preserves, allowing humans to safely construct dwellings and commercial buildings in the area.

Heavy metal pollution is the severe toxic substance pollution of the soils. Of these, arsenic is a toxic metalloid of global concern which can be intensified by human activities such as applications of pesticides and wood preservatives, min-ing and smelting operations and coal combustion. Bioremediation of arsenic-con-taminated soils and groundwaters shows a great potential for future development due to its environmental compatibility and possible cost-effectiveness. It relies on microbial activity to reduce, mobilize, or immobilize arsenic through sorption, biomethylation, complexation, and oxidation–reduction processes. Microbially mediated redox reactions involving organic carbon, Fe, Mn and S are the basic underlying mechanisms affecting arsenic mobility. Microorganisms have evolved biochemical mechanisms to exploit arsenic oxyanions, either as an electron accep-tor for anaerobic respiration, or as an electron donor to support chemoautotrophic fixation of carbon dioxide (CO2) into cell carbon. A number of investigations have been performed to remediate arsenic-contaminated soils and groundwater using biologically based method. Plant growth promoting Rhizobacteria combats heavy-metal stress through the processes of: exclusion – the metal ions are kept away from the target sites; extrusion – the metals are pushed out of the cell through chromosomal / plasmid-mediated; accommodation – metals form complexes with the metal binding proteins (e.g. metallothienins, a low molecular weight proteins) or other cell components; and bio-transformation – toxic metal is reduced to less toxic forms and methylation and demethylation (Jaiswal, 2011).


REMEDIATION STUDIES ON REMOVAL OF TOXICITY IN SOIL OF NEPAL

In Nepal very few studies have been carried out on the remediation of toxic substances from the soil. As water resources are of great priority for human beings in Nepal, as in other industrializing countries, most of the toxicity reme-diation research and projects are focused on the drinking-water sector. Some investigations have been carried out on the effect of pine litter in compost to soil acidification on red soils originating from phyllitic parent materials and brown (non-red) soils from quartzitic materials. The soil was analyzed for pH, exchangeable cations, carbon, and available phosphorous (Bray-1) using stan-dard procedures. No acidification was detected after the first year, but in second year soil acidification was taking place. Initially the rate of acidification was higher in the non-red soils than the red soils but acidification was significant in both soils in the second year. While the carbon and calcium content improved with pine litter addition, the pH decreased. In the non-red soils, the available phosphorous content increased. This suggests that pine litter is acidifying the soils and the phosphorous availability in the red soils. These results suggest that the addition of other types of litter is needed to have a positive impact on nutri-ent management.

A test has also been conducted on the effects of applying lime to the acidic soils of the Jhikhu Khola watershed. Eight sites with low soil pH were selected. A recommended dose of lime was applied to five ropanis of land (1 ropani = 508 m2) by each of three farmers (three sites) in Lamdihi to test its effects on maize. For example: 120, 230 and 294 kg of lime per ropani on clay loam were used with soil pH of 6.0, 5.5 and 5.2, respectively. Likewise, lime was applied to five vegetable farming sites (cauliflower, potato, tomato and brinjal). At each site, control plots were established, and soil pH and production before and after the lime application was studied. The results show that there is a slight increase in soil pH (by 0.1–0.3) after one crop season following lime applica-tion. Interestingly, the production of potato increased by about 50% in plots where lime was applied. A few research farmers pointed out that it was easier to till the land after lime application. The effect of lime on soil pH and production demands much more intensive scientific study including cost–benefit analysis, proper design, and accuracy of measurement.

CONCLUSIONS

Nepal is a mountainous country with distinct altitudinal, topographical, climatic and vegetational characteristics. The deficiency of nutrients such as phosphate, potassium, sulphur, manganese, zinc, cadmium, lead, calcium, magnesium, boron and molybdenum in the soils is widespread in different regions of the country. Population growth, industrialization and unsustainable urbanization are leading to an accumulation of heavy metals in the soils. Bioremediation of arsenic-contaminated soils and groundwater shows a great potential for


future developments in Nepal. Plant growth promoting Rhizobacteria seem to be fruitful for combating heavy-metal stress through the process of exclusion, extrusion, accommodation, bio-transformation, methylation and demethylation. Since Nepal is a rice-consuming country, heavy rice cultivation may also lead to arsenic and nitrate pollution; therefore, at least nitrogen-efficient genotypes can be used to reduce nitrate pollution in the country (Hakeem et al., 2011).

There is a need for further research on assessment of soil nutrients and other toxic pollutants present in soil throughout the country. Very few remediation studies of soil toxicity and remedial practices have been carried out in Nepal. Degradation of soil fertility and toxicity is causing nutritional, health and sanita-tion problems in addition to food adequacy; therefore, there is an urgent need for studies on the phytoremediation technology (Ashraf et al., 2010a, b; Ozturk et al., 2008, 2012).

REFERENCES

Andersen, P., 2007. A review of micronutrient problems in the cultivated soil of Nepal. An Issue with Implications for Agriculture and Human Health. Mountain Research. Dev. 27 (4), 331–335. http://dx.doi.org/10.1659/mrd.0915.

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