Keywords

14.1 Introduction

Soil salinity and sodicity is a serious issue of land degradation worldwide and is predicted to become more of a problem in the future (Wong et al. 2008). Soil salinity refers to the high salt concentration in the soil, and sodicity is the presence of high concentration of just sodium ions (Na+) among all of the other cations present in the soil such as magnesium (Mg2+), calcium (Ca2+), etc. (Bernstein 1975). Saline soil is one of the important soil resources in the world, and coastal saline soil is one of the main types of saline soil. Sandy loam and silt soil are examples of soil types present in coastal saline region (Li et al. 2016). Salt-affected soil is distributed in 831 million hectares of land worldwide including 397 and 434 million hectares of saline and sodic soil, respectively (FAO 2000). The characteristic features and principles involved in the identification, reclamation, and management of salt-affected soil are similar throughout the world. Some of the factors vary regionally such as soil characteristics, climatic condition, water availability, farm management efficiency, available resources, and economic status that lead to differences in methods and reclamation potential (Abrol et al. 1988). Various methods have been established for reclamation purposes but are associated with certain limitations. In this regard, phytoremediation can be seen as an effective, low-cost, and environmentally safe technology. It is a plant-based technology which enhances soil quality and productivity potential, thereby reducing pollutants or contaminants responsible for impaired plant growth. Some plants have more capacity to remediate degraded sodic or saline soil. It is important that the plants that are selected possess the highest potential of phytoremediation. If those plants also have the potential for high biomass production, they can be used for bioenergy generation. Bioenergy is a renewable source of energy from biological materials that produces heat, electricity, and fuel and their coproducts (Yuan et al. 2008). Among various energy sources, bioenergy is the most abundant and versatile renewable energy in the world (Zhuang et al. 2011; Edrisi and Abhilash 2016). Bioenergy is termed as the conversion of biomass into energy (McKendry 2002a). Biomass is the typical form of renewable energy that has been widely utilized as source of energy for domestic purposes since quite long ago (McKendry 2002b). Biomass can be produced by growing dedicated energy crops such as short rotation coppice (SRC), perennial grasses, forest residues, sludge from organic industrial wastes, and organic domestic wastes (McKendry 2002a). According to Ni et al. (2006), resources produced from biomass used to convert into energy have been classified into four categories:

  • Energy crops: herbaceous energy crops, woody energy crops, industrial crops, agricultural crops, and aquatic crops.

  • Agricultural residues and end products: crops waste and animal-produced waste.

  • Forests wastes and leftover: mill wood, logging residues, trees, and shrubs residues.

  • Industrial and municipal wastes: municipal solid waste, sewage sludge, and industrial effluent waste.

Biomass is used for the purpose of heating, cooling, and producing electricity and liquid biofuels. Burning fossil fuels, deforestation, and human activities have led to the emission of greenhouse gases into the atmosphere. The usage of biomass for biofuel reduces greenhouse gas emissions, making it carbon neutral (Kraxner et al. 2013). Energy produced from biomass has been categorized into two groups: biomass produced from food crops such as corn grain, sugarcane, soybean, oil seed, etc.; and biomass produced from cellulosic feedstock such as starch, sugar, fatty acid, or cellulose (Ghosh 2016). The physical quantity of biomass has enough potential for worldwide bioenergy production (Altman et al. 2015). The new renewable energy obtained through wind, solar, and biofuel is growing fast continuously and contributing to global renewable energy supply. Bioenergy is one of the alternative sources for fossil fuel, particularly for those used in transportation. Presently, commercially available biofuels are produced from starch or sugar-rich crops for bioethanol and from oilseeds for biodiesel production (Popp et al. 2014).

There are numerous plant species that are capable of cleaning up the soil. These plants are also used to obtain useful by-products such as biofuel (biodiesel or bioethanol), fiber, wood, charcoal, alkaloid, bioplastic, etc. (Tripathi et al. 2016). In India, the 1970 oil crisis has led to the establishment of bioenergy promotion (Rabindranath et al. 2010; Edrisi and Abhilash 2016).

14.1.1 Saline and Sodic Soil: Origin, Characteristics, Distribution, and Parameters for Salinity/Sodicity Measurement

14.1.1.1 Origin of Saline and Sodic Soil

Salinity is caused by natural weathering of parent material, deposition of sea salt carried by wind and rain, inundation of coastal land by tidal water, and anthropogenic activities such as excessive irrigation by underground water resulting in a rise of the water table, irrigation by salt-containing water, poor drainage, etc. (Munns 2005; Manchanda and Garg 2008; Hasanuzzaman et al. 2013; Hasanuzzaman et al. 2014). Salts present in the upper surface of the soil profile undergo hydration, hydrolysis, oxidation, solubilization, and carbonization through chemical weathering. The salts solubilize and are transported away from the origin sites through soil surfaces or groundwater. Salts in the groundwater are gradually concentrated when the water moves to more arid areas (Abrol et al. 1988).

14.1.1.2 Characteristics

Saline and sodic soil bears several features which make them unsuitable for agricultural practices. Saline and sodic soil possesses poor physical properties and fertility problem that adversely affect the growth and yield of various crops (Sumner 1993; Naidu and Rengasamy 1993; Qadir and Schubert 2002; Qadir et al. 2005). It has been reported that saline soil has electrical conductivity with value ranging from 2 to more than 32 dS/m (Richards 1954; Farifteh et al. 2008). Saline soil consists of many ions like chlorides, sulfates, nitrates and bicarbonates of sodium, calcium, magnesium, and potassium (Bul 2013). Most of saline soil also consists of some proportion of gypsum (CaSO4) (Abrol et al. 1988). Sodic soil consists of sodium carbonate, sodium bicarbonate, and sodium chloride as dominating components. The physicochemical characteristics of saline and sodic soil has been depicted in Table 14.1.

Table 14.1 Physicochemical characteristic of saline and sodic soil
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14.1.1.3 Distribution of Saline and Sodic Soils

According to Szabolcs (1974), salt-affected soils can be found in North America, Mexico, Central America, South America, Africa, Southern Asia, North and Central Asia, Southeast Asia, Australia, and Europe. Several states of India such as Uttar Pradesh, West Bengal, and Gujarat are largely salt affected (Edrisi and Abhilash 2016).

14.1.1.4 Parameters for Salinity/Sodicity Measurement

Sodium absorption ratio (SAR) is the ratio of soluble sodium to the sum of the square root of divalent cations, usually calcium (Ca++) and magnesium (Mg++), divided by 2:

$$ SAR=\frac{Na}{\sqrt{\left( Ca+ Mg\right)/2}}\kern0.5em (\mathrm{Harron}\kern0.5em \mathrm{et}\kern0.5em \mathrm{al}.\kern0.5em \mathrm{1983}) $$
(14.1)

An equivalent proportion of sodium remaining present in the cation exchange complex when expressed in terms of percentage is referred to as exchangeable sodium percentage (ESP) (Mau and Porporato 2015):

$$ ESP=\frac{100\left({E}_{Na}\right)}{CEC}\kern0.5em (\mathrm{Qadir}\kern0.5em \mathrm{et}\kern0.5em \mathrm{al}.\kern0.5em \mathrm{2007}) $$
(14.2)

Cation exchange capacity (CEC) is the quantity of adsorbed cations on the unit mass of soil (Sposito 2008). Cation exchange capacity of a saturated saline soil paste can be analyzed by the sodium acetate method (Harron et al. 1983).

14.2 Methods for Bioenergy Generation

Biomass can be converted into bioenergy by different processes associated with various sources of biomass, conversion processes, their application, and infrastructure used (Mckendry 2002a). Biomass can be converted into bioenergy by means of producing three types of products: electrical/heat energy, transportation fuel, and chemical feedstock (Mckendry 2002b).

Biomass-based energy production processes are divided into two categories (Ni et al. 2006): thermochemical conversion and biochemical conversion. A third technology for bioenergy generation is mechanical extraction (with esterification) that produces biodiesel (McKendry 2002a).

There are three thermochemical processes (Ni et al. 2006):

  • Combustion: Biomass burnt in air is combustion. Equipment includes stoves, furnaces, boilers, steam turbines, and turbogenerators used for the conversion of chemical energy stored in biomass into heat, mechanical power, or electricity. At a temperature range from 800 to 1000 °C, hot gases are produced by combustion of biomass. Bioenergy production efficiency by power plant is 20–40% produced mainly by pyrolysis (Verma et al. 2011). Temperature ranging from 650 to 800 K is used to convert biomass heated in the absence of air at a pressure of 0.1–0.5 Pa into biofuel such as liquid oil, charcoal, and gaseous compounds (Ni et al. 2006).

  • Liquefaction: Biomass is converted to liquid hydrocarbon under low temperature and at higher hydrogen pressure (Warren Spring Laboratory 1993).

  • Gasification: The gasification process is suitable for producing fuel and electricity using gas engines by gasification of biomass. This is operated either by simple technology based on a fixed-bed gasifier or fluidized bed technology.

Biochemical conversion processes are of two main types (Mckendry 2002a):

  • Anaerobic digestion: This is the direct conversion of biomass into gas through anaerobic digestion. Biogas products are methane and carbon dioxide, along with a few other gases in smaller quantities such as hydrogen sulfide (EU 1999). Biogas is used in spark-ignition gas engines and gas turbines and can be upgraded to finer quality gases by removing carbon dioxide (Mckendry 2002a).

  • Fermentation: In the process of fermentation, ethanol is produced from sugar crops such as sugarcane, sugar beet, and starch crops (maize and wheat). The material biomass is crushed, and starch is converted to sugar by enzymatic activities using yeast, and finally sugar is converted to ethanol (Mckendry 2002a).

Microbial fuel cell is another emerging technology where microorganisms are placed in an electro-biochemical chamber without air to oxidize organic matter and release electrons and protons, thus producing electricity (Pant et al. 2010; Hernandez-Fernandez et al. 2015) (Fig. 14.1).

Fig. 14.1
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Procedure for conversion of chemical energy of substrate into electrical energy by bacteria through microbial fuel cells (Hernandez-Fernandez et al. 2015)

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14.3 Phytoremediation Potential of Energy Crops

Jatropha curcas has the potential to phytoremediate lindane and fly ash-contaminated sites by accumulating these contaminants in root followed by stem and leaf (Abhilash et al. 2013; Jamil et al. 2009). There are some species and their hybrids such as Populus and Salix known for their phytoremediation potential of contaminated sites (Zalesny et al. 2007). Populus is known to remediate the landfill sites, petroleum sludge, salts, heavy metals, pesticides, solvent, explosives, and radionucleotides (Burken 2001; Erdman and Christenson 2000; Gordon et al. 1997; Neill and Gordon 1994; Thompson et al. 1998; Zalesny et al. 2007). Salix is known to phytoremediate dairy effluent, wastewater sludge, municipal wastes, and cadmium from the contaminated sites. Plants such as Pistacia chinensis, Sapium sebiferum, and Xanthoceras sorbifolium are distributed in different parts of China and considered as oil-yielding plant cultivated under different range of environmental conditions (Shaoa and Chu 2008). In Greece, experimentation with Eucalyptus was done for wood and biomass production (Panetsos and Alizoti 1996). The result indicated that six species of Eucalyptus, namely, E. bicostata, E. cladocalyx, E. viminalis, E. saligna, E. camaldulensis, and E. dalrympleana are considered as dedicated energy crops (Panetsos et al. 1981). Pinus taeda has rapid growth on soil that is poorly or moderately drained (Coyle et al. 2008). Liquidambar styraciflua has potential to remediate soil contaminated with uranium (U) and thorium (Th) (Saritz 2005). Glycine max, Panicum virgatum, and Helianthus annuus are considered as biofuel crops grown on marginal soils such as brownfield sites (Smith et al. 2013). A study on legumes and trees for fuelwood production on sodic wasteland has been reported by Goel and Behl (2001). The legumes referred were Acacia auriculiformis, A. nilotica, Albizia lebbeck, A. procera, Dalbergia sissoo, Leucaena leucocephala, Pongamia pinnata, Prosopis juliflora, and Pithecellobium dulce. The trees studied were Azadirachta indica, Eucalyptus tereticornis, and Terminalia arjuna. Among the species studied, P. juliflora ranked first as the most promising species for biomass production, and Acacia nilotica ranked second most promising species for biomass production on degraded sodic land (Goel and Behl 2001). Table 14.2 depicts the potential of pollution remediation and bioenergy production.

Table 14.2 Pollution remediation and bioenergy potential of energy crop
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14.4 Reclamation of Saline and Sodic Soil

Reclamation of sodic and saline soils requires removal of most of the exchangeable cations and its replacement by Ca2+ ions and solubilized salts from the root zone, which can be done by various methods (Abrol et al. 1988). Before amelioration of a specific site, important factors should be considered such as soil depth to be ameliorated, presence of a dense solid layer in the subsoil, salt constituent in the soil, availability of water for leaching, nature and depth of groundwater, topography of land, type of crops to be grown after amelioration, and climatic condition of the region (Qadir et al. 2000). Chemical amelioration of sodic land for their reclamation can be divided into three categories: gypsum and calcium chloride as a soluble calcium salt; acid-forming compounds such as sulfuric acid, iron sulfate, aluminum sulfate, lime sulfur, and pyrite; and less soluble calcium salts such as limestone (Abrol et al. 1988). Chemical amelioration using gypsum provides a source of Ca2+ ions directly to the soil that replaces excess Na+ ions while dissoluting calcite (CaCO3) in the soil (Shainberg et al. 1989; Gupta and Abrol 1990; Oster et al. 1999; Qadir and Oster 2002 and Qadir et al. 2002). Methods to ameliorate saline/sodic soils include leaching salts from the upper surface of the soil and transporting them to lower depths, flushing of salts from the salt crusts at the surface and also in the shallow water table, etc. Biological amelioration involves sequestration of salts by the aerial (i.e., completely exposed in air) or shallow depth parts of plants that can be harvested and thus removes the salts from the soil (Qadir et al. 2000). Plants can improve chemical properties of soils, decrease soil pH, add organic matter, and dissolute lime (Ilyas et al. 1997). Remediation of saline soil is a critical global issue that requires multidisciplinary ways to remediate salt-affected land including agricultural practices, varieties of salt-tolerant crops, and phytoremediation.

14.5 Phytoremediation of Saline and Sodic Soil by Energy Crops

Phytoremediation is considered a cost-effective and environmentally safe technology for saline soil remediation (Hasanuzzaman et al. 2014). Phytoremediation involves various processes such as phytoextraction, phytodegradation, rhizofiltration, phytostabilization, and phytovolatilization (Fletcher 2006). According to Qadir et al. (2005), the two main advantages of phytoremediation are: i. no financial outlay needed for purchase of chemicals (for chemical amendment), and ii. salt-resistant crops generate high-value by-products. Roots of plants maintain soil structure and enhance drainage through the formation of macropores (pores greater than 0.08 mm in diameter) at deeper depths (Czarnes et al. 2000). Phytoremediation utilizing bioenergy crops/plants is one of the best technologies for remediation of saline and sodic soil because the harvested biomass can be used to produce biofuel or other commercial by-products while ameliorating the soil. The best bioenergy crops for soil amelioration should have high biomass production, be cost effective, have low contaminant content, have less nutrient and water requirements, be carbon neutral for the whole life cycle, and do not lead to the “food versus fuel” issue (Singh and Singh 2016).

Lal and Pimentel (2007) reported that several species of plant can produce abundant, good quality forage during summer, including warm season grasses such as switchgrass (Panicum virgatum), big bluestem (Andropogon gerardii vitman), and Indian grass (Sorghastrum nutans). Salt-tolerant grasses include Guinea grass (Panicum maximum), elephant grass (Pennisetum purpureum), and Kallar (also called Karnal) grass (Leptochloa fusca). Some of the short rotation woody perennials such as poplar (Populus spp.), willow (Salix spp.), and black locust (Robinia pseudoacacia L.) produce 10–20 tons of dry weight of biomass per hectare. Some important halophytes that grow in brackish water containing salt concentrations up to 30,000 ppm include pickle weed (Salicornia bigelovii), salt grass (Distichlis palmeri), salt brushes (Atriplex spp.), and few algae (e.g., Spirulina geitleri). Some non-edible oil-yielding plants include Jatropha (Jatropha curcas), Pongamia (Millettia pinnata), and Madhua (Madhuca longifolia). There are few energy crops in the world (Miscanthus, Ricinus, Jatropha, and Populus) that possess phytoremediation potential and act as carbon sinks, thus contributing profit through carbon tax credits (Bauddh and Singh 2012a, b; Bauddh and Singh 2015a, b; Bauddh et al. 2015a, b; 2016a, b; Pandey et al. 2016). Vetiver (Chrysopogon zizanioides) and lemongrass can tolerate and grow in saline soil. Live and dry biomass of vetiver and lemongrass has economic importance through their oil production (Maiti and Kumar 2016). India is the largest producer of lemongrass oil with production of 300–350 tons year−1, of which 80% is exported to developed countries of the world (Lal et al. 2013). Mesquite (Prosopis juliflora) grows in Africa, Argentina, Australia, Brazil, Cameroon, Caribbean, Central America, Egypt, Ethiopia, Hawaii, India, Kenya, Nigeria, Pakistan, Paraguay, Peru, Ecuador, Portugal, Senegal, Spain, Sri Lanka, Sudan, Uganda, the United States, and Yemen. Mesquite is known to rehabilitate degraded saline and sodic land and concomitantly increases soil fertility by adding soil organic carbon, nitrogen, and phosphorus and decreases exchangeable Na+ levels, pH, and electrical conductivity (Prasad and Tewari 2016). It can also be used for the production of charcoal, bioethanol, timber, fuelwood, and antibiotics (Prasad and Tewari 2016). It is estimated that crop residues and lignocellulosic residues from cereals can produce 4 billion Kg and 3 billion Kg of ethanol per year, respectively. One Mg (megagram) of corn stover can produce 280 L of ethanol, and 1 Mg of corn grains can produce 400 L of ethanol. One Mg of biomass produces about 18.5 GJ of energy. Three billion Mg of residue can produce 840 billion L of ethanol or about 56 × 109 GJ of energy (Lal 2008). Presently, global bioenergy consumption is 50 EJ yr.−1 and is expected to reach to 80–160 EJ yr.−1 (Pandey et al. 2016). The total biofuel production in India increased from 27.3 million L in 2007 to 46.4 million L in 2011, including 34.8 million L for bioethanol and 11.6 million L for biodiesel (Edrisi and Abhilash 2016). Marrison and Larson (1996) estimated that total bioenergy production from Africa by 2025 will be 18 EJ per year on the basis of planting crops on 10% of available land except forest, agricultural, and wilderness areas. Halophytes are being considered as potential new agricultural crops to reclaim salt-affected land. A species of halophyte (Salicornia bigelovii) can withstand high salinity and produce biomass and seeds of 2 tons hectare−1, yielding 28% oil, 31% protein, 5% fiber, and 5% ash (Glenn et al. 1999).

Gharaibeh et al. (2011) assessed the potential of Atriplex halimus in reclamation of calcareous saline sodic soil. Cultivation of this plant significantly increased soil properties. The electrical conductivity (ECe) was reduced from 5.8 to 3.7 dSm−1 (Fig. 14.2); however, ESP was found to be decreased. ECe value after plantation of Atriplex halimus in salt-contaminated areas was the indication of removal of Na+ ions from the soil.

Fig. 14.2
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Effect of plantation of Atriplex halimus in salt-contaminated areas (Gharaibeh et al. 2011)

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Plantations of some plant species on degraded sodic land (D-SL) with rehabilitated Terminalia arjuna (R-TA), rehabilitated Prosopis juliflora (R-PJ), reference Tectona grandis (Ref-TG), rehabilitated mixed forest (R-MF), and reference mixed forest (Ref-MF) improved soil physicochemical characteristics and soil particle distribution, exchangeable sodium percentage, and microbial enzyme concentration (Singh et al. 2012). Bulk density for D-SL was 1.62 g cm−3. After rehabilitation with R-TA, the bulk density reduced to 1.24 g cm−3 (29% decrease). With rehabilitated Prosopis juliflora (R-PJ), it showed a 24% decrease, and with R-MF, a 21% decrease in bulk density has been reported. Water holding capacity (WHC) percentage increased to 52.00 ± 2.75% in Ref-MF as compared to D-SL having WHC of 36.33±2.65%. Na+ ion concentration decreased in the following order: R-TA (3.45 cmol kg-1) <R-PJ (3.43 cmol kg-1) <Ref-MF (1.96 cmol kg-1) <R-MF (1.47 cmol kg-1) <Ref-TG (0.80 cmol kg-1). The mean values of K+, Ca2+ and Mg2+ ion increased to 2.48, 20.35, and 5.50 cmol kg-1 respectively in Ref-MF. Figure 14.3 depicts the role of afforestation in improvement of physicochemical characteristics of soil.

Fig. 14.3
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Effect of plantation on bulk density (BD), water holding capacity (WHC), and soil particle distribution of degraded sodic land, rehabilitated land uses, and reference plantation and forest (Singh et al. 2012)

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14.6 Conclusion

Saline and sodic soils are distributed in around 831 million hectares of land worldwide and possess various adverse features such as high pH, high exchangeable sodium percentage, high sodium adsorption ratio, and low cation exchange capacity that make a soil infertile. At the same time, fuel and energy needs are increasing globally, leading to greater emissions of greenhouse gases which results changes in climate. Chemical treatment using gypsum shows rapid amelioration of saline and sodic soil, but is a costly and non-eco-friendly approach. The emerging technology of phytoremediation using specific plants that reclaim sodic and saline land may be used to produce energy. Some of the energy crops like Miscanthus, Ricinus, Jatropha, and Populus are extensively used worldwide. Reclamation of saline and sodic soils utilizing such plants is preferable because of its applicability and sustainability. Phytoremediation using bioenergy crops could be adopted as a better approach for mitigation of major environmental concerns like land degradation, pollution, energy crisis, and climate change.