Keywords

14.1 Introduction

The projected world population by 2050 is from 7 billion (current estimated) to 8.9 billion approximately (Singh et al. 2011), and the food security to this burgeoning number is a paramount concern. The rapid increase in environmental contamination and global climate change is another important threat toward sustainable agriculture. Farmers are under pressure to use excess amounts of chemical fertilizers and pesticides to enhance the agricultural productivity. The application of synthetic fertilizers not only deteriorates the soil health but also enhances the greenhouse gases into the atmosphere as well as contaminates the surface and groundwater. Traditional cropping systems infuse a variety of toxicants to the soil and ultimately enter into the food chain. Abiotic stresses like salinity, acidity, drought, heavy metal, flood, high temperature, freezing, chilling, etc. also damage the agriecosystems, which results in decreased crop yield. These stresses decrease crop productivity by decreasing the photosynthetic pigments, protein denaturation, production of reactive oxygen species (ROS), lipid peroxidation, etc. (Fig. 14.1) (Chodak et al. 2015; Isayenkov and Maathuis 2019; Sallam et al. 2019).

Fig. 14.1
figure 1

Different abiotic stresses, their impacts and mitigation strategies (Chodak et al. 2015; Bauddh et al. 2016; Isayenkov and Maathuis 2019; Sallam et al. 2019)

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Salinity, drought, and heavy metals are considered as major abiotic stress and reduce a significant portion of agricultural productivity. Plant growth is dependent on the rate and level of photosynthesis, hence environmental stress affecting photosynthesis directly impacts the growth of plant. Salt stress can also influence the synthesis of abscisic acid which when moves to the guard cells, closes the stomata. As a result of the closure of stomata, the rate of photosynthesis declines. The ability of germination and vigor is the most important and primary process in the production of any plant and the adverse impacts of abiotic stress on these parameters are well reported (Bhaskar et al. 2009; Kumar et al. 2009; Singh et al. 2010; Bauddh and Singh 2009, 2011, 2012). Germination being important in the reproduction often influenced by the environment created due to the stress induced by water-scarce conditions (Leila 2007). Water being absorbed by the seed is the first stage in the process of germination. The quantity of water absorbed, mainly dependent on the type, size, and chemical composition of the seed (Rahmani 2006). Generally, soil moisture level lesser than soil’s field capacity is considered favorable for the germination of the seed but in the water-scarce conditions where there is less than desirable water absorption in these seeds are incomplete which can result in diminishing or even stopping the germination process (Leila 2007). Several strategies have been developed to mitigate the impacts of abiotic stresses, especially salinity, drought, and heavy metals. The tools like cultivation of stress-tolerant varieties, application of organic amendments, genetic manipulation, application of microbial inoculants, etc. have been found significantly effective to reduce the adverse impacts of majority of abiotic stresses. Application of biochar has been proven an efficient potential to reduce the adverse impacts of drought, salinity, and heavy metals. Biochar have several other beneficial properties like it retains moisture in soil, balance soil pH, use to make fuel, used in metal remediation from the soil, etc. (Yuan et al. 2019). Several studies have been found that application of biochar to the soil enhances the physicochemical and biological properties of soil and overcome the negative effects created by the stressed conditions.

In the present chapter, the efforts have been done to explore the adverse impacts of three major abiotic stresses, i.e., drought, salinity, and heavy metals. Further, the application of biochar to overcome the impacts of these stresses during the cultivation of crops has also discussed thoroughly.

14.2 Drought, Salinity, and Heavy Metal Stresses

14.2.1 Drought Stress

One of the major concerns of the present time is to cope up with the condition of water scarcity that is arising throughout the world. Water scarcity is among the most catastrophic stress that can severely affect human life both directly and indirectly arising from several other stresses. Drought can be defined as an event of prolonged shortage of water due to insufficient precipitation usually for more than one season (Trenberth et al. 2014). In most of the regions, the phenomenon of drought is a periodic condition and other places can remain in the state of drought for several years. Whenever an area experiences drought, it has a massive effect on life and activities in these areas. Drought also has an impact on hydration of soil, which slowly deteriorates soil quality and also severely affects the animals living in these regions. Some of the climatic factors that are precursors of drought include high wind speeds, high mercury concentrations, etc. Apart from the abovementioned drought, there are other types of drought such as meteorological drought, hydrological drought, and agricultural drought which are described below.

14.2.1.1 Meteorological Drought

It is defined as the drought condition arising due to absence of precipitation in a particular area for a very long time span. However, there is no common consensus among the researchers about a particular value that will define the drought condition like amount of rain or time span of absence of rain. Due to the varying climatic condition, even deserts have drought but their condition is different from the condition of a grassland’s drought.

14.2.1.2 Hydrological Drought

Hydrological drought may be defined as the water deficiency in the surface and subsurface regions, which may affect the supply of water to the water bodies such as freshwater streams, lakes, ponds and most importantly confined water reservoirs and groundwater. Hydrological drought is not a seasonal phenomenon, it occurs and develops over an extended period of time.

14.2.1.3 Agricultural Drought

Agricultural drought is a condition when the water is in such scarce condition that it becomes difficult to sustain the grown crops. This condition usually arises when the expected rainfall is not achieved during essential growth period of crops (this period varies in different crops) and may result in the destruction of crops ranging from mild-to-absolute crop loss. This type of drought can develop very fast if expected rain does not arrive or there is an extensive heat wave, especially during essential growing periods of the crops.

It is a well-known fact that drought condition usually begins with the below-average rainfall in any particular region, although the scarcity of water supply is not the only reason for causing the drought situations. Any kind of activities that deplete the capacity of soil to hold water can be one of the main reasons in the commencement of water scarcity in and around any area. These activities may include over farming of a particular area and extensive deforestation. Both of these activities exploit and deplete the nutrients present in the soil and loosens the soil increasing the soil erosion and destroying the water holding capacity of the soil. This is the prime reason resulting in the desertification of that particular area. On top of that deforestation has been proven to be the reason for reduced rainfall, which adds up to the drought condition already prevailing. When there is a smaller number of trees around any area, vast area of land remains uncovered, which increases water loss directly from the land. All of this can lead to the complete destruction of any kind of forest on its own which further deteriorates the environmental conditions.

14.2.2 Salinity Stress

Salinity is simply the presence of salts such as sodium, magnesium and calcium and bicarbonates in the soil while sodicity refers specifically to the amount of sodium present in the soil. Saline soil may be defined as the soil with an electrical conductivity of at least 4 dS m−1 by measuring the saturated soil extract while sodic soil can be defined as the soil with an exchangeable-sodium percentage (ESP) which is more than 15 (United States Salinity Laboratory Staff 1954). Salinity is of two types; primary salinity and secondary salinity. The salting that results from anthropogenic activities, mainly land development and agriculture is known as secondary salinity. Unrestrained cutting of trees, overgrazing of economic land by animals, and the use of chemical fertilizers are some of the other contributing factors to soil salinity (Lakhdar et al. 2009). Salinity is not bound to rural areas only. Urban salinity occurs generally by the combination of both faulty irrigation-based salinity and due to dry land. Saline soils are mostly dominant in arid and semiarid regions throughout the world, and many productive lands are converted into salt-affected wastelands (Qadir et al. 2000). Globally, the extent of primary salinity is about 955 M ha, whereas the secondary salinization affects 77 M ha, where 58% of the area is affected by irrigation (Metternicht and Zinck 2003). The global estimate of the land affected by salinity is 400 million hectares, which makes up 6% of the total world land area (Arora et al. 2017). The distribution of saline lands is not uniform. The continent of Australia has the highest salinity surface area with 963 million hectares of saline land followed by Asia with about 923 million hectares of saline land (ICARDA 2002). Fifty percent of the arable land is expected to encounter a serious salinity problem by 2050 (Meng et al. 2016).

Classification of salt-affected soil is done on the basis of pH, electrical conductivity (EC), sodium absorption ratio (SAR), and exchangeable sodium percentage (ESP) of the saturated extract (Bohn et al. 2001). Salt-affected soils are thus categorized into saline soil, sodic soil, and saline–sodic soil (Table 14.1).

Table 14.1 Characteristics of salt-affected soil (Amini et al. 2015)
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14.2.3 Heavy Metal Stress

The rampant industrialization and urbanization along with biological and behavioral variations in human, lead to histrionic changes in the environment and climatic patterns. Haggag et al. (2015) discussed feed of devastating population and their food security increased the biotic and abiotic stresses on crop productivity and the environment. Minhas et al. (2017) emphasized on abiotic stresses like salinity, drought, flooding, metal toxicity, nutrient deficiency, high and low temperatures, UV-exposed photo inhibition, air pollution, wind, hail, and gaseous pollutants caused severe damage to the crop productivity and eventually reduced the crop yields. The global per capita food supply and sustainable agricultural food production are the major challenges (Tilman et al. 2002). The technological advancement used an excessive amount of chemical fertilizers such as new cultivars, mineral fertilizers, pesticides, synthetic fertilizers like DDT (1,1′-(2,2,2-trichloroethane-1,1-diyl) bis (4-chlorobenzene)) and polyaromatic hydrocarbons (PAH), etc. these activities increased the concentration of heavy metals in agricultural soil particularly Cd, Pb, As, Zn, Co, Ni, Mn, etc. (Atafar et al. 2010; Chibuike and Obiora 2014). According to Connor et al. (2018), a national soil quality survey in China showed about 19.4% of nations’ agricultural land has been affected by heavy metal contaminations. Heavy metals are naturally present in the soil and geologic as a useful concentration but anthropogenic activities increased the concentration of these elements to amounts that are harmful to plants, animals, and living organisms (Chibuike and Obiora 2014). Plants growing on these heavy metal-containing soils show a reduction in growth, performance, decrease crop yield with quality, and metal accumulations in crops can lead to a potential threat to human health. Chromium toxicity in Bangladesh is increased by the continuous advancement in industrial activities cause cancers to humans through direct contact and other health problems such as respiratory tract and skin problems (Uddin et al. 2017). Some heavy metals are toxic even at low concentrations such as As, Cd, Pb, Cr, and Ni and also carcinogenic by nature (Jan et al. 2015; Mishra and Bharagava 2016 Mishra et al. 2018).

14.3 Adverse Impacts of Drought, Salinity, and Heavy Metal on Soil Quality

14.3.1 Effects of Drought

Drought is one of the worst abiotic stresses which vastly affects the fertility of the soil. It greatly affects the microbial activities, soil respiration, waste decomposition, etc. (Hoogmoed and Sadras 2016; Schmidt et al. 2016). Microbial community and their function in the soil ecosystem plays a very significant role in deciding the fertility and improving the structure of any soil (Wall et al. 2012; Arafat et al. 2017). For example, microbial communities generally mass-produce enzymes that are main reason for 90% of soil organic matter decomposition which in turn helps in C mineralization, this whole process is responsible for the cycling of the nutrients in the soil (Swift et al. 1979). In comparison to fungal properties, the bacterial population is more sensitive to the stress caused due to scarcity of the water and in turn, their reaction adversely impacts the fertility of the soil and soil structure (Bardgett and Wardle 2010).

14.3.1.1 Soil Enzymes

Soil enzymes and the related activities that occur are known to be the indicators of the soil health as they are directly dependent and can give information about microbial and physicochemical status of the soil (Baum et al. 2003). These enzymes are generally synthesized by fungi and bacteria present in the soil and may contain enzymes like ureases, pectinases, phosphatases, and proteases. All these enzymes are very important part and influence soil fertility. Drought is one of the major stresses that can directly and most frequently affect the soil enzymes. Soil enzyme is one of the first affected characteristics during water scarcity thus giving drought the control over nutrient availability and ultimately soil fertility (Sinsabaugh et al. 2013).

14.3.1.2 Microbial Activity

Water un-availability and its stress is one of the primary limiting factor in the survival of microbial community through induced osmotic stress, starvation, and resource competition, thus making the presence of water to be an important factor in the functioning of soil microbial communities and the effect of the stress can be very severe (Griffiths et al. 2003; Sowerby et al. 2005). It has been proven by the scientists from all over the world that soil fertility and its stability is greatly influenced by the microbial community present and they are very sensitive indicators of the environment to stresses like drought (Zornoza et al. 2007). Bacteria are considered to be a major part of any soil microbial community and very important to the fertility of the soil. During the conditions of drought, bacterial community is greatly affected thus limiting the decomposition capacity of the bacteria which may result in a change of structure of soil (Hueso et al. 2012). The availability of water also influences substrate availability and other soil characteristics like its physical characteristics that will affect the population and overall performance (Hueso et al. 2012)

14.3.1.3 Nutrient

Under drought conditions, nutrient uptake through roots as well as transport of nutrients from the roots to shoots is greatly affected mainly due to reduced transpiration rate and the impairment of permeability of membranes (Alam 1999). The drought condition also results in the reduction of the diffusion rate of the nutrients within the soil to the roots (Pinkerton and Simpson 1986).

Nitrogen is one such element which is among the most important mineral for plant growth and is required in subsequent amount as it is a common component of nucleic acid and amino acid. Therefore, the deficiency of nitrogen will immediately cause slow growth in plants. Various researchers have conducted study to establish a relationship between the availability of water and fertilizing property of nitrogen. It has been seen that under water-scarce condition nitrogen fertilizer does not yield good crops and at the same time, increasing water without sufficient supply of nitrogen fertilizer also inhibits growth (Smika et al. 1965). Scientists and researchers have also proven that under drought/water-scarce condition nitrogen-fixing property of leguminous plant is reduced (Streeter 2003). Phosphorus, another important element is a component of phospholipids, dinucleotides, nucleic acids, adenosine triphosphate, and phosphoproteins. Therefore, phosphorus is essential for processes like photosynthesis, transfer and storage of energy, transport of carbohydrates, and regulation of some enzymes. It has been proven that under drought conditions, there are more requirements for adding phosphorus and increased phosphorus uptake into the soil. It is a known fact that under drought or water-scarce condition, phosphorus uptake by plants is reduced (Pinkerton and Simpson 1986), i.e., the transfer and uptake of phosphorus and translocation of phosphorus to the shoots from roots are greatly affected during mildly dry soil conditions (Resnik 1970). In a study by Turner (1985) concluded that in drought-affected areas, phosphorus deficiency is one of the earliest symptoms observed in soil and plants grown in these soils. Therefore, the application of phosphorus fertilizers is required to increase the yield of crops under drought conditions (Garg et al. 2004). Potassium is essential in protein synthesis, photosynthesis, synthesis of glycolytic enzymes, maintaining turgor movements, and also cell expansion (Marschner 1995). Potassium becomes an essential element for the turgor pressure and turgor movement in plants in dry soil conditions. Kuchenbuch et al. (1986) in a study concluded that dry soil reduces the potassium uptake in onion plants. Calcium plays a very important role in maintaining several physiological changes that control growth and other responses to environmental stresses. Some of these physiological changes include solute and water and stomatal closure in a plant. Although calcium uptake decreases under drought conditions same as other elements, but overall calcium uptake by plants is only slightly decreased in comparison to other major elements like nitrogen, phosphorus, potassium, etc. There is not much work done for the effect of drought on elements like magnesium and sulfur. However, studies have shown that drought conditions reduce both magnesium and sulfur uptake (Scherer 2001).

14.3.2 Effect of Salinity on Different Properties of Soil

14.3.2.1 Physical Properties

High level of exchangeable sodium ion can structurally deteriorate the soil that results in the decline of pore volume and also in salt-affected soil it can disrupt the relation between soil–air and soil–water (Rengasamy and Olsson 1991). In sodic soil, slaking, dispersion, and clay swelling are the chief mechanisms that are involved in the breakdown of aggregates as sodicity in the soil affects soil hydraulic conductivity and its infiltration rate (Rengasamy and Sumner 1998). Slaking process causes permanent blockage of the pores even though it is a reversible process while dispersion is an irreversible process that can cause translocation of individual soil particles (Rengasamy and Sumner 1998). This mechanism of soil pore system and its structural stability can be explained by diffuse double layer (DDL) theory, which suggests that the attraction and repulsion between ions in soil particles is a result of intermolecular and electrostatic forces working on them (Quirk 1994). The increase of sodium ion on the exchange site of the clay or soil particles results in the increase of repulsive force which increases the inter-particulate distance turn the breakdown of soil aggregates affecting the soil structure (Oster and Shainberg 2001). Swelling and dispersion in sodium dominated soil bring about changes in the hydraulic properties of the soil.

Low salinity and high sodicity lead to a substantial reduction of hydraulic conductivity and infiltration because of the induced dispersion and swelling (Dikinya et al. 2006). Ensuing slaking and dispersion in the surface layer of sodic soils, a surface crust is formed, which is a thin layer formed after drying. This crust makes the surface soil prone to intense erosion as well as waterlogged conditions (Shainberg et al. 1992).

14.3.2.2 Chemical Properties

Saline and sodic soils have high values of SAR, ESP, EC, and pH. Saline soils normally suffer from deficiencies of NPK while their high pH has an adverse effect on the availability of micronutrients like Al, Fe, Zn, Cu, and Mn (Lakhdar et al. 2009). Salt toxicity, high osmotic pressure as well as soil degradation lead to decline in vegetation growth and decrease in carbon inputs in the soil further deteriorating the physicochemical properties of the soil (Wong et al. 2009). It is also observed that irrigation with sodium-rich water increases the soil sodicity, which results in the decrease of total C and N (Chander et al. 1997). The dispersion of soil aggregates uncovers the confined organic matter, thus increasing the decomposition rate by the soil microbes.

Increase in salinity decreases the mineralization of nitrogen and increases the loss of gaseous NH3 (Paliwal and Gandhi 1976). It was further discovered that in the arid soils with increasing salinity and sodicity even with organic amendment leads to a decrease in the mineralization of C and N (Pathak and Rao 1998). The ability of plants to take up water is reduced by the high ion concentration, primarily Na and Cl ion. This, in turn, affects the plant growth and plant cell (Muneer and Oades 1989).

14.3.2.3 Biological Properties

The changes in soil chemistry negatively affect the soil microbial and biochemical processes, which play a crucial role in maintaining the soil’s ecological functions (Rietz and Haynes 2003). Increasing soil salinity adversely impacts microbial growth and activity, as increased concentration of salt in the soil causes osmotic stress resulting in microbial cell dehydration (Wichern et al. 2006). Other factors, in addition to salt stress, that contributes to the decrease in microbial population and activities in salt-affected soils are Na+ toxicity; nutritional deficiency like Ca2+ deficiency; increase in the level of toxicity of ions such as bicarbonate, carbonate, and chloride; and also significant loss in organic matter resulted by structural degradation (Nelson and Oades 1998). A study was conducted in the arid region of southeast Spain to analyze the effect of salinity on the composition of microbial community in the soil. It was found that the increase in salinity had a negative impact on the microbial community of soil (Garcia et al. 1994). The enzymatic activity, as well as the rate of mineralization of C and N, decreased at high levels of salinity. In a study, the nitrification rate is increased by up to 83% with increase in salinity to 20 dS m−1, which stimulates volatilization of ammonia (Wichern et al. 2006). Soil biochemical and microbial activities are negatively affected by salinity and sodicity induced by irrigation (Rietz and Haynes 2003). Many other studies have also reported that soil microbial biomass and enzyme activities significantly decrease with salinity and sodicity (Tripathi and Srivastava 2006). A shift in the microbial community has been observed with increasing salinity; an adaptive trait to reduce salt stress by lowering the metabolism is noticed (Yuan et al. 2007). Microbial and biochemical activities significantly affect soil ecological function. They play a central role in enhancing the structure of soil as well as in the stabilization of the soil aggregates (Six et al. 2005).

14.3.3 Impact of Heavy Metals

14.3.3.1 Impacts on Soil Physicochemical Quality

Metals exist either as separate entities or in combination with other soil components. Odueze et al. (2017) reported that the soil physicochemical properties affected by heavy metal concentrations present in different layers of soil locations and depth. According to a survey in China by Wang et al. (2018) emphasized on the effects of heavy metal contaminations in agricultural soils concerned to soil physicochemical properties and cultivation age of crop species. Similarly, in Zambia, the physicochemical characteristics of local forest soil were contaminated by the copper mining waste including heavy metals that decreased the pH level (5.05), affect the bulk density (1.24 g/cm3), total organic carbon (2.24%), total nitrogen (0.07%), available P (4.9 mg/kg), K (37.8 mg/kg), Na (49.0 mg/kg), Ca (99.0 mg/kg), and Mg (81.2 mg/kg) (Mutale et al. 2019). As discussed by Obasi et al. (2012), the physicochemical properties of soil are mainly influenced by the mobility and bioavailability of heavy metals. They emphasized that the physicochemical parameters of the soil help to support microbial diversity, metabolic changes, and lastly growth of plant species. The effects of heavy metal on soil physicochemical quality with different levels are shown in Table 14.2.

Table 14.2 Effect of heavy metals on soil physicochemical quality
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14.3.3.2 Impact on Soil Biology (Microbial and Enzymatic Activities)

Soil microbes such as bacteria, fungi, algae and protozoa have good metabolic activities for depolymerization and mineralization of organic matter in the forms of N, P, and S. Soil enzyme activities are good indicators of heavy metal toxicity in the soil for bioavailability assessment (Xian et al. 2015). Sobolev and Begonia (2008) reported the effects of Pb on soil microbial community in low (1 ppm) and high (500–2000) concentrations. Kouchou et al. (2017) observed heavy metal contaminations in alkaline soils of the region of Fez (Morocco) and showed adverse impact on actinomycetes and fungi. Atafar et al. (2010) indicated that the microbial biomass in the soil contaminated by Cu, Zn, Pb, and other heavy metals were inhibited drastically. Shi and Ma (2017) observed that Cd contamination affects the microbial activity like respiratory intensity, urease activity, and catalase activity in forest soil and garden soil.

Soil organic matter is the dominant factor contributing to the soil microorganisms and measurements of free soil enzyme activities that can serve as useful indicators of microbial metabolic potential as explained in Table 14.3 (Hagmanna et al. 2015). The research done by Jin et al. (2015) depicted that the heavy metal contaminations adversely affect the soil biological functions, including the size, activity, and diversity, of the soil microbial community and the activity of various enzymes involved in the transformation of C, N, P, and S. Some findings in the review done by Chibuike and Obiora (2014) showed the heavy metal affects the soil microorganism’s quantity, diversity, density, and their activities. The metal toxicity on microorganisms is dependent on several aspects such as pH, soil temperature, organic matter, clay minerals, inorganic anions, and cations, and chemical forms of the metal (Wuana and Okieimen 2011; Chibuike and Obiora 2014). Therefore, the effects of the heavy metals on soil microbial and enzymatic activities are mainly dependent on the soil physiological properties, the stimulus of heavy metals on soil microbial activity, the end product on soil enzyme activity, and the composition of soil microbial community (Khan 2000). Similarly, Khan et al. (2010) found that heavy metals (Pb and Cd) have been an inhibitory impact on soil enzyme activities and as well as microbial community structure. Thus, heavy metals significantly impact on soil enzymes, they restrict the growth and reproduction of microorganisms, reduce the synthesis and metabolism of the microbial enzymes (Chu 2018). The effects of heavy metal on soil microbial diversity may be assessed more comprehensively by using methods such as microbial biomass, C and N mineralization, respiration, other enzymatic activities, etc. (Oijagbe et al. 2019).

Table 14.3 Effects of heavy metals on soil biology (microbial and enzymatic activities)
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14.4 Effects of Drought, Salinity, and Heavy Metals on Growth and Productivity of Crops

14.4.1 Effects of Drought

Drought is a kind of stress that may affect the plants at individual level and even at whole crop level. This extreme stress can cause permanent damage to the plants and has an obvious symptom, e.g., wilting of the leaves. All plants require a particular amount of water for their survival throughout their lifecycle to give better productivity. Plants are well known to possess drought resistance. Other symptoms of drought can be seen by reduction in leaf area, increased leaf drop, reduced leaf growth, sunken stomatal development, yellowing of leaves, and wilting which all lead to the death of the plant (Nakayama et al. 2007).

Water stress inhibits cell enlargement more than cell division. It reduces plant growth by affecting various physiological and biochemical processes, such as photosynthesis, respiration, translocation, ion uptake, carbohydrates, nutrient metabolism, and growth promoters (Jaleel et al. 2008a, b, c, d, e; Farooq et al. 2008).

14.4.1.1 Effect of Drought on Seed Germination

Ability of seed germination and vigor is the most important and primary process in the production of any plant. Germination being important in the reproduction often influenced by environment created due to the stress induced by water-scarce condition (Leila 2007). Water being absorbed by the seed is the first stage in the process of germination. Under drought conditions, osmotic and matric potentials are dramatically decreased which does not allow the required amount of water to reach into plant tissues. The negative effect of drought on plant has been proven by several scientists again and again over time. Zareian (2004) stated that scarcity of water can significantly reduce the germination rate; however, different plant responds differently depending on the stage of germination. Shekari (2000) conducted a study on millet to assess the effect of drought on germination, root and shoot length and concluded that with increasing drought conditions the rate and percent increase in parameters observed reduces considerably. Drought affects many plant growth parameters affecting its overall germination, growth, yield, and production of dry matter.

14.4.1.2 Effects of Drought on Morphological Characters of Plants

Considering plant physiology, scarcity of water causes serious stress on plant growth, yield, and survival. Ghodsi et al. (1998) studied the effect of induced drought on plant and documented 30–50% decrease in yield due to drought and concluded that plant growth is severely affected as a result of high temperature, low humidity, high sun intensity, and high evapotranspiration. Similarly, Bagheri (2009) stated that high temperature due to high light intensity increases plant respiration, photosynthesis, and enzyme activity. Under drought condition, the increased light intensity from the sun increases the photoreaction processes such as photosynthesis and production of free oxygen radicals that ultimately leads to plant death.

Dryness causes the nutrient content of topsoil to be completely absorbed by the plants, which gets stored in fruits whereas salts and other ions get accumulated into the top soil due to increased drought conditions. The increase in ions and salts around the roots of the plants results in osmotic stress, which ultimately causes ion toxicity and death of plants. Under such conditions, the primary response of plants usually is biophysical responses. These responses after drought conditions include loosening and widening of cell walls, decrease in cell volume, decrease in cell pressure that also affects the potential of cell development and if this potential is reduced then growth rate also decreases (Bagheri 2009).

It has been proven by several researchers throughout the world that drought stress is among the worst stress which turns out to be an important limiting factor during the times of development of plants, especially during initial phases of germination (both expansion and elongation) and establishment (Anjum et al. 2003; Kusaka et al. 2005; Bhatt and Rao 2005; Shao et al. 2008).

Most affected plant crops are usually submerged plant crops such as rice and are considered more susceptible to drought in comparison to other plants. In an experiment with soybean plants, stem length was found to be decreased under water-scarce conditions (Specht et al. 2001). In several studies concerning height of the plants, Wu et al. (2008) concluded that water-scarce conditions reduced the plant height of citrus seedlings by almost 25%. Heuer and Nadler (1995) stated that length of the stem is significantly affected due to drought conditions for potato plant and similar kind of results was observed for Petroselinum crispum (parsley) (Petropoulos et al. 2008), (soybean) (Zhang et al. 2004), Vigna unguiculata (Cowpea) (Manivannan et al. 2007), and Abelmoschus esculentus (Okra) (Sankar et al. 2007, 2008).

Under water-scarce conditions, cell growth and cell expansion are reduced due to extremely low turgor pressure whereas increase in osmotic regulation helps in maintaining cell turgor to help the plant grow under water-scarce conditions in plants such as pearl millet (Shao et al. 2008).

Bhatt and Rao (2005) conducted a study on the plant species Abelmoschus esculentus under dry conditions and concluded that reduction in the cell enlargement was responsible for the reduction of average plant height and also for the leaf area and size reduction. A similar study was conducted for assessing the leaf area change in the Populus plant under water-deficit condition by Wullschleger et al. (2005). It was concluded that water stress reduces the leaf growth thereby reducing leaf area in the plant and since the growth of leaf area is important for adequate photosynthesis and production/yield, the result was an overall reduction in the growth rate. Same results, i.e., reduction in growth was observed for Glycine max (soybean) (Zhang et al. 2004) and many other species (Farooq et al. 2009). The leaf growth was more sensitive to water stress in wheat than in maize (Sacks et al. 1997), Vigna unguiculata (Manivannan et al. 2007), and sunflower (Manivannan et al. 2007, 2008).

14.4.1.3 Effects of Drought on Yield

The main purpose of harvesting any crop in the world is to get a good amount of harvestable yield. Any specie of crop requires a minimum amount of water to sustain and produce a good harvestable yield. Plantings of sunflower with abundant water supply, the yield increase was associated with both an increase in grain number and in individual grain weight (Soriano et al. 2002). Water scarcity primarily affects the dry matter yield and is the most important factor in determining the yield (Petropoulos et al. 2008). Soriano et al. (2002) concluded that in the sunflower harvest water-deficient condition was an important factor in the decrease of the yield and production and that presence of adequate amount of water is important throughout the season from establishment to harvesting period (Soriano et al. 2002). In another study by Edward and Wright (2008) on the wheat plant, they concluded that grain size and number decreased dramatically under drought-induced conditions.

In a similar study on maize, water-deficit stress vastly decreased the final yield, which was attributed to the extent of defoliation caused by water scarcity mainly during the early growth period (Monneveux et al. 2006). Drought conditions decrease seed production and yield in soybean mainly as a response to water-scarce conditions, which generate fewer seeds and pods in an area (Specht et al. 2001). Whereas in a comparitive study between water-stressed soybean seeds and controlled seeds, the yield of stressed seeds was found to be much lesser in comparison to water-controlled plants (Specht et al. 2001).

Tahir and Mehid (2001) conducted a study to know the effect of drought on sunflower plant and found that water-deficit condition greatly reduced the head diameter, dry weight, and harvestable yield per plant per unit area. The study observed a negative correlation between head diameter and fresh root and shoots weight, whereas a positive correlation was observed between dry roots and shoot weight and achene yield per plant per unit area under water-scarce conditions. Water-scarce conditions prevailing for more than 12 days at flowering and grain filling stage was found to be the most damaging one in decreasing the achene yield in sunflower plant (Reddy et al. 2004), in common bean, and green gram (Webber et al. 2006), maize (Monneveux et al. 2006), and Parsley (Petropoulos et al. 2008).

14.4.1.4 Effects of Drought Stress on Pigment Composition

Photosynthetic pigments are important component mainly present in leaves of the plant and phytosynthetic bacteria. Chlorophyll is the most important pigment among several and drying of soil has a great effect on chlorophyll “a” and “b” which in turn affect the health of the plant (Farooq et al. 2009). However, during times of drought, carotenoids play a major role and help plants to withstand adverse effects of drying soil. Some of the changes that occur in the pigments during water-scarce conditions are given below.

14.4.1.4.1 Chlorophylls

Researchers from around the world have proven in a number of experiments that chlorophyll and carotenoid contents are reduced due to the drying of the land (Farooq et al. 2009). The decrease in chlorophyll and carotenoids have been greatly observed in cotton plants (Massacci et al. 2008), sunflower (Kiani et al. 2008), Vaccinium myrtillus (Bilberry) (Tahkokorpi et al. 2007), etc. It has also been proven that the overall foliar photosynthesis rate of plants decreases with the decrease in leaf water potential (Lawlor and Cornic 2002).

Apart from this, there is not a common consensus among researchers whether drought limits photosynthesis through stomatal closing or pigment or metabolic disfigurement (Anjum et al. 2003; Lawson et al. 2003). Some mixed results have also been observed in cases such as that of Ashraf et al. (1994) where they stated that chlorophyll b content was observed to increase in two lines of ladyfinger plants, whereas chlorophyll a was observed to have remained unaffected resulting in the reduction of chlorophyll a:b ratio in water-scarce conditions.

14.4.1.4.2 Carotenoids

Carotenoids are class of photosynthetic pigments mainly composed of yellow, orange, and also red pigments, including carotene that is mainly responsible for giving color to the plants and its parts such as autumn leaves and ripe tomatoes. They also protect chlorophyll pigments from photodamage. These pigments are mainly synthesized by all photosynthetic and some nonphotosynthetic organisms as well (Wilson et al. 2008).

The carotenoid play a more important role during times of drought by playing the protective role of β-carotene through direct quenching of triplet chlorophyll, preventing the production of any singlet oxygen thereby protecting plant tissue from oxidative damage occurring due to water-deficit conditions (Farooq et al. 2009).

14.4.1.5 Effect of Drought on Stomata

Stomata are minute pores usually found in the epidermis of leaves and stems whose main functions are water and gaseous exchange. Stomata are mainly found in the lower side of the leaves where generation of new stomata takes place continuously throughout the growth period of the leaf (Zhao et al. 2015). Active stomatal functioning decreases the rate of transpiration and is among the most important processes that maintain the water balance in the plant tissues. Stomatal functioning/closure is also responsible for reducing cell growth and cell expansion, which affects the overall decrease in the rate of biomass production and yield (Rauf et al. 2015; Nemeskeri et al. 2015). It has been proven by many scientists that stomatal closure is among the primary response that a plant show in case of water scarcity to prevent or reduce water loss due to transpiration (Clauw et al. 2015; Nemeskeri et al. 2015). Closure of stomata is an active mechanism employed by plants to avoid dehydration (Osakabe et al. 2014; Clauw et al. 2015).

14.4.1.6 Drought-Associated Plant Diseases

Dry and hot conditions help the spreading of infections and diseases through pathogens and make plants susceptible to these diseases. Normally some diseases may be less harmful or lethal for the plant depending on the type. Some of the diseases that affect the plant during dry and water-scarce conditions are aflatoxin contamination, Aspergillus ear rot, etc. In general, certain diseases are prevalent while others are less severe or do not occur. With drought conditions, we anticipated yield losses due to soybean cyst nematode (SCN) and charcoal rot, as well as corn grain as a consequence of Aspergillus ear rot, charcoal rot, caused by Macrophomina phaseolina, etc. (Al-Kaisi et al. 2013).

14.4.2 Adverse Impacts of Salinity on Growth and Productivity of Crops

One of the most significant factors that limit the growth and production of crop is salinity. Salt stress generally affects the plants in two ways:

  1. 1.

    Osmotic stress, a high concentration of salt makes extraction of water arduous for roots.

  2. 2.

    Specific ion toxicity where the high levels of salt inside the plant pose harm to it (Munns and Tester 2008; Hussain et al. 2008).

Salinity has a number of damaging effects on physiological processes in plants like increase in the rate of respiration, ion toxicity, change in the metabolism of C and N, mineral distribution, membrane instability and permeability, reduced leaf area, dry mass, stomatal conductance as well as decreased efficiency of photosynthesis leads to the decrease in the productivity of plant (Sudhir and Murthy 2004; Kim et al. 2004; Hayat et al. 2012a, b).

14.4.2.1 Effect of Salt Stress on Plant Growth

Salinity stress has a significant impact on the growth and development of plants (Table 14.4). The effect of salt stress on processes such as germination, vegetative growth, and seedling growth and vigor has an adverse impact that ultimately leads to poor plant growth (Sairam and Tyagi 2004). Osmotic phase inhibits the growth of young and new leaves while the ionic phase accelerates the senescence of mature leaf and causes them to fall off. In soil when there is an excess concentration of salt, not only is the plant growth inhibited but this salt stress even causes death. The connection between the increase in the concentration of sodium chloride and the decrease in the plant length is evident in the result of various studies (Houimli et al. 2008; Rui et al. 2009; Memon et al. 2010). The influence of salinity on plants is such that the number of leaves decreases with an increase in the concentration of salt (Gama et al. 2007; Ha et al. 2008). It is also observed that the leaf area gets negatively affected as well with the increase in the concentration of NaCl (Rui et al. 2009).

Table 14.4 Effect of salinity on growth and productivity of crops
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14.4.2.2 Effect of Salt Stress on Photosynthesis Process

The inhibition of photosynthesis takes place when high concentrations of Na+ and Cl− are accumulated in the chlorophyll which is a major content for the photosynthesis to take place. This is directly correlated with the health of the plant (Zhang et al. 2008). It was observed that photosynthetic gas exchange parameters are reduced significantly due to salt stress conditions in Solanum melongena (Shaheen et al. 2012). Rate of photosynthesis, transpiration, and stomatal conductance is reduced significantly due to salt stress in Cucumis sativus as well as in Lycopersicon esculentum (Ahmad et al. 2012). It has been noted that the photosynthetic rate decreases with increase in salinity. This takes place due to certain factors (Iyengar and Reddy 1996):

  1. 1.

    Osmotic stress caused by high salt concentration dehydrates the cell membrane, which causes reduction in permeability to CO2.

  2. 2.

    Toxicity caused by Na+ and Cl−. Cl− inhibits N uptake by the roots, hence inhibiting the photosynthetic rate as well (Banuls et al. 1991).

  3. 3.

    The closure of stomata causes reduction in CO2 supply restricting carboxylation reaction as well as the reduction in loss of water through transpiration affecting light-harvesting and energy conservation system (Brugnoli and Björkman 1992).

  4. 4.

    The alteration of the cytoplasmic structure causes a change in enzymatic activity.

  5. 5.

    Salinization enhances the leaf senescence rate and reduces sink activity.

14.4.3 Effect of Heavy Metals

14.4.3.1 Effect of Heavy Metals on Growth and Productivity of Crops

The heavy metals are available for plant uptake as soluble components in the soil solutions (Asati et al. 2016). At trace level some heavy metals like Zn, Ni, Cu, Co, etc. are beneficial for the plant growth. However, high concentrations of these metals in soil become toxic and reduce the growth and productivity of plants. Reduction in growth as a result of changes in physiological and biochemical processes such as decreased growth rate, chlorosis, necrosis, altered stomatal action, leaf rolling, reduced flow rate of cations, decreased water potential, alterations in membrane functions, inhibition of photosynthesis, respiration, altered metabolism, ultimately leads to food insecurity (Ashfaque et al. 2016; Hasanuzzaman et al. 2018). The direct effects of high metal concentration include inhibition of cytoplasmic enzymes and affect cell buildings due to oxidative stress, resulting in cellular damage (Dubey et al. 2018; Syed et al. 2018). Several researchers found that the accumulation of heavy metals in crop plants is mostly concerned with the probability of food contamination (Nazir et al. 2015). Though several heavy metals like Cd, As, Hg, etc. are not essential for plant growth, they are readily taken up and accumulated by plants. Arif et al. (2016) found the presence of heavy metals in both forms, i.e., essential and nonessential in the environment. Essential heavy metals such as magnesium (Mg), iron (Fe), manganese (Mn), and zinc (Zn) play a beneficial role in plant growth and development. A trace level of these beneficial elements improves the plant’s nutritional level and also several mechanisms for the normal growth, productivity, and better yield of plants.

Connor et al. (2018) reported that the pH of paddy soils in southern/eastern China has decreased due to excessive use of fertilizers and consistently increased available heavy metal concentrations and thus, greater contaminant enrichment in crop production. The bioavailability determines contaminant availability to a receptor and the level of risk in land management. Toxic heavy metals are widespread throughout the world and cause acute and chronic toxic effects on plants grown in such contaminated soils (Yadav 2010). For example, Cu is a vital metal for the growth and development of plants; however, it becomes potentially toxic at higher concentrations (Asati et al. 2016). Su et al. (2014) reported when copper content in soil is more than 50 mg kg−1, it affects citrus seedlings and if soil copper content is 200 mg kg−1, wheat suffers from wilting. On the other hand, Zn is the main functional elements to produce plant chlorophyll and also helps in metabolic, growth and development, generation of oxidative properties (Arif et al. 2016; Andresen et al. 2018). A study conducted by Anwaar et al. (2015) indicated that exogenous Si application improved the growth and development of cotton crops suffering from Zn toxicity stress by restricting Zn bioavailability and oxidative damage. The high levels of Hg in agricultural lands may result in Hg toxification in the entire food chain. The Hg always exists in the forms of HgS, Hg2+, Hg°, and methyl Hg. Therefore, the ionic form of Hg is always predominant in agricultural soil. Cr has been reported to cause oxidative stress that involves stimulation of lipid peroxidation in plants resulting in abrupt damage to cell membranes (Hayat et al. 2012a, b). The Pb uptake by plant roots reduced the crucial forms of precipitates with phosphates, sulfates, calcium, and chemicals in the rhizosphere (Fahr et al. 2013). Ni is also toxic to the plants, especially in ionic form like Ni+2 in soil and caused various physicochemical alterations like inhibition in growth and developments, photosynthesis rate, decreased chlorophyll content, and diverse toxicity symptoms such as leaf chlorosis, necrosis, and wilting in different plant species, including rice (Sachan and Lal 2017). Farid et al. (2015) examined the Cd, Pb, and Ni concentrations in soil 0.111 ppm, 0.87–8.97 ppm, and 0.017–1.72 ppm, respectively, at 0–15 cm while 0.88 ppm, 0.43–6.77 ppm, and 0.055–0.852 ppm, respectively, at 15–30 cm. Cd, Pb, and Ni concentrations in the plants ranged 0.00–2.25 ppm, 1.11–5.29 ppm, and 1.51–4.96, respectively, also transferred to the plant tissues ranged 0.00–2.25 ppm 1.11–5.29 ppm, and 1.51–4.96, respectively. Pb and Ni concentrations were found below permissible levels but Cd was found above the permissible levels in plants as well as in groundwater. Considerably, heavy metal toxicity varies from different plant species, concentration, definite metal, chemical forms, and soil solutions and other soil properties (Shah et al. 2010; Ackova 2018). Table 14.5 shows the impacts of heavy metals on different plant productivity and their growth.

Table 14.5 Effect of heavy metals on growth and productivity of crops
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14.5 Application of Biochar for Mitigation of Abiotic Stress

14.5.1 Mitigation of Drought Using Biochar

Restoring or improving soil water holding capacity is one of the only few options to improve soil productivity to achieve good crop production during the times of drought. Choosing a better management strategy among numerous others is very important as it can play a great role in restoring soil’s water and nutrient holding capacity (Baronti et al. 2014). A recent advancement has been made in the field of biochar that can effectively achieve the task of improving soil characteristics such as its water holding capacity. Using biochar has been considered one of the most practical and sustainable way to achieve and increase fertility of the soil, including water holding capacity. Several advantages of biochar such as long residence time in the soil large surface area in comparison to other amendments to improve soil quality make it much more preferable than other options throughout the world (Lehmann et al. 2011). Biochar are usually prepared in the absence of oxygen and under high pressure and temperature (above 250 °C), where the end product is a carbonaceous material with very high stability mainly attributed to recalcitrant aromatic structures of biochar molecules (Wardle et al. 2008; Biederman and Harpole 2013). The biochar has been proven to increase the moisture-holding capacity of the soil mainly attributed to its larger surface area and to recalcitrant aromatic structures of biochar molecules which also makes it suitable for improving agronomic performance under various climatic conditions including dry conditions (Biederman and Harpole 2013). It also possesses smaller pore size that allows them to hold capillary water for longer period of time making it suitable for application in drought-affected areas. Biochar are considered to have high potential to change soil characteristics including pH, nutrient retention, and overall fertility; however, the capacity of change varies greatly depending on the physicochemical properties of biochar itself including feedstock material, pyrolysis temperature and pressure, specific surface area, and method of application in the field (Baronti et al. 2014). As mentioned previously, improving soil water holding capacity and moisture retention capacity are mainly attributed to the porosity and large surface area of the biochar and its application in the drought prone areas, there is a lack of common consensus among researchers of a typical type of biochar to be used in these areas as moisture retention capacity of applied biochar mainly depends upon the manufacturing procedure.

The effect of biochar also improves soil quality and makes it suitable for plant growth during various periods including germination to adulthood thereby increasing overall plant growth as it improves the accessibility of roots to soil water and air. The application also improves response of soil to soil moisture, size of aggregate, texture, soil matrix, dynamics, cation retention, and permeability making it a much more preferable option.

14.5.1.1 Application of Biochar with Microorganisms to Mitigate Drought Stress

Egamberdieva et al. (2017) reported an increase in biomass, growth, nodulation, and nutrient uptake (phosphorus and nitrogen) in lupin (Lupinus angustifolius) under water-scarce conditions when biochar is applied with microorganisms in comparison to solely applied microorganism’s inoculation. The study also showed a considerable increase in relative water and chlorophyll content in comparison to the controlled ones. Similarly, Liu et al. (2017) observed that application of biochar with Rhizophagus irregularis decreased the water use efficiency, nutrient uptake, biomass, and overall growth as compared to control in potato. However, in contrary to the above findings, Pressler et al. (2017) concluded that application of wood-derived biochar with microorganisms had no significant effect on soil biota (including protozoa, bacteria, fungi, and nematodes) under low irrigation conditions.

Mickan et al. (2016) observed that inoculation of biochar with Arbuscular mycorrhizal fungi enhances drought tolerance of the plants by improving its physiological mechanisms such as nutrient uptake, and biochemical properties including osmotic adjustment, hormonal activities, and antioxidant systems. However, application of biochar with Arbuscular mycorrhizal fungi to the agricultural soil encouraged the growth of extra-radical hyphae and also increased mycorrhizal colonization of roots. Water potential was observed to be the same with and without biochar application.

Previous studies done by several researchers have proved that the application of biochar with microbial inoculation may prove itself to be far better helpful in mitigating and preventing drought stress in plants. However, there is no common consensus about the effectiveness and type of biochar to be used in such conditions, so there is a vast possibility of work in the near future and discover more advantages of biochar over other applications for drought mitigation.

14.5.2 Mitigation of Salinity Stress Using Biochar

Biochar has the potential to increase plant biomass and crop yield. Recent findings have shown that biochar treatment can increase crop yield by an average of 10% (Jeffery et al. 2011). Studies have also demonstrated that crop yield after several years of the application still show a significant increase after single treatment of biochar (Major et al. 2010). Salinity affects the total land area of about 7% of the world. It has been estimated that 30% of the irrigation land has been adversely affected by salinization (Chaves et al. 2009; Wicke et al. 2011). If no action is taken to prevent land degradation, up to 69% loss in revenue is estimated with time (Munns and Tester 2008). Charcoal and activated charcoal have been utilized in the industry for desalinization processes as it has high capacity to sorb a variety of salts (Bartell and Miller 1923; Zou et al. 2008). Decrease in biomass leads to lower carbon input that further deteriorates the soil. This is a major effect of salinity (Wong et al. 2009).

14.5.2.1 Effects of Biochar on Saline Soil Properties

Improvement of soil physicochemical and biological properties which is related to the removal of sodium such as leaching, absorption ratio as well as electrical conductivity has been observed with the application of biochar, which in turn assists in the reduction of salt stress (Sun et al. 2016; Drake et al. 2016). Soil enzymatic activity in saline soil varies with the rate of application of biochar, soil enzyme type as well as incubation time. The study shows that the soils’ physicochemical properties have improved significantly (Wang et al. 2014; Bhaduri et al. 2016). It has also been observed that the application of biochar (30 g mm−2) in salt-stressed soil though did not affect the pH of the soil, increased the electrical conductivity of the soil (Thomas et al. 2013). Application of furfural biochar in saline soil resulted in the decrease in pH on the other hand it increased the soil organic carbon as well as cation exchange capacity and available phosphorus in soil (Wu et al. 2014). It has been observed that composted biochar lead to the increase in cation exchange capacity and organic matter, while it decreased the exchangeable sodium and also the pH of the soil (Luo et al. 2017). The application of biochar has been observed to improve soil properties, such as increasing soil moisture, the Na binds with the biochar which results in the decrease in root sensitivity to osmotic stress.

14.5.2.2 Effect of Biochar on Plant Growth under Salinity Stress

As discussed in Table 14.6, many researchers have proven that amendment of biochar in fairly saline soils improves growth of plants, biomass as well as the rate of photosynthesis (Akhtar et al. 2015). In the tidal land soils containing high concentrations of soluble salt as well as exchangeable sodium, the application of rice hull biochar increased the growth of maize as well as its biomass (Kim et al. 2016). Application of biochar under saline irrigation (3.6 dS m−1) increased the growth of tomato as well as increased the total biomass (Usman et al. 2016). A study conducted to estimate the effect of biochar derived from Fagus grandifolia on the growth and biomass production of two herbaceous species namely, Prunella vulgaris and Abutilon theophrasti, under saline condition. It was found that the biomass of both plants increased under salt stress. The effect on carbon profit through photosynthesis, water use efficiency, and chlorophyll fluorescence in both species did not have any significant impact under salt stress (Thomas et al. 2013). Amendment of biochar along with suitable microbial inoculation has been reported to improve growth and biomass production of the plant under saline conditions as compared to the control (Nadeem et al. 2013; Fazal and Bano 2016). The results obtained by Akhtar et al. (2015) depict that application of biochar increases leaf area, shoot, and root biomass, which increases with bacterial inoculation. The studies emphasize that the effect of biochar can be enhanced with the application of symbiotic microorganisms.

Table 14.6 Effect of biochar on plant growth under salinity stress
Full size table

14.5.3 Mitigation of Heavy Metal Stress by Using Biochar

Several organic fertilizers like compost, vermicompost, manure, organic fertilizers, farmyard manure, etc. have been suggested to restore and improve soil quality and crop productivity (Mas-Carrió et al. 2018). Soil amelioration with biochar is the pre-eminent to promote sustainable agriculture because it has ability to nourish the soil and provide vital nutrients to the plants resulting in improved plant productivity. As described in Table 14.7, the application of biochar has capacity to reduce metal bioavailability and immobilized other contaminants also (Yuan et al. 2019). Similarly, several researchers explained that different types of biochar have potential and efficiency to adsorb the toxic agrochemicals and reduce the bioavailability of heavy metals and their uptake by plants (Abbas et al. 2017). Nie et al. (2018) assessed the bioavailability of Cd, Cu, and Pb and health of soil microbial population in the contaminated soil, by using sugarcane dry pulpy residues-derived biochar and they found that application of biochar enhanced growth of Brassica chinesis L. in terms of root and shoot development, control the bioavailability of metal and also enhanced the microbial population. Similarly, Zheng et al. (2015) investigated the effect of biochar (beanstalk and rice straw) on the bioavailability of Cd in contaminated soil and their accumulation into rice crop and they observed that biochar decreased the phytotoxicity of Cd (35–81 %). Hayyat et al. (2016) found that application of biochar in metal contaminated soil showed a significant decrease in metal toxicity and enhancement in the soil nutrient/fertility, plant growth, crop yield, carbon content, etc. Zheng et al. (2017) examined the effects of biochar (rice straw) on Cd-containing soil and their accumulation in lettuce plant. They observed that the level of Cd reduced by 57 % while increasing biochar rates (0, 6, 12, 18 t ha−1) increased shoot length and yield of lettuce plant. Khan et al. (2013) conducted an experiment to check the effects of biochar (derived from sewage sludge) on the rice plants and they found that the biochar increased the shoot biomass, grain yield, and the bioaccumulation of P and Na, decreased the bioaccumulation of N (excluding in grain) and K and decreased the bioavailability of metals (As, Cr, Co, Cu, Ni, and Pb); however, Cd and Zn concentrations were not affected in the plant parts by the application of biochar. Meng et al. (2018) found that the application of biochar (rice straw and swine manure) decrease the heavy metal bioavailability from contaminated soil in the order Pb > Cu > Zn > Cd. Kim et al. (2015) observed that the rice hull biochar significantly reduced the phytoavailability of metal present in contaminated soil and metal uptake by lettuce plant decreased with the increasing biochar application. Xu et al. (2018) found the adding of macadamia nutshell biochar (5% w/w) to the soil decreased Cd and Pb toxicity and increased total microbial phospholipid fatty acids (PLFAs), microbial respiration rate, biomass carbon and microbial availability.

Table 14.7 Mitigation of heavy metal stress by using biochar
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14.6 Conclusions

Along with other conventional and biotechnological interventions, the application of organic manures may also be used to overcome the adverse impacts of abiotic stresses. The application of biochar as an organic amendment has proved to mitigate from drought, salinity and heavy metals. Application of biochar is an environment friendly and economically viable. Its application improves soil physicochemical and biological properties that help the plants to fight with adverse conditions of the environment. It has been found that biochar enhances water holding capacity, increase the nodulation, nutrient uptake, microbial diversity in the soil and thereby increases the growth and productivity of the plants. In the same way, biochar has potential, and efficiency to adsorbs the toxic agrochemicals and reduces their bioavailability, especially heavy metals and ultimately reduces their uptake by plants.