Remediation of Saline Soils by Application of Biochar, A Review

Journal of Environmental and Agricultural Sciences (JEAS). Zaib et al., 2022. 24(3&4):29-36 

Open Access – Review Article

Remediation of Saline Soils by Application of Biochar: A Review

Muhammad Zaib ^1, ⃰, Usama Farooq ¹, Muhammad Adnan ¹, Saleem Sajjad ¹, Zaheer Abbas ¹, Kamran Haider ¹, Noreen Khan ¹, Roaid Abbas ¹, Awon Shahzeb Nasir ², Muhammad Furqan Muhay-Ul-Din ^3
1 Department of Soil and Environmental Sciences, University College of Agriculture, University of Sargodha, Sargodha, Punjab, Pakistan
² Department of Plant Breeding and Genetics, University College of Agriculture, University of Sargodha, Sargodha, Punjab, Pakistan
³ Department of Agricultural Extension, University College of Agriculture, University of Sargodha, Sargodha, Punjab, Pakistan

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Abstract: Globally, agriculture is the backbone of farmers and plays a crucial role in food security, for a world with increasing population and food demand. As we know that the world’s population is increasing rapidly every year, so the supply of food for the alarmingly increasing world population has become a serious problem. Food insecurity is aggravated by changing climatic conditions, soil degradation, and loss of arable lands due to various abiotic stresses, including salinity. Salinity greatly affects the world’s agricultural lands due to various factors such as low availability of fresh and salt-free water, high temperature, etc. The salinity is caused by both primary and secondary processes. Primary salinity is mainly caused by many natural processes while secondary is mainly caused by human activities. Salinity is a land degradation process, characterized by a high concentration of soluble salts in the soil. It can suppress crop growth by influencing various functions and processes of plants, ultimately leading to significant yield reduction. Biochar is an organic-based material that helps to remediate soil salinity by various mechanisms in the soil system. Biochar has the potential to enrich microbial diversity and enhance activity which plays a vital role in the improvement of soil physical, chemical, and biological activities enhancing the productivity of the crops. The present review contains extensive details about salinity and its remediation using biochar.
Keywords: Abiotic stress, biochar, carbon-rich material, carbon sequestration, food security, microbial activity, plant growth, salinity, soil salinity, sustainable agriculture, water availability to plants.

*Corresponding author: Muhammad Zaib, email: zaibch767@gmail.com

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Cite this article as Zaib, M., U. Farooq, M. Adnan1, S. Sajjad, Z. Abbas, K. Haider, N. Khan, R. Abbas, A.S. Nasir, M.F. Muhay-Ul-Din. 2022. Remediation of Saline Soils by Application of Biochar: A Review. Journal of Environmental & Agricultural Sciences. 24(3&4): 29-36. [Abstract] [View Full-Text] [Citations]

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Copyright © Zaib et al., 2022. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium provided the original author and source are appropriately cited and credited.

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1. Introduction
In the global landscape, agriculture dominates with around 4.7 billion hectares (i.e., 38% of the global land surface area) area under agriculture. Approximately one-third (around 1.6 billion hectares) is used for crop cultivation (FAO, 2022). Area under cultivation is rapidly decreasing due to urbanization, industrialization, soil degradation, climate change etc. (Winkler et al., 2021). Increasing food demand is mainly due to population increase, limited arable land, competition for resources, industrialization, market demand and other factors that led to the extensive prevalence of monoculture in agriculture (Calicioglu et al., 2019; Falcon et al., 2022).

Monocultures, dominate the global arable land and have resulted in various disadvantages, degradation of natural resources (e.g., soil), a buildup of diseases and pests, and productivity and crop yield decline. Moreover, Monocultures are major contributors to chemical contamination, deforestation, depletion of natural resources, loss of biodiversity, soil degradation, greenhouse gas emission (Bennetzen et al., 2016; Tubiello et al., 2022) vulnerability to climate change and overall global environmental changes (Bogužas et al., 2022; Bourke et al., 2021; de Groot et al., 2021). Recent estimates show that food production is responsible for up to 29% of global greenhouse gas emissions (Panchasara et al., 2021; Xu et al., 2021).

Increasing crop yields to meet increasing food demand is becoming a challenging task due to various abiotic stress (i.e. salinity, drought, heat, cold, etc.). It is important to reduce the effects of various stresses that affect crop productivity to meet increasing food demands (Falcon et al., 2022; Winkler et al., 2021).

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2. Salinity
An increased concentration of salts, beyond a certain threshold, in both water and soil, is called salinity. It can cause water stress for the plants, which in turn affects their growth and development (Askari-Khorasgani et al., 2021; Eswar et al., 2021). Globally saline soils are a major challenge for agriculture, affecting crop yields and compromising the ability of farmers to grow crops in affected areas. Arid and semiarid areas are characterized by high temperatures, low precipitation, and high evapotranspiration, salinity can be a major problem for crop production (Hassani et al., 2020; Hopmans et al., 2021).

2.1. Sources of Salinity
Salinity can accumulate in soils through a variety of mechanisms, including irrigation with saline water, the use of salt-based fertilizers, and natural processes such as weathering and erosion (Olson et al., 2022; Sabino et al., 2020). Salinity problems are divided into two categories, primary and secondary (Eswar et al., 2021). In primary salinity, various natural resources are responsible for the formation of saline soils such as parent materials from which salty soil is devolved, salt pans, salt lakes, salt precipitation, salt marshes, and salt flats (Hopmans et al., 2021; Ondrasek and Rengel. 2021).

2.1.1. Natural Sources of Salinity
Changing climatic conditions have caused a remarkable increase in salinity (Rahman et al., 2018). Rising temperature and melting of glaciers have caused significant anomalies in the hydrological cycle and increased the water flow and altered the timings of water availability (Cunillera-Montcusí et al., 2022; Lassiter, 2021). Moreover, climate change has caused increased intensity, frequency and duration of extreme hydrological events (droughts, floods) (Tabari et al., 2021; Yang et al. 2021). These changes are causing seal level rise leading to salinity in coastal areas by altering the balance between saltwater and freshwater. Rising sea levels can shift the boundary between saltwater and freshwater upstream, causing seawater intrusion, imbalanced nutrient accumulation and freshwater salinization making it less suitable for human consumption or agriculture (Kaushal et al., 2021; Heiss et al., 2022).

2.1.2. Anthropogenic Activities and Salinity
Human activities have the greatest impact on the soil by reducing various soil properties and thereby altering the hydrological balance between different water uses. The process of soil degradation by human activities is called secondary salinization. Human activities play a major role in salinization (Mohanavelu et al., 2021). Ondrasek and Rengel. 2021). Land use-land cover change, such as urbanization, deforestation, and land clearing for agriculture can alter the natural water cycle, increases the rate of evaporation and lead to soil degradation and salinization of soil and water resources (Bhardwaj et al., 2019; Glatzle et al., 2020; Maertens et al., 2022; Yin et al., 2022). Irrigation with saline water can cause salt accumulation. If this practice continued for a longer period, it would aggravate the salinity issue. Moreover, inefficient drainage, over-irrigation and runoff from agricultural lands can also contribute to the salinity development of nearby water bodies (Zhao et al., 2020).

Industrial processes such as mining, and chemical manufacturing can release salts into the environment through wastewater discharge, or leaks (Cooper et al., 2022; Echchelh et al., 2018). Municipal wastewater discharge can contain high levels of salt, which can increase the salinity of water bodies and if used for irrigation purposes, it can deposit salts and other chemicals in agricultural soils (Bekir et al., 2022; Ezugbe et al., 2021; Ondrasek and Rengel. 2021).  More than six percent of the world’s land is affected by salinity. Of the world’s 230 million hectares of arable irrigated land, 45 million hectares are currently affected by salinity and of the 1,500 million hectares under dryland agriculture; 32 million are salt-affected to changing amounts (FAO, 2022). The proportion of different countries in the world affected by salinity is shown in Table 1. Serious efforts are required to protect soil and aquatic resources from continued salinization (Hints et al., 2022; Schuler et al., 2019).

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3. Effect of Salinity
3.1. Effects of Salinity on Soil
Under saline field environments, the nutrient interactions at the soil–plant level are complicated and regulated by several factors including soil water movements, accessibility of nutrients, structural stability of soil, humus content, organic matter, redox potential and pH in connection to the dissolution of salts in the rhizosphere (Rengasamy, 2016). Soil salinity can affect soil structure, reduce water holding capacity, disturb overall soil stability, and reduce soil organic content and water infiltration (Gonçalo et al. 2019). Salinity has various effects on soil, but increases in sodium absorption ratio (SAR), pH, exchangeable sodium percentage (ESP) as well as reductions in soil microbial activity and cation exchange capacity (CEC) indicate this condition of the soil (Roy and Chowdhury. 2020). Moreover, hyperosmotic stress in plants occurs when Na⁺ and Cl^– are high in plants and this condition leads to plant water deficit. Soluble salts can irritate plant roots by disrupting water absorption (Maathuis et al. 2014).

3.2. Effects of Salinity on Plants
Plants show various physiological, biochemical, molecular and morphological modifications in response to the increased salt concentrations in their environment (Meng et al. 2018). Salinity negatively influences almost all plant processes including plant growth and plant structure (Singh, 2022), through biochemical and physical disturbances, ionic imbalance and toxicity, nutrient deficiencies, and osmotic and oxidative stresses (Arif et al., 2020). These, salinity-induced abnormalities, causes reduced nutrient and water uptake, and compromised photosynthetic efficiency leading to decreased crop yields are potential hazards associated with salinity (Naz et al., 2021; Yasir et al., 2021). Around 500 species (i.e., 0.14% of the global floristic diversity), can survive saline conditions (Isayenkov, 2019). Among these salt-tolerance species, a few crops are also included, like quinoa, triticale, oats, wheat, barley and sorghum (Barros et al., 2021; Meng et al., 2018).

The presence of salts can affect plant reproduction and productivity due to the complex interactions between soil physico-chemical properties and plant morphological and physiological characteristics (Akbarimoghaddam et al. 2011). Salinity reduces soil water potential, hindering water and nutrient uptake. Plants collect salts together with the water they use and often accumulate Na⁺ and Cl^– ions in plant cells to toxic levels (Maathuis et al. 2014). Furthermore, cellular enzyme activity can be disrupted due to ionic imbalances, nutrient toxicity and water stress (Naz et al., 2021). These factors lead to different responses in plants, manifested by a variety of symptoms in cells and organs (Figure 1).

Fig. 1. Soil salinity classes based on EC (electrical conductivity) of the soil. Non-saline (EC 0-2 dSm^-1), slightly saline (EC 2-4 dSm^-1), moderately saline (EC 4-8 dSm^-1), strongly saline (EC 8-16 dSm^-1), extremely saline (EC >16 dSm^-1) (based on Richards 1954).

A reduction in respiration characterizes stressed plants, which also show an alteration in the distribution of assimilations, a process of inhibited photosynthesis and a weaker production of new leaves. At the same time, there are increased morphological changes in the organs (thickening and succulence of the leaves, reduced length of the internodes), wilting, drying and even necrosis of the organs and entire plants.  In addition, growing crops in saline areas can be detrimental to their appetite (Kumar and Verma, 2018). Rogers (1997) tested the

The impact of salinity on plants can be classified in three categories: first of all, salts impair water absorption from the soil, exposing plants to water stress which can result in retarded growth and decreased yield. Secondly, excessive absorption of certain salt ions like Na+ and CI- can cause toxicity in plants and consequently damaging to the internal structures of plants, affecting physiological processes and frequently reducing plant growth, leaf burn, or even plant demise. Thirdly, imbalanced ions due to increased uptake of salt ions might inhibit the availability of other key nutrients (e.g. potassium, magnesium, nitrogen, phosphorus). Thus salt influence on plant growth through various systems, increasing the soil osmotic stress, hyperaccumulating certain ions in plant tissue, and changing the plant nutrient dynamics (Askari-Khorasgani et al., 2021; Tanu, 2022).

3.3. Effect of Salinity on Microorganisms
Soil salinity is a major problem that can reduce soil microbial community, enzymatic activity, soil respiration rate, and the bacteria growth of the soils (Naz et al., 2021; Tripathi et al. 2006). Osmotic and specific ion effects are the two basic processes of soluble salts that can affect soil microbes. Soluble salts enhance the osmotic capacity of soil water, which pull out water from the cells. Osmotic effect and specific ion effect are the two basic processes of soluble salts that can affect soil microbes. Earth’s osmotic potential increases due to soluble salts. Leaking water from the cell causes the cell to shrink and can kill microbes to get water out of the soil. If the osmotic potential is low, then that condition can also make it difficult for the plant roots and microbes to get water from the soil (Grossiord et al., 2020).

Microorganisms play a vital role in plant productivity, soil structure and various function of soil. Microbes of soil comprise bacteria, fungi, archaea, viruses and protozoa (Mahmood et al., 2021). Microorganisms participate in ammonification, oxidation, nitrification, nitrogen fixation and other activities which lead organic matter to decompose and deliver nutrients to plants to maintain their life cycle (Zhang et al., 2019). Microorganisms can also accumulate carbon and other nutrients in their biomass which are converted into inorganic material which can enhance soil fertility after cell death by living microbes (Yan et al., 2015).

Excessive salt concentrations can affect the structure and composition of soil microbiota (Liang et al., 2022). Since the salt-affected soils have low osmotic potential, different microbial genotypes can survive on low osmotic potential. Most of these die at low osmotic potential (Llamas et al. 2008). Fungi is more susceptible to osmotic stress than bacteria (Mandeel, 2006). Salt stress has a negative impact on soil microbial community (Mahmood et al., 2021; Zhang et al., 2019).

Soil microorganism plays a vital role in the regulation of ecosystem functions (Singh and Gupta, 2018). Microbial biodiversity in saline soils is most affected by other factors such as pH and nutrient availability (Lozupone and Knight, 2007). Respiration is frequently used to measure the microbial activity in the soil (Yan and Marschner, 2013), according to the rate of degradation of organic matter, an increase in the rate of respiration in the soil causes stress (Mamilov and Dilly, 2002).

3.4. Effect of Salinity on Humans Health
Salinization of land and water has been increasing over the last few years due to rising sea levels (Hassani, et al., 2020; Lassiter, 2021; Singh, 2022). Salinity has become a major problem for people living in coastal areas (Rahman et al., 2018; Tanu, 2022). Increasing salinization of water cause problems such as lack of drinkable water, irrigation, agricultural problems, high blood pressure, and kidney malfunctions by increased salt consumption through drinking saline water (Kaushal et al., 2021). Therefore, they are at higher risk of high blood pressure and many other diseases than people who live in cities (Rasheed et al. 2014). According to the WHO, many people in coastal regions take more sodium than the desirable daily intake (>5 g/day) (Rasheed et al. 2014). There is a strong relationship between blood pressure and Na (Aburto et al. 2013). One-third of the world’s deaths and three-quarters in economically weaker countries are associated with cardiovascular diseases, mainly due to increased consumption of soluble salts (World Health Organization, 2015). Higher salt concentrations in the groundwater also significantly negative impact on human health. People living in coastal areas have money to spend, but there are no fresh fruits or vegetables in the markets. As a result, coastal communities are forced to eat locally grown food, with high salt concentrations (Rasheed et al. 2014).

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4. Soil Amendments
A variety of organic and inorganic amendments can be used to maintain or improve soil productivity. Organic amendments are known to provide macro and micronutrients that are essential for plant growth. Animal manure, crop rotation and plant residues are essential to maintain soil fertility and this phenomenon can support the soil to provide various nutrients (Khan et al., 2022). These organic amendments can provide long-term soil N, especially at the rate of mineralization of the organic matter incorporated. Both macro and micronutrient content are highly dependent on the organic sources and quality of organic amendments (Quilty and Cattle, 2011).

However, due to world population growth, necessary to meet increasing global food requirements, many organic amendments have been replaced by inorganic fertilizers, which are more expensive than organic amendments. Synthesized fertilizer helps plants store their own nutrients, also helping plants obtain the nutrients they need from the soil. Organic amendments can be easily adopted on a small scale (Jensen et al. 2011). However, excessive application of fertilizers leads to soil degradation, can also affect biodiversity and is also detrimental to the environment. However, mostly, when various chemicals such as nitrate, potassium and phosphate, etc. can cause groundwater contamination, which is harmful to humans (Mohanavelu et al., 2021).

4.1. Biochar and Soil Characteristics
Biochar is a major source of carbon, derived from the burning of plant-based materials at high temperatures in the absence or little presence of oxygen. Application of biochar in soils has become an important source to provide carbon-rich material. Since biochar is a major source of important nutrients that are essential for plant growth and soil structure, biochar has been used as a soil amendment to increase soil fertility for the past few decades (Guo et al. 2020). Utilization of biochar under various abiotic stresses is a beneficial and important source of nutrients to enhance fertility and maintain various morphological processes of plants (Kim et al. 2016). The physicochemical properties of the soil have also been upgraded using biochar (Niazi et al. 2016).

Biochar is obtained by pyrolysis, in the absence or low presence of oxygen, of plant or animal-based materials at a high temperature of 300-600℃. Once incorporated into the soil, this material increases soil carbon storage as the material is thermally transformed (Sakhiya et al. 2021). An internal structure like graphite biochar is considered a high-quality biochar that can store carbon in soil over thousands of years (Sakhiya et al. 2021). Biochar is applied to soil as a soil conditioner because it can be used as a plant life promoter in the soil (Lehmann et al 2011). Field experiments with biochar application in soil have benefited agricultural productivity.

4.2. Biochar‑based Materials
There is a growing interest in the application of biochar for soil amendments, soil fertility enrichment and carbon sequestration (Haider et al., 2022; Karim et al., 2022; Saifullah et al., 2018). Biochar is a plant-based material that comes into existence by the burning of different plants based materials at a high temperature in the presence or little existence of oxygen. In the past, researchers have done a lot of work on increasing the efficiency of biochar, and these efforts are currently ongoing. This review will focus on different types of biochar that remove pollutants from soil and water. There are various materials that are used to enhance the performance of biochar such as compounding, doping and chemical activation of the material. Biochar is produced from plant-based materials like wood, grasses, crop residues etc. at a high temperature by the process of pyrolysis. There are various sources of biochar such as animal manure, twigs, fruit pits, crop straw and residues, forestry waste and some bag and food waste (Parthasarathy et al., 2022; Rahman et al., 2020; Sigua et al., 2014).

Biochar is produced by biomass and is a low-cost product used in arable lands to improve soil physiochemical properties of soils (Shakoor et al., 2021). Different organic materials made biomass such as agricultural residue, forest residues, sludge, and food waste. This waste material is readily available all over the world and is found everywhere. Disposal of this type of bio-waste is a problem, so converting this type of waste into biochar is a viable strategy. Two methods of biochar production are hydrothermal and pyrolysis. The pyrolysis method is mainly used in the production of biochar. The quality of biochar depends on the properties of biomass used for biochar preparation (Premarathna et al., 2019).

Mostly biochar is produced from plant-based materials which are cheaper, easily available and nutrient-rich (Singh et al., 2015). Millions of tons of residual waste are generated every year due to the influence of human activities. Low-cost biochar is produced due to the presence of large amounts of cellulose and lignin in biomass. Biochar can decrease pollutants through the porous structure of biochar (Haider et al., 2022).

Salt-infested soils often suffer from malnutrition and low use of organic compounds. As a result, this problem can decrease crop yield (Mahmoud et al. 2019). The presence of phosphorus in salt-affected soils has reduced the productivity of fields (This reduction in plant yield is due to the precipitation of phosphorus with Ca ions) (Penn and Camberato, 2019). So, the presence of P in saline fields and how to maintain it is a serious problem today. Most of the arid and semi-arid regions of the world are highly affected by mineral salts and these salts are also known as plant stress (Most of the hot regions of the world are more affected by salinity due to less rainfall and less water availability for leaching of salts from the root zone). A report estimated that more than 7% of the world’s land or 1.1 Gha is highly affected by salinity (Wicke et al. 2011).

Salinity causes various soil disturbances such as uptake of nutrients, membrane permeability and water absorption. Salinity causes various changes in plant metabolism and nutrient balance, hormonal balance, production of reactive oxygen species (ROS) and gas exchange (Munns, 2002). This salinity-induced soil disturbance causes various changes in cell growth and cell division, plant reproduction and plant growth, resulting in plant death (Rezaei and Razzaghi, 2018). Salinity affects the internal processes of various plants as well as the external structure. Salinity disrupts plant processes such as seed germination, nutrient availability, water retention and nutrient uptake, eventually leading to plant death (Ali et al. 2017).

The application of biochar in saline soils improved the physicochemical properties and soil structure of the soil, the application of amendments also improved the organic content and nutritional status of the degraded soil by salinity, also increasing cation exchange capacity (CEC) by supplying Ca to the soil solution and displacing Na from exchange sites (Mohanavelu et al., 2021). Therefore, biochar improves soil physical properties to enhance water-holding capacity and maintain soil porosity. Moreover, in saline soils, biochar improves activities of soil microorganisms to recover saline soils (Amini et al. 2016). Application of this material in salt-affected soils accelerates the leaching process and thereby this material thus reduces the time required to reclaim salt-affected soils and alleviate the negative impact of salinity on crop production (Yue et al. 2016).

Addition of biochar in the soil increases the stability of organic molecules and improves organic carbon which will support attached soil aggregates with each other for longer periods than other organic amendments with easily degradable molecules (Wu et al., 2017). Moreover, studies have shown that the addition of biochar as an organic amendment to saline soils helps to improve the physiochemical properties of saline soils (Zhang et al. 2019) and biological (Zheng et al. 2022) soil properties are interrelated to each other to remove sodium-like as electrical conductivity (EC), sodium leaching and sodium adsorption ratio (Saifullah et al. 2018). Application of biochar to saline soils in potato fields can reduce salt stress on crops (Mohanavelu et al. 2021). Biochar use in saline soils is more reliable as it can absorb more salts (Na⁺) (Saifullah et al., 2018).

4.3. Impact of Biochar on Soil EC
Massive amounts of water are required for washing out salts from the rhizosphere. A high volume of water used for irrigation avoids the accumulation of salts in the root zone (Mohanavelu et al., 2021). The rate of leaching of soluble salts from salt-affected soils can be enhanced by various organic and inorganic amendments (Palansooriya et al., 2019). Many reports have suggested that biochar application can facilitate salt leaching from salt-affected soils and reduce soil EC (Yue et al., 2016; Lashari et al., 2015). It was reported that the application of biochar in different trials decrease the EC of salt-affected soils. A decrease in EC accelerates the leaching of salts and improves soil hydraulic conductivity and porosity of soil (Yue et al., 2016). Application of amendments in salt-affected soils maintains soil physicochemical properties. Hence, the use of different amendments in salt-affected soils facilitates the leaching of soluble salts. It was reported that EC was reduced by 42% using both poultry manure and biochar (Lashari et al., 2015).

Modification in the EC of salt-affected soils depends entirely on the aging, amount and types of biochar.  In some cases, application of biochar can increase the EC of salt-affected soils if the EC of applied biochar is higher than salt-affected soils. Moreover, the EC of water used for irrigation is also important. If irrigation water has a higher EC, it will lead to the accumulation of salts rather than salt leaching (Yue et al., 2016). Contrarily, the application of biochar with EC lower than the soil EC will facilitate the leaching of salts from the rhizosphere (Mohanavelu et al., 2021).

4.4. Impact of Biochar on Soil pH, SAR
Biochar is an amendment used to improve soil structure. It has been reported in many experiments and there is a large amount of data available on increasing the pH of salt-stressed soils with the application of biochar (Nath et al., 2022). Most of the experiments have been conducted in soils with low pH using biochar (Liu et al., 2017), also reported that in high pH soils, the results of using low pH biochar would be just the opposite (Liu et al., 2017). While various previous studies have shown that applying biochar to saline soils reduces the pH of saline-affected soils (Wu et al., 2017) the mechanisms for lowering pH are unclear. It is due to the exchangeable sodium percentage (ESP) associated with saline-sodic and high pH of sodic soils. Application of biochar in saline soils reduces the ESP of salt-affected soils, which is responsible for pH reduction (Lashari et al., 2015). Moreover, the initial pH of applied biochar is very helpful for the pH of salt-affected soils pH and this initial pH of biochar is very useful in various changes that occur by the application of biochar in salt-affected soils. It has been observed that soil pH and ESP can be modified by the application of different types of biochar in salt-affected land (Sun et al. 2016) (Table 2). Also, in salt-affected soils (pH 8.4) a measurable change in pH has been observed with biochar (low pH 3.1) application. Therefore, the main reason for the change in the pH of the affected soil is the difference between the pH of the salt-affected soil and the pH of the applied biochar (Murtaza et al., 2021; Nath et al., 2022).

On the other hand, many previous studies show the beneficial effects of biochar on soil sodium absorption ratio (SAR) and exchangeable sodium percentage (ESP). Several trials reported that the application of biochar under salt conditions can reduce the SAR/ESP of affected soils (Sun et al., 2017). The interaction of Ca and Na in soil depends on the value of SAR and the type of biochar that can increase the content of Na and Ca. The effect of biochar on SAR is controlled by the rate and type of biochar (Kim et al., 2016). High levels of Na-containing biochar can increase SAR or ESP of salt-affected soils (Zheng et al., 2022). Consequently, before the incorporation of biochar in salt-affected soils it is recommended that the pre-testing process for Na content (Murtaza et al., 2021).

4.5. Influence of Biochar on Soil Nutrient Pool
Due to chemical, physical and biological degradation of salt-affected soils, such soils suffer from nutrient deficiency, imbalanced nutrient status, and ion toxicity (Arif et al., 2020). High concentrations of Na in salt-affected soils can reduce the uptake of essential nutrients by plant roots (Maathuis et al. 2014). Salt-affected soils are generally low in organic matter from plant biomass and sodium, phosphorus and potassium for various reasons (Rengasamy, 2016).

Application of soil nutrients under salt stress conditions can improve crop productivity (Olson et al., 2022; Sabino et al., 2020). As biochar is produced by burning plant-based materials, it contains various plant nutrients that are applied to soil after being deposited in salt-affected soil (Ali et al., 2017; Amini et al., 2016). Thus, the application of biochar in salt-affected soils can increase nutrient status and meet plant nutrient requirements (Karim et al., 2022).  Phosphorus (P) is an essential macronutrient and a limiting nutrient for plants (Ros et al., 2020). In soil, P supply is pH dependent, its availability is maximum near pH 6.5. As pH increases it resulted in higher P-fixation by calcium, whereas lower pH causes p-fixation aluminum. As salt-affected soils have higher pH levels and lower organic matter content than optimal soils, high pH limits plant growth due to low availability of P (Penn and Camberato, 2019).

Biochar is used as a source of P and soil organic carbon in salt-affected soils. Therefore, application of biochar in the saline environment increased the availability of P nutrients for the growth of plants (Glaser and Lehr. 2019; Lashari et al., 2013), or can rise the growth of P-solubilizing bacteria in the rhizosphere (Liu et al., 2017). Carbon-rich materials are the result of the decomposition of plant and animal materials under aerobic or anaerobic conditions and are used as a nutrient supplement that enhances soil properties (Mohanavelu et al., 2021). Biochar effects directly soil physical, chemical, and microbial diversity (Sun et al. 2017; Rizwan et al. 2016). Biochar has significant potential to convert hydrogen ions, and the capability to interchange anions or cations, and it is considered a profitable, biologically accessible, and efficient adaptive absorber (Huang et al. 2020). It is a soil modifier and can be used to bind contamination; it can also use to enhance microbes in the fields (Mohan et al., 2014).

Long-term management efforts are required to ameliorate salt-affected soils; hence, transitional or temporary policies to manage salinity can be a suitable possibility to enhance field returns. There are several benefits to using Biochar in small amounts in water (Sun et al. 2017). Since salt stress is known to suppress the productivity of arable lands, we can increase tolerance in plants by using biochar (Fig. 1). Biochar is used as a soil amendment to increase soil fertility. Table 1 summarizes the valuable effects of biochar.

The use of biochar is beneficial in salt-affected soils, which significantly improves the chemical and physical properties of soil by reducing the effect of salts on plants through the release of sodium (Sun et al. 2017). Biochar can improve soil properties in salinity, water retention, drought and temperature stress (Rizwan et al. 2016). reported that the application of poultry manure biochar in maize crop can increase microbial biomass C in saline soils and increase the activity of enzymes that produce ammonia and CO₂ from urea and can also increase invertase enzymes activity in the formation of fructose and glucose (Bhaduri et al., 2016).

The properties of salt-affected soils can also be enhanced with the use of carbon-based amendments (Mohanavelu et al., 2021; Zheng et al., 2022). Though, there is little record of the effects of biochar in salt-affected soils. Soil pH can’t be disturbed by the addition of biochar 30 g/m². However, the use of biochar increases the EC of salt-affected soils as compared to low (Zhang et al., 2019; Zheng et al., 2022). Also, furfural biochar in salt-affected soils reduced H⁺ ions, while increasing the soil organic carbon and cations exchange capacity and accessible phosphorous in the soil (Wu et al. 2017). Furthermore, the current details show that biochar is rich in calcium and magnesium ions and also can improve their accessibility when introduce into the soil as substances that can increase soil properties and ameliorate soil health (Rajkovich et al. 2012). Aggregation of soil particles and soil structure can be improved with the application of biochar, which will increase soil water and nutrient holding capacities, reducing water and nutrient losses (O’Connor et al. 2008).

Application of carbon-rich black material with a pyroligneous solution can reduce pH level, sodium and sodium chloride content in salt-affected soils. Biochar supplementation can also increase soil organic carbon and phosphorus content in salt-affected soils. Biochar can be used to reduce the effects of soluble salts (salinity) or exchangeable sodium (sodicity) through the adsorption of sodium ions (Lashari et al., 2013). In potatoes grown under salinity stress, biochar application can reduce salinity stress with the help of high Na⁺ adsorption capacity of biochar materials (Saifullah et al., 2018; Lashari et al., 2014).  Biochar can stimulate microbial activity, as it is a source of carbon for microbial activity, and it is more degradable. It can support small amounts of soil microbes. However, the release of short-term labile pyrolysis products can stimulate topsoil organic activity (Lehman et al. 2011).

Biochar is a good carbon source used for carbon enrichment or carbon storage. Another viable purpose is to supply a rich source of carbon in the soil for living organisms for the significant enhancement in microbial activity in saline fields after the incorporation of biochar. Earlier statements on the effects of carbon-rich material on soil microbes carbon are inconsistent. In certain trials, amendment with carbon-rich material had no significant impact whereas the introduction of carbon-rich material (biochar) remarkably can reduce microbe’s carbon (Palansooriya et al., 2019; Zheng et al., 2022).

Carbon-rich biochar has significant impacts on soil microbes. Even microbial biomass carbon reproduces some variations in soil organic material and decay. Therefore, any methods and resources that enhance or reduce soil carbon material can disturb biomass and MCA. Therefore, biochar is a resource of carbon that can store extra carbon for soil microbes and enhance the total carbon of soil microbial activity (Zhang et al. 2019).

Table 2. Effects of biochar on plants grown under saline conditions

4.6. Role of Biochar in Carbon Sequestration and Climate Change Mitigation
Emissions of greenhouse gases from various human activities have increased over the past few decades. Production and application of carbon-rich materials in soil have achieved great significance (Hong et al., 2021; Woolf et al. 2010). Biochar is a highly porous material with significant carbon sequestration potential, also for long periods of time. These properties make it a potential tool for climate change mitigation (Gross et al., 2021; Lehmann et al. 2006; Xu et al., 2021). The carbon sequestration potential of biochar depends on the type of material used for its production, production process involved, and the soil conditions where it is applied. Improved soil fertility, water retention, and enhanced plant nutrient uptake through biochar application can reduce dependence on synthetic fertilizers, and thereby reduce the emission of greenhouse gases (Khadem et al., 2021; Sarfraz et al., 2019; Zhang et al., 2019). 

4.7. Impact of Biochar on Microbial Community
High-stability of biochar is key for its prolonged retention and delayed decomposition in the soil through the activity of soil microorganisms (Wu et al. 2017). These soil microbes play an important role in the soil such as nutrient cycling, and fixation, P-solubilization, etc. Soil microbes perform important ecological services including recycling different fractions of organic waste. These microbes play an important role in the process of mineralization, immobilization, etc. The community and diversity of soil microorganisms have a significant impact on soil quality, structure, soil health, soil productivity, soil fertility, and ecosystem function (Khadem et al., 2021; Zou et al. 2017). Biochar has a direct effect on the soil microbial community. It can increase the diversity, number, and activity of soil microbiota in carbon-rich soils (Liu et al., 2017; Zhang et al. 2019). Scientists are interested to enhance the properties of biochar which are essential for soil health (Sun et al. 2017; Rizwan et al. 2016).

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5. Conclusion
Salinity is the most emerging problem all over the world. Soil salinity has various negative effects on performance and productivity. Various inorganic and organic amendments are used to treat salt-affected soils. Biochar is a promising choice for soil reclamation, its application to soil can improve the properties of saline soils and promotes plant growth. This method is inexpensive and can be easily adapted on both small and large scales. Biochar is a rich source of nutrients, essential for the proper growth of plants, maintaining the photosynthesis process, improving nutrient uptake and reducing sodium uptake. Biochar, a carbon-rich material, enhances the microbial community in saline soils. More studies are required to evaluate the role of biochar under both drought and salt stress conditions. Soil salinity can be reduced by avoiding over-irrigation, using good quality water for irrigation and mulching the soil to maintain soil temperature which helps soils retain water for longer periods and reduce evaporation rates.

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Competing Interest Statement: None of the authors have any competing interests.

List of Abbreviations: EC, Electrical conductivity; SAR: Sodium Adsorption Ratio

Acknowledgment: Authors would like to thank Dr. M. Imtiaz, Soil and Environmental Biotechnology Division, National Institute for Biotechnology and Genetic Engineering (NIBGE), Faisalabad, Punjab, Pakistan, for his help in writing and reviewing the manuscript, and moral support to complete this manuscript.

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