Keywords

1 Introduction

Agriculture has faced numerous challenges in the twenty-first century. It needs to produce more food and fibre to feed a growing population with shrinking natural resources. The world’s population has exploded in the recent century, reaching 8.6 billion in 2030 and 9.8 billion in 2050, with Africa playing a large role (Foresight 2011). The vast bulk of the world's population (60%) lives in Asia (4.4 billion people), followed by Africa (1.2 billion people), Europe (738 million people), Latin America and the Caribbean (634 million people) and the rest of the world (5%) (Table 1). China (1.4 billion) and India (1.3 billion) are the world's two largest countries, accounting for 19% and 18% of the global population, respectively (UNDESA 2015).

Table 1 Population of the world by region (UNDESA 2015)
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Food security is a relevant and important issue for many countries, with a particular focus on developing countries. There is growing concern about the world's future food consumption and supply. The global food system is under an unparalleled convergence of forces, which is expected to worsen by 2050 (Foresight 2011). Increased food production will necessitate more inputs, such as land, water or energy, or a combination of these. By 2050, demand for cereals for food and animal feed is expected to reach 3 billion tonnes, up from almost 2.1 billion tonnes currently. According to the forecasts, feeding a world population of 9.1 billion people in 2050 will necessitate a 70% increase in overall food production between 2005/07 and 2050. In poorer countries, production would have to nearly quadruple. This would result in considerable increase in the production of a number of important commodities. Annual cereal production, for example, would need to increase by about one billion tonnes, and meat production by more than 200 million tonnes, to a total of 470 million tonnes in 2050, with 72% of that in developing nations, up from 58% today. To sufficiently feed the world's population, it would be necessary to produce the types of foods that are currently unavailable in order to provide nutrition security (UNEP 2014).

India (24.3%) and China (16.7%) account for a major portion of world food demand (Dotaniya et al. 2022a). Because of rising incomes, emerging countries will seek more animal origin foods in the future. The rate of increase in global cereal demand will slow until 2050. Cereals account for the majority of global food demand (49%) and will continue to do so until 2050. Global cereal demand grew at a constant rate of 1.3% per year from 1969 to 2007 and is expected to dip further to 1.2% in 2030 and 0.9% in 2050, despite a large increase in world cereal demand from 940 million tonnes in 2005/2007 to 3 billion metric tonnes by 2050. Developing countries will account for nearly all of the increase in cereal consumption. The top 20 nations account for about 77.5 of global CE food demand (Table 2).

Table 2 The top 20 countries that influence global CE food demand (Emiko and Will 2017)
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2 Soil Fertility Status of Indian Soils

Increased soil erosion, declined soil fertility and reduced biodiversity at the local level, depletion and pollution of groundwater and eutrophication of surface waters at the regional level, and changes in atmospheric composition and climate at the global level are all known negative effects of agricultural intensification. Ramamurthy and Bajaj (1969) produced the first systematic soil fertility map of Indian soils in 1967. Around 4% of samples were rich in available P at the time. According to a soil fertility map released in 2002 (Motsara 2002), roughly 20% of soil samples have high levels of accessible P. Muralidharudu et al. (2011) used GIS to create district-level soil fertility maps in India, which revealed that 57% of districts had low available nitrogen, 36% had medium and 7% had high. In terms of available P, around 51% of districts had low soils, 40% had medium soils and 9% had high soils. The soils of roughly 9% of districts had low available K status, 42% had medium available K status and 49% had high available K status. Farmers’ indiscriminate use of phosphatic fertilizers has resulted in excessive P levels in some soils. Because Indian soils have a low-to-medium organic matter level, the nitrogen deficit is likely to persist. The three estimations of soil fertility for K show that the percentage of samples that test high has risen over time (Ramamurthy and Bajaj 1969; Ghosh and Hasan 1980; Motsara 2002).

Different soils have their own set of difficulties that prevent them from obtaining long-term high production. Acidity in hill and laterite soils; nutrient leaching in sandy soils; nutrient fixation in red, laterite and clayey soils; obstructed drainage in swell-shrink soils; N volatilization losses from calcareous soils; Zn deficiency in low organic matter, sandy, high pH, high-water table soils; and B deficiency in highly calcareous leached acid soils. Zn deficiency was found in 49% of the soil samples tested, followed by S (41%), Fe (12%), Cu (3%), Mn (4%) and B (4%) (32% in some selected areas such as Bihar). In the case of P (15–20%), N (30–50%), S (8–12%), Zn (2–5%), Fe (1–2%) and Cu (1–2%), the current state of nutrient usage efficiency is fairly low (1–2%). In India's intensive cropping systems, fertilizer partial factor productivity is dropping.

Soil organic matter is essential for maintaining soil fertility. Organic matter status of soil in Alfisols of Ranchi deteriorated over time in the soybean-wheat system due to a lack of balanced fertilizer input. In the soybean-wheat system at Jabalpur, balanced fertilization with NPK and NPK + FYM increased the organic matter status in Vertisols. During the last three and a half decades, the partial factor productivity of fertilizers has decreased, from 15 kg food grains/kg NPK fertilizer in 1970 to 5 kg food grains/kg NPK fertilizer in 2005. In the rush to increase production, little thought was given to long-term soil quality and high productivity. As a result, main crop annual compound growth rates have decreased from 3.36% in 1981–85 to 0.11% in 2001–05. Deterioration in soil quality causes a drop in partial factor productivity and compound growth rates of key crops in intensive cropping systems as well as low nutrient use efficiency. Continuous cropping reduces organic C levels by 50–70%, bringing them closer to the climatic and precipitation-driven equilibrium values. The following are the primary causes of soil quality degradation: a large nutrient gap between demand and supply; high nutrient turnover in the soil–plant system combined with low and imbalanced fertilizer use; emerging deficiencies of secondary and micronutrients in soils; soil acidity; nutrient leaching in sandy soils; nutrient fixation in red, laterite and clayey soils; and obstructed drainage in Swedish soils.

3 Heavy metal

Metals are necessary for the survival of all living species on the planet. These are required for the plant, animal and human life cycles to be completed. Deficiency of these metals in a living body can cause a variety of symptoms or limit an organism's capacity to develop and operate (Minhas et al. 2021; Dotaniya et al. 2022b). Some metals are required for life, but if their concentration in the body exceeds their required concentration, they become poisonous (Dotaniya et al. 2014a). These metals are divided into groups depending on their toxicity, origin and concentration in the living body, among other factors. Several metals have been found harmful to human health, and their introduction into the human body via food chain pollution requires scientific supervision. Heavy metals have a geogenic origin and are distributed by anthropogenic activities such as industrial waste, geochemical structure, agriculture and mining (Table 3). These metals form compounds with other ambient elements, making them more harmful to living things (Singh et al. 2020). The toxicity of a metal is determined by the metal's type, chemical form, climatic variables and other factors. Heavy metals including As, Cd, Cu, Pb, Ni and Zn are prevalent contaminants that originate from both natural and anthropogenic sources (Dotaniya et al. 2018a).

Table 3 Sources of hazardous metals (CPCB 2009)
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3.1 Adverse effects of heavy metals on Plant

According to Arnon and Stout's (1939) essentiality criterion, a plant's heavy development necessitates 17 essential plant nutrients. Micronutrients such as zinc, copper, manganese, iron and nickel are regarded as important plant nutrients, yet larger concentrations of these metals are phytotoxic. Other heavy metals/toxic metals that are known to cause harm to humans and animals include Cd, Cr, Hg and Pb. These metals do not appear to play a defined role in plant physiology, and at high concentrations, they cause plant toxicity symptoms and reduce plant biomass (Dotaniya et al. 2018b, 2019a) (Table 4). Few plants in nature have the ability to modulate heavy metal uptake patterns from the soil–water system, as well as tolerance to a specific metal or combination of metals (Dotaniya et al. 2020b). However, these metals have a specific tolerance level, beyond which plants exhibit the same toxicity as common plants. Heavy metal toxicity resulted in the production of reactive oxygen, and the plant's enzyme cofactors are replaced (Sytar et al. 2013). Heavy metal toxicity, according to Saha et al. (2017), has a negative impact on transcription and antioxidative processes, cellular redox imbalance, ionic trafficking, DNA synthesis and damage, and amino acid synthesis. According to Haider et al. (2021), cadmium toxicity changed the uptake pattern of Ca, Mg, P and K from soil and cause water scarcity in plants, resulting in low growth and production. According to Panday and Sharma (2002), heavy metals (Co2+, Ni2+, Cd2+) reduce chlorophyll content, Fe activities and associated enzymes (catalase, peroxidase), as well as chlorophyll heme biosynthesis in cabbage. Ni toxicity was more than Cd and Co toxicity in this experiment (Pandey and Sharma 2002).

Table 4 Adverse effects of heavy metals on plants
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3.2 Adverse effects of heavy metals on human

Toxic metals are found to have a negative impact on the cell membrane as well as mitochondria, endoplasmic reticulum, nuclei and numerous enzymes associated with metabolism, detoxification and damage repair in biological systems (Table 5) (Wang and Shi 2001; Dotaniya et al. 2014b, 2017). DNA damage and conformational changes caused by metal ions have been reported to affect cell cycle regulation, cancer or apoptosis when metal ions interact with cells (Kasprzak 2002; Beyersmann and Hartwig 2008). The production of reactive oxygen species (ROS) and oxidative stress have been implicated in the toxicity and carcinogenicity of metals like As (Tchounwou et al. 2004a, Yedjou and Tchounwou 2004b), Cd (Tchounwou et al. 2001), Cr (Patlolla et al. 2009; Dotaniya et al. 2019b) and Pb (Tchounwou et al. 2004b), among others (Sutton and Tchounwou 2007). Due to their extreme toxicity, these five elements are considered priority metals with substantial public health consequences. Toxins in the system have been proven to damage several organs at low levels. In addition, the United States Environmental Protection Agency (US EPA) and the International Agency for Research on Cancer (IARC) classify these metals as “known” or “probable” human carcinogens based on epidemiological and experimental studies that demonstrate a link between exposure and cancer incidence in humans and animals.

Table 5 Effects of heavy metals on human
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4 Heavy metal pathway in human being

Heavy metal concentrations in soil and their absorption by plants are increased when low-quality inputs are used, impacting the crop's yield and level of heavy metal in soil. Heavy metal concentrations have a negative influence on soil biota and other features (Muchuweti et al. 2006). Health risks are associated with elevated levels of heavy metals in soil and plant components, which can harm humans (Meena et al. 2020a). Toxic heavy metals can enter the human body from the soil through plants (Solanki et al. 2020). This means that identifying the numerous methods by which heavy metals are transported from soil to plants is vital. There are a variety of factors that influence how heavy metals accumulate in agricultural plants (Dotaniya et al. 2020b). Some plants are better at absorbing metal particles than others. For the transfer coefficient, the bio-concentration factor and accumulation factor are utilized (Rattan et al. 2005; Dotaniya et al. 2020c). It is more likely that heavy metal contamination will arise in peri-urban areas with sewage or polluted water (O’ Connor et al. 2018a, b). Heavily contaminated vegetables provide a greater threat to human health than other crops (Dotaniya et al. 2020a; Meena et al. 2020b). Toxic metal ingestion inhibits the operation of other metal ions as well as the immune system, resulting in stunted growth, poor mental development as well as other human impairments (Fig. 1) (Iyengar and Nair 2000; Türkdogan et al. 2003).

Fig. 1
figure 1

Heavy metal pathway in human

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5 Role of Biochar in Agriculture

Farmers in developing countries burn more than 10 billion tonnes of crop waste in their fields each year. This contributes to the release of 16.6 billion tonnes of CO2, 11.2 billion tonnes of CO2e, 1.1 billion tonnes of smog precursors and 65.7 million tonnes of PM2.5 into the atmosphere. Crop waste burning produces enough CO2 and CO2e emissions to equal the annual emissions of 714 coal-fired power plants. Instead of burning the waste, converting it into biochar removes three tonnes of CO2 from the atmosphere for every tonne produced; when added to fields as a soil amendment carbon is permanently sequestered. The use of biochar in soil remediation provides a number of environmental benefits that are consistent with GSR (Shen et al. 2019). For example, (1) biochar can be made from agricultural, household or industrial waste; (2) co-products of biochar pyrolysis, such as syngas and bio-oil, can be used as green energy sources; (3) biochar can help increase soil fertility by adding nutrients or improving soil structure or pH and (4) biochar production converging.

Climate change is threatening global food security. Farmers who use biochar as a soil amendment will benefit in terms of higher yields, healthier soil, lower acidity, better water retention, stronger plants, richer soil life, less contamination, higher fertility and better seed germination (Novotny et al. 2015). Because of its high moisture adsorption capacity, it is extremely useful in reducing mildew in damp areas. Biochar adsorption properties make it an ideal solution for soil pollution remediation (Tripathi et al. 2016). The long-term benefits of producing biochar include a significant reduction in greenhouse gases, which contribute to global warming (Sullivan and Ball 2012). It is widely acknowledged as a powerful solution for reducing global warming.1

5.1 Properties of biochar

5.1.1 Larger surface area

Biochar is a porous carbonaceous material that can hold massive amounts of water. The porous structure of biochar also serves as an excellent habitat for soil microbes. Larger surface areas are helpful for regulating the nutrient concentration in soil solution (Cao et al. 2018; Sakhiya et al. 2020). The pore size of biochar varies greatly, with nanopores (0.9 nm), micropores (2 nm) and macropores (>50 nm). When pyrolysed at 800 °C, biochar made from malt wasted rootlets had a surface area of 340 m2/g and a porosity of 0.21 cm3/g (Manariotis et al. 2015). Manure and biosolid biochar have a substantially lesser surface area (5.4–94.2 m2/g) than plant biochar (112–642 m2/g) such as wheat, oak wood, maize stover and pine needles (Li et al. 2017).

5.1.2 pH value

Plants prefer soil with a pH of 6.5–7, but most soils in the developing world are acidic to very acidic (4–5.5). Even if nutrients are present in the soil, most plants cannot absorb them in acidic soils. The addition of biochar to such soils raises the pH by up to a whole point. More nutrients become available to crops as the pH rises.

5.1.3 Cation exchange capacity

Composting of biochar with calcium, iron, magnesium, phosphorous, potassium and sulphur may enhance the nutrient efficiency. Biochar provides long-lasting, slow-release nutrition effect on crop. Biochar in a heavy metal-contaminated field can make cadmium, lead or mercury chemically bound to it (adsorbed), where plants will no longer take them up and water will no longer wash them away. The cation exchange capacity (CEC) of biochar is an important property for improving soil nutrient retention and reducing fertilizer runoff. However, CEC estimations for biochar in the literature are highly diverse, ranging from 5 to 50 cmol (+)/kg and even reaching as high as 69–204 cmol (+)/kg (Munera-Echeverri et al. 2018).

5.1.4 Carbon substrates in soil

Plants cannot consume their elements in their natural state. They cannot just suck up nitrogen, no matter how much they need it; they need microbes to digest it first and release it out as nitrates or nitrites. Because of its high carbon content (60–90%) (McGlashan et al. 2012), the incorporation of biochar into soils is regarded as a significant and long-term approach to sinking atmospheric CO2 in terrestrial ecosystems. Aside from the benefits of reduced emissions and GHG sequestration, biochar has a number of positive effects on soil quality (Lehmann and Joseph 2015). Adding biochar to soils boosts crop production by improving soil physico-chemical and biological properties such as water retention, pH and microbial activity (Ahmad et al. 2014). Furthermore, because of the lower fertilizer requirements, biochar can help to reduce agricultural emissions from fertilizer use (McGlashan et al. 2012).

6 Remediation methods of heavy metal from ecosystems

On the basis of practicality and resources, several types of remediation technologies are being used to remove heavy metals from the environment (Solanki et al. 2020). Metal concentration, shape and kind, as well as moisture in the soil, have a key role in the removal of heavy metals from ecosystems (Meena et al. 2020b). A brief description is given in Table 6.

Table 6 Heavy metal remediation methods
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6.1 Role of biochar in heavy metal remediation

Biochar comes under chemical remediation strategy for heavy metals removal. It is an environment-friendly method of removing heavy metals from the soil. In addition to metal removal, it also adds nutrient-rich compounds to the soil that enhances crop output (Meena et al. 2021).

Recent biochar research basically explains two methods of adsorption by biochar: first, direct adsorption, and second, improving the physico-chemical properties of soil such as pH, CEC, mineral and organic matter (OM) content (He et al. 2019a, b; Wang et al. 2021). Physical sorption, ion exchange, electrostatic interaction, precipitation and complexation are some of the mechanisms involved in controlling the removal of heavy metals from polluted soils via direct adsorption by biochar (Dotaniya et al. 2016; Inyang et al. 2016). The surface of biochar possesses various functional groups, including hydroxyl, carbonyl and carboxyl (Tan et al. 2015), and their abundance is the most important factor for regulating the sorption-based heavy metal stabilization (Guo et al. 2020). Organic matter in the soil can assist reduction of trace metal availability in the soil solution, which can harm agricultural plant nutrition and growth (Zhang et al. 2017).

Researchers have suggested utilizing biochar as a binder to immobilize heavy metals from soil systems. It is used as a soil conditioner because it is high in organic carbon. Plants that have a higher capacity for water absorption have a better ability to transfer nutrients around. There is also an increase in cation exchange capacity as well as the ability to absorb heavy metals, resulting in a less contaminated soil solution. Biochar functions as a climate change mediator and enhances soil carbon sequestration, according to Hayyat et al. (2016).

For soil microbial populations, biochar application increases the amount of soil organic carbon and food sources that are available to them. By altering microbial community composition (Igalavithana et al. 2017), increasing variety (Cheng et al. 2018), and therefore stimulating particular microbial activities, soil biochemical cycles can be improved, increasing nutrient absorption and crop yield (Hayat et al. 2010). Rhizobacteria (bacteria belonging to the groups; Azospirillum, Enterobacter, Klebsiella and Pseudomonas) can be directly influenced by biochar-induced alterations in plant development (Tu et al. 2020). It has been observed that biochar reduces the accessible heavy metal concentration by 55.5% (Ahmad et al. 2014), while another study found that it reduces acid-soluble Pb2,3+ and Cu2+ by 18.8–77.0 and 19.7–100.0%, respectively.

Pyrolysis temperature of biochar and the chemical composition of the biomass have an impact on the adsorption capacity of biochar (Uchimiya et al. 2011; Xu et al. 2014). Biochar is commonly added to heavy metal-contaminated soil to raise its pH (Seneviratne et al. 2017; Soudek et al. 2017). This is largely due to variations in raw material that results in varying amounts of alkaline ash. Biochar made from mineral-rich raw materials that are pyrolyzed at high temperatures have a high ash content that results in soil alkalization (Cao and Harris 2010; Lehmann et al. 2011). In general, adding charcoal and raising the soil pH reduce the competition for sorption sites between H+ and metal cations (Mn+), lowering the mobility and availability of heavy metals in the soil (Gomez-Eyles et al. 2013).

The efficiency of bioavailability is affected by the biochar's source material, pyrolysis temperature, metal form and concentration and soil characteristics. Biochar incorporation creates various carbon–metal complexes in soil and aquatic systems, lowering metal bioavailability. The coordination of metal electrons to C=C (-electron) bonds in peat moss biochar decreases the mobility and bioavailability of copper, cadmium and lead (Park et al. 2016). Heavy metal bioavailability is reduced due to the presence of biochar functional groups and their affinity for heavy metals. Table 7 lists some of the biochar and their effects on availability of heavy metals in soil.

Table 7 Type of biochar and their effect on bioavailability of heavy metals in soil
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Cheng et al. (2018) demonstrated that an increase in soil microbial richness and diversity, caused by a rise in pH from using biochar, results in favourable physico-chemical changes of the soil as well as an increase in the supply of accessible nutrients. Some of the positive and negative effects of biochar on soil properties are listed in Table 8.

Table 8 Biochar effect on soil properties
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Limitations of biochar applications:

  1. 1.

    High CEC may help in adsorption of heavy metal in biochar and release it during the high crop growth stages by release of root exudates.

  2. 2.

    The present experiments of use of biochar for heavy metals are limited to lab scale. It is critical to make biochar for practical field use. Future research should focus on improving the biochar pyrolysis process to fully exploit its potential for treating metal-contaminated environment.

  3. 3.

    Biochar aged in soil inhibits the growth of earthworms and/or fungi.

  4. 4.

    Use of biochar at high rates (15 t/ha) induces increased weed growth and immobilized plant nutrients.

  5. 5.

    Higher application of biochar may also disturb the soil organic matter decomposition rate by mediating the soil microbial population and diversity.

7 Conclusions and future prospects

A rise in industrial pollution has caused negative impact on soil health and food crop productivity. One of the biggest challenges is heavy metal toxicity, as these are very harmful for the health of environment. Biochar improves soil fertility for sustained crop production and it is an option worth considering to maintain the health of soil. Biochar has a high carbon-sequestration capacity and is climate robust. Larger surface area and CEC improve adsorption capacity, resulting in increased heavy metal immobilization. It also improves soil fertility and nutrient mineralization over time by regulating the variety and count of soil microbes. The stable C pool increased and CO2 emissions from the soil are reduced under biochar application. More study should be done to know the effects of biochar immobilized heavy metals on the rhizospheric characteristics over time.