Biochar-based fertilizers and their applications in plant growth promotion and protection

Abstract

Soil is an integral part of the ecosystem because it serves as a habitat for various microorganisms and lays the foundation for supporting plant growth and development. Therefore, factors such as increased anthropogenic activities hand by hand with other natural processes that harm the ecosystem may eventually lead to a decline in soil quality and fertility, hindering the growth of plants and soil microbial communities. Given the current global scenario of increasing human intervention, it is essential to find effective measures and reliable technologies to restore soil quality. Biochar is an emerging soil ameliorant employed for soil health restoration and is primarily generated through the anoxygenic pyrolysis of biomass. The biochar application in soil remediation may be beneficial due to biochar’s unique physicochemical properties, including high carbon and metal fixation abilities. In addition, biochar possesses abilities to reduce the plant's environmental stress injuries. This review briefly overviewed the ingredients and mechanism of biochar productions. We then emphatically reviewed the advances in biochar applications in soil bioremediation, soil microflora growth stimulation, and the alleviation of various biotic and abiotic stresses in plants.

Introduction

In the coming years, global climate change will become a severe issue following the increase in the human population estimated to cross the margin of 9 billion by 2050 (FAO 2009). Subsequently, an elevation in anthropogenic activities will be recorded, and the present fossil fuel-based economy, which is inevitably going to sustain, will affect the human population at large. Other ecosystem components such as plants as well as the soil ecosystem will also have to face the harsh reality of climate change (Titirici et al. 2015). The combination of stresses, including biotic and abiotic such as pathogen attacks, drought, heavy metal contamination of soil and water, nutrient-deficient soil, high salinity etc., will ensue with the changing climate. These stresses will adversely affect plant growth and development, limiting agricultural productivity and food security (Parihar et al. 2015; Thalmann and Santelia 2017). Elevated atmospheric CO₂ concentrations resulting from rapid fossil burning and utilization has set an alarming bell for climate change. If this rate of increase in CO₂ emissions is not stopped or overlooked, then the CO₂ concentration is likely to cross 590 ppm with an average rise of the global temperature by 1.9 °C by the end of the twenty-first century. This has been reported by the Intergovernmental Panel on Climate Change (IPCC) (IPCC 2007), eventually leading to the collapse of the entire ecosystem.

In the advent of rapid industrialization and elevated anthropogenic activities, the formulation of new techniques for effective remediation of contaminated soil, wastewater and other ecosystem components will have a crucial role to play concerning environmental management and sustainable development. Many techniques have been devised to produce clean energy to mitigate stressful environmental conditions. For example, the production of biofuels through the conversion of organic biomass using several thermal and biological methods (Huang et al. 2013; Luo et al. 2012). The production of biochar is also of potential interest in this regard and gaining much attention from the scientific community. It is a renewable form of energy that can be used as a sustainable, cost-effective tool for carbon sequestration, thereby reducing the atmospheric carbon levels (Fig. 1) (Schmidt et al. 2021).

Fig. 1.

Application of biochar and its beneficial role for plant growth and development. Its amendment to soil exhibit positive effects including lowering of atmospheric carbon levels, stimulation of soil microbes, increase in soil carbon content, imparting drought salinity and metal toxicity tolerance to plants, heavy metal ion sorption in soil, enhancing bioremediation activities in soil and increasing soil fertility for enhanced agricultural productivity

Biochar, also known as bio-coal, is generally produced by pyrolysis of organic material such as biomass and waste derived from forest and agricultural resources in oxygen limiting conditions to yield charcoal-like substances rich in carbon. The application of biochar is not only limited to improving the organic carbon content of the soil, but its application has shown remarkable results in alleviating the adverse effects of drought and salinity stress from plants (Haider et al. 2015; Hammer et al. 2015). Using Biochar, heavy metal can be effectively stabilized and immobilized in soil, particularly in agricultural fields where extensive use of chemical fertilizers takes place along with waste dumped sites where soil heavy metal concentration usually remains high (Rees et al. 2014). Studies conducted to examine biochar’s role on soil microbiota have also revealed the growth-stimulating effects of biochar on soil microbial communities, thereby opening the door to developing cost-effective strategies to enhance soil quality (Yilu et al. 2018). Biochar can influence and modify the physicochemical properties of the soil along with the inhabitant biological components (Garciá et al. 2016; Zhang et al. 2017a). Besides its use in soil remediation, biochar has also shown remarkable results in the decontamination of aqueous solutions, e.g., wastewater (Pi et al. 2015) (Fig. 2). Although before its application, it is crucial to evaluate and characterize the physicochemical properties of biochar for better results of soil health and wastewater management. This review critically examines the role of biochar application in maintaining soil quality and remediation of soil pollutants.

Fig. 2.

An integrated role of biochar in environmental management. From removal of soil contaminants and increasing its remediation to promoting plant growth and development through the stimulation of plant growth promoting soil microbes, soil carbon sequestration, increasing soil organic matter, upgradation of plant tolerance level to combat abiotic and biotic stresses, to wastewater treatment etc., biochar has a wide applicability towards mediating different ecosystem services

Biochar-based fertilizers: an overview of its role in agriculture

Conventional fertilizers threaten the environment in many ways, such as nutrient leaching, global warming, and volatilization from fertilized croplands. Shi et al. (2019) obtained a granular biochar mineral urea composite (Bio-MUC), which was able to retain nitrogen more than the urea fertilizer (UF). Also, the Bio-MUC promoted the growth of the plants by enhancing the shoot growth by 14% and root growth by 25%. Qian et al. (2014) showed that Biochar compound fertilizers (BCF) made from different feedstocks (pig manure compost, maize straw, peanut husk and municipal waste) was much more efficient in improving nitrogen (N) use efficiency and reducing greenhouse gases emissions in rice production when compared with conventional chemical fertilizer. Biochar based fertilizers have also been applied to study the interactions between biochar and the rhizosphere. This type of study was done by Chew et al. (2020), whereby the addition of biochar to crop fields caused significant changes in electron status of the rhizospheric soils, increasing the potential difference between the rhizospheric soil and the root membrane by 65 mV, thereby resulting in increased plant nutrient content and biomass by 67%. This was due to the reduced requirement for free energy for nutrient accumulation. Thus, it can be concluded, Biochar based fertilizers have several agronomic benefits involving nutrient uptake (nitrogen and phosphorous) and increasing plant biomass, respectively.

Biochar production: overview

Biochar production is eco-friendly, cost-effective, and reusable; these properties of biochar gained an attraction from researchers for use in removing contamination from the environment (Mishra et al. 2020; Gupta et al. 2021; Yaashikaa et al. 2020). The main abiotic parameter that affects biochar production is the temperature (Kambo and Dutta 2015). Pyrolysis, gasification, hydrothermal carbonification, flash carbonization, and torrefaction are the standard thermochemical techniques used in biochar production (Van Der Stelt et al. 2011; Yuan et al. 2017; Ng et al. 2017). The thermal decomposition of organic materials under the temperature range of 250–900 °C in an oxygen-limited environment is called pyrolysis (Ahmad et al. 2014; Osayi et al. 2014). Depending on the temperature and the retention time, this process can be further dived as slow, intermediate and fast pyrolysis (Mohan et al. 2006). The different stages of pyrolysis for the production of biochar are (i) drying and conditioning, (ii) torrefaction, (iii) exothermic pyrolysis, and (iv) endothermic pyrolysis. There are four modes of pyrolysis, out of which slow and fast pyrolysis techniques are the traditional approaches that are followed for biochar production. Apart from these conventional pyrolysis approaches, modern techniques have been devised, including flash pyrolysis, vacuum pyrolysis, microwave pyrolysis, electro-modified biochar, magnetic biochar, etc. Biochar production depends on the characteristics of biomass used and the processing parameters such as temperature, heating time and residence time etc. (Yaashikaa et al. 2020). A recent study on the assessment of Biochar production methods showed that the pyrolysis conditions are the key to biochar production (Amalina et al. 2022). During the pyrolysis, lignocellulose components (cellulose, hemicelluloses, lignin) undergo chemical changes such as cross-linking, depolymerization, and fragmentation under high temperature and produces different states of product, i.e., solid, liquid, and gas (Collard and Blin 2014). Among these final products, waste solids and liquid comprise biochar and bio-oil, respectively, while the gaseous products released as syngas have carbon monoxide, carbon dioxide and hydrogen, etc. (Wei et al. 2019; Yaashikaa et al. 2020) (Fig. 3).

Fig. 3.

Production of biochar through the process of pyrolysis. Organic biomass derived from different sources such as agricultural, forestry, agro-food industry etc., are converted to a carbon rich product (biochar) through thermal decomposition in a pyrolysis chamber. Biochar gets settled at the bottom of the chamber in the form of carbon residues and the pyrolysis vapour at the top can be further processed to produce bio-oil. The solid product derived, i.e., biochar, has been potentially used as a soil amendment

Mechanism of biochar production

The production of biochar from plant-based biomass depends on the underlying different reactions and mechanisms of Lignocellulose (cellulose, hemicellulose and lignin) breakdown during pyrolysis (Yaashikaa et al. 2020) (Fig. 4). Thus, each component, namely cellulose, hemicellulose, and lignin, have their respective individual carbonization decomposition behaviours dependent on the temperature and heating rates. This difference in reactivity determines the carbonaceous structure of the biochar produced.

Fig. 4.

An overview of the steps of mechanism of lignocellulosic biomass thermal decomposition through the process of pyrolysis(slow/fast). The components of lignocellulose (cellulose, hemicellulose and lignin) undergo various chemical reactions to form intermediate products through the depolymerization of their constituent polymers, decarboxylation, dehydration, fragmentation, and subsequent intramolecular rearrangement, aromatization, repolymerization, etc., ultimately giving rise to by-products with high carbon content including biochar (residual char), condensed liquid product (tar/bio-oil) and other incondensable gases

Cellulose decomposition

Cellulose starts to decompose at about 315 °C and gets completed by 400 °C. The cellulose decomposition can take place via two pathways, namely, the exothermic (anhydrocellulose) and endothermic (via levoglucosan) (Lehmann et al. 2011; Lehmann and Joseph 2012). The end products of the exothermic pathway are biochar and a few non-condensable gases. In the endothermic pathway, the levoglucosan undergoes dehydration to produce biochar and hydroxymethylfurfural, which decomposes to produce bio-oil and syngas. Reversibly, the hydroxymethylfurfural can proceed through several other reactions to produce solid biochar again (Yaashikaa et al. 2020).

Hemicellulose decomposition

In the plant biomass, hemicellulose is the first component that is decomposed. The decomposition initially begins at 220 °C and gets completed by 315 °C (Lehmann and Joseph 2012). Next, the hemicellulose decomposes into oligosaccharides which proceed through a series of reactions to produce biochar, syngas and bio-oil (Huang et al. 2012; Yaashikaa et al. 2020).

Lignin decomposition

Lignin decomposition starts at 160 °C and completes by 900 °C (Lehmann and Joseph 2012). The decomposition of lignin is more complex as compared to cellulose and hemicellulose. Firstly, the β-O-4 lignin linkage breaks, resulting in free radicals, which capture the protons from other species, resulting in decomposed compounds which further processed as biochar, bio-oil and syngas (Yaashikaa et al. 2020).

Biochar application and soil health management

Understanding the relationship of biochar with soil health is essential as numerous studies have reported a range of effects biochar has on soil quality and soil microbiota. This section of the review particularly illustrates the effects of biochar on the physicochemical properties of soil, its impact on soil microbial communities and microbial responses to biochar in combination with heavy metal contaminated soils.

Effects of biochar application on the physicochemical properties of soil

Soil organic carbon (SOC) is crucial for sufficient yields due to its water and nutrient retaining capability, which improves the soil structure and provides a suitable habitat for the microbiota. However, as the population is increasing globally, the need for food directly impacts the environment with growing pollutions, including soil contaminations with increased soil infertility and decreased SOC. Several methods have been employed to combat these challenges, such as inorganic fertilizers, but excessive use of such fertilizers also harm the environment. Thus, bio-char has come here to play a role as it has natural carbon-sequestering property. The addition of biochar increases the quality of soil through remedying and improving its physicochemical as well as biological properties such as pH, electrical conductivity (EC) of soil particles, cation exchange capacity (CEC), organic carbon (OC), organic matter and the total nitrogen content along with providing essential inorganic minerals (K, P, Fe, Zn, Ca, Mg) to the soil (Busscher et al. 2010; Van Zwieten et al. 2010). Different types of biochar based on the feedstocks have varied effects on different soil types. For example, the wood charcoal feedstock applied on anthrosol soil type increased the C-content, pH value and phosphate in the soil (Chan et al. 2007). Eucalyptus log and maize stover feedstock used in clay-loamy and silt-loamy soil increased the total N-derived from the atmosphere by up to 78% (Güereña et al. 2012). Therefore, an important aspect of biochar amendment involves the characterization and evaluation of biochar properties and the soil type.

The effect of biochar application on soil and plants depends on the duration of the application of biochar on crop fields. Short-term application of biochar (1–3 weeks) increases the dissolved organic carbon (DOC), cations and anions in the soil solution (Silber et al. 2010), which consecutively increases the electrical conductivity and pH of the soil (Joseph et al. 2015). After 1–6 months of biochar application, biochar's physical and chemical properties change significantly, involving the increase in the surface area and porosity of the soil (Schreiter et al. 2020). Long-term exposure to biochar on soil causes fragmentation of the biochar particles and oxidation of the biochar surfaces exposed through segregation of micro-agglomerates (Wang et al. 2020).

Effects of biochar application on growth of soil microbiota and plants

Different types of fungi and their mycorrhizal forms, such as arbuscular mycorrhizal (AM) fungi, ectomycorrhizal (ECM) fungi, etc., are essential soil-microorganisms forming symbiotic associations with plant roots. This association regulates the dynamics of soil carbon and terrestrial carbon fluxes through carbon sequestration and other microbial activities (Read et al. 2004; Rillig and Mummey 2006; Schmidt et al. 2021). The roots of plants usually remain inaccessible to the mineral nutrients (e.g., phosphorus) present in the biochar matrix because of its larger diameter of roots that cannot effectively find their way into the micro-sites of biochar. In such circumstances, the fine microfilaments (hyphae) of AM fungi find their application in foraging the microsites of the biochar matrix, absorbing essential nutrients and making them available for plant utilization (Mickan et al. 2016; Schnepf et al. 2008). Therefore, stimulating the growth of such microbes using particular soil amendments can prove beneficial for plants (Rillig 2004). For example, increased proliferation of AM spores in the rhizosphere and increased AM root colonization of the Cacao (Theobroma cacao L.) plant were observed upon bamboo biochar application (Aggangan et al. 2019). Growth improvement of AM further mediated Cacao plant growth and development by aiding in improved nutrition acquisition (Aggangan et al. 2019). Moreover, biochar addition influences the community structure of bacterial populations in soil. Bacteria involved in nitrogen-fixation (e.g. Rhizobium, Azospirillum, etc.), nitrification (ammonia-oxidising and nitrite-oxidising bacteria) (e.g. Nitrospora, Nitrobacter) as well as methanotrophic bacteria (e.g. Methylobacterium) were reported to increase in soil amended with biochar (Abujabhah et al., 2018). Thus, biochar improves the biological nitrogen fixation processes by stimulating bacterial nitrification rates and increasing the nitrogen available for plant uptake.

Biochar application also alleviates the adverse effects of abiotic stress on microorganisms. Abiotic stress, such as drought, imposes stressful conditions for microorganisms to survive in their ecological niche. For example, a reduction in AM fungi population and its root colonization abundance of chickpea (Cicer arietinum) was reported under drought stress. However, amending soil with biochar derived from woody branches of button mangrove improved growth and root colonization by inducing favourable conditions for these fungi to produce spores, mycelium, vesicles, and arbuscules (Hashem et al. 2019). So, biochar not only acts as an agent for restoring soil quality but also stimulates and enhances the growth of soil microorganisms (Table 1).

Table 1.

Types of biochar inducing growth stimulating effects on different soil microbial communities

S. No.	Biochar parent material	Soil type	The temperature of pyrolysis (°C)	Residence time	Application rate % (w/w)	Microorganism’s growth stimulated	Reference papers
1	Yeast biochar; Glucose derived biochar	Eutric fluvisol, Cambisol	850	–	–	Fungi; Gram-negative bacteria	(Steinbeiss et al. 2009)
2	Jarrah (Eucalyptus marginata Sm.) feedstock	Sandy loam Grey Orthic Tenosol	600	24 h	2	Arbuscular mycorrhizal (AM) fungi	(Mickan et al. 2016)
3	A mixture of spruce and pine wood; Chicken manure	–	550	12 h	–	AM fungi (Rhizophagus irregularis, etc.)	(Hammer et al. 2014)
4	Coniferous wood chip	Albic Luvisol	500	5 h	1.37	AM fungi	(Hammer et al. 2015)
5	Woody branches of button mangrove (Conocarpus erectus L.)	–	450	2.5 h	3	AM fungi	(Hashem et al. 2019)
6	Eucalypt green waste	Black clay soil, Red loam, and Bown sandy loam	650–750 °C	< 3 min	2.5, 5 and 10	Nitrogen-fixing bacteria, Nitrifying bacteria and Methanotrophs	(Abujabhah et al. 2018)

Biochar stimulates soil microbial response to heavy metal contaminations

Many factors work as anti-ecosystem, of which heavy metals pose a severe threat to the environment and even cause severe life-threatening diseases in humans such as Minamata disease, itai-itai disease etc. (Taty-Costodes et al. 2003). Cadmium is one such heavy metal with characteristic features of high solubility, mobility, and bioaccumulation has adverse effects on soil and water quality (Belhalfaoui et al. 2009). The concentration of these pollutants increases in environmental zones where industrial manufacturing of products such as nickel–cadmium containing batteries, metal coatings, incineration of soil wastes and agricultural effluents gets accumulated, poisoning the groundwater/surface and other water bodies (Basheer 2018).

Amending soil with biochar derived from different pyrolyzed biomass to remediate soil and immobilize heavy metals is an area of research that has gained attention in the present scenario (Table 2) (Houben et al. 2013). The microbial communities thriving in a healthy state is required in soil for transforming and mobilizing organic carbon, but soil deterioration with heavy metals such as Lead (Pb), Arsenic (As), Cadmium (Cd), Zinc (Zn), Copper (Cu) etc., suppress the growth of resident microbes. This suppression decreases soil microbial phospholipid fatty acids (PLFAs) concentration, including the microbial carbon-use efficiency.

Table 2.

Representation of specific biochars used for remediation of different soil heavy metals

S. No.	Biochar parent material	Pyrolysis temperature (℃)	Residence time (hours)	Application rate (optimum)	Biochar attributes to soil properties	Soil heavy metal remediated	Reference papers
1	Coffee husk	500	3	15t/ha	Increased soil electrical conductivity, total nitrogen, organic carbon, exchangeable cations, available phosphorus	Lead	(Bayu et al. 2016)
2	Wheat straw	350–550	–	40t/ha	Increased soil organic carbon, cation exchange capacity pH; decreased chemical mobility of Cd and stabilized it	Cadmium	(Cui et al. 2011)
3	Rice straw	500	4	5% (w/w)	Increased soil organic carbon, the abundance of Fe reducing bacteria, enhanced Arsenic methylation and volatilization by bio stimulating microbes	Arsenic	(Chen et al. 2017)
4	Rice straw	700	2	3% (w/w)	Increased soil pH, microbial biomass, nutrients, immobilized Cd (precipitation with Fe and Mn oxides) onto biochar surface	Cadmium	(Bashir et al. 2018c)
5	Chicken manure	700	4	–	Increased soil pH (alkalinity), mineral content (rich in phosphorus), ion exchange reactions, complexation and precipitation of heavy metals	Cadmium	(Huang et al. 2018)
7	Rice bran	700	6	0.625 g/L	Increased organic and mineral components are leading to the complexation of heavy metal ions	Cadmium	(Yiliang and Chen 2014)
8	Sewage sludge	900	–	0.2% (w/v)	Increased Cd adsorption by ion exchange with alkaline earth cation (Ca); increased pH mediating precipitation of Cd forming insoluble cadmium compounds	Cadmium	(Chen et al. 2015)
9	Macadamia nutshell	465	–	5% (w/w)	Reduced Cd and Pb bioavailability; increased soil pH mediating metal precipitation with carbonates; improved microbial carbon use efficiency	Lead, Cadmium	(Yilu et al. 2018)

Biochar is described as an effective tool for soil remediation because of the physicochemical properties of this carbon-rich substance that can adsorb heavy metal contaminants onto its surface and instead reduce the bioavailability of these metals to plants and soil microorganisms (Mackie et al. 2015). For example, cadmium bioavailability was significantly reduced upon applying biochar derived from rice straw and rice hull, considerably contributing to the increase in soil microbial biomass, soil nitrogen and organic matter (Bashir et al. 2018a, b, c).

Different feedstocks used for biochar manufacturing have a range of effects on metals' mobilizing capacity, rendering different ion adsorption mechanisms concerning the intrinsic property of the final pyrolyzed product. It was found that animal and plant-derived manures with their characteristic surface functional groups (–OH, –COOH) mediated the chelation of Cd²⁺ ions (Han et al. 2017; Xu et al. 2014). Sewage sludge and animal-derived biochars act on Cd²⁺ ions by precipitating them with minerals present in biochar (PO₄³⁻ and CO₃²⁻) (Chen et al. 2015; Xu et al. 2013). While certain plant-based biochars are derived from wheat straw, eucalyptus stem mediates the immobilization and adsorption of Cd²⁺ cations by exchanging them with other cations such as K⁺, Ca²⁺, Mg²⁺, Na⁺ available in the biochar matrix (Trakal et al. 2014).

Arsenic (As), another heavy metal, is known to cause several health implications in humans. Arsenic is converted into less harmful forms through methylation and volatilization activities by various soil microorganisms such as fungi, bacteria and yeast utilizing multiple biological processes (Wang et al. 2014; Zheng et al. 2013). However, biostimulation techniques are required to enhance the metabolic activities of these soil microbial communities for efficient arsenic methylation and volatilization (Gao and Burau 1997). A recent study reported that biochar, combined with genetically modified Pseudomonas putida, increased arsenic methylation and volatilization into the atmosphere; moreover, the soil type also plays a crucial role in this remediation process (Chen et al. 2017). Therefore, biochar with its microbial biostimulation capacity represents an effective future technology for the remediation of metal-contaminated sites.

Application of biochar for mitigation of biotic stress in plants

Soil-borne pathogens cause a wide range of diseases in plants in different setups of nursery, field or greenhouse environment affecting crop productivity, quality, and longevity (Katan 2017). Therefore, methods for comprehensive disease control measures are required that are cost-effective and economically sustainable. Many studies have reported biochar as having the qualities of a vital soil amendment capable of showing positive results against both foliar and soil-borne pathogens of plants (Frenkel et al. 2017).

Plants are the support system for all living organisms, so it is essential to explore every possible way of saving them from pathogen or pest attacks. Therefore, it is necessary to analyse how amending soils with biochar can alleviate biotic stress in plants as studies concerning this topic are limited and need proper investigation. Several studies have reported that biochar amendment possibly confers induced resistance in plants against pathogenic organisms (Table 3) (Elad et al. 2010; Harel et al. 2012). There can be two types of induced resistance depending on the resistance acquiring mechanisms by specific elicitors and regulatory pathways in plants. One is induced systemic resistance (ISR), and the other is systemic acquired resistance (SAR). The pathogenic attack causes induced systemic resistance pathway but is mainly promoted by beneficial microbes colonizing the roots of the plants such as plant growth-promoting rhizobacteria (PGPR) and fungi (PGPF) and involves phytohormones including jasmonic acid (JA) and ethylene (ET) (Azami-Sardooei et al. 2010; Van der Ent et al. 2009). In contrast, systemic acquired resistance involves the phytohormone salicylic acid (SA) mediating the synthesis of pathogenesis-related proteins (PR), activated upon a confined hypersensitive reaction against a plant pathogen (Durrant and Dong 2004). A significant increase in the relative expression of defence-related genes correlated with the ISR or SAR pathway after biochar amendment to soil was shown in the leaves of strawberry plants (Harel et al. 2012). Similarly, a study into the molecular insights of the resistance gaining mechanism of tomato plants against a soil-borne pathogen revealed a significant upregulation of genes related to JA biosynthesis and signalling after biochar treatment (Jaiswal et al. 2020).

Table 3.

A representation of the types of biochar used and their effects on biotic (pathogen/pest) stress amelioration from plants

S. No.	Biochar material	Pyrolysis temperature (℃)	Application rate (optimum)	Signalling mechanism	Pathogen/Pest	Plant species infected	Disease amelioration		Reference paper
							( +)	(−)
1	Citrus wood	–	1–5% (w/w)	ISR	- Foliar fungal pathogens: Botrytis cinerea (grey mold) and Leveillula taurica (powdery mildew) - Broad mite pest: (Polyphagotarsonemus latus)	Tomato and Pepper	+		(Elad et al. 2010)
2	Pinewood	550–600	5% (v/v)	ISR	Phytophthora cinnamon and Phytophthora cactorum	Quercus rubra (L.) and Acer rubrum (L.)	+		(Zwart and Kim 2012)
3	Holm wood	650	1.2% (v/v)	ISR pathway involving ET activation and H2O2 accumulation	Root-knot nematode (RKN) Meloidogyne graminicola	Rice (Oryza sativa)	+		(Huang et al. 2015)
4	Greenhouse waste	450	1 and 3% (w/w)	ISR pathway; JA signalling	Botrytis cinerea	Tomato	+		(Mehari et al. 2015)
5	Greenhouse pepper plant waste	350	3% (w/w)	ISR pathway; JA biosynthesis and signalling	Soil-borne pathogen: Fusarium oxysporum f. sp. radicis lycopersici	Tomato	+		(Jaiswal et al. 2020)
6	Maple bark	700	1,3, 5% (w/w)	Possible priming of SA instead of JA	Soil-borne pathogen: Rhizoctonia solani (Foliar blight disease)	Soybean		−	(Copley et al. 2017)

The ( +) and (−) sign under the section “Disease amelioration” in the table depicts if the given biochar has a positive or negative impact towards stress mitigation, respectively

Possible ways by which the amendment of biochar can aid in defence up-gradation in plants have been described in many papers (Graber et al. 2014; George et al. 2016). For example, an investigation into the use of biochar for managing Fusarium ear rot in maize showed that poultry faecal waste and sawdust biochar proved beneficial in suppressing the infection caused by Fusarium verticillioides through effective management of the resident soil pathogens (Akanmu et al. 2020). Furthermore, experiments were conducted on tomato (Solanum lycopersicum) plants inoculated with Botrytis cinerea (necrotrophic fungus) and soil amendment with greenhouse waste biochar in the tomato-Botrytis cinerea pathosystem reported the reduction of infection and disease severity by up to 50% (Mehari et al. 2015). Another study showed that spelt husk biochar managed to reduce the infection rates of Pratylenchus penetrans (root-lesion nematode) on carrot (Daucus carota) plants (George et al. 2016).

However, biochar may not always prove beneficial in reducing disease severity. Furthermore, its role as a biotic stress reliever can change in different pathosystems depending upon the biochar type and the concentration applied. For example, the susceptibility of soybean to a foliar disease was reported to worsen under maple wood biochar application (Copley et al. 2017). Therefore, it is imperative to assess particular biochar in terms of the biochar dose-dependent plant ecophysiological responses before soil treatment.

Biochar mitigating abiotic stress in plants

Drought, salinity and heavy metal toxicity are harmful abiotic stresses for plants affecting their metabolic activities and limiting their growth. Furthermore, increased anthropogenic activities have set a course of changes in climatic conditions, elevating such stresses. Therefore, exploring methods to protect plants from these abiotic stresses is essential, and studies have reported biochar may be a helpful agent in abiotic stress mitigation in plants (Abbas et al. 2018; Yoo et al. 2020) (Fig. 5). Here we explain the different uses of biochar to mitigate abiotic (Drought, salinity and heavy metal) stress and the underlying plant responses upon its amendment to the soil (Joseph et al. 2021).

Fig. 5.

An illustration depicting how biochar amendment to soil influences the soil physicochemical as well as biological properties producing a range of affects with outcomes that proves fruitful for improving plant growth and agricultural productivity under the impact of different abiotic (drought, metal toxicity, salinity) and biotic stresses (microorganisms/pest)

Plant responses to drought and salinity under biochar application

Drought (water-limiting) conditions is one of the main abiotic factors harming the plant physiology, reducing its photosynthetic efficiency, stomatal conductance, along with several morphological limitations to the plant, including plant height, root development, growth and yield (Anjum et al. 2011; Heuer and Nadler 1995). Particularly in arid and semi-arid climatic conditions, plants usually face harsh environmental constraints, such as low-soil water content, low soil organic matter, and nutrition for proper growth and development. Biochar implementation in such areas has an immense scope because of its high nutritional value (Zahedifar and Najafian 2017) and can provide plants with essential nutrients (Table 4).

Table 4.

A representation of the types of biochar used and their beneficial effects on different plant species under drought and salinity stress

S. No.	Biochar material	Abiotic stress	The temperature of pyrolysis (℃)	Application rate % (w/w)	Plant species	Plant physiological and morphological responses	References
1	Stems of Lantana camara	Drought	450	1	Lady’s finger (Abelmoschus esculentus)	Increase in: • Plant height • Photosynthesis • Plant biomass, Maintained stomatal conductance	(Batool et al. 2015)
2	Cattle manure	Drought	600	1.5	Soybean (Glycine max)	Increase in: • Plant water-use efficiency • Straw fresh matter yield • Straw dry matter yield • Pod length • Plant height, etc	(Gavili et al. 2019)
3	Woody branches of button mangrove (Conocarpus erectus L.)	Drought	450	3	Chickpea (Cicer arietinum L.)	Increase in: • Chlorophyll synthesis • Net photosynthetic efficiency • Relative water content (RWC) • Membrane stability index • Nitrogen-fixing ability	(Hashem et al. 2019)
4	Chips of wood (Picea abies (70%) + Fagus sylvatica (30%)	Drought	550–600	1.5 and 3	Maize (Glycine max)	Improvement in: • Plant-soil water relations (Water-use efficiency) • Plant photosynthesis • Plant biomass • Plant nitrogen use-efficiency • PSII photochemistry efficiency	(Haider et al. 2015)
5	Hardwood and softwood biochar derived from oak and pine, respectively	Salinity	–	5	Eggplant (Solanum melongena)	• Enhanced Physiological processes • Increased root and shoot growth • It increased plant yield • It reduced electrolyte leakage	(Parkash and Singh 2020)
6	Corn straw	Drought and salinity	500	5	Quinoa	Increase in • Plant height (11.7%) • Shoot biomass (18.8%) • Grain yield (10.2%)	(Yang et al. 2020)
7	Wheat straw	Drought and salinity	550	–	soyabean	Increase in • Plant biomass • Grain yield	(Zhang et al. 2020)
8	Dried leaves material and saw dust	Salinity	> 350	1 and 2	wheat	Increase in • Root and shoot length (23%) • Leaf water potential (16%) • Osmotic potential (10%)	(Kanwal et al. 2017)
9	Cattle manure	Drought	600	1.25	spinach	Increase in • Stomatal conductance (SC) • Saturated hydraulic conductivity (Ks) • Porosity	(Gavilli et al. 2018)

Animal manure biochars may have an advantage over plant biomass-derived biochars in arid and semi-arid areas owing to factors including ease of access, higher nutritional composition and cation exchange capacity, etc. (Rizwan et al. 2016a, b; Singh et al. 2010). Gavili et al. (2019) found that biochar obtained from cattle manure when applied at low levels (1.25%) manifested positive effects on the Soybean plant’s physiological and morphological traits with an increase in the plant water-use efficiency (WUE), straw fresh matter yield, straw dry matter yield, pod length etc. However, when applied at higher rates (2.5% and 5% wt) showed adverse effects on plant traits. Therefore, application rates, soil type, the feedstock used, plant species, etc., play an essential role in determining the biochar effect on water-stressed plants. Increasing plant WUE through soil amendments is being focused on, and studies have demonstrated that biochar application holds the potential to ameliorate water scarcity stress from plants as it exhibits water retaining capacity and also provide essential nutrients (Laird et al. 2010; Novak et al. 2009a, b).

Salinity is also very prevalent in arid and semi-arid areas. Under high salinity conditions, osmotic and ionic stress are the two significant stresses a plant has to face. The application of biochar in high salt content soil effectively reduces the soil salinity as it has the power of high salt adsorption (Thomas Sean et al. 2013). High salt adsorption results from higher surface areas and cation exchange capacity of biochar (Parkash and Singh 2020). Even biochar helps to maintain the Na⁺/K⁺ ratio in plants through soil ion adsorption mechanisms. However, the effectiveness of biochar in mitigating the salinity stress vary among the different biochars, along with the plant species, application rates and the type of soil, and therefore should be tested among other crop plants to evaluate the rate of mitigating salinity stress (Hashem et al. 2019).

Plant responses to heavy metal stress under biochar application

Heavy metal stress poses a severe threat to plant growth and development, inducing morphological, physiological, and biochemical changes in plants, leading to decreased plant biomass and yield (Rizwan et al. 2016a, b; Younis et al. 2016). Numerous studies have reported that the addition of biochar at optimum levels proves beneficial for soil as well as for plants by providing favourable conditions of increased soil nutrient availability and decreasing the plant uptake of heavy metals by complexing and immobilizing the metal ions onto its surface (Carter et al. 2013; Schmidt et al. 2021). Furthermore, Biochar amendment to the soil to reduce solubility, mobilization and bioaccumulation of heavy metals in toxic concentrations in the succeeding food chains of plants, animals and humans provides an alternative to costly metal extraction techniques.

The presence of functional groups (COO⁻, COH, and OH⁻) on the biochar surface induces a net electronegative charge favouring the increase in soil alkalinity (Li et al. 2016). Carter et al. (2013) showed that the inclusion of Rice husk biochar at an application rate of 50–150 g kg⁻¹ had potentially beneficial effects on Lettuce (Lactuca sativa) and Chinese cabbage (Brassica chinensis) growth, the impact of which lasted for three cropping cycles (Carter et al. 2013). Agricultural fields contaminated with heavy metals retards plant growth and exposes the succeeding food chains to metal contaminants (Chaney et al. 2004). Cadmium is one such heavy metal that is toxic at high concentrations and inhibits plant growth in terms of root and shoot biomass, nutrient uptake and damage to ultra-structural components (Rizwan et al. 2016a, b). It was found that wheat straw biochar, when added to soil, significantly increased the soil pH value along with the soil organic carbon (SOC) content, thereby reducing the Cd bioavailability to plants and decreasing rice grain Cd concentration (Cui et al. 2011).

Chicken manure and green waste biochar were also potent in immobilizing heavy metals such as Cd, Pb, and Cu, thereby reducing their phytoavailability to plants and providing it with essential nutrients like phosphorus and potassium (Park et al. 2011). Lu et al. (2014) showed that an application of rice straw biochar reduced the concentration of Cu and Pb in Sedum plumbizincicola by 46% and 71%, while application of bamboo biochar managed to reduce the concentration levels of Cd in the plant shoots by 49%. This reduction involved a potential technique of metal complexation onto biochar surface and reducing its phytoavailability to plants (Lu et al. 2014). The maximum adsorption capacity of biochars for metal ions also differs between feedstock and the pyrolytic conditions (Table 5). Therefore, characterization of the physicochemical properties of different biochars, including modified biochars, is crucial regarding the development of biochar materials for environmental management.

Table 5.

Various biochars and their maximum adsorption capacities for heavy metals

S. No.	Biochar material	Pyrolysis temperature (℃)	Heavy metal	Maximum adsorption capacity of biochar (Qmax) (mg/g)	Reference papers
1	Rice bran	700	Cd2+	18.00	(Yiliang and Chen 2014)
2	Chicken manure	700	Cd2+	149.55	(Huang et al. 2018)
3	Rice straw	700	Cd2+	158.77	(Huang et al. 2020)
4	Canna indica	500	Cd2+	188.80	(Cui et al. 2016)
5,	Swine manure	250	Cd2+	81.32	(Han et al. 2017)
6	Spent Pleurotus ostreatus substrate	700	Pb2+	326	(Wu et al. 2019)
7	Spent shitake substrate	700	Pb2+	398	(Wu et al. 2019)
8	Rape straw biochar modified with KMnO4 impregnation	600	Cd2+	81.10	(Li et al. 2017)
9	Sewage sludge biochar with CaCO3 nanoparticle modification	500	Cd2+	36.5	(Zuo et al. 2017)
10	Chaenomeles sinensis seed	450	Cr6+	93.19	(Hu et al. 2019)
			Cu2+	105.12

Agronomic and environmental risks related to biochar use

All the new technologies have got their pros and cons. Apart from describing the beneficial aspects of biochar application, we should also consider the unintentional consequences of biochar use on soil geochemistry, microflora and fauna, crop yield and greenhouse gas effluxes (Sparkes and Stoutjesdijit 2011). Application of biochar in agricultural fields have been observed to either increase or decrease the micro-flora in the soil. Decrease in AM fungal abundance have been observed after the addition of biochar (George et al. 2012). The potential drawbacks related to biochar affecting the soil geochemistry involves the following:

• (i)The binding and deactivation of agrochemicals like herbicides and nutrients in the soil were observed by Yu et al. (2009).
• (ii)The negative impact on plant germination is due to some phytotoxic compounds present in biochar (Rogovska et al. 2012). The most common toxic observed in biochar is polycyclic aromatic hydrocarbons (PAHs), (Freddo et al. 2012; Hilber et al. 2012; Hale et al. 2012).
• (iii)The unavailability of nutrients is present in the soil and increases soil electrical conductivity (EC) (Novak et al. 2009a, b).
• (iv)The wide pH range of the biochar (4–14), depending on the various factors of biochar production (feedstock, pyrolysis temperature, degree of oxidation), affects the availability of metal contents in the soil and vice versa the soil’s total microflora (Steiner et al. 2007).

The drawbacks open questions for future research which can be done to overcome such problems.

Nanomaterial based modified biochar and their application

The composition of biochar is very much dependent on the raw feedstock material and the pyrolytic conditions set for production (Ghassemi-Golezani et al. 2021). Although biochar has an excellent nutritional composition, many key nutrient elements might be lost during pyrolysis (Ding et al. 2016; El-Naggar et al. 2019). Therefore, using advanced technologies like nanotechnology, biochar can be fortified to enhance its nutrient compositions as well as other essential properties. Redefining biochar with the incorporation of nanomaterials significantly improves the physicochemical properties of biochar, such as an increase in pore numbers, enhanced cation exchange capacity, heavy metal immobilization, specific surface area and nutrient composition etc. (Liu et al. 2015; Tan et al. 2016). This improvement in physicochemical properties enhances biochar performance towards facilitating plant growth, soil fertility, and available nutrients in the soil. Modified biochars with nanoparticles such as magnetic iron oxide particles, graphene, chitosan, carbon nanotubes, nanoscale zero-valent iron and nano-metal oxides/hydroxides etc., have found broad applicability across a range bb of environmental remediation technologies (Tan et al. 2016). Ghassemi-Golezani et al. (2021) showed that application of biochar modified with nanocomposites (metal oxides) of magnesium (Mg) and manganese (Mn) significantly alleviated salinity stress in Safflower (Carthamus tinctorius) plants and increased the availability of key nutrients such as Mg and Mn thereby improving the physiological and biochemical functioning (Ghassemi-Golezani et al. 2021). In terms of waste-water treatment, biochar based nanocomposites have gained wide functionality as they have a high potential to adsorb contaminants from the polluted water compared to raw biochar (Chen et al. 2011; Pi et al. 2015). Nanobiochar (biochar with catalytic nanoparticles) could be a promising technology for removing organic pollutants such as methylene blue, phenol and sulfamethoxazole from the contaminated water. Biochars offering properties such as high electrical conductivity, surface functional groups with unique surface properties, and chemical stability proves advantageous for blended catalytic nanoparticles (e.g. biochar loaded with TiO_2, etc.). Biochar not only improves the stability of the blended nanoparticles but also enhances its adsorption and photocatalytic degradation capacity of organic pollutants in wastewater compared to nanoparticles alone (Cuong et al. 2019; Kumar et al. 2017; Zhang et al. 2017b). The performance of biochar-based nanomaterials for soil restoration and carbon sequestration has also been studied. Liu et al. (2020), showed that the application of nanobiochar proved beneficial for alleviating the phytotoxic effects of soil heavy metal (Cd²⁺) contamination and increased the soil microbial biomass. It has been reported that the use of Ni/Fe biochar in the soil–plant system helped in the adsorption of polybrominated diphenyl esters (PBDEs) and reducing the phytotoxicity, and also in reducing the accumulation and translocation of PBDEs (Wu et al. 2018). Creamer et al. (2018) used MgO nanoparticles stabilized in the biochar structure to capture CO2. These researchers found that the biomass: nanoparticles ratio significantly affected the capability of the nanocomposite to sequester carbon dioxide. Therefore, the practical application through the combination of nanomaterial with biochar remediation technology can come up as an effective, reliable and low-cost technology for environmental remediation.

Conclusion and future perspectives

Strategic approaches and the development of novel techniques for environmental remediation are essential for the increasing environmental pollutants. These pollutants have negative implications on biotic life, be it microorganisms, plants, animals or humans. Factors threatening plant survival, in turn, question the survivability of organisms dependent on plants for food and nutrition. In such circumstances, the role of biochar in environmental management has received much attention due to its physicochemical and biological properties. Biochar features a sustainable, easy, and cost-effective technique whose substrate material is unwanted organic waste pyrolyzed into the carbon-rich compound. Furthermore, due to the rich carbon content and high residence time in soil, biochar improves soil fertility owing to its sequestered carbon and mineral composition.

Furthermore, biochar application has proven effective for mitigating different biotic and abiotic stresses in plants. Although there are certain disadvantages of using biochar, mainly concerned with its application rate as it can reduce herbicide availability in agricultural fields through surface adsorption. Evaluating the properties of biochar and their characterization through advanced and novel techniques can reveal unique traits specific to feedstock material. This can define the application possibilities and limitations to other disciplines, including use as gas adsorbents, supercapacitors, energy storage, fuel cell systems, pharmaceutical and polyaromatic hydrocarbon (PAHs) contaminant removal, etc., where the use of biochar is still in its early stages. Moreover, the recent advances in nanotechnology for the modification of biochar physicochemical properties have attracted the attention of researchers owing to its improved traits for decontamination of soil and wastewater compared to normal biochar. Considering the pros and cons, the development of biochar as an effective technology-based environmental management tool needs detailed investigation. The investigation should be carried out to characterise lignocellulosic components of feedstock material, pyrolysis mechanisms including process parameters, and through intensive field studies.

Thus, biochar has become a promising tool for waste management. Modified biochars have been extensively used for different environmental applications, including soil fertilizers which have opened a new dimension in improving soil quality. For future directions, new biochar preparation and modification methods to see the long-term effect on the soil property should be studied to get the maximum benefit. To avoid the toxic effects of using such fertilisers in crop fields, a deep analysis should also be done on the biochar production from contaminated wastes. For better use of biochar, a database should be created that will provide complete information about the raw material for biochar-based fertilizer preparation, properties of biochar and its impact on the soil system so that the application of biochar-based fertilizers would benefit a variety of crops.

Author contribution

HA, VHK and AM compiled the manuscript and having equal contribution. SB: helps in in the preparation of diagram. PKS help in the editing of the manuscript and NCJ contributed to conceptualizing and guiding the manuscript writing.

Funding

This work was supported by DST-SERB start-up- grant (file no. SRG/2020/002237) from gov of India, awarded to Dr Naveen Chandra Joshi.

Declarations

Conflict of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Ethical approval

Ethical statement not applicable.