Abstract
Ensuring global food security under present and futuristic scenario of climate change accompanied by expanding population is of major concern worldwide. Researchers across the world are focusing on sustainable agronomic practices to combat climate change induced issues like desertification, reduction in crop yield, pest outbreaks, and reduction in soil health. Biochar has demonstrated positive effects on plant growth, development, fruit quality, carbon capture and sequestration. Studies indicate that biochar amendments to soil can enhance soil health, plant growth and development as well as help in combating abiotic stressors. Biochar has several properties that make it an attractive soil amendment, including high porosity, surface area, water-holding capacity, and the ability to increase soil microbiota, which in turn can promote nutrient uptake and reduce soil erosion. Environmentally, biochar’s ability to degrade organic pollutant as well as sequester carbon makes it a very attractive addition to sustainable agriculture. Nonetheless, biochar has also been reported to have potential hazardous impacts on human health associated with possible leakage of toxic elements and particulate matter pollution. However, lack of exhaustive studies that detail all the major aspects of biochar application in agroecosystem impedes decision making on its overall usability in agriculture. There exists a need for comprehensive review that cover all the important aspects of biochar supplementation. This review provides a thorough overview of the multifaceted impacts of biochar, encompassing its positive effects on soil, plant growth, development and environment, while also exploring the limitations along with potential toxic effects associated with its application.
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1 Introduction
The serious and pressing issue of global warming has captured the attention of researchers, policymakers, and concerned citizens worldwide. IPCC (Intergovernmental Panel on Climate Change) reported a 1.5 °C increase from pre-industrial levels in global temperature between 2030 and 2052 [1]. This coincides with the continued increase in global population especially of global south creating not only an alarming scenario of a vast vulnerable population facing natural disasters but also, possibly, an acute shortage of food grain production. Food production is at peril not only due to a decrease in arable lands, an increase in soil salinity, groundwater depletion, and land desertification but also because of water scarcity [2]. In this situation, amidst the search for solutions to decrease water requirements of crop fields and as a soil amendment, biochar has been the highlight of the debate. According to the International Biochar Initiative, biochar is a product of the thermochemical transformation of any biomass in an environment of limited oxygen. Throughout the world, biochar can be found as a result of subsurface fire, burning of vegetation, and traditional soil management practices. Glaser and Birk [3] discovered that Amazonian dark soil (terra-preta), rich in biochar, was actually man-made by pre-Columbian civilizations and is still highly fertile. As a result of its high stability, biochar remains in the soil for hundreds of years, thus eliminating the need for periodic application. In fact, wood-derived biochar has a reported time of 1000–1200 years before complete degradation [4]. Biochar enhances fertility of soil and promotes the growth and development of plants, making it a sustainable alternative to conventional fertilizers [5] (Fig. 1). When incorporated into the soil, biochar improves its physical, chemical, and biological properties [6]. It acts as an excellent soil amendment, enhancing water retention and reducing nutrient leaching, thus mitigating the negative effects of drought and nutrient deficiency [7, 8] (Fig. 1). Porous structure of biochar provides a habitat for beneficial microorganisms, promoting soil biodiversity and enhancing nutrient cycling [7] (Fig. 1). The ability of biochar to trap carbon from the atmosphere is one of the main factors contributing to its significance. Carbon dioxide (CO2) is successfully removed from the atmosphere when biomass is converted into biochar (stable form of carbon), and persists in the soil for many centuries [9]. Because of its capacity to sequester carbon, biochar is a useful instrument in the quest against climate change, helping to lower atmospheric CO2 levels and offset greenhouse gas emissions [9].
An overview of the effect of biochar on soil and plant. Feedstock in the form of manure, wood and farm residues can be used for biochar manufacture. Biochar positively influences plant properties like height, number of leaves, leaf area, total biomass, yield, pigments, flavonoids, nutrient uptake and soil properties like pH buffering capacity, WHC, CEC and soil microbiota
Abiotic stress refers to the detrimental effects of inanimate things on plants, hindering their growth, development, and productivity. These stressors include but are not limited to extreme temperatures, drought, salinity, and heavy metal contamination. As climate change intensifies, the frequency and severity of abiotic stressors are on the rise, necessitating sustainable solutions to safeguard global food security. Because of its effectiveness against abiotic stress, biochar is a versatile prospective choice for contemporary agriculture. Mainly due to its physical properties of high porosity and surface area it combats water stress [7]. Increased availability of water in rhizosphere also reduces salinity stress and promotes soil aeration and health. Additionally, by adsorbing heavy metals and other gaseous pollutants like CH4 (methane) and N2O (nitrogen oxide), biochar can treat polluted soils by lowering their bioavailability and inhibiting plant uptake [10]. Although biochar has shown potential in amelioration of organic pollutants from environment [11], lack of sufficient studies hampers derivation of a satisfactory conclusion. Beyond agriculture, biochar holds promise in a range of other applications like construction material [12], adsorbent of air pollutants [4], and anaerobic digestion catalysts [13]. In forestry, biochar can be used to restore degraded lands, enhance reforestation efforts, and improve the survival rates of tree seedlings [14]. In wastewater treatment, biochar effectively removes pollutants and contaminants, offering a cost-efficient and sustainable solution for water management [15]. Biochar also has potential as a source of renewable fuel in the energy sector, as it can be used in biomass combustion or gasification processes to generate heat and electricity [16]. The significance of biochar goes beyond its environmental and agricultural applications as its production can also contribute to waste management by converting biomass residues, such as agricultural waste and forestry by-products, into a valuable resource. By diverting organic waste from landfills and utilizing it to produce biochar, we can reduce methane emissions and promote a circular economy, closing the loop on waste disposal.
But to understand intricacies of its effect on agricultural systems as well as environment, we need to compile existing literature that also spots gaps in scientific studies. This paper holistically covers biochar’s influence on soil structure, soil quality, plant growth, defense against abiotic stressors, yield, fruit quality and on greenhouse gases emission in agriculture. It highlights sustainability of biochar application in agro-ecosystem on environment while also highlighting its reported toxic effects as well as inefficacy. We have also addressed literature gaps where they exist and scientific researches that can address those gaps.
2 Methodology
2.1 Search methodologies
Using the databases of Google Scholar, Scopus, and Web of Science, a literature search was carried out to gather pertinent papers published in peer-reviewed journals between 2010 and 2023. The following search keywords were used: biochar, plant growth, soil fertility, soil microbiota, soil amendments, abiotic stressors, Greenhouse gases, and crop plant. These keywords were used separately and also together using conjunctions (and, for) and prepositions (in, on, after, with) to find out relevant content.
2.2 Selection criteria
Articles in English language and published between 2010 and 2023 after searching with keywords were considered for this review. Papers that were holistically focused on each aspect of this review were prioritized.
2.3 Screening and suitability assessment
Papers generated after initial searches were filtered on the basis of relevance to the topic of this review and subsequently, total of 350 papers were considered. A further exclusion of 88 papers was done to remove papers not directly connected or having insufficient information for this topic. 262 articles were studied completely and 196 papers completely met the criteria of inclusion. 185 papers were then included in this review. The primary focus in this article is on the biochar’s effect on different soil properties, plant growth, biochemical properties of plants, and development. Biochar’s effect on abetting abiotic stresses and greenhouse gas pollution was also addressed in this review.
3 Factors affecting biochar’s properties
Biochar formation involves three key stages: drying, pyrolysis, and cooling. Firstly, the biomass feedstock is dried to eliminate moisture content, allowing for a more efficient pyrolysis process. The dried biomass is then heated to high temperatures during the pyrolysis stage (normally ranging from 350 to 700 °C) in a low-oxygen environment [17]. This lack of oxygen prevents complete combustion, converting the biomass into solid carbon and other byproducts such as bio-oil and syngas. The last step involves gradual cooling down of biochar to ensure it retains its desired properties [17]. Choice of feedstock can vary from cereal husk, crop residues, manure to wood. Biochar’s characteristics are most significantly influenced by temperature [18]. Mostly, it is because the volatile components, H (Hydrogen), O (Oxygen), and N (Nitrogen) contents decrease with an increase in temperature [18]. O:C and H:C ratios of biochar along with humus content also decrease with an increment in pyrolytic temperature [18] (Fig. 2).
Major factors affecting biochar’s properties. Feedstock such as Manure, wood and crop residues influence lignin content and surface chemistry of biochar. Pyrolytic temperature influence Humus contents, volatile contents, H:C and O:C ratio. Type of soil such as clayey, sandy, saline acidic or calcareous and soil deficiency affects efficacy of biochar
High pyrolysis temperatures and longer residence times result in biochar that is often more stable, has a higher surface area and a greater carbon content. However, surface area does not show a linear relationship with increase in temperature as very high temperatures of 800–1000 °C diminishes surface area [19]. One possible mechanism behind increase of surface area with increase in synthesis temperature might be release of volatile gases from biomass during pyrolysis, resulting in increased porosity [18]. Biomass with a high lignin content tends to produce biochar with greater porosity and surface area, enhancing its effectiveness in soil applications [20]. Interplay of factors directing biochar’s characteristics are complex, for example while hemicellulose and cellulose decompose at 200–300 °C, lignin might require high temperatures of > 850 °C [18], thus depending on organic constituents of biomass, synthesis temperature must be carefully selected. Figure 2 illustrates that the choice of feedstock along with the conditions of pyrolysis, influence the biochar properties such as cation exchange capacity (CEC), nutritional content, chemical makeup, and surface area [18]. The chemistry of biochar's surface is affected by the functional groups present in the raw material (Fig. 2). These functional groups can be polar or non-polar, and affects the biochar’s interactions with nutrients and pollutants in the environment, thus determining its effectivity in remediating soil contaminants [21]. Biochar generated at lower pyrolytic temperatures (350 °C) have VOCs (Volatile Organic Compound) including furans, ketones and short carbon chain aldehydes sorbed onto its surface. However, longer carbon chain hydrocarbons and sorbed aromatic chemicals frequently predominate in biochar generated at higher temperatures (> 350 °C) [21]. Additionally, sorbed VOCs were decreased by oxygen during pyrolysis. These observations are significant because VOCs are known to generate mixed reactions in plants and rhizospheric microbial community. While some can decline plant productivity [22] others have reported an interesting observation where influence of biochar on bacterial population is taxa specific [23]. Khodadad et al. [23] noted increased abundance of Actinobacteria and Gemmatimonadetes, reflecting the complex mechanisms behind various factors affecting biochar’s efficacy. While possibility of creating biochar with specific characteristics, appropriate for different applications by tailoring choice of feedstock and conditions of pyrolysis exists, our review found existing studies to be inadequate for such attempt. A further elucidation of mechanisms that influence biochar is needed to come to a conclusion.
4 Effects of biochar on soil
Soil health is of paramount importance in agriculture and environmental sustainability. Different soil properties that are considered agriculturally significant are soil CEC, fertility, microbial communities, water holding capacity (WHC), and pH. Biochar has been reported to have mostly beneficial effects on all of these properties (Fig. 3). Its ability to improve soil structure, nutrient retention, microbial biomass, WHC, and CEC, and increase pH buffering capacity offers an encouraging avenue towards a sustainable and resilient farming practice [6]. In the following sections, we have examined available literature that is currently available on the implications of biochar on soil properties.
Beneficial effects of biochar on soil properties. Biochar reduces uptake of heavy metals such as Pb (Lead) on amending soil. Biochar increases cation Exchange Capacity (CEC), pH buffering capacity, soil organic content and microbial growth
4.1 Soil water holding capacity
Soil’s WHC is one of the key factors that determine growth of plant and productivity while being influenced by a range of factors, such as soil structure, organic matter quantity, texture, and management practices [24]. The WHC of biochar is due to its porous nature. Research by Laghari et al. [25] (Table 1) reported biochar application on sandy soil of thar desert resulted in increased WHC by 14%. Similarly, Zhang et al. [26] observed increased WHC of loamy soil by 22% after biochar application, which allows it to retain water and release it slowly over time. This property is particularly beneficial in sandy soils with low WHC. Various studies have reported an increase in WHC of sandy loam soil with biochar addition. Rehman et al. [27] reported a 1.5% increase in WHC of sandy loam soil, for successive 1% biochar addition up to 10%. Biochar addition beyond 10% was not observed to be agriculturally significant as it causes damage to the plant [27]. Biochar can also improve soil structure by reducing soil compaction and improving soil aggregation, which in turn can improve water infiltration and WHC [28].
Moreover, biochar application can increase soil organic matter content, which is positively correlated with WHC (Table 1) [29]. Biochar’s effect on WHC may vary depending on type of biochar and soil. For instance, Jeffrey et al. [30] noticed biochar made from wheat straw significantly improved WHC in sandy soil compared to rice husk biochar. Similarly, a study by Kloss et al. [31] noted that the biochar’s effect on soil WHC was more remarkable on loamy soil than on sandy soil. In conclusion, biochar has the capacity to enhance WHC by increasing the amount of soil organic matter, improving structure of soil, as well as providing a porous medium for water retention. Wang et al. [32] conducted a neutron imaging analysis of biochar-amended soil and reported that biochar increases the moisture content in the surroundings of the soil. However, it was also established that intraparticle structure has a bigger role than interparticle structure in moisture retention [32]. Thus, as biochar ages, its utility in water retention decreases.
4.2 Soil fertility and microbial communities
By making more nutrients available in the soil, biochar has frequently been observed to enhance soil fertility and reduce nutrient leaching [33] (Fig. 3). High surface area along with a porous nature of biochar allows it to hold nutrients such as phosphorus, nitrogen and potassium in the soil [34]. The CEC increasing ability of biochar also allows it to retain cations, which can be released to plants when needed [35]. Biochar also contains various minerals beneficial for plant growth, such as calcium, magnesium, and iron [36]. Moreover, biochar application can reduce soil acidity by increasing soil pH [37]. Under biochar application availability of nutrients in the soil, like nitrogen and phosphorus increases but results are inconsistent for potassium [38]. The majority reports have reflected that biochar amendments can increase microbial biomass and activity [7, 8], while others have observed no effect [33, 39]. Biochar’s effects on soil microorganisms also depend on the biochar used, the soil type, and the management practices employed. For example, biochar produced from different feedstock like wood, manure, or farm residues can have varying chemical properties and microbial impacts [40]. Variation in beneficial impacts of biochar on soil microbial communities are dependent on soil properties and its inherent microbiota [41]. Additional investigations have revealed that biochar amendments in soil can increase the abundance of soil bacteria, such as Bacillus, Pseudomonas, and Actinomycetes, which are essential for nutrient cycling and disease suppression [7, 8, 42, 43]. Biochar’s effects on soil microorganisms may depend on various factors, including biochar properties (e.g., pyrolysis temperature, feedstock), properties of soil [e.g., pH, texture] and application rate [8, 41, 44]. For instance, Chatterjee et al. [44] noticed biochar generated at high pyrolysis temperatures (600–700 °C) to have greater pores and micro surface area than biochar prepared at low pyrolysis temperature. High pore volume and surface area are positively correlated with increased microbial activity and biomass [45].
4.3 Soil pH
Biochar’s impact on soil pH is dependent on multiple variables, including the biochar feedstock type, the pyrolysis temperature and amount of biochar applied. Generally, biochar raises soil pH, although the magnitude of the effect can vary [46]. Biochar has high pH buffering capacity, reflecting that it can resist changes in soil pH when subjected to acidic or alkaline conditions (Fig. 3) [47]. Consequently, adding biochar to soil may be helpful in stabilizing pH levels, preventing drastic fluctuations be detrimental to plant growth.
Because the number of acidic functional groups found on biochar's surface diminishes with oxygen loss as the pyrolytic temperature rises, the pH of soil gradually shifts from neutral or acidic to alkaline [48]. Biochar can adsorb acidic cations, such as aluminum (Al3+) and hydrogen (H3O+), thus neutralizing soil acidity. Biochar also increases the CEC of soil which helps to bind and hold cations, such as calcium (Ca2+) and magnesium (Mg2+) (Fig. 3), consequently increasing soil pH [49]. The concentration of biochar added to the soil and its frequency of application are crucial factors determining its effect on soil pH. Low application rates may have minimal pH-altering effects, while high application rates could significantly impact soil pH over time [50]. Alkaline components of biochar like calcium (Ca2+), magnesium (Mg2+), and potassium (K+), work as liming agents when added to acidic soils. These chemicals interact with soil acids, releasing hydroxide ions (OH), which balance the acidity of the soil [51]. Biochar derived from alkaline-rich materials, like wood ash, is often more alkaline in nature than biochar made from acidic sources [51]. Consequently, the choice of feedstock and pyrolysis conditions can influence how biochar interacts with soil pH [21]. In a meta-analysis of 111 investigations on the biochar’s impact on various soil parameters, reflected that biochar application is consistently responsible for the increased soil pH across different soil types and geographic regions [30].
5 Effects of biochar on plants
5.1 Growth and morphology
The positive effects of biochar on plant growth have been widely studied viz. plant height, shoot and root biomass, and leaf area [52]. For instance, Carter et al. [53] reported that contrasted with no biochar applications, in Brassica chinensis and Lactuca sativa, the biochar treatments enhanced the end biomass, plant height, root biomass, and leaf count in every cropping cycle. Carter et al. [53] also observed that soils without fertilization had the largest biomass increase due to the addition of biochar (903%), in comparison to fertilized soils (which had a 483% rise in biomass). Similarly, Jeffery et al. [40] reported that adding biochar to soils resulted in an overall benefit with a 10% statistically significant boost in agricultural production. Jeffery et al. [40] attributed this increase to various soil improvements like rise in soil pH, reduction of soil compaction and improvement in soil nutrient availability. These improvements of soil can lead to improved plant growth and biomass production. Xiang et al. [54] observed that root biomass, root volume, and surface area rose significantly after applying biochar by an average of 32%, 29%, and 39%, respectively. Additionally, the application of biochar raised the number of root tips, root length, and root diameter on average by 9.9%, 52%, and 17%, respectively [54].
In addition to its effect on plant growth, biochar has also influenced plant morphology. For example, Zhang et al. [55] conducted research on the impacts of biochar on super japonica rice roots and yield, as well as the usefulness of biochar in rice cultivation, and observed that the application of biochar in the early phase of growth enhanced the primary root’s length, volume, and fresh weight, thus resulting in increase of total and active absorption areas of the roots. Delayed root senescence along with higher root activity was observed under biochar application during late growth stages in rice [55]. Biochar treatments demonstrated higher physiological activity in roots as well as more root activity during the growing season and due to increase in panicle number/hill, grain number/panicle, rate of seed-setting, the biochar treatments had higher average yield (25.28%) than control [55]. Hence, the study concluded that in order to maximize the rice root system and physiological traits, biochar is advantageous. Interestingly, Paneque et al. [56] observed that at 1.5-ton ha−1 biochar, there was no change in the development of biomass production of the plants. But 15-ton ha−1 biochar application promoted plant growth and caused the sunflower plants to have larger leaves, longer plant stems, and wider inflorescences than those raised on the unaltered soil, thus showing the response of biochar on the plant to be dose-dependent. Based on research conducted on two cultivars of soybean (Glycine max (L.) Merr.) seedlings, Zhu et al. [57] observed that when compared to the control at ten days after germination (DAG), biochar significantly boosted total root length and root surface area, with maximum increases of 48.4% and 27.4% (P < 0.05) reported at an application rate of 1.5%. However, effects were localized mostly on fine roots (0.5 mm). Furthermore, the vitality of the roots and the quantity of soluble sugar in the leaves were considerably enhanced by both 0.75% and 1.5% biochar supplementation [57]. Conversely, at 1.5% biochar rate and at 7 and 10 DAG, the root/shoot ratio significantly decreased by 32.3% and 23.5%, respectively [57]. Hence, it was suggested by Zhu et al. [57] that regardless of the P-efficient cultivars used, biochar had a good impact on the growth of soybean seedlings by enhancing root shape and vitality.
Additionally, despite comparable numbers and sizes of nodules, it has been demonstrated that biochar-amended soil had greater levels of nitrogenase activity in individual root nodules of clover than unamended soil [58], suggesting that biochar can alter plant morphology by enhancing root growth and branching, which can improve nutrient uptake and overall plant performance. Jabborova et al. [59] demonstrated on basil (Ocimum basilicum) that, in comparison to the control, biochar concentrations (2%,3%) substantially enhanced plant height (38%, 48%), leaf length (15%, 24%), leaf number (15%, 27%), and leaf width (36%, 50%), respectively. The overall root length increased noticeably (30%, 46%, and 61%) with increase in amounts of biochar (1%, 2%, and 3%), respectively [59]. Also, in comparison with control, the treatment with 3% biochar dramatically increased root surface area (47%), root diameter (37%), and root volume (45%) [59]. Biochar’s water retention capacity was credited to reduce leaf transpiration and boost photosynthetic rate, which increases carbon fixation [56, 60].
5.2 Biochemical properties of plants
When incorporated into soil, biochar significantly influences the biochemistry of plants. The exact mechanism by which biochar beneficially alters biochemical properties, i.e., pigments, phenolics, flavonoids, lipids is yet to be elucidated. However, several possible reasons have been cited by researchers, including providing key nutrients, additional carbon, elevating functions of enzymes such as sucrose synthase, starch synthase, glutamine synthetase, glutamate synthase, improving rhizosphere and reducing water stress [33, 61, 62].
5.2.1 ROS generation and Antioxidative defense system:
As a result of cellular metabolism, reactive oxygen species (ROS) are generated naturally, and plants have evolved complex antioxidant defense systems to maintain ROS at optimal levels [63]. These molecules play dual roles in plants—they act as secondary messengers in signaling pathways that regulate growth, development, and stress responses [64]. At the same time, excess ROS can lead to oxidative stress detrimentally affecting DNA, proteins, and lipids in cells [65]. Figure 4 illustrates that biochar amendments can modulate ROS levels by affecting the plant's antioxidant defense system [66]. Antioxidant enzymes including catalase (CAT), peroxidase (POD), and superoxide dismutase (SOD), which play crucial roles in neutralizing excessive ROS can all be stimulated significantly by biochar [60] (Fig. 4). The porous structure of biochar provides a habitat for beneficial microorganisms that aid in nutrient cycling and contribute to a balanced soil environment, indirectly influencing ROS homeostasis in plants (Fig. 4). The mechanisms by which biochar reduces ROS generation in plants are multifaceted. Firstly, biochar can adsorb heavy metals and organic pollutants, preventing their uptake by plants [67]. This reduction in pollutant exposure helps alleviate oxidative stress and ROS generation. Secondly, biochar-amended soils have improved water and nutrient retention capabilities, leading to better plant health and reduced susceptibility to stress-induced ROS production [68]. Additionally, the stable carbon structure of biochar can indirectly enhance soil microbial activity, fostering a symbiotic relationship that indirectly reduces ROS levels [69] (Fig. 4).
Effects of biochar on ROS generation and antioxidative system of plants. Biochar causes stimulation in production of antioxidative enzymes such as Superoxide dismutase (SOD), Ascorbate peroxidase (APX), Catalase (CAT), Glutathione reductase (GR) and Peroxidase (POX) which minimizes oxidative stress caused by Reactive Oxygen Species (ROS) generated due to abiotic stressors such as heat, ozone, salinity. Biochar also increases microbiota and availability of nutrients like Mo (Molybdenum), Fe (Iron), Mn (Manganese), Mg (Magnesium), N (Nitrogen), P (Phosphorus), K (Potassium), B(Boron), Zn (Zinc), Cu (Copper) and Ca (Calcium)
5.2.2 Pigments
The biochar’s effect on chlorophyll content has been reported to be mixed. Some studies have indicated that biochar has potential to increase chlorophyll content, while other researches have reflected that it’s effect can be insignificant or even decrease chlorophyll content. According to Abid et al. [70], biochar boosted chlorophyll and carotenoid content in tomato plants. The study used rice husk biochar applied at rates of 0, 10, 20, and 30 g for every kilogram of soil and reported that chlorophyll content increased with increasing amounts of biochar up to 20 g per kg of soil but at 30 g per kg of soil, chlorophyll content decreased slightly, thus, establishing that beyond a threshold beneficial effect of biochar saturates and in fact shows toxic effects [70].
The increase in pigments of plants is mainly an outcome of multiple factors like an increase in nutrient bioavailability, water availability, decrease in ROS production, and boosting of plant resistance to biotic and abiotic stressors [71, 72]. A significant contributing factor to the rise in chlorophyll may be the increased availability of Mg, a component of chlorophyll. Other than Mg, increased S and N availability also positively correlates with photosynthetic rate [73]. Chickpeas grown in soil amended with biochar derived from branches of button mangrove plants resulted in a 3% increase in total chlorophyll, chlorophyll a, chlorophyll b, and carotenoids [74]. Jabborova et al. [59] on basil (Ocimum basilicum) demonstrated that at 2% and 3% biochar supplementations, the contents of chlorophyll, total sugar, flavonoids, and soil enzyme activity were all significantly enhanced. In comparison to the control, 10-ton ha−1 biochar application enhanced the vitamin C (39%), TSS (29%), total acidity (33%), and lycopene (24%) in tomato, according to Almaroai and Eissa [75].
Nevertheless, Ali et al. [76] observed that biochar decreased chlorophyll content in wheat plants in a study using wheat straw biochar. Biochar supplementation (0, 10, 20, and 30 g per kilogram of soil resulted in chlorophyll content decrease with increasing amounts of biochar up to 30 g per kg of soil) [76]. Contradictions in the literature suggest that the effects of biochar on chlorophyll content are contingent upon several parameters, such as the kind of soil, the raw materials used to make the biochar, and the concentration used.
5.2.3 Secondary metabolites
Secondary metabolites are plant compounds that have no direct role in development or growth, but they are crucial for plant defense against pests and pathogens, and in producing aromas and flavors in fruits and vegetables [77]. Table 3 depicts the impact of biochar application on secondary metabolite production in various plants. Biochar has been shown to affect secondary metabolite production in plants, although the results have been mixed. In a study by Petruccelli et al. [78], applying biochar to soil increased the production of phenolic compounds in tomato plants, which have antioxidant properties (Table 2). However, the results differed with different biochar feedstock. Deng et al. [79] on Cyclocarya paliurus in a pot study reported variations in secondary metabolite concentrations depending on pyrolysis temperatures of biochar, with the best results obtained at 500 °C of pyrolysis (Table 2). Some studies have contrasted the effect of biochar supplementation with other amendments [80, 81] (Table 2). Biochar’s efficacy was observed to be intermediate between mycorrhiza and rice husk compost in terms of reduction in proline content by Ahmadabadi et al. [80].
5.3 Nutrient uptake and concentration in plants
Biochar can also improve plant nutrient uptake by enhancing the availability of nutrients in soil [82]. Because biochar is porous, it creates an extensive network of spaces within the soil matrix, which serves as habitat for helpful soil microbes [83]. These microbes including bacteria and mycorrhizal fungus, form symbiotic relationships with plants, aiding in the uptake of essential nutrients. The enhanced microbial activity in biochar-amended soils leads to increased availability of nutrients like nitrogen, phosphorus, potassium, and micronutrients, all of which are crucial for plant biochemical processes [82].
Further, biochar has the ability to absorb nutrients and release them gradually, such as phosphate and nitrogen, lowering the risk of leaching and improving plant uptake. Uptake of nitrogen (74–80%) and phosphorus (76–95%) by tomato plants was significantly increased by biochar amendment, resulting in the increase in tomato yield (> 50%), even when the nitrogen fertilizer applied was significantly decreased [84]. Durukan et al. [85] noticed that with increasing biochar applications, Mg, Ca, K, and N concentrations in sugar beet plants increased. Additionally, Zn, Mn, and Cu microelement concentrations grew, while Fe concentrations decreased under conditions of drought stress. Nigussie et al. [86] in a study on impact of biochar application on specific aspects of chromium-polluted soils and uptake by lettuce, concluded that adding biochar considerably improved the absorption of nitrogen, phosphorus, and potassium. In soils with high Cr contamination (20 ppm), applying biochar resulted in a substantial decrease in the uptake of Cr [85]. It shows that using biochar is crucial to enhancing nutrient uptake, improving Cr-polluted soils, and lowering the carbon dioxide produced by burning wood [86]. When poultry dung biochar was pyrolyzed and then amended with NHO3 or NHO3 + H3PO4, the water-soluble P, K, Ca, Mg, Fe, Zn, Cu, and Mn rose in concentrations [86]. This had a favorable impact on the amount of nutrients made available to plants and nutrients taken up by plants [86]. At a P treatment rate of 25 kg/hm2, biochar supplementation improved the wheat root mycorrhizal colonization by 70% [87]. Additionally, soil pH and biochar’s mycorrhizal colonization value increased plant P uptake and stem growth [88]. Syuhada et al. [89], based on their corn plant study, concluded that adding biochar greatly improved the uptake of N and K by corn, but not the uptake of Mg and Ca. Pavlikova et al. [90] assessed the impact of biochar on the mustard (Sinapis alba L.) and spinach (Spinacia oleracea L.) and demonstrated that Ca, Mg, and Na content in plants was reduced by biochar, whereas K content rose in all plants.
Biochar application was also observed to increase the nitrogen, phosphorus, and potassium concentration in wheat plants [91] and micronutrient concentration in crop plants [37]. Asai et al. [92] noted an increase in iron (Fe3+) and zinc (Zn2+) concentrations in rice plants after biochar application. This was attributed to the increased availability of Fe3 and Zn2+ in the soil, increasing their concentration in rice plants [92]. Micronutrient uptake and grain yield increase in rice plants was also corroborated by Jatav et al. [93].
Nonetheless, biochar’s effect on nutrient concentration in crops may vary depending on the biochar type used, the soil type, and crop species according to Jeffery et al. [30]. For example, nutrient concentration in tomato plants was higher when biochar produced from hardwood was used than that produced from softwood [30].
5.4 Plant yield
Conclusive studies have shown that biochar can improve plant yield [75, 94, 95]. Wu et al. [96] treated cotton crops with biochar and reported with 1% biochar, the 75 mg/kg K2O treatment produced the best yield, proving that biochar can boost the yield of cotton and, as a result, reduce the need for chemical K fertilizer and lower the threats that chemical fertilizer poses to the environment. In red ferrosol and redox-hydrosol, biochar dramatically increased peanut biomass and pod yield up to two and threefold, respectively [94]. Gebremedhin et al. [95] noted that wheat grain and straw yields were dramatically boosted by biochar with respect to the control treatment by 15.7% and 16.5%, respectively. In addition, there was a 20% increase in root biomass, demonstrating that biochar helps increase wheat productivity by holding onto nutrients and water [95]. In metal-contaminated soil, compared to the control treatment, 5 and 10-ton ha−1 biochar application considerably enhanced tomato fruit yield by 20 and 30%, respectively, as per Almaroai and Eissa [75]. Also, the availability and uptake of crucial nutrients (N, P, K) were improved with amendment of biochar, while toxic element levels were decreased [75]. Agbede et al. [97] showed that biochar application increased the yield of sweet potatoes. The increase in crop yield with biochar application was attributed to its ability to enhance soil structure, WHC, nutrient availability, and microbial activity [97].
Uzoma et al. [98] examined the impact of applying biochar on yield of maize and demonstrated that applying biochar at 15 and 20 t/ha amendment resulted in increased maize yield by 150% and 98%, respectively. The researchers attributed this yield increase to the improvement of soil fertility and nutrient availability, which were enhanced by the biochar application [98]. Yooyen et al. [99] assessed the effect of applying biochar on soybean yield and observed increased seed weight by 28% and 36.8% in biochar treatments of 20 t/ha and 30 t/ha, respectively, in reference to the control group [99]. It was observed that adding biochar improved the pH, organic matter content, and availability of soil nutrients, contributing to the increase in soybean yield [99]. Dong et al. [100] noted straw biochar to be more effective in increasing paddy yield than bamboo biochar, thus, establishing a clear relation between raw materials of biochar and its effect on crop plants. Mousavi et al. [101] also reported an increase in the yield of cowpeas after biochar application in a two-year field experiment with sugarcane biochar. However, Yeboah et al. [102] observed the method of biochar supplementation to be a significant factor in affecting cowpea yield, growth, and nutrient uptake and reported spot and ring method of biochar application to be more efficient than the broadcasting method. Thus, recent research has highlighted that the use of biochar may significantly increase agricultural output.
5.5 Fruit quality
Several factors, including color, firmness, weight, dimensions, carbohydrate content, protein content, vitamin content, etc. can describe fruit quality. In addition to improving the nutritional and sensory quality of fruits, biochar can also help to protect fruits from pests and diseases. Simiele et al. [103] in a study noted that biochar increased the vitamin C content of tomatoes by 20%, the sweetness of tomatoes by 10%, and a reduction in fruit rot incidence in tomatoes by 50%. Another study noted that biochar increased the anthocyanin content of blueberries by 30% [104]. Akhtar et al. [105] noted an increase in lycopene and titratable acidity in tomatoes. Additionally, biochar increased the acidity of strawberries by 15% and reduced the incidence of powdery mildew by 30% [106]. In comparison to other soil amendments like lime, biochar had better effects on fruit quality [107]. Citrus fruit quality in terms of peel, edibility, and soluble solids also showed improvements as per Zhang et al. [108]. These improvements in taste and flavor can be attributed to the better growth (Shoot and root height, girth), biomass accumulation, increase in photosynthetic rate, increased nutrient content of the fruits, soil fertility (total C and organic matter), and protection against plant pathogens after biochar supplementation [53, 57, 109].
6 Efficacy of biochar against abiotic stresses
Abiotic stresses like drought, salinity, and heat are threats to the sustainability of global agroecosystem. These abiotic stresses are also increasing with increase in global warming [110]. Biochar, with its highly porous structure, helps in water retention over dry periods and its CEC-increasing ability helps in adsorbing excess cations [111].
6.1 Ozone
Elevated levels of ozone in the atmosphere are particularly harmful to crops and vegetation, leading to reduced yields, stunted growth, and increased susceptibility to diseases [60]. Ghosh et al. [60] used biochar at concentrations of 2.5% and 5% in a pot experiment on wheat cultivar HD 2967 exposed to elevated ozone exposure. One key mechanism through which biochar operates is by activating the plant’s antioxidant defence systems. Biochar application at both low (2.5%) and high (5%) concentrations resulted in an increase in antioxidative enzyme activities under ambient as well as elevated ozone conditions [60] (Fig. 5). A decrease in plant nutrient uptake especially of N, P, and K under elevated ozone was also observed to be ameliorated due to increased phytoavailability of nutrients [60]. Ghosh et al. [60] observed overall a beneficial impact of biochar addition on soil pH, CEC, plant height, biomass, and consequently yield. However, with respect to the grain yield of wheat, no significant increment was noticed under biochar application at 5% compared to 2.5% [60]. However, we could not find more studies detailing impact of pyrogenic carbon on ozone induced damage to plants. Although, Zhou et al. [112] ascribed biochar’s adsorbent nature as probable factor to act as ozone-remediating but methodical findings are still needed to make conclusions. Hence, this aspect needs to be explored by further studies as tropospheric ozone is a serious secondary air pollutant projected to cause heavy crop damage in coming decades [113, 114].
Abiotic stressors generate oxidative stress in plants which decreases their growth and yield. Fertilization with biochar at appropriate doses can improve soil structure and Photosynthetic activity that helps in alleviating oxidative damage and consequently Plant yield
6.2 Drought
Drought, a prolonged period of abnormally low precipitation, poses a significant threat to ecosystems worldwide, affecting both natural and cultivated vegetation. Drought stress decreases seed vigour, germination percentage, total chlorophyll, carotenoids and elevates proline content in plant [115] (Table 3). In addition, the scarcity of water prompts plants to allocate resources strategically, often prioritizing essential functions for survival [115]. Application of biochar at 10 tons/hectare and 20 tons/hectare resulted in decrease of proline content of stressed plants and improvements in germination percentage, total chlorophyll and carotenoid content in soybean (Glycine max) [115]. However, Hafeez et al. [115] demonstrated 20 t ha-1 to be most effective concentration, majorly for providing defence against drought stress. Hafeez et al. [115] also observed no significant effect of biochar fertilization on protein content of soybean leaves. Nonetheless, Biochar supplementation was identified to be suitable for increasing nitrogen mineralization, growth parameters and yield in Wheat (Triticum aestivum L.) [116]. A healthy and diverse soil microbiome is essential for supporting plant health, especially during periods of environmental stress. In wheat, improved condition of soil organic carbon, dissolved organic nitrogen, soil moisture and soil enzymes at grain filling stage were associated with better yield [116]. However, beneficial effects of biochar on growth of plants, chlorophyll content, net photosynthesis rate, WUE, Soil WHC peaked at 2.5% in tall reed (Phragmites karka) against drought and heat stress, showing a threshold limit of biochar fertilization [117]. A significant room exists for further understanding as long term studies detailing biochar’s exhaustive effect on plants against drought stress are insignificant.
6.3 Salinity
Salt stress is one the major risks to food security due to poor agronomic practices and change in weather cycles induced by climate change. The deleterious effects of salt stress are particularly pronounced in arid and semi-arid regions where irrigation practices and poor water management contribute to soil salinization. Saline soils have been reported to cause physiological drought in plants, creating difficulty in water uptake [118]. Sodium ions, prevalent in salt stress, can accumulate in plant tissues, displacing essential ions like potassium and calcium. This ion imbalance disrupts the membrane potential and enzyme activities, leading to cellular dysfunction and damage [118]. Biochar addition at 5% was observed to ameliorate toxic effects on wheat by reducing plant Na+ uptake, increasing soil moisture content, and increasing nutrient availability [118] (Table 3). The high cation exchange capacity (CEC), enables pyrogenized carbon to bind and retain positively charged ions, including sodium [118] (Table 3). By reducing the availability of excess sodium in the soil solution, biochar helps prevent the uptake of harmful ions by plant roots. Biochar at rates of 0%, 15%, 30% and 45% w/w increased soil pH and germination percentage respectively in wheat [Yannong-19] crop against salinity stress [88]. Increase in soil pH buffering capacity, nutrient availability and plant growth resulted in increase of plant biomass [88]. Biochar acts as reservoir of nutrients by supplying them to plants [88]. Increase in pH buffering capacity allows soil to maintain a more favourable pH levels for ionic uptake [88]. Ability to act as sponge and avail greater quantity of water to plant’s roots is very useful character of biochar that becomes significant under salt stress. In a study by Cakmakci et al. [119], under saline water irrigation, nutrients [Ca, K, Mg, P, Cu, and Mn], relative water content and chlorophyll content (SPAD) of the leaves also increased. A significant improvement of Mn, Fe and K content allows better photosynthetic functioning in plants [119]. In addition, leaf water potential which is a marker of soil water stress was also observed to be improved under biochar application [118]. Therefore, biochar application in salt-irrigated water circumstances has a significant chance of reducing the severity of salinity stress exposure in plants.
6.4 Heavy metal
As industrialization continues to progress, the discharge of heavy metals into the environment has become a significant concern. Heavy metals like lead, cadmium, mercury, arsenic, and zinc are among the most common pollutants, and their accumulation in soil poses a threat to plant life. Heavy metal toxicity is responsible for degradation of chlorophyll molecules, stunted growth and inducing oxidative stress in plants [120]. Heavy metals often compete with essential nutrients for uptake by plant roots. This interference can result in nutrient imbalances, affecting the plant's metabolic processes [120]. Younis et al. [121] observed that biochar application reduced harmful effects of Cd toxicity in Spinach (Spinacia oleracea) in terms of reduction in malondialdehyde content and improvement in photosynthesis. Biochar at 3.0% and 5.0% w/w decreased oxidative stress in rice plants while increasing plant growth, soil pH and Electrical conductivity under Cd stress [122]. Abbas et al. [123] applied biochar at 3.0% and 5.0% w/w on wheat subjected to Cd and drought stress and noted a decrease in oxidative stress, increase in antioxidative enzymes and plant growth. By supplying a stable source of organic matter, enhancing soil fertility and promoting the proliferation of beneficial microorganism biochar helps in combating stress conditions induced by heavy metal contamination [123]. Positive effects of biochar application were attributed to reduced phytoavailability of Cd and improvement in soil conditioning [122, 123]. Biochar has large number of functional groups like hydroxyl, carboxyl and phenolics on its surface, which increases its adsorption capacity for heavy metals and decreases their phyto-uptake [124]. Lately, a combination of biochar with ferrate, KOH and Tin(IV) Sulfide is used for heavy metals removal from environment [125]. These composites elevate electrostatic interactions and heavy metal binding on the surface of biochar. Biochar can facilitate chemical transformations in the soil, influencing the speciation of heavy metals. Its functional groups, can alter the pH and redox conditions, leading to the conversion of toxic metal ions into less harmful forms. This transformation helps in reducing the potential harm to plant roots and improves overall soil health. Aromatic carbon, quinones, and O-containing functional groups present on surface are also noted to be involved in reduction of toxic Cr(VI) to less harmful Cr(III) form [125]. Doping of biochar with suitable elements to induce hole defects can be an appropriate strategy to enhance surface reactions and reduce bioavailability of heavy metals.
Table 3 provides a summary of the application of biochar at varying rates to protect plants from a range of abiotic stressors, such as heat, drought, salt, ozone, and heavy metal toxicity. Overall, the effects were positive although a saturation was observed at higher rates of biochar application. This threshold rate of saturation was observed to be dependent on the type of crop, abiotic stress, and feedstock of biochar.
7 Efficacy of biochar in environmental pollution
7.1 GHGs
In recent decades concerns have been raised repeatedly about significant greenhouse gas emissions from agriculture [126, 127]. Biochar’s potential to effectively reduce the emissions of greenhouse gases like carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) from agroecosystems is well documented [128]. This is because biochar has a high surface area, which allows it to adsorb gases and reduce their release into the atmosphere [33] (Fig. 6). Wang et al. [129] reported that applying biochar to agricultural soil resulted in lowered N2O emissions but no significant decrease in CO2 emissions. Biochar application also reduced CH4 emissions from livestock manure by up to 70% [130]. A meta-analysis of 14 studies conducted by Cayuela et al. [131] observed a reduction in nitrous oxide emissions by an average of 49% after biochar application (Fig. 6).
Effects of biochar on decreasing Green House Gases (N2O, CH4) emission. Biochar decreases methanogenic bacteria, increases methanotrophic bacteria, reduces denitrification by increasing soil aeration and adsorbs N2O and CH4
Similarly, Clough et al. [46] noted that biochar reduced nitrous oxide emissions by 70–80% in a maize cropping system. Various explanations were provided for soil N2O reductions including decreased denitrification due to soil aeration and surface adsorption of gases on biochar surface [104, 132]. Qi et al. [133] reported biochar application reduced methanogenic archaea populations in paddy fields. Researchers have often explained methane reductions in terms of inhibition of methanogenic activity, increase in methanotrophic activity, increased porosity and adsorption after biochar application [104, 134, 135] (Fig. 6).
Overall, the use of biochar can potentially mitigate greenhouse gas emissions and improve soil quality significantly. By trapping carbon and limiting the amount of greenhouse gases generated by waste management systems, livestock, and the soil, biochar has the potential to play an important role in mitigating climate change.
7.2 Soil pollution and organic pollutant
Iron is an essential micronutrient for plant growth and plays a crucial role in various biochemical processes. However, the availability of iron in soil is highly dependent on its complex cycling dynamics. Iron can exist in different oxidation states (Fe2+ and Fe3+), and its solubility varies with changes in pH, redox potential, and microbial activity. Huang et al. [136] demonstrated that biochar, through its impact on soil aeration and leading abundance of iron-reducing bacteria, can lower soil redox potential, promoting the conversion of ferric iron to more soluble ferrous forms (Fig. 7B). In addition, conversion of Fe3+ to Fe2+ was accompanied by generation of .OH radicals which were linked with degradation of organic pollutant. This property of biochar influencing iron cycling in soil was observed by Wang et al. [137] to decrease Cd2+ uptake by paddy plants. It was observed that by formation of iron plaques, biochar induced Cd immobilization in soil.
A Pretreatment with KOH and SLS can increase micropore volume and surface area. Increased surface area and micropore volume promote CO2 capture. B. Effects of biochar on soil pollution and organic pollutant removal. Increase in pH, Iron reducing microorganism and reduction in redox potential leads to conversion of Fe3+ to Fe2+ [KOH = Potassium hydroxide; SLS = Sodium lignosulfonate]
Wang et al. [138] attempted to find out effect of various biochar concentrations on fluoride (F) contaminated tea plants. Biochar was observed to significantly reduce F contents in tea leaves under 8% supplementation, in addition to enhancement in polyphenols and caffeine contents. A correlation analysis by Wang et al. [138] showed that decrease in exchangeable Al3+ ions and Ca2+ content helped in lowering F enrichment in tea leaves. This F immobilizing property of biochar was also corroborated by Ghassemi-Gholezani and Farhangi Abriz [139] in Safflower seedlings, where biochar fertilization decreased fluoride solubility and fluoride content in plant. The observations of Sadhu et al. [140] details promising potentials of biochar in remediating fluoride contamination in potable water. This potential can be due to high adsorbing capacity of pyrogenic carbon on account of presence of functional groups that present electrostatic attraction.
Organic pollutants, arising from various industrial, agricultural, and urban activities, pose a threat to ecosystems and human health. Chen et al. [141] investigated the adsorption capacity of biochar derived from rice straw for the removal of polycyclic aromatic hydrocarbons (PAHs) from water. The results showed a significant reduction in PAH concentrations, highlighting the potential of biochar as a remediation tool. Biochar addition cause increased water availability, nutrient enrichment and microbial activity; thus, it results in a favorable environment for microbe-mediated organic matter degradation [142]. Huong et al. [143] investigated biochar activated peroxymonosulfate based degradation of organic pollutants (bisphenol A and tetracycline) in wastewater and noted increased BPA and tetracycline degradation at higher biochar application. Imidacloprid is an insecticide that is found to be toxic to human and animal cardiorespiratory and neural system. It was observed that biochar application led to imidacloprid degradation primarily due to increased hydroxyl radical production [136]. Gasim et al. [144], in a review on application of biochar to remediate organic pollutants in water, noted that doping of biochar with heteroatoms (N, S, P) increased its active sites and hence, adsorptive capacity (Fig. 7A).
This topological defect-induced increased adsorption was also supported by work of Miao et al. [145], who listed increased electron donating ability as major factor for increment of catalytic activity. This catalytic activity was observed to be significant for tetracycline degradation [144]. Gasim et al. [144], ascribed aromaticity of pyrolyzed carbon to generate major beneficial impact on organic pollutant degradation by increased catalytic power (due to π–π* transitions) and immunity from oxidative events that are not intended.
7.3 Carbon sequestration
As human activities continue to release large amounts of greenhouse gases into the atmosphere, finding effective ways to capture and store carbon has become imperative. Carbon sequestration refers to the capture and long-term storage of carbon dioxide from the atmosphere to prevent its release into the atmosphere. Recalcitrance of biochar, i.e. its resistance to decomposition renders it useful in carbon sequestration [3]. Liu et al. [146], in a review on potential of biochar as carbon-capture material in construction industry, noted biochar-cementitious composite to be significant candidate for sequestering of carbon. In addition, property of high adsorption is also highly helpful in carbon capture [125]. The characters of large surface area, presence of active functional groups and porous structure which we discussed as useful in GHGs removal are also highly beneficial in carbon capture. Fang et al. [125] performed a correlation analysis and demonstrated a correlation of CO2 adsorption with surface area and basicity of biochar. For instance, KOH is known to increase micropore volume and surface of charcoal and hence its pretreatment can be utilized to increase CO2 adsorption [147].
The reusability of biochar in CO2 capture is a very novel feature that increases its functionality. Also, since doped biochar has increased surface area and micropore volume, Zhang et al. [147] found sodium lignosulfonate infused biochar to have increased efficiency in CO2 capture. It was also noted that micropore volume and surface area were critical to absorb CO2 and VOCs. Likewise, doping with basic metals can also promote CO2’s chemical adsorption; nevertheless, doping to a great extent could lead holes to plug thereby preventing adsorption. Associated to the aforementioned, increased surface area and surface basicity of bamboo biochar treated with KOH resulted in higher CO2 acquisition [148] (Fig. 7A). However, the degree of sequestration of biochar also differs with feedstock. Tan et al. [149] listed computer- aided planning and tools to design biochar especially for agro-industrial purpose with goals of developing negative emission additive. Therefore, although biochar is a viable option for locking carbon, there are still challenges that need to be addressed, including variations in the material's characteristics, optimal methods to implement it, and potential adverse effects. Additionally, the scalability of biochar adoption on a global scale necessitates further research and development. Regardless, biochar remains highly effective in carbon sequestration.
8 Limitations of biochar
Although biochar application has been reported by majority of studies as beneficial to agroecosystems, some concerns have been raised about possible side effects [33]. Factors governing production of biochar including its raw materials may contribute to its potential toxicity. While biochar’s inhibitory effect on N2O emissions has been well documented, NOx emissions may increase during synthesis of biochar due to NOx predecessor’s formation [150]. A potential release of NH3 has also been documented by Liu et al. [150] which can contribute to atmospheric hazing and particulate matter pollution. Besides, biochar contains particulate carbon and during handling it can cause health hazards like respiratory problems. Because it might contain hazardous organic chemicals and heavy metals, biochar can also cause adverse effects on plant development [33]. Choice of feedstock is a very vital step in biochar synthesis as biochar sourced from waste products may contain toxic elements like Arsenic (As) and Pb [151]. This becomes alarming if use of biochar is to be recommended on large scale for farmers, especially if biomass used for biochar generation is not carefully sourced. For example, biochar sourced from Mischanthus was observed by Oleszczuk et al. [152] to have greater concentration of trace metals and Polycyclic Aromatic Hydrocarbon (PAHs). According to Rombola et al. [153], biochar supplementation can also increase PAH, which can cause oxidative stress in plants. This concentration of PAH occurs because during pyrolysis, several aromatic ring formations happens in biomass. However, Wang et al. [154] established an incremental lifetime cancer risk assessment of PAH induced in plants grown in biochar amended soil and noted no significant hazard to human health. Thus, with multiple factors directing biochar’s effect renders unpredictability to its applicability, care becomes more vital in selection of feedstock. In a germination test, Liao et al. [155] produced biochar from wheat, corn, and straw at different temperatures of 200, 300, 400, and 500 °C. Wheat, rice, and maize seedlings could not establish roots or stems when treated with biochar made from rice straw produced at 500 °C [155].
Another case of toxicity of biochar was reported by Novak et al. [156] when, after using biochar for 25 days, it was discovered that the amount of nitrate in the soil leachate decreased, which was proportional to the amount of biochar used. This demonstrated that N could be adsorbed onto the biochar’s surface, inhibiting plant growth by lowering the amount of inorganic N readily available. Biochar is also documented to contribute in soil leachate of phosphate and organic carbon, reducing its phyto-availability [157]. Dissolved organic carbon is harmful for phytoplankton population of aquatic ecosystems [158]. Increase in soil pH due to biochar can also hamper NH4+ uptake [159]. Additionally, while charcoal has been reported to disturb gram negative bacterial cell–cell communications via N-acyl-homoserine lactone [160], a similar hazard does exist in biochar amended soil (Fig. 8).
Toxic effects of biochar supplementation. Biomass extracted from waste grown vegetation might contain toxic metals. Aromatic ring formation during pyrolysis can lead to PAH formation. PM Particulate matter, PAH Polycyclic Aromatic Hydrocarbon, DOC Dissolved Organic Matter
Furthermore, biochar amendment at high doses might contribute to soil salinity causing nutrients to precipitate [161]. Besides, the biochar’s effect on crops in temperate climate still remains uncertain [30], thus establishing a climate dependent benefit to agroecosystem. Additionally, Weyers and Spokas [162] have reported harmful effects on earthworm populations with biochar amendment. Recommendation of biochar to farmers will also require some serious practical considerations such as access to consistent supply of biomass for biochar feedstock and possible loss of biodiversity that could happen if woody plantations are taken on large scale. Although, use of Prosopis juliflora, an invasive woody tree is increasingly used as source for wood-derived biochar, efficacy of agricultural residues-based biochar needs to be thoroughly investigated to achieve a more sustainable goal towards circular economy. Therefore, application of biochar without pre-assessment of its feedstock, soil deficiencies and crop requirements could lead to no or even toxic effects.
8.1 Future prospects
Most of the present studies are focused on short-term biochar applications. However, to develop an in-depth insight into biochar’s benefits and its usability by farmers a need for long-term field study exists. Future research should focus on ascertaining the optimal rates and application mechanism of biochar for various crops and soils, along with its long-term effects on health of soil and plant. The biochar’s effect has been reported to augment antioxidative enzymes [60], but their molecular expression is yet to be researched. Whether biochar leads to any effect on gene expression is a question yet to be answered. Also, since different sources as well as pyrolysis temperatures affect biochar properties, studies should focus on the effect of husk, wood, and manure derived biochar prepared at different temperatures on one crop to have a comparative analysis. Thus, there exists an identified necessity to standardize biochar field studies. Major loophole that exists in existing literature is lack of comprehensive studies that details biochar’s effect on growth, yield, biochemistry of plants, soil ecosystem and on target as well as non-target microbiota that will help in weighing biochar’s benefits as well as possible harms.
Data availability
Not applicable.
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Acknowledgements
The authors are thankful to the Head, Department of Botany, Banaras Hindu University. For providing the facilities required to perform a comprehensive literature review, the authors are grateful to the Co-ordinators, Institute of Eminence, Centre of Advanced Study (CAS) in Botany and ISLS, Banaras Hindu University. Vineet Upadhyay is highly appreciative of University Grants Commission (UGC), New Delhi, India for providing Junior Research Fellowship (JRF). Krishna Kumar Choudhary is grateful to Science & Engineering Research Board, New Delhi (P-07/1311) and Seed grant, Institute of Eminence (IoE - Dev. Scheme no. 6031), Banaras Hindu University, Varanasi, India for the 253 financial assistance. Shashi Bhushan Agrawal is also thankful to CSIR Emeritus Scientist research Project (Project code no. P-25/0878) for financial support.
Funding
This work was supported in the form of JRF [UGC-Ref No.:211610058595] from University Grants Commission [UGC], New Delhi, India and the CSIR Emeritus Scientist research Project (Project code no. P-25/0878).
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Vineet Upadhyay: conceptualization, visualization, writing- original draft preparation. Krishna Kumar Choudhary: visualization, writing-review and editing, supervision. Shashi Bhushan Agrawal: conceptualization, visualization, writing- review and editing, supervision. All authors read and approved the final manuscript.
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Upadhyay, V., Choudhary, K.K. & Agrawal, S.B. Use of biochar as a sustainable agronomic tool, its limitations and impact on environment: a review. Discov Agric 2, 20 (2024). https://doi.org/10.1007/s44279-024-00033-2
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DOI: https://doi.org/10.1007/s44279-024-00033-2