1. Introduction

Climate change negatively impacts crop growth. Salinized, weak alkaline soils; temperature fluctuations; and latitudinal factors require urgent scientific attention [1, 2]. Soil, a complex ecosystem, plays a vital role in agriculture, ecosystem services, climate change mitigation, landscape protection, and human development [3, 4]. Long-term cultivation can lead to degradation, resulting in reduced soil organic matter, severe erosion, and diminished aggregate stability [5, 6]. Soil quality directly impacts ecosystems, affecting crop production, biota, and human well-being, contributing to a healthy environment [7, 8]. Various factors, including pollution levels, bacterial activity, climate, and land use, can alter soil characteristics [9]. Green approaches, like soil amendment, offer effective means to reduce salinity by enhancing physical and chemical properties and nutrient distribution. These approaches also promote microorganism growth and restore soil health [10, 11, 12, 13]. The application of additives is a promising method to enhance soil characteristics. In recent years, biochar (B) has gained significant interest as a versatile soil fertilizer, owing to its high porosity, abundant surface functional groups, strong adsorption capacity, and recalcitrant carbon content [14].

Biochar (BC) is plant-derived charcoal used to absorb carbon dioxide when incorporated into soil [15, 16]. It plays a crucial role in long-term climate change mitigation [17] and enhances soil biochemical properties [18]. BC also facilitates large-scale disposal of waste biomass [19]. Studies highlight its significance as a carbon source and soil enhancer, sequestering pollutants and enhancing fertility [20]. Its elemental composition includes essential elements like C, N, and H, alongside lower-nutrient elements [21]. Adding BC improves soil properties such as porosity, bulk density, and water availability [22]. Pyrolysis releases volatile compounds, increasing BC’s surface area and creating pores akin to honeycombs, enhancing water and nutrient retention [23, 24].

BC incorporation enhances soil structure, aeration, nutrient availability, and alters microbial communities, fostering crop growth and yield [25, 26, 27]. It raises soil pH and enhances cation exchange capacity [28, 29]. Amended soils exhibit heightened fertility and nutrient retention [30], with significantly boosted extractable nutrient levels [31]. BC increases hydraulic conductivity, base saturation, and nutrient levels and reduces erosion rate [32, 33]. It halts the leaching of nitrate nitrogen from soils [34]. The diverse pore sizes of BC influence microorganism habitats, promoting beneficial bacterial survival and enhancing bacterial structure [35, 36, 37, 38, 39, 40, 41]. BC’s effects on soil microorganisms are influenced by nutrient availability, microbial community composition, plant–microbe signaling, and habitat creation [42].

Climate change disrupts the vital carbon storage in soil, impacting global food production and greenhouse gas emissions [43, 44, 45, 46]. Methane (CH₄) and nitrous oxide (N₂O), potent greenhouse gases, primarily from agricultural land, have a significant warming effect [47]. Biochar is pivotal in curbing emissions and averting climate change by enhancing soil quality, reducing pollution, and stabilizing carbon [48, 49, 50]. It is technically and economically feasible for carbon capture and storage [51], disrupting the carbon cycle and preventing emissions from reentering the atmosphere [52, 53]. This fosters soil health, lowers emissions, and boosts carbon sequestration [54, 55, 56].

2. Literature review methodology

A set of thematic keywords were used for exploring the relevant research articles and review papers, such as biochar, feedstock, soil quality, soil fertility, soil pH, soil physicochemical properties, biochar amendments, soil organic matter, carbon sequestration, the role of biochar amendment, plant growth and yield attributes, crop performance, and so forth. In this regard, five different databases were used, namely, Research Gate, Google Scholar, Science Direct, Web of Science, and Scopus. These databases comprise extensively used huge collections of related mainstream research articles. Following this, multiple research and review articles were selected within the era 2000–2022. Furthermore, related research articles referenced in the abovementioned papers were also reviewed.

3. Application of biochar in soil quality improvement

Biochar is considered as a potential soil amendment, as its addition improves the C accumulation (60–80%) in the soil, which is central to the enhancement in the soil properties, and hence, biochar technology could be very effective for soils having low organic matter contents (OM) [22]. The following sections discuss the role of biochar as a potential amendment to enhance soil quality.

3.1 Improvement in soil bulk density

Soil bulk density, a critical indicator of soil physical properties, is closely linked to soil tightness [57]. It reflects soil compaction and health and influences crucial aspects of the plant life cycle [58]. Enhanced soil porosity and aeration, facilitated by biochar, lead to reduced bulk density [59]. Biochar’s lower bulk density contributes to decreased soil bulk density and improved water holding capacity due to its larger surface area [60, 61, 62]. Numerous studies have reported a significant negative impact of biochar addition on soil bulk density [63, 64]. Głąb et al. [65] found a 35% reduction in bulk density with the incorporation of 4% biochar. Similarly, Qin et al. [66] documented a decrease in bulk density with an increase in overall soil porosity after introducing biochar. The influence of biochar on soil bulk density is closely tied to soil texture; coarser-textured soils experience more pronounced reductions compared to finer-textured soils [67]. This may be attributed to the higher bulk density of coarse-textured soils (~1.6 g cm⁻³) compared to biochar (~ 0.6 g cm⁻³), allowing interactions between the biochar and soil particles, resulting in a decline in the final bulk density of the soil. Additionally, biochar’s high porosity and sandy soils’ lower porosity contribute to decreased bulk density [68]. Furthermore, the nature of feedstock material and pyrolysis temperature significantly influence bulk density, with higher pyrolysis temperatures leading to lower bulk density [69]. The rate of biochar application also affects bulk density, generally showing a negative correlation with application rate [58].

3.2 Improvement in surface area and soil porosity

Biochar significantly influences soil porosity through its pore distribution, particle size, and connectivity [57]. Studies have reported an increase in soil porosity following biochar addition, ranging from 14% to 64% [22]. Higher pyrolysis temperatures enhance soil porosity, potentially due to organic matter decomposition and the formation of micropores [70, 71]. The impact of biochar on soil porosity depends on soil texture and type, with coarse-textured soils showing a greater increase compared to fine-textured soils, likely due to their lower inherent porosity [72]. In sandy soils, biochar improves porosity by enhancing mechanical interactions with soil particles, leading to notable improvements [73]. Increased porosity positively affects soil microbial activities, and different pore size distributions may influence specific microbial species [74]. Additionally, Seyedsadr et al. [75] found that biochar addition increased total porosity and reduced bulk density compared to compost. They suggested several mechanisms for improved soil porosity, including direct pore effects within biochar, accommodation pores formed between soil aggregates and biochar, and enhanced persistence of soil pores through increased aggregate stability [76].

3.3 Improvement in soil aggregate stability

Soil aggregate stability is crucial for resisting mechanical stresses like surface runoff, water erosion, and precipitation effects [58, 77]. Disintegration of aggregates leads to fine particles susceptible to erosion, potentially forming a soil crust upon re-sedimentation [78]. Biochar application significantly impacts aggregate stability through interactions with soil mineral surfaces, facilitated by oxidized carboxylic groups on biochar particles [79]. This interaction fosters aggregate formation, creating stable microaggregates and larger pores, ultimately enhancing stability and structure [80]. Studies have reported an increase in soil aggregate stability ranging from 6 to 217% after biochar addition [32, 81]. This improvement may be attributed to the high carbon content of biochar, forming bonds with various oxides, and organic matter providing a food source for microorganisms. This leads to increased microbial activity and the secretion of mucilage, contributing to stable aggregates [82]. Additionally, biochar may enhance aggregate resistance against clay swelling and slaking [83].

3.4 Improvement in soil consistency

Soil consistency attributes, such as plastic limit, liquid limit, and plasticity index, are crucial for designing stable slope systems [84]. Understanding soil consistency is vital for engineering applications and agronomic contexts, including tillage operations and compaction. Additionally, biochar application can enhance soil consistency by increasing organic carbon concentrations [85]. Different types of biochar applied to sandy soils at various rates resulted in significant improvements in plasticity index (48–99%) and liquid limits (8–22%) [85]. This improvement is primarily attributed to biochar’s larger surface area and increased porosity [86, 87]. Laboratory experiments by Choudhary et al. [88] showed that biochar addition improved soil consistency limits up to a specific weight limit, leading to enhanced physical properties of the soil.

3.5 Improvement in cation exchange capacity (CEC)

The CEC indicates the ability of soil to absorb, retain, and exchange cations, and enhancing the number of exchange sites for cations can augment the soil CEC contents. Soils having a high CEC can readily absorb K⁺, NH₄⁺, Mg²⁺, and Ca²⁺ and improve the utilization of nutrient ions in soils [89]. The CEC is largely pH-dependent particularly on tropical soils, and some biochars can raise the pH of the soil as well as soil CEC [90, 91, 92]. Use of peanut shell-derived biochars in an acidified soil increased the soil pH, CEC, SOM, as well as plant biomass [93]. Biochar application increases soil charge and cation exchange capacity (CEC) by 20–40% compared to control conditions [94]. Non-wood feedstock biochar exhibits higher CEC than wood-derived biochars [95, 96]. Feedstock type, application rate, and pyrolysis temperature are crucial factors influencing soil CEC regulation [97].

3.6 Biochar application on soil EC and pH modulation

Soil acidity is a significant challenge for upland agriculture, affecting approximately 30% of potential arable lands globally [98]. Factors such as aluminum (Al) toxicity, as well as deficiencies in calcium (Ca), magnesium (Mg), and phosphorus (P), act as constraints on crop production in acidic soils [99]. The alkaline properties of specific biochars play a crucial role in enhancing crop productivity in acidic and highly weathered soils [100]. However, the addition of pine sawdust biochar in sandy desert soil has been observed to decrease soil pH [101]. Hence (Figure 1), it is imperative to exercise caution in selecting the appropriate acidic or alkaline biochar capable of modifying the soil rhizosphere accordingly for optimal plant growth. Electrical conductivity (EC) is closely related to the concentration of salts dissolved in water and is a critical factor influencing upland crop growth (Table 1). Biochar derived from agricultural waste and woody feedstock typically exhibits low to moderate EC, while manure-derived biochar tends to have higher EC [116, 117, 118]. Biochar application has been shown to improve the growth and yield of crops in saline conditions, attributed to higher concentrations of essential ions in the biochar [118]. Most biochars have higher soluble salt content and, consequently, higher EC compared to agricultural soils [119]. However, excessive salt content is detrimental to plants due to reduced osmotic potential. Therefore, maintaining low soil EC is crucial for optimal nutrient availability and plant growth. It is worth noting that the EC of soil may increase with higher application rates of biochar [102, 103, 120, 121]. Rice husk biochar, however, did not significantly impact soil EC [122]. Biochar pH ranges from 4 to 12 and directly influences soil pH upon application. Biochar interacts with aluminum ions (Al³⁺) and hydrogen ions (H⁺) in soil, leading to reduced ion concentrations [123]. Higher pyrolytic temperatures can result in biochar with alkaline pH [124]. In acidic soils, biochar additions raise soil pH to varying degrees [92, 125, 126]. The pH increases gradually with higher biochar application rates, but the impact on alkali soils is less pronounced [94]. Biochar additions lead to a 25% increase in crop yield in tropical regions, attributed to lower acidity, whereas effects are less significant in temperate zones [127]. Biochar addition also affects soil pH and the activity of essential ions involved in phosphorus (P) complexation and sorption [128]. Research by Lu et al. [129] demonstrated that various types of biochars enhance soil pH and buffering capacity, correlating with improved soil pH buffering capacity.

Figure 1.
Conceptual scheme on the effect of biochar amendment on the physicochemical, biological properties, and greenhouse gas emissions (adapted from Li et al. [102, 103]).

Experiment	Feedstock	Temperature (° C)	Application rate	Plant	Effect on soil properties	References
Pot	Rice husk	750	0, 2, and 4% w/w	Oryza sativa	Improved soil properties observed	[104]
Pot	Woodchip biochar, straw biochar, and vineyard-pruning biochars	525	3% w/w	Sinapis alba, Hordeum vulgare, and Trifolium pratense	Lowered bulk density and enhanced aggregate stability	[63]
Field	Poultry manure	450	5 t ha⁻¹	Zeya mays	Increased water retention (3.3–31.3%) after biochar application	[105]
Pot	Coffee ground and coffee husk	530	4, 8, 12, and 16 Mg ha⁻¹	Zeya mays	Improved nutrient and water retention, boosting water use efficiency by 50% and enhancing carbon content	[106]
Pot	Sludge and straw biochars	500	2 and 4% w/w	Triticumaestivum	Decreased bulk density by 17–18%, tensile strength, and soil surface cracks and increased shear strength.	[107]
Field	Barley straw	400	10 t ha⁻¹	Brassica rapa	Decreased the bulk density, enhanced soil porosity and s pH, total N, CEC, and available P	[108]
Field	Cattle manure	600	0, 1.5, 2.5, and 5% w/w	Spinacia oleracea	Increased total porosity by 2–12%, and water use efficiency	[109]
Pot	Wood, bamboo, rice straw, and Chinese walnut shell	500	5% w/w	Phyllostachy pubescens	Improved the soil EC, and decreased the soil pH.	[110]
Pot	Soft wood biochar made by gasification procedure	500 and 600	4 and 10% w/w	Lollium perenne and Eruca sativa	Increased water retention decreased in the pH	[111]
Field	Peanut shell	220	5 t ha⁻¹	Nicotiana tabacum	Decline the bulk density, increased total N, total porosity, water contents, and nutrient holding capacity	[112]
Greenhouse	Wheat straw	350–550	0, 25, and 50 t ha⁻¹	Solanum lycopersicum L.	Decreased the bulk density, enhanced soil porosity, and improved soil 3-phase composition	[44, 45, 46]
Greenhouse	Southern yellow pine	400	5, 10, 15, and 20% w/w	Vitis rotundifolia	Decreased the bulk density (40%), increased soil porosity (50%) and soil pH buffering capacity	[113]
Field	Rice husk	—	0 and 2 t ha⁻¹	Brassica oleracea	Improved infiltration rate, hydraulic conductivity, maximum water holding capacity, and aggregate stability.	[114]
Pot	Maize residues	450–950	0, 2.1, 4.2, and 8.3 g kg⁻¹ w/w	Phaseolusmultiflorus	Improved the soil aggregation (188%) and soil water retention (128.9%)	[115]

Table 1.

Effect of biochar addition on soil physicochemical properties and plant growth.

3.7 Improvement in soil organic matter (SOM)

Soil organic matter (SOM) is a vital factor reflecting soil fertility and providing nutrients for soil life [130]. Biochar application increases SOM, with effectiveness dependent on biochar quantity and stability [131, 132]. Biochar contains essential plant nutrients, enriching soil nutrient levels upon application [57]. It also catalyzes the formation of soil organic matter from small organic molecules [82, 133]. Biochar enhances soil fertility and crop yields by boosting SOM content [134]. Elevated SOM content improves crop growth and productivity due to increased soil porosity and nutrient availability [135]. In coastal regions, biochar.

addition increases corn yields while reducing soil salinity [136]. It also enhances root microbe activity and organic matter uptake by plant roots. Chen et al. [133] found that applying biochar at 15 and 30 t ha⁻¹ increased SOM content. Additionally, Liu et al. [137] showed that biochar application reduced organic matter mineralization by inhibiting the β-glucosidase enzyme.

3.8 Improvement in soil biological properties

The effects of biochar application on soil microbial biomass are diverse, with some studies indicating positive impacts [138] and others showing no significant changes [139]. Additionally, biochar can enhance soil microbial biomass carbon, nitrogen, C-mineralization, and enzyme activities [140, 141]. Barman et al. [142] observed an increase in different enzyme types with freshly prepared biochar. Moreover, biochar has been found to mitigate toxic contaminants and modify soil enzymes [143]. Furthermore, biochar application has been associated with increased root colonization and spore germination of mycorrhizal fungus, attributing to improvements in soil properties [144, 145]. It has also shown potential to enhance biological nitrogen fixation in various crops [146, 147, 148]. Shifts in microbial community structures and functions after biochar supplementation are influenced by physicochemical attributes of biochar, organic matter content, nutrient availability, and water retention capacity [149]. Additionally, dominant bacterial groups like Alphaproteobacteria, Betaproteobacteria, Proteobacteria, Gammaproteobacteria, and Rubrobacteridae have been reported to increase after biochar addition [150]. Tang et al. [151] observed that soil microorganisms, including those in the rhizosphere, utilize various carbon sources after biochar amendment. In a 30-month experiment, Somerville et al. [152] found that the addition of organic matter, including biochar, improved soil biological properties, attributing it to increased porosity supporting microbial biomass, particularly mycorrhiza [153]. Kolton et al. [154] noted that biochar has the potential to enhance the diversity and metabolic potential of microbiota in the tomato rhizosphere. Pei et al. [155] reported that biochar amendment increased C mineralization, microbial respiration, and microbial communities. Additionally, nutrient-enriched biochar stimulated microbial activities responsible for pesticide degradation [156, 157], indicating a close interaction between microbial activities and pesticides in soils [158]. Biochar application also enhanced the alpha diversity of various microbial communities, particularly fungal species [159]. While numerous studies have focused on improving soil biological attributes with biochar application (see Table 2), there is still much to learn about the dynamics of soil microbial communities and the enhancement of soil biological properties in relation to biochar application.

Experiment	Feedstock	Temperature (° C)	Application rate	Plant	Effect on biological properties	References
Pot trial	Pine needle and Lantana biochar	—	2 and 5 t C ha⁻¹	Triticumaestivum and Oryza sativa	Increment in the soil biological activities	[160]
Field	Red gram, Maize stalk, cotton stalk, and mesquite wood	350–400	2.5 and 5 t ha⁻¹	Arachishypogaea	Increment in the population of actinomycetes, bacteria, and fungi	[161]
Field	Poultry litter	300	2.25 and 5 t DM ha⁻¹	Pasture grass	Reduced soil toxicity to Heterocyprisincongruensand Vibrio fischeri	[162]
Pot	Mixture of maple, oak and birch woody biomass	—	1 t ha⁻¹	Zea mays	Increment in the soil potential microbial activities	[163]
Field and white peat	Holm oak	650	3% w/w	Lactuca sativa and Fragariaananassa	In strawberry grown in white peat, biochar application altered the rhizosphere microbiology	[164]
Field	Local timber harvest	450–550	20 t ha⁻¹	Cucurbita maxima	Increased microbial biomass C, enzyme activities, and phosphatase enzyme	[165]
Pot	Orchard pruning biomass	500	65 g kg⁻¹	Lactuca sativa	Biochar in combination with compost enhanced the population and diversity of microorganisms	[166]
Field	Wood biochar	450–550	20 t ha⁻¹	—	A shift in the bacterial dominated microbial communities was observed after the biochar addition for 3 months	[167]
Pot	Hardwood lumber scraps	500–550	20% w/w	Solanum melongena	Biochar alone and in combination with compost altered the soil microbial communities and their functional diversity	[168]
Growth chamber/Pot trial	Maize	210 and 600	2% w/v	Cicer arietinum	Improved the symbiotic relationship with Mesorhizobiumciceriand improved the nodule number by 52%	[169]
Field	Rice straw	450	1500 kg ha⁻¹ a⁻¹	Oryza sativa	Improved the microbial biomass C and microbial biomass N	[170]
Field	Waste eucalypt wood	550	10% v/v	Corymbia maculata	Improved microbially mediated OM decomposition, increased microbial activities	[152]
Pot	Platanus orientalis derived	650	0, 0.5, 1, 2, and 4% w/w	Brassica chinensis	Increased sucrase, urease, and catalase activities	[171]
Field	Yellow pine	350	11.2 and 22.4 t ha⁻¹	Paspalum distichum	Compost amendment delivered more promising results as compared to biochar amendments in terms of microbial biomass	[172]
Pot trial	Cherrywood, wood, and maize	450, 600, and 850	2% w/w	Lactuca sativa	Improved soil enzyme activities (N and P cycling) and plant interaction with microbial inoculants	[173]
Field	Senna siamea	485	2000 kg ha⁻¹	Zeya mays	Improvement in the dehydrogenase activities, fungal, bacterial population and microbial biomass C	[174]

Table 2.

Effect of biochar prepared from different sources on the soil biological properties.

4. Effect of biochar amendment on soil fertility

Soil fertility refers to a soil’s ability to provide essential plant nutrients for sustained crop productivity [175]. Modern agricultural practices often lead to soil degradation and nutrient depletion [175]. Biochar addition improves soil properties by increasing organic carbon and crucial mineral nutrients like nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur [176]. However, biochar’s nutrient composition depends on raw material source and pyrolysis conditions, which may not always meet diverse soil requirements [177]. Biochar also enhances nutrient retention and utilization in soil. After incorporation, nitrate and ammonium nitrogen retention increases significantly [178]. Additionally, biochar treatment leads to substantial increases in grain biomass and nitrogen and phosphorus utilization efficiency compared to using nitrogen fertilizer alone [45]. Research on biochar production and applications is rapidly expanding [179, 180]. While carbon, nitrogen, and phosphorus cycling influences soil microbial activity, other essential elements like sulfur, potassium, magnesium, and calcium are crucial for soil fertility and quality [181, 182]. Biochars rich in exchangeable base cations support plant growth [183], and a strong correlation exists between oilseed rape yield and soil potassium content [184]. Comprehensive assessment of soil fertility considers interconnected interactions, with integrative indicators like soil pH, electrical conductivity, and cation exchange capacity providing a more holistic understanding [181, 182].

5. Improvement in carbon (C) sequestration

The rise in ambient temperature due to anthropogenic activities has led to a 0.88°C increase in global surface temperature since the nineteenth century [185, 186]. Biochar application offers an eco-friendly solution for soil organic carbon (SOC) sequestration [11, 187, 188, 189]. It enhances SOC recalcitrance under warming conditions, promoting carbon sequestration [190]. Biochar also inhibits SOC mineralization by increasing biochemical recalcitrance or microbial activity [191, 192]. Field experiments over three years demonstrate significant improvements in soil carbon sequestration with biochar addition [193]. Various studies confirm biochar’s efficacy in enriching the SOC pool and reducing GHG emissions in agricultural soils [194, 195, 196, 197, 198]. Feedstock and pyrolysis conditions are critical factors influencing biochar’s effectiveness [199]. Zhang et al. [200] found that biochar addition increases recalcitrant carbon in soil, reducing carbon cycling enzyme activity while enhancing phosphorus, nitrogen cycling enzyme, and oxidase activities. They suggest combining biochar with nitrogen fertilization for enhanced carbon sequestration. Rahman et al. [201] report that biochar application from sugarcane bagasse improves SOC retention and other soil quality parameters. Han et al. [38] demonstrate significant increases in carbon sequestration and reductions in net global warming potential with apple wood-derived biochar application, offering promise for mitigating global warming concerns related to apple production.

xudong Ma

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