Revitalizing Degraded Soils: The Role of Biochar in Enhancing Soil Health and Productivity

Abstract

Biochar (BC), a carbonaceous material derived from biomass pyrolysis, exhibits a wide range of physicochemical properties, including a high cation exchange capacity, porosity, and specific surface area, which make it a highly valuable amendment for soil enhancement and environmental sustainability. As BC has shown strong potential to remediate soils, enhance their fertility, and increase crop productivity, it can successfully be used as a soil remediation factor. Additionally, it can play a critical role in carbon sequestration and climate change mitigation, revealing a high sorption capacity, multifunctionality, and long-term persistence in soils, where it can remain stable for hundreds to thousands of years. The present systematic review aims at presenting the dynamics of BC when incorporated into a soil system, focusing on its pH, water-holding capacity, aeration, microbiota, and carbon and nutrient availability across various case studies, particularly in acid, saline/sodic, and heavy metal-contaminated soils. Given the variability in BC performance, robust, long-term field-based research is essential to validate the current findings and support the development of targeted and sustainable biochar applications.

1. Introduction

Soils are directly and indirectly the basis for the supply of ecosystem services, including food production, which is indispensable for human prosperity [1,2]. According to the Food and Agriculture Organization (FAO), 95% of the world’s food is produced directly or indirectly through soil-based processes [3]. High-quality soil supports nutrient cycling, agricultural production, and biodiversity conservation [4]. However, one-third of this valuable resource has degenerated to a considerable extent because of erosion–desertification, nutrient replenishment, loss of organic carbon, compaction, acidification, and salinization [3].

In addition, a range of anthropogenic activities such as pollution, waste discarding, and land use/land cover (LU/LC) changes further deteriorate the properties of the soil. Population growth, sectoral developments, and shifts in the waste produced, reported during epidemics, have resulted in a dynamic increase in environmental pollution [5]. The degradation of soil has intensified due to the input of rising quantities of pollutants such as polycyclic aromatic hydrocarbons (PAHs), persistent chemicals, heavy metals, and engineered nanomaterials [6,7]. An additional environmental and economic burden enhancing the nutrient imbalance of soils is the ongoing “fertilizer crisis”. The excessive use of chemical fertilizers and the lack of knowledge on their optimal application have worsened soil degradation and caused severe spot and diffuse pollution, which can be critical on a catchment scale [8,9]. Moreover, chemical fertilizers often exhibit low efficiency since nutrients are not in a form directly accessible to plants. Additionally, their production demands high energy input while failing to compensate for the loss of organic matter caused by humus degradation.

An indicative example of the extent of soil degradation nowadays is demonstrated by the European Soil Data Centre (ESDC) Dashboard, where it is presented that 62% of European soil is degraded [10]. According to the above, a number of countries in Central and Southern Europe face threats to the biological functions of their soil, with a peak value of 77.5% in the Netherlands, where nearly 60% of the land is used for agriculture. Additionally, phosphorus deficiency (<20 mg kg⁻¹) is found in large regions of Southern Europe, including Portugal (>64.4%), Spain, Slovenia, and Greece (>53.5%). On the other hand, nitrogen surplus (>50 kg ha⁻¹) is a major issue in Ireland (>79.6%), the Netherlands (>87.4%), and Luxemburg (>85.6). Nutrient imbalance in soils strengthens the link between soil degradation and fertility loss through limited nutrient use efficiency (NUE, PUE, KUE, etc.) by plants [5,11]. Moreover, a recent study reported that unamended drought-tolerant millet plots observed a 37% yield reduction per cm of topsoil loss [12,13].

Beyond direct human activities, climate change can lead to soil erosion, degradation, and desertification, endangering the sustainability of several ecosystems. Therefore, along with farming practices unfavorable to soil conservation, land use/land cover (LU/LC) changes, and pollution, the human-induced climate crisis also aggravates soil quality, afflicting soil fertility and crop productivity. The encapsulated soil organic carbon (SOC) that is liberated with soil erosion can be emitted into the atmosphere in the form of carbon dioxide (CO₂), increasing the concentrations of greenhouse gases (GHGs). The aforementioned highlights the necessity for sustainable solutions for soil amendment.

One emerging solution is biochar (BC), a carbon-based material produced by pyrolysis of biomass under anaerobic conditions, which has attracted considerable attention as a soil amendment. Its composition includes elements such as nitrogen (N), potassium (K), and phosphorus (P), which serve as nutrients for plant growth, while its carbon content ameliorates the organic matter concentration of soils. BC is very stable and recalcitrant in soil [6]. Its physicochemical properties are favorable for barren soil mitigation and microbiota increase. These properties place biochar as a promising option to enhance soil health and productivity of degraded soils [14,15].

Although previous reviews have addressed various aspects regarding biochar use in soils, this review attempts to gain insight into the complex dynamics accompanying BC application to soil by reviewing the latest advances in the literature and the emerging developments on former limitations and providing discernment of its multiple benefits and potential. This review aims to move beyond generalizations and offer insight for agricultural and agroforestry managers, as well as policymakers, in adopting good practices and stipulating guidelines regarding the establishment of BC as a sustainable soil amendment.

2. Materials and Methods

The methodology of this review is based on the criteria of a systematic review, often followed in the waste treatment field and broader environmental research [16,17], developed by Boaz et al. [18]. The review methodology incorporates the following steps: Firstly, the survey of the literature was based on specific criteria to ensure that only relevant and high-quality studies were considered. Research articles were retrieved from trustworthy sources such as the following scientific repositories: Web of Science (https://www.webofscience.com/wos/woscc/smart-search, accessed on 5 February 2025); Science Direct (https://www.sciencedirect.com/, accessed on 5 February 2025); Scopus (https://www.scopus.com/pages/home?display=basic&zone=header&origin=sbrowse#basic, accessed on 6 February 2025); PubMed (https://pubmed.ncbi.nlm.nih.gov/, accessed on 6 February 2025); and European Commission data (https://ec.europa.eu/eurostat/data/database, accessed on 7 February 2025). The advances in the subject matter (i.e., biochar as a soil amendment) were elaborated based on up-to-date studies where BC implementations are discussed. The retrieved studies were then scrutinized, and their methods and references were also checked, to ensure their validity. A large part of the articles employed was obtained through the snowball method, ensuring that they fulfilled the criteria described. A number of articles were discarded during the process, when one of the criteria was not met. Figure 1 depicts the main steps of the review process.

The next step was the determination of the research questions of the review, to organize the thematic axes as paragraphs: “What are the latest environmental applications of BC?” “What are the recent advantages of BC implementation in soil?” “Is there quantifiable evidence that the described BC treatment is beneficial to soil and plants?” “What are the limitations or side effects of BC treatment and what are the reasons behind them?” “What are the emerging mechanisms and interactions that should be taken under consideration when apply BC aiming at a beneficial use?” “What are the policies and global strategies BC fits in?”

The following step was identifying the studies employed. Specifically, 171 peer-reviewed articles were studied. These studies were cross-referenced, and those with similar objectives were combined to form a cohesive synthesis within thematic paragraphs. Any conflicting results were included in each thematic paragraph. The subsequent step was to discuss the main advances towards the sustainable future employment of BC presented in the thematic paragraphs. Verification regarding the deductions of the review was achieved with multiple up-to-date references, and lastly, emphasis was placed on maintaining the relevance of the incorporated information by structuring thematic paragraphs that provide the latest developments in each subject and promote this ability sufficiently.

3. Biochar Meets European and Global Roadmap Targets

3.1. Degraded and Problematic Soils Across Europe

Problematic soils in Europe are defined by their poor physicochemical and biological properties that hinder or prevent plant growth [19]. These soils are often the result of long-term degradation processes, including erosion, pollution, and radical changes in their initial composition, mainly driven by anthropogenic activities. The European Soil Data Centre (ESDAC) monitors heavy metal pollution, nutrient imbalance, (harvest, tillage, wind, and water) erosion, loss of organic carbon, loss of biodiversity, salinization, and (soil) sealing of European land [10,20]. The European areas with problematic soil are depicted in Figure 2 [10,20,21,22,23,24,25,26,27,28,29].

Erosion is another critical factor affecting soil health. Using a sustainable baseline of >2 t ha⁻¹ yr⁻¹ for all types of erosion, the ESDAC estimated high erosion rates exceeding 5 t ha⁻¹ yr⁻¹ (>5.1%) in some areas [26,27,28]. Such extensive soil loss not only reduces the quality of land but also changes how water moves through the land and increases vulnerability to desertification. Degradation due to pollution is obvious in Ireland, Slovenia, and France, with elevated levels of heavy metals such as Cd, As, and Cu, followed by Poland and Spain [20,21,22,23]. Overall, it has been found that 89% of agricultural land and 35.7% of forest areas have unhealthy soil, based on degradation processes [20]. These data highlight the urgent need for sustainable soil management strategies across Europe and worldwide.

3.2. Biochar in Global Mitigation Strategies

3.2.1. BC Application Meets the Targets of European and Global Strategies

Soil quality is considered a matter of the utmost importance on the EU agenda. The recent mission “A Soil Deal for Europe” has set the ambitious goal for a radical transition from the current 62% problematic soils to 75% healthy soils “for food, people, nature and climate” by 2030 [30]. Soil mitigation is undoubtedly the linchpin of all individual strategies of the EU (

Table 1. European and global strategies in which widespread BC use could substantially contribute.

Strategies	Key Points	References
EU Soil Mission.	Goal: increase healthy soils from 38% to 75% by 2030. Slogan: “Caring for soil is caring for life”.	[30]
European Green Deal.	Soil mitigation is a core strategy.	[31]
Common Agricultural Policy (2023–2027).	Focuses on restoring degraded land and sustainable nutrient management.	[32]
Farm to Fork Strategy.	Aims to reverse biodiversity loss and ensure healthy food.	[33]
Zero Pollution Action Plan.	Targets carbon sequestration and zero soil pollution.	[34]
Circular Economy Action Plan.	Prioritizes reducing soil sealing and rehabilitating contaminated fields.	[35]
Fertilizer Code (FAO).	Addresses nutrient imbalances and soil pollution; suggests alternatives like biochar (BC).	[36]
Voluntary Guidelines (VGGT).	Framework for responsible governance of land, forest, and fisheries tenure.	[37]
Paris Agreement.	Target: cut net CO2 emissions by 43% by 2030 and achieve net zero by 2050.	[38]
COP29 UN Climate Conference.	Requires inclusive climate plans across all sectors and GHGs.	[39]

). The transition to sustainable crop nutrition, soil management, and restoration is not only a European but a global concern. Our planet has lost one-third of its arable land during the past 40 years due to erosion and pollution [19]. Therefore, nutrient imbalances and soil pollution are key targets of global guidelines (Table 1) where BC can play a pivotal role (elaborated in Section 6, Section 7, and Section 9).

3.2.2. BC in Agriculture Reduces GHG Emissions

BC could play a pivotal role in a global strategy to achieve the 2030 and 2050 targets, not only by capturing and sequestering carbon, but also by reducing GHG emissions from soils following its application (

Table 2. The positive effect of BC treatment in agriculture on GHG emissions.

Key Points	Description	Reference
Projected GHG emission increase from agriculture.	7.5% expected increase; 86% of the emissions come from livestock.	[40]
Potential global GHG reduction via biochar (BC).	Over 6% annual reduction in CO2-equivalent emissions.	[41]
Regions with high BC removal potential *.	Eastern Europe; South America; Northwestern Africa.	[41]
GHG reduction by BC application to soil.	12–50% reduction in non-CO2 GHG emissions.	[42]
SOC enrichment and GHG reduction by BC in China.	SOC ↑ ** by 1.9 Pg C; CH4 ↓ ** by 25 Mt CO2-eq/year; N2O ↓ ** by 20 Mt CO2-eq/year.	[43]
Major GHGs from agriculture.	CO2; CH4; N2O.	[44]
Highest CO2 emissions source.	Inorganic fertilizer.	[44]
Lowest CO2 emissions.	BC combined with inorganic-fertilizer-amended fields (lowest); purely BC-amended fields (second lowest).	[44]
BC’s effect on GHGs from rice–wheat cultivation.	CH4 ↓ ** > 20%; N2O ↓ ** > 36%; NO ↓ ** > 28%.	[45]
Effect of BC re-application.	Increased carbon sequestration without further enhancing crop yield or reducing GHGs/N-oxides.	[45]
Emissions in vermicomposting.	Among various waste by-products, lowest CH4, N2O, and NH3 were achieved when using BC.	[46]
Effect of BC on greenhouse gas emissions in Cd-contaminated paddy soil ***.	CO2 ↓ ** by ~8%; CH4 ↓ ** by ~2%.	[47]

Note: * removal potential relative to national emissions; ** ↑ stands for increase and ↓ stands for decrease; *** i.e., intentionally flooded soil for rice cultivation.

).

In recent studies, as summarized in Table 2, BC application has been associated with a 12–50% decrease in non-CO₂ GHG emissions, depending on the soil type and management practices [42]. Moreover, combining BC with inorganic fertilizers [44] results in the lowest CO₂ emissions from soil. Raza et al. [46] studied the use of biochar in waste management, particularly in vermicomposting, which led to notably lower emissions of CH₄, N₂O, and NH₃. Overall, these findings underscore biochar’s role as a practical tool for reducing agriculture emissions globally.

4. Biomass Influence on Biochar Characteristics

4.1. Differentiations in Properties

A crucial parameter determined by the target purpose is the type of biomass feedstock used for BC production [48]. As presented in Table 3, both the content and physicochemical properties of BC depend on the raw material used. These properties face alterations induced by pyrolysis temperature [19].

The selection of the raw material requires deep insight into several mechanisms and stresses that soils undergo, the responses of different plants to the same BC, and complex relations among physicochemical and biological variables [42]. For example, BC originating from plant residues (e.g., from rice husk and sugar cane) tends to have high ash content and alkalinity, raising the soil pH and making it suitable for acidic or heavy metal-contaminated soil [49]. In contrast, BC produced from livestock biomass (manure, etc.) contains more functional groups, improving sorption capacity, and is typically more acidic (Table 3). Moreover, corn straw and waterweed biomasses produce BCs with high CEC values, which enhance nutrient retention and microbial activity, particularly in degraded soils [19,42]. Woody feedstocks, on the other hand, like pecan shells or wood pellets, are rich in carbon and appear to have improved water retention [42]. Therefore, optimizing BC properties through appropriate biomass and pyrolysis temperature is essential for maximizing the benefits in polluted soils.

Table 3. Biomass properties’ effects on the produced BC.

Biomass Source	Key Properties	Pyrolysis Temperature	Effects on BC
Tea Waste	High nitrogen (N) content; beneficial for promoting soil fertility.	300 and 500 °C	N content: 2.46–2.61% [48]
Sugar Cane	Elevated ash content.	600 °C	Ash content: 15% [5]
Rice Husk	High ash content.	400 °C	Ash content: 49% [5]
Corn Straw	Cation exchange capacity (CEC) improves with pyrolysis temperature.	300 °C	CEC at 300 °C: 183 cmol kg−1 [19]
		700 °C	CEC at 700 °C: 210 cmol kg−1 [19]
Waterweeds	High CEC; suitable for improving soil nutrient retention and fertility.	500 °C	CEC: 509 cmol kg−1 [7,19]
Wood Chips	High nitrogen (N) content and low CEC, which limits cation retaining ability.	550 °C	N content: 480.3 g kg−1; CEC: 9 cmol kg−1 [19]
Wood Pellets	High carbon (C) content; long-lasting benefits to soils.	500 °C	Carbon content: 800 g kg−1 [19]
Pecan Shells (Pyrolyzed)	High carbon (C) content; suitable for improving soil organic matter and water retention.	700 °C	Carbon content: 834 g kg−1 [42]
Sewage Sludge (SSB)	Contamination concerns (pathogens, heavy metals, PAHs) limit widespread use without proper treatment.	High temperature and time reduce PAHs	Can help in neutralizing acidic soils; improving EC; and increasing microbial and enzymatic activity [50,51,52,53]
Co-pyrolyzed SSB + Ash	Increased bioavailable phosphorus (P); enhanced surface area; ideal for agricultural applications; P recovery; SS recycling.	600 °C	P bioavailability reached 92 wt%; potential as P and K slow-release fertilizer; improved adsorption properties compared to SSB alone [51,54]
Pyrolyzed SSB + Carrier Gas	Carrier gas during pyrolysis and cooling phase eliminates micropollutants (below their detection limits) except Cu and Zn.	650 °C	The produced SSB adheres to the guidelines set by the International Biochar Initiative (IBI) [55]

Table 3. Biomass properties’ effects on the produced BC.

Biomass Source	Key Properties	Pyrolysis Temperature	Effects on BC
Tea Waste	High nitrogen (N) content; beneficial for promoting soil fertility.	300 and 500 °C	N content: 2.46–2.61% [48]
Sugar Cane	Elevated ash content.	600 °C	Ash content: 15% [5]
Rice Husk	High ash content.	400 °C	Ash content: 49% [5]
Corn Straw	Cation exchange capacity (CEC) improves with pyrolysis temperature.	300 °C	CEC at 300 °C: 183 cmol kg−1 [19]
		700 °C	CEC at 700 °C: 210 cmol kg−1 [19]
Waterweeds	High CEC; suitable for improving soil nutrient retention and fertility.	500 °C	CEC: 509 cmol kg−1 [7,19]
Wood Chips	High nitrogen (N) content and low CEC, which limits cation retaining ability.	550 °C	N content: 480.3 g kg−1; CEC: 9 cmol kg−1 [19]
Wood Pellets	High carbon (C) content; long-lasting benefits to soils.	500 °C	Carbon content: 800 g kg−1 [19]
Pecan Shells (Pyrolyzed)	High carbon (C) content; suitable for improving soil organic matter and water retention.	700 °C	Carbon content: 834 g kg−1 [42]
Sewage Sludge (SSB)	Contamination concerns (pathogens, heavy metals, PAHs) limit widespread use without proper treatment.	High temperature and time reduce PAHs	Can help in neutralizing acidic soils; improving EC; and increasing microbial and enzymatic activity [50,51,52,53]
Co-pyrolyzed SSB + Ash	Increased bioavailable phosphorus (P); enhanced surface area; ideal for agricultural applications; P recovery; SS recycling.	600 °C	P bioavailability reached 92 wt%; potential as P and K slow-release fertilizer; improved adsorption properties compared to SSB alone [51,54]
Pyrolyzed SSB + Carrier Gas	Carrier gas during pyrolysis and cooling phase eliminates micropollutants (below their detection limits) except Cu and Zn.	650 °C	The produced SSB adheres to the guidelines set by the International Biochar Initiative (IBI) [55]

4.2. Sewage Sludge Biochar (SSB)

Agricultural remnants have been thoroughly discussed as BC feedstocks. The use of sewage sludge biochar (SSB) as a soil amendment has recently attracted attention. In both Europe and the US, treated sewage sludge is utilized in the construction material industry, agriculture (reuse), and landfills (disposal) [50,56,57]. SSB, or alternatively biosolid-derived biochar (BDB), has been employed as a cement replacement material [58]. Utilizing SSB as a source of phosphorus (P) for soil revitalization would be an upgraded step in waste management in the frame of the circular economy.

Studies on SSB’s impact on soil and crops still present inconsistent findings. Although large-amount SSB application was associated with inhibitory effects on corn plant growth in alkaline soil due to high background nutrient levels, a moderate rate of 1500 kg hm⁻² was found to be significantly beneficial [59]. This rate broadens the usage of SSB for soil amendment and crop enhancement, minimizing health risks from corn consumption. Notably, crop productivity increased by over 50% compared to the fertilized control, while the immobilization of inorganic P in SSB was worth noting [59]. There are studies that report significantly decreased metal (Cd, Zn, Cu, and Pb) bioavailability in soil and an increase in crop yield after SSB application [60,61]. However, the heavy metals from the SS are retained in the SSB, and this is still the main concern regarding its large-scale usage. Schlederer et al., 2024 [55], deduced that specific treatments (i.e., pyrolysis temperature of 650 °C and the use of a carrier gas during the phases of pyrolysis and cooling) can be used to produce BC which conforms to the limits set by the International Biochar Initiative (IBI) and the European Biochar Certificate (EBC) regarding all micropollutants, except for Cu and Zn. The main methods in the literature for SSB production are pretreatment, high-temperature pyrolysis with a carrier gas, co-pyrolysis, and BC fabrication [52,55]. Therefore, scaling up SSB’s application as a soil amendment prerequires optimized and standardized production methods [62]. As a solid waste rich in P, SS constitutes an important renewable resource. The product of pyrolysis of SS, SSB, exhibits reduced water-extractable P, with slow release [63]. It is worth mentioning that BC application in medium- to fine-textured soils was reported to increase the available P content of the soil by 324%, with more pronounced effects at higher application rates (>20 t ha⁻¹) [63]. In addition, BC application was found to improve the crop productivity of maize and wheat by more than 50% compared to unamended controls [63]. Therefore, SSB is a promising slow-release P fertilizer which provides long-term benefits for soil health and productivity.

5. Production Methods of Biochar

BC can also be produced by gasification, pyrolysis (slow, fast), or hydrothermal carbonization [64]. Gasification is a thermochemical process that favors syngas production, while a small percentage (usually around 10%) of BC is formed. Among them, pyrolysis is the most widely used process.

Table 4. Production methods of biochar.

Production Treatment	Specific Method/Interesting Pyrolysis Application	Characteristics of Method	References
Thermochemical Conversion	Gasification	Favors syngas production; BC yield ~10%; high surface area and porosity; could remediate acidic soils; however, unsuitable for soil amendment due to not meeting NTC 5167 standards (CEC, TOC, pH, ash content) *.	[64,65,66]
Hydrothermal Treatment	Hydrothermal Carbonization	Uses 160 °C pretreatment followed by 500 °C pyrolysis; considerably improves nutrient content, pH, surface area, and CEC compared to raw biomass; phytotoxicity makes it a non-appropriate amendment.	[56,67]
Pyrolysis	Slow Pyrolysis	Occurs at 300–700 °C; slow heating (5–10 °C/min); long residence time (hours to days); preserves diverse surface functional groups; limited thermal degradation of lignin retains 40–70 wt% carbon; production ratio of BC to bio-oil and syngas increases; physicochemical properties improve over time.	[15,28,68,69,70,71]
Pyrolysis	Fast Pyrolysis	Faster heating and shorter duration; lower carbon retention than slow pyrolysis.	[68]
Pyrolysis	Medium/High-Temperature Pyrolysis	At 600–900 °C with 22% oxygen content; increases elemental content (P, K, Ca, etc.); however, higher temperatures reduce element bioavailability; surface area up to 323.33 m2/g depending on conditions (i.e., 900 °C, 500 W, and 120 min); suitable for a wide range of carbon-based applications.	[19,72,73]
Pyrolysis	Low-Temperature Pyrolysis	At ~400 °C; better for soil amendment and fertilizer applications due to higher element bioavailability.	[73]

* Note: NTC 5167 standard: CEC > 30 meq/100 g, TOC > 25%, ash content < 60%, and pH between 4 and 9 [67].

presents the latest production methods, grouping their main characteristics.

Both the temperature and duration of pyrolysis are determined based on the target purpose [68]. BC accumulates approximately half of the carbon content of its original feedstock. Due to its high content of aromatic carbon, it is recalcitrant to microbial decomposition, unlike uncharred biomass. The fact that BC is a refractory material makes it persist in soil for long periods (e.g., 1000–10,000 years), serving as an organic carbon sink [74], as presented in Figure 3.

The potential soil type trend after BC treatment was another unclear parameter in the literature. Purakayastha et al. [79] investigated the effect of three BC types on three different soil orders: Inceptisol (alkaline soil with low SOM), Mollisol (alkaline soil with high SOM), and Alfisol (acidic soil with SOM). They carried out an incubation experiment for 290 days to acquire long-term results. No trend was found for pyrolysis temperature across soil type and biomass regarding the priming effect and nutrient availability, whereas the pH and EC were ameliorated. After a thorough review of the literature, the authors deduced that, in most cases, BC treatment raises the soil pH by between 0.1 and 2.0 units, for a wide variety of soil types with different native pH values [79].

6. Biochar and Soil Revitalization

According to Joseph et al. [42], BC undergoes three main transformation stages after its incorporation into soil: the dissolution stage (1–3 weeks), the stage of reactive surface development (1–6 months), and the aging stage. During these phases, BC contributes significantly to soil improvement in both physical (Figure 4) [5,80] and biological (Figure 5) aspects.

Its high specific surface area and porous structure, especially the abundance of macropores, not only aid water retention but also create favorable microhabitats for soil microbiota [81].

Biologically, BC plays a key role in stimulating microbial activity. The macroporous structure supports heterotrophic microorganisms by facilitating access to organic matter, thereby enhancing SOC mineralization [82]. Additionally, the application of alkaline BC facilitates the neutralization of acidic soils, thereby enhancing soil enzymatic activity and fostering a more diverse and active microbial community [81,83].

The positive effects of BC are more pronounced in coarse-textured soils (sandy soils), where it significantly enhances soil properties compared to its application in finer-textured soils (loamy or clay soils). El-Naggar et al. [84] treated soils with three types of BC. The CEC of the sandy soil increased by 906%, 180%, and 130%, whereas the only amelioration for the sandy loam soil was by 13% for the paddy straw BC. The former indicates that the application of BC enhanced the availability of cations in the sandy soil, based on the following mechanism: The untreated sandy soil exhibits a low pH. At a low soil pH, most of the cation exchange capacity comes from pH-dependent charges located on the edges of 2:1-type clay minerals and organic matter [84]. These sites hold onto H⁺ (and Al³⁺) more strongly in acidic conditions, meaning fewer base cations (Ca²⁺, Mg²⁺, K⁺) are present. After BC application, the pH of the sandy soil increases, and more sites are deprotonated, allowing Ca²⁺, Mg²⁺, K⁺, etc., to bind. Thus, the pH-dependent charges contribute more to the CEC [84]. A slight rise in the pH was reported in the sandy loam soil after the application of BC, attributed to the relatively high buffering capacity. Therefore, BC treatment did not cause significant alterations in exchangeable cations in the sandy loam soil [84]. These different responses to the same BC type highlight that the selection of BC should consider the specific site traits.

Meta-analyses concluded that the plant-available water content in coarse-textured soils increased by 33–45%, while in fine soils, the increase was only 9–14% [85,86]. Elevated soil organic matter (SOM) was identified as an indicator of reduced pesticide concentrations due to its enhanced sorption capacity [87]. In the same vein were the results for herbicide concentrations after BC amendment. For example, in BC-amended soils, the dissipation of fenoxaprop-ethyl was slower, resulting in the formation of fewer metabolites. However, these metabolites degraded significantly faster than in untreated soils [88]. In cases of hillslope erosion, a high rate (5%) of BC was reported to halt the aggravation of soil characteristics, as the alterations due to riling were balanced by BC application to the soil [89].

6.1. Biochar Effects on Saline and Sodic Soils

Among various stress conditions, soil salinity and sodicity pose important stresses to soil fertility in arable croplands [90]. BC properties are beneficial to salt-affected soils, especially in terms of cationic nutrients (e.g., K, Ca, Mg, Zn, Mn).

As shown in

Table 5. BC’s effect on salt-stressed soils.

Key Points	Description of Effects	References
Soil Salinity and Sodicity.	Major stressors to soil fertility in croplands.	[90]
Causes of Salinization.	Irrigation and arid conditions lead to salinization, reducing nutrient uptake and causing imbalances.	[92]
Annual Salinity Increase.	10% per year, indicating a growing challenge.	[92]
Benefits of BC for salt-stressed soil.	Improves salt-affected soils by enhancing CEC, surface area, porosity, and water-holding capacity. Facilitates retention of multivalent cations and Na-Ca exchange (i.e., Na sorbed onto BC surface, whereas Ca liberated in soil solution).	[91]
Na and K Interaction.	High Na hinders K uptake. BC raises K availability in soil, supporting plant growth.	[90,91]
Reduction in ESP and SAR.	BC sorbs Na, lowering exchangeable sodium percentage (ESP) and sodium absorption ratio (SAR) in stressed soils.	[90]
Effect on Salt-stressed Paddy Fields.	BC reduces bulk density, Na+, Cl−, and EC; BC improves crop performance.	[90]
Effect on Evaporation.	BC enhances evaporation after irrigation, promoting salt accumulation in the upper 2 cm of soil, which is removable. After multiple irrigation–evaporation cycles, salinity lowers in root zones. Eventually, BC alters salt distribution and reduces weekly evaporation (by 35% at 5% BC rate and 43% at 10% BC rate); BC decreases the water consumption needed for soil desalinization.	[91]
Effect on Silt Loam Soil.	BC pyrolyzed at 300 °C reduced salinity and sodicity and Na+/K+ and Mg2+/Ca2+ ratios and enhanced aggregate stability. Improved soil properties (except pH) and increased leaching potential, facilitating soil restoration; improved aggregate stability and raised larger-pore proportion (>300 µm).	[93]
Hydrogeological Benefits to Silt Loam Soil.	Enhanced pore connectivity and elongated pore structure improve hydraulic conductivity, water penetration, and least-limiting water range (LLWR).	[93]

, BC treatment affects physicochemical processes, resulting in the alleviation of salinization and sodification, hence ameliorating soil properties. More specifically, in salt-stressed fields, BC reduced exchangeable Na⁺, Cl⁻, and electrical conductivity, leading to improved crop performance [90]. It also reduced the negative effects of high sodium, increasing K availability for plants to grow [90,91]. Thus, these results highlight the potential of biochar to mitigate the adverse effects of salinity and sodicity by enhancing soil health and supporting long-term agricultural productivity.

6.2. Biochar for Remediation of Heavy Metal-Contaminated Soils

BC has shown strong potential in remediation of heavy metal-contaminated soils, primarily due to its high cation exchange capacity (CEC), functional groups on the surface, and porous structure.

As presented in

Table 6. BC’s effect on heavy metal soil reclamation.

BC Effects	Effect Description/Qualification	References
Pollutant Adsorption.	BC is suitable for chemical adsorption due to surface functional groups, active sites, and increased CEC.	[81]
CEC Effects (Pyrolysis Temperature and Time).	High CEC enhances remediation of metal-contaminated soils and reduces fertilizer runoff; high pyrolysis temperatures reduce functional groups (e.g., –OH, –COOH, –CH, –C=O), lowering initial CEC of fresh BC; fresh BC does not immediately raise soil CEC; over time, ion sorption and retention ability of BC increases, as new functional groups form on its surface → ↑ * CEC and ↑ * AEC **.	[6,94,95]
Heavy Metal Uptake Reduction.	BC limits plant uptake of heavy metals (Pb, Cd, Cu, Ni, Zn, As) via its large surface area and fine particle size; negative functional groups facilitate metal ions’ immobilization, reducing their bioavailability.	[6]
Factors Affecting Metal Absorption/Uptake Reduction.	Biomass material and pyrolysis temperature are the key factors, e.g., pig carcass BC is more effective than tobacco stalk BC; Cd and Zn uptake reduced by >64% and >94%, respectively; rice straw BC at 600 °C reduced methyl mercury in rice grains by 92%.	[6]
	Decrease in Zn and Cu concentrations in plants due to lowered bioavailability; decrease in total concentrations of Pb (21%) and Cd (19%) was observed in native BC-treated soil, and by 38% for Ni in contaminated BC-treated soil; increase in CEC by 12–19%.	[96]
Sunflower Shell BC on Paddy Soil.	Increased SOC by 9% and lowered Cd bioavailability by 24%.	[47]
Low-Dose BC and Earthworm Synergistic Effect.	0.1% BC with E. fetida improved BC diffusion; increased pH and microbial populations’ modification; Cd fixation efficiency > 56%.	[97]
BC with Macrobiota.	The presence of BC helped macrobiota alleviate Cd and 2,4-dichlorophenoxyacetic acid toxicity, improving soil fertility.	[98]
nZVI/BC Composite Effect.	Reduced Cd in pore water by 26–73% after 140 days of exposure; BC composites show promise for heavy metal soil reclamation.	[99,100]

Note: * ↑ stands for increase, ** AEC stands for anion exchange capacity.

, BC effectively immobilizes cationic contaminants (e.g., Cd, Zn, Pb) [6] and contributes to reduction in fertilizer runoff. The pyrolysis temperature and feedstock type significantly influence BC performance, which can achieve metal reductions up to 90% [6].

Moreover, synergistic effects, such as BC combined with earthworms or macrobiota, have been shown to enhance the metal removal efficiency and promote soil microbial health [97,98]. Overall, these findings underline biochar as a sustainable solution for the remediation of heavy metal-polluted soils.

7. Biochar’s Effect on Soil Fertility

7.1. Fresh BC

It is of utmost importance that BC provides nutrients in forms directly available for plant uptake, as happens with K, which is conserved unaffected in BC from the raw biomass [49].

Table 7. BC treatment effects on soil fertility.

Effects	Description of Effects	References
Nutrient availability from BC.	BC conserves K in a plant-available form from raw biomass.	[49]
Greater nutrient uptake by BC-amended than by chemically fertilized (at same nutrient levels) plants and seedling growth.	BC created ~−160 mV potential at the root epidermal cell layer of the plant, compared to the root surface. This potential difference lowered the free energy required for root nutrient accumulation, enhancing nutrient uptake.	[101]
	Increased concentrations of N, P, K, and Fe found in rice seedlings; increased silicon compounds and iron oxide nanoparticles found in their roots; the latter led to elevated seedling growth.	[101]
Nano-BC advantages over conventional BC.	Enhances nutrient uptake and alleviates abiotic stress through redox reactions and cation–π/electrostatic interactions with pollutants in the rhizosphere; nano-BC adsorbs organic pollutants and heavy metals.	[102]
K release from BC vs. chemical fertilizers.	BC-based fertilizers achieve slower release of K than chemical fertilizers (KCl).	[103]
P availability from BC.	BC increases P availability on average by a factor of 4.6; crop yields increased annually by 10–42% (median 16%).	[42]
Role of BC in drylands.	The greatest yield responses were observed either in treatments with fertilized BC or in soils with low fertility and high P sorption capacity (e.g., acidic tropical soils; sandy soils).	[59,104]
Role of BC in soils poor in nutrients—severely weathered tropical soils.	Water-holding capacity and nutrient retention improved; slow release of phosphorus from SSB or manure BC led to efficient phosphorus utilization; DOM released from BC reduced P sorption by soil.	[59]
DOM (Dissolved Organic Matter) interaction.	BC increased the humification of soil DOM by releasing intrinsic DOM and selectively adsorbing low-molecular-weight DOM fractions; BC pyrolyzed at 300 °C ↑ * DOM via release; BC at 700 °C ↓ * DOM via adsorption. Effect depends on pyrolysis temperature and native DOM properties.	[105]
SOC (soil organic carbon) and yield increase in Chinese farmlands.	BC increased SOC storage by 1.9 Pg C and crop yields by ~19% → significant potential for large-scale agriculture.	[43]
Effect of BC on alfalfa in polymetallic-contaminated soil.	The formerly halted development exhibited length and root/shoot growth (length ↑ 11–23%, root weight ↑ 24–46%, shoot weight ↑ 14–38%) after 60 d treatment; antioxidant stress markers reduced; chlorophyll content increased.	[96]
BC limitations.	BC is less effective when its nutrient content is low or soil is nutrient-rich; compost + BC is better in these cases: the faster-degrading compost biomass causes consistent nutrient flow for plant uptake until the relatively slow release from BC begins. Most research was conducted in temperate/tropical soils; boreal impacts of BC are understudied.	[6,49,106]
Adverse effects of BC pyrolyzed at high temperature.	BC usage at high rates can reduce P and N availability.	[42]
SOC priming effect.	BC induces negative priming of SOC by 3.8% (range −21% to +20%) via physical and metabolic barriers to microbial decomposition. Reduction in SOC mineralization is caused by limited access to microbiota (which cannot reach the organic substrates to metabolize due to physical barriers) and metabolic limitation (unfavorable conditions that hinder microbial catabolic activity).	[42,107]
SOC decomposition variability.	Effects depend on BC type/raw material; positive/negative effect of BC on SOC mineralization can shift over time if conditions alter; BC characteristics like low-temperature pyrolysis, cellulose content, and surface properties influence mineralization rates.	[74,108,109]
Carbon persistence.	BC carbon remains in soil 10–100 longer than unpyrolyzed carbon (long-term C sequestration); reduces mineralization of both native and newly added carbon.	[110]
General plant and soil benefits.	BC improves plant traits (height, weight, leaf area/number, biomass, chlorophyll, flavonoids), nutrient uptake, and soil properties (pH, water retention, CEC, microbiota activity).	[111]

Note: * ↑ stands for increase and ↓ stands for decrease.

summarizes the latest findings regarding BC complex impacts on soil fertility.

As presented in Table 7, BC application in soil may affect the decomposition of native SOC, depending on the type of BC raw material. The accurate way in which BC treatment affects SOC mineralization remains unsettled in the literature, as the positive or negative effect of BC on SOC mineralization could shift over time if management or climate conditions alter [108]. There has been an attempt to address this uncertainty in the literature with updated models such as MIMICS-BC, which depicted the authors’ observations regarding the SOM response after BC treatment [109]. The effect of BC on native SOC decomposition varies, probably because of their different carbon stability, CEC, and pore structure. BC with a high cellulose content, BC produced at low temperatures (<400 °C), and fresh BC, as well as BC with a reduced surface area, a lower aromatic content, an elevated ash content, more hydrophobic surfaces, and a higher O/C ratio, may achieve faster mineralization and a prominent (positive) priming effect [74]. It must be underlined that most of the current literature investigates BC effects on temperate soils, followed by tropical soils. The response to BC employment in boreal environments is rather undocumented [6].

The total nitrogen (TN) in BC varies depending on the raw material and the pyrolysis temperature. The typical TN content ranges between 0.1 and 9.5% across different feedstocks and temperatures [112]. BC’s contribution is not to add N into soil but to improve N retention in soil by reducing leaching and gaseous loss. Moreover, it improves phosphorus availability by reducing the leaching process in the soil [113]. The impact of BC application on other nutrients can be inconsistent; however, it has been found that BC conserves K in a plant-available form from raw biomass [49,113]. Therefore, selecting feedstocks or altering the raw biomass via physicochemical/biological approaches can lead to BC with desirable properties. The engineered or modified BC regulates the stability of macroaggregates and soil inorganic N transformation, thereby enhancing plant responses under N deficiency (or even N toxicity) in the soil. BC affects biological N fixation by enhancing enzymatic activity that stimulates the conversion of N₂O to N₂, hence reducing denitrification and N₂O release [114,115]. As a result, BC ameliorates the nutrient use efficiency of plants.

BC, in contrast to hydrochar, generally enhances germination and seedling growth [116]. BC mitigates root growth inhibition in acidic soils [117]. BC amendment alleviated heavy metal phytotoxicity (Medicago sativa) in polymetallic-contaminated soil [96]. It has been reported that BC amendments reduce Al phytotoxicity via liming and adsorption. They also reduce Cd and Zn phytotoxicity and downregulate antioxidant enzyme transcript levels (MnSOD, CAT, and GR expression) by lowering reactive oxygen species (ROS) [118]. ROS are produced in plants during various processes associated with abiotic stress [119]. Furthermore, BC influences soil microorganisms, depending on the feedstock, pyrolysis temperature, and soil type. It has been found that wood chips and mechanically deboned meat (MDM) BC affect microbial diversity and enzyme activity differently in different soil types, such as in Cambisols (typically found in temperate and boreal regions) and Luvisols (typically found in temperate regions) [120]. A concern of extended use is the localized accumulation of organic contaminants, although BC reduces their bioavailability. Since BC’s impacts on soil and microbiota are multifactorial, site-specific trials are recommended before extended BC application [121].

Overall, BC has a beneficial impact on physio-morphological and functional plant traits such as height, leaf area and number of leaves, total biomass, root and shoot weight, crop yield, pigments such as chlorophylls and flavonoids, nutrient uptake, and soil properties like pH, water-holding capacity, CEC, and soil microbiota activity [111].

7.2. Aged BC

Aged biochar in soil BC as a soil conditioner is not a brand-new concept. Its origins can be traced back to ancient Amazonian cultures that converted bio-waste into black, carbon-rich soil with perpetual fertility, known as Terra Preta [122]. The key component of Terra Preta, which provides long-term persistence of organic matter and enduring fertility, is black carbon. Crop productivity on Terra Preta soils is almost double that on adjacent unamended soils, providing evidence that BC aging does not hinder soil fertility at least for several centuries [123].

Table 8. Aged BC positive effects and concerns.

Aged BC Traits	Description	References
BC and Fertilizer Interaction	Yield response is highest in the third year after single application of BC with fertilizer, showing improvements over time.	[106]
Soil Integration	BC becomes part of soil aggregates, stabilizing rhizodeposition and microbial products.	[106]
Longevity in Soil	BC can persist for hundreds to thousands of years.	[42]
Regional Variations	BC effects differ across temperate, tropical, and boreal regions.	[124]
Adverse Biological Effects—Toxicity	In tropical soils, aged BC negatively affected earthworm growth and fungal populations and reduced root biomass of Solanum lycopersicum and Oryza sativa. Soil acidity potentially contributed to adverse effects; potential toxic influence on plant roots.	[124]
Soil Thermal Properties	BC reduces soil thermal diffusivity due to its own low thermal diffusivity and the associated reduction in soil bulk density → potential impact on heat balance of topsoil.	[125]
Microbiota Support	Aged BC generally fosters growth and diversity of soil microbiota via nutrient accumulation in its pores and niches → supports ecosystem stability and diversity.	[102,126]
Plant-Beneficial Microorganisms	Promotes nitrogen-fixing bacteria and mycorrhizal fungi; serving as a substrate, aged BC expedites nutrient cycling and supports mutualistic microbe–plant interactions.	[127]
Long-term Nutrient Cycling	Enhances microbial enzyme production accompanied by cellulose/lignin degradation → SOM breakdown and nutrient mineralization → improve long-term nutrient availability and cycling.	[102]

shows that aged BC generally has positive effects on ecosystem stability and diversity, through long-term soil health, nutrient cycling, and microbial interactions. However, some studies noted a potential toxic effect in tropical soils. Therefore, its use must be tailored to the specific soil conditions.

8. Biochar–Earthworm Interactions in Soil System

Understanding the interaction between biochar and earthworms is essential for evaluating the long-term behavior and ecological impact of BC in soil systems. Earthworms are soil macrofauna well-known as “soil engineers” since their activities promote soil health [128]. Moreover, they are employed in vermicomposting, an eco-friendly process whereby earthworms biodegrade organic waste, converting it into compost. On the one hand, integrating BC into vermicomposting can significantly upgrade the overall quality of the product (by expediting organic matter degradation, humus formation, etc.) [129]. On the other hand, the effect of BC on earthworm populations is debatable. There are contradictory findings in the literature regarding earthworm responses to BC amendments.

Table 9. Negative and positive/neutral effects of BC on earthworm populations.

Effect Type	Observed Effect	References
Negative	Eisenia fetida showed halted growth and reduced antioxidant enzyme activity after 28-day BC exposure; SSB produced at a lower pyrolysis temperature (300 °C) and applied at 10% rate had a stronger negative impact on E. fetida survival than SSB produced at a higher temperature (700 °C).	[125,130]
Negative	High application rates of BC caused weight loss and elevated mortality in earthworms.	[131]
Negative	Nano-sized BC particles, even at a small dose (0.1%), caused harm to Eisenia fetida.	[97,132]
Neutral/Positive	Application of BC at 5%, 10%, and 15% did not significantly impact earthworm populations, showing no harmful effects.	[133,134]
Positive	Macro-BC particles at 0.1% had synergistic effects with Eisenia fetida, improving soil quality.	[97,132]
Positive	Treatment of wood-based BC (high ash content) with dairy slurry over two years, across varying rates, showed no effect on earthworm populations in a cold, humid temperate region.	[135]
Positive	BC addition at 1–3% in Pb-contaminated soil reduced earthworm weight loss and mortality, attributed to BC’s toxicity suppression effects on soil.	[136]
Positive	Long-lasting positive effects on earthworm activity, reproduction, and abundance were observed with BC application.	[137,138]
Positive	BC–earthworm coexistence promoted crop yield, soil remediation, and synergistic effects, likely via stimulation of nutrient cycling enzymatic activity (soil type plays a critical role).	[139,140]
Positive	BC enhanced earthworm accumulation ability and physiological and metabolic resilience under Cd and Pb contamination.	[100]
Positive	Earthworms helped distribute BC particles further in topsoil and transfer aggregates deeper into the soil, facilitating soil improvement and carbon binding.	[87]

summarizes the main negative and positive effects reported in the latest literature.

Avoidance behavior tests are usually employed to assess whether BC addition into soil is beneficial or risky to the earthworm population [133]. This variability in effects is probably due to the differentiated toxicity of different BC types, determined by feedstock composition and pyrolysis temperature. Specifically, BC produced from plant residues or at lower pyrolysis temperatures usually presents lower toxicity than that derived from livestock waste or produced at higher pyrolysis temperatures. However, as shown in Table 9, SSB produced at 300 °C had a stronger negative impact on Eisenia fetida survival than SSB produced at 700 °C. This result is likely attributed to the high dose (10%) used in the experiment. Moreover, earthworms can only ingest small BC particles (e.g., <0.5 mm); therefore, smaller particles are responsible for stress [135]. Overall, field-based long-term research is considered the most secure practice in evaluating earthworm responses to BC applications.

9. Discussion

9.1. BC as a Soil Amendment

Soil quality and fertility are closely linked to food chains and sustainable ecosystems, directly impacting food safety and accessibility. In this context, BC amendments could play a pivotal role in agroforestry and smallholdings by utilizing local available feedstocks, such as wood ash, poultry litter and manure, across regions from SE Asia to Sub-Saharan Africa. Chemical fertilizers, compost, and microbial fertilizers were employed to increase the humus content, aiming at facilitating the development of fruit tree root systems and nutrient uptake. They underlined that both SOM and nutrients of the slope plots followed the same trend. Increased porosity and SOM supplement along with mitigation of soil agrochemical contamination could be efficiently addressed by BC amendments. However, the required alterations in biomass for achieving soil remediation cannot be accomplished in the short term [141]. Aged BC has proven beneficial to the soil by stimulating nutrient availability and expediting nutrient cycling in the long term [102].

BC improves the water-holding capacity by reducing bulk density and enhances the nutrient holding of coarse-textured soils such as sandy soils, since these soils usually drain too fast and have low fertility [142]. Moreover, BC application in sandy soils increases tensile strength and enhances water infiltration [142]. Fine-textured, clayey soils may benefit in terms of an aeration improvement and a decrease in compaction; however, the efficiency of BC in these soils is less remarkable [142]. In loamy soils, the effects are generally moderate, since these soils are already balanced. In soils with low organic matter, BC provides a habitat for microbes and helps in the retention of nutrients, fostering fertility. The benefits are smaller in soils with high organic matter since these soils already support microbial activity and nutrient cycling [143]. BC (usually alkaline) can raise pH of acidic soil and improve nutrient availability, decreasing Al/Mn toxicity, which is a result of high solubilization and competition of Al and Mn with essential nutrients in acidic soils [144]. Podzolic soil, for example, is an acidic, sandy, and barren soil type that forms in cool, humid regions. A significant difference was reported with 2% biochar amendment in all the soil mixtures that were examined by Saha et al. [145], while BC treatment in newly converted boreal Podzols can significantly improve soil fertility and agricultural productivity [146]. In alkaline soils, additional liming may either not promote soil quality or cause negative effects [147]. Sandy soils or acidic soils can benefit considerably from BC with a high ash content (or low carbon content), boosting plant productivity [148]. BC improves water retention in dry soils and can improve aeration and microbial activity in waterlogged soil. In degraded soils, BC stimulates biological activity more noticeably. In the case of soil subject to erosion, incorporating BC into the topsoil boosts soil structure and stability and suppresses surface runoff, hence minimizing soil erosion rates [149]. Moreover, co-utilization of BC with organic additives such as compost or manure has been reported to enhance soil aggregation and water-holding capacity by creating a soil matrix less vulnerable to erosion [150]. The former also promotes BC efficiency and productivity in tropical weathered soil [151]. The efficiency of BC depends not only on BC characteristics but also on different soil types and the physicochemical composition and genesis of the soil. BC is particularly beneficial to soils with low water-holding capacity, such as sandy soils, acidic or saline soils, podzolic soils of boreal regions, and weathered tropical soils. In these cases, BC significantly improves soil revitalization and crop yields.

9.2. BC’s Side Benefits in Agriculture

Soil treated with BC presented enduring elevated susceptibility to erosion, greater than the corresponding runoff loss [152]. BC contributes to climate-resilient and sustainable agriculture also by its ability to achieve long-term carbon dioxide removal (CDR) [153]. It exhibits an elevated technology readiness level as a climate change mitigation alternative and has the potential to be deployed for nationwide climate objectives, combined with conventional site-specific benefits. In the same frame, the European Green Deal and the derived strategies intend to diminish P losses and fertilizer usage in agriculture [154]. P is a limiting factor for plant growth, but excessive P fertilization aggravates ecosystems [155]. A surplus of P is a characteristic mainly of the Northern EU and UK, whereas bioavailable P depletion is pronounced in the Mediterranean zone [140]. The evaluation of the current soil degradation and soil compaction extent in the European territory renders concern [155,156]. Nevertheless, soil degradation is not only about biodiversity loss or Europe. It is a global matter of major economic importance. Sartori et al. [157] estimated an average increase in soil erosion rates of between 30 and 66% under different climate–economic scenarios over the period 2015–2070. Subsequently, they used the projected land productivity losses as inputs into an economic global simulation model. The latter projected a global economic recession reaching USD 625 billion by the year 2070. Vulnerable regions are expected to face a serious challenge in terms of food security and food accessibility. Global primary agricultural production could lose more than 350 million tons under the worst-case scenario by the year 2070 [157]. Therefore, soil remediation and fertility enhancement should constitute a top priority, and BC could substantially contribute to this cause.

9.3. Advanced Uses of BC

Due to its properties, BC has great potential to be established for soil remediation and soil fertility enhancement. Its positive effects are enhanced when the soil type, feedstock and target application are well aligned. BC should be exploited more efficiently, with novel directions that fit in with and enhance the cyclic economy model. For instance, according to the EU, about 80% of the plastic waste in agriculture is due to plastic mulch films [158]. To a considerable extent, they end up as microplastics on land. Co-pyrolysis of livestock feedstock with waste plastic mulch films (plastichars) promotes and stabilizes soil enzyme activities without harming earthworms. These plastichars presented superior enzyme binding capacities compared to the livestock-based BC [159] while also preventing microplastic pollution in agricultural fields. Additionally, BC composites produced from feedstock–plastic blends have a higher carbon content [160]. It has been also documented that BC can accelerate the composting process and lead to a superior end-product [79]. In addition, BC can indirectly enrich soil through manure amelioration and can be used as a supplement in livestock feed [161].

Moreover, BC production exhibits economic feasibility and reduces the footprint of both the production and use of chemical fertilizers. For example, co-composting with organic materials has been put forward as one of the most efficient BC application practices for barren soils [79]. To maximize the economic benefit, the cost of BC application should be minimized by identifying a valid minimum BC application rate. In addition, the slow release of nutrients by BC has recently been employed for the synthesis of eco-friendly BC-based fertilizers aiming to overcome nutrient excess in soils and nutrient loss from chemical fertilizers [162]. However, before the policy is stipulated in detail, further research should be conducted on application rates, long-term effects, and potential limitations reported in the literature on BC treatment.

9.4. Future Perspectives

It is necessary that agricultural and waste research, and BC research, exploits technological affordances. Hyperspectral imaging (HSI) is a simple though cutting-edge technology based on the electromagnetic spectrum that can acquire spectral and spatial information for material identification in environmental monitoring. Sanchez-Hernandez et al. [159] employed an extended visible/near-infrared (EVNIR; 600–1700 nm) mobile hyperspectral imager with quartz–tungsten–halogen (QTH) light sources that produce visible and NIR outputs. They captured the temperatures at which gases were produced during pyrolysis. After employing statistics (principal component analysis; PCA), they managed to classify the BC samples (i.e., fresh BC, BC from non-disturbed soils and the burrow walls and plastichars). The affordances of HSI in BC research should be further investigated since HSI could technologically upgrade and facilitate BC identification and monitoring.

According to Wang et al. (2024) [163] and Rex et al., 2023 [164], BC production is still in the phase of “pilot scale production to primary demonstration” (TRL 2–5), which is less mature than bio-oil production (TRL 6–9). Therefore, efforts could focus on developing engineered BC with advanced properties and on optimizing methods to maximize its effectiveness in improving soil health and plant growth. Machine learning (ML) could play a significant role in this attempt, addressing the complexity of these interactions. Recently, ML algorithms, such as ANNs and Random Forests have been used to predict the yield and composition of BC products under different feedstock inputs and pyrolysis conditions, as well as to determine the optimized rate of BC for capturing pollutants from water bodies [165,166]. Random forest has been also employed to predict biomarkers of the BC response. The model provided insights into how BC amendment affected the soil microbial community [167]. Moreover, the RBF_SVR model revealed the behavior of gas diffusion in BC-amended soil [168]. So far, this review has mostly focused on predicting the adsorption efficiency of BC for water quality improvement [166,169]. It is suggested that the models presented in this manuscript can be trained with sequential datasets and incorporate multiple factors (e.g., soil type and properties; BC types and properties; plant species, properties, and fertilizing needs; environmental conditions; application rate; aged BC; etc.) for the evaluation of tailored BC usage scenarios on stressed soils. Another suggestion in the literature is that BC dynamics should be incorporated into Earth System Models (ESMs) in a way that is sufficiently fast and explicit, where machine learning could assist to suppress the computational load [87].

The abundance of soil- and crop-beneficial properties is the strong point of BC [49]. BC is a multifunctional material that has the potential to address agricultural challenges. Efforts should be geared towards the combination of BC types with sustainable management practices such as agroforestry, diversification of crops, deployment of climate-tolerant cultivars, and tree-based farming systems which favor bio-amelioration and carbon sequestration [170,171]. A paradigm shift in the production model, incorporating tailored BC utilization, seems imperative, aiming to secure biodiversity and resource conservation and enhance productivity.

10. Conclusions

Biochar presents a broad range of advantageous properties that make it a promising tool for enhancing soil health and addressing environmental challenges. Its ability to supply cationic nutrients (e.g., K, Ca, Mg, Zn, Mn), increase the CEC, enhance the specific surface area, and significantly increase the porosity and water-holding capacity contributes significantly to soil fertility. BC also raises the pH of acidic soils and favors microbiota, as it serves as a shelter and a nutrient sink. Therefore, BC enhances microbial abundance and soil enzymatic activity and improves soil properties. In addition, BC persists in soil for hundreds to thousands of years. Aged BC continues to support microbial activity and diversity selectively and fosters beneficial species growth. It supports the symbiotic microbe–plant relationship and promotes ecosystem resilience.

Furthermore, BC has been used for soil restoration. BC is a promising material in the removal of heavy metals from contaminated soil, favorably affecting plant growth. It acts as an effective amendment in problem soils. BC’s effect on soil reclamation and fertility is determined by multiple factors such as raw biomass quality (composition and contaminant content), pyrolysis characteristics (mostly temperature), and soil attributes. Long-lasting positive effects on earthworm activity have been reported with tailored BC application; additionally, it facilitates distribution of BC particles into the topsoil and promotes transportation of aggregates to lower soil layers. Advanced BC composites such as plastichars have attracted attention due to their elevated properties and ecological side benefits.

State-of-the-art technologies and modeling approaches, such as machine learning, hyperspectral imaging (HSI), and Earth System Models (ESMs), could contribute to a better understanding of BC dynamics. A current limitation is that most of the studies are conducted at the laboratory scale and are short-term. This fact raises the uncertainty of the claims, as underlined by numerous researchers. Consequently, robust, long-term field research is essential before shaping global biochar-related policies. The current literature indicates BC as a promising tool for sustainability. Although it is sometimes referred to as “black gold,” its widespread adsorption requires a careful and evidence-based approach that accounts for both promises and limitations. With continued research, BC has the potential to contribute meaningfully to soil regeneration and the circular economy.