Biochar Amendments for Soil Restoration: Impacts on Nutrient Dynamics and Microbial Activity

Abstract

Biochar is increasingly recognized as a multifunctional soil amendment that improves soil fertility, nutrient cycling, and crop productivity. Studies across field, greenhouse, and incubation settings show that biochar enhances nutrient retention, reduces leaching, and regulates carbon, nitrogen, and phosphorus cycling. Its effects are shaped by intrinsic physicochemical properties and interactions with soil minerals, microbial communities, and enzymatic processes. Short-term benefits of biochar applications often include improved nutrient adsorption and water regulation, while long-term applications support stable soil organic matter formation, root development, and fertilizer use efficiency. Biochar also reshapes soil microbial diversity and activity. Beneficial bacterial groups such as Proteobacteria and Actinobacteria, along with fungi such as Mortierella, respond positively, enhancing nitrogen fixation, phosphorus solubilization, and organic matter decomposition. Meanwhile, biochar applications could suppress pathogens. Enzyme activities, including urease and phosphatase, are typically stimulated, driving nutrient mobilization. Yet outcomes remain context-dependent, with biochar feedstock, application rate, soil conditions, and crop type influencing results; excessive use may suppress enzymatic activity, reduce nutrient availability, or shift microbial communities unfavorably. Practically, biochar can improve fertilizer efficiency, restore degraded soils, and reduce greenhouse gas emissions, contributing to climate-smart agriculture. Future work should prioritize long-term, multi-site trials and advanced analytical tools to refine sustainable application strategies.

1. Introduction

The excessive application of chemical fertilizers has triggered multiple environmental challenges, including soil degradation caused by acidification, nutrient leaching, deterioration of soil structure, decline in beneficial microbial populations, and an increase in greenhouse gas emissions [1]. Biochar, a pyrolyzed carbon-rich solid produced under oxygen-limited conditions from diverse biomass sources, has emerged as a sustainable amendment due to its capacity to improve soil properties and foster beneficial microbial activity [2,3]. Because of its high carbon content, porous structure, negatively charged surface, and alkaline properties, biochar can greatly change soil physicochemical properties, such as lowering bulk density, increasing pH, and improving nutrient retention and availability, water holding capacity, cation exchange capacity, and aggregate stability [4,5]. Nonetheless, these effects are influenced by multiple factors, including feedstock type, pyrolysis conditions, biochar application rate, soil type, and climate [6,7].

A growing body of evidence highlights its effectiveness in lowering greenhouse gas emissions [8,9,10]. A critical review by Lyu et al. [8] demonstrated a general inhibitory effect of biochar across various environments, with the impact on greenhouse gases decreasing in the order: N₂O > CH₄ > CO₂. This suggests that biochar modifies the physicochemical characteristics of soil and compost under diverse conditions, thereby influencing gas emissions. While relatively few, studies have also shown that biochar has no significant effect on or may even increase greenhouse gas emissions. A smaller review by Atilano-Camino et al. [11] on total greenhouse gas emissions from soils with and without biochar amendment found that the average CO₂ (7.7 t/ha) and CH₄ (4.9 t/ha) outputs were higher in soils amended with biochar than those from soils without biochar, whereas N₂O emissions (1.6 t/ha) were lower than those in the untreated control. Additionally, biochar has been reported to promote long-term crop productivity by fostering synergistic interactions between biochar, soil, and microbial communities [12,13]. Soil is a highly intricate ecosystem that sustains diverse microorganisms essential to ecosystem functioning. The incorporation of biochar has been shown to influence these functions by enhancing enzyme activity and restructuring microbial community composition [14]. Soil enzymes are critical for mobilizing recalcitrant nutrients, thereby ensuring a continuous energy supply for soil microbiota [15]. Moreover, biochar-induced increases in the pH of acidic soil promote crop growth by releasing phosphorus (P) bound to iron (Fe) and alleviating aluminum (Al) toxicity, ultimately regulating key aspects of soil nutrient dynamics [16].

The nutrient composition of biochar is largely influenced by the type of feedstock and the pyrolysis conditions. For instance, biochar produced from manure and biosolid feedstocks generally contains higher levels of nitrogen (N) and P compared to that generated from straw or wood materials [17,18]. During pyrolysis, an increase in ash content (inorganic residues) elevates the concentrations of P, calcium (Ca), magnesium (Mg), silicon (Si), and potassium (K), whereas the N content declines at higher pyrolysis temperatures due to gaseous losses [19,20].

Biochar also functions as a nutrient reservoir, effectively retaining macronutrients such as N, P, and K, as it can store and hold these nutrients within its structure and on its surface and slowly release them, keeping them available in soil longer [21,22]. However, excessive application of biochar may raise soil pH above the optimal level for crop growth, which can alter nutrient availability and ultimately diminish soil fertility and crop productivity [23]. A study reported that potato yields dropped by over 6% with mineral and 10% with organic fertilization, likely due to biochar-induced alkalinity that raised soil pH and reduced nutrient availability [24]. Although less reported, nutrient losses, particularly N, may occur through leaching and nitrous oxide emissions. For instance, at a high application rate of 30 t/ha, biochar increased nitrate and dissolved organic nitrogen leaching by 38% and 170%, respectively, relative to the control. This treatment resulted in the highest N losses through both nitrous oxide emissions and leaching, indicating that application rate plays a crucial role in N loss [25]. Nonetheless, a meta-analysis has shown that, at application rates of up to 120 t/ha, biochar generally reduces nitrous oxide emissions, as larger quantities can limit N availability for denitrification through the adsorption of nitrate. This could raise N use efficiency [26]. Yu et al. [27] noted that the nutrient retention capacity of biochar is primarily governed by its porosity and surface charge, which determine its ability to adsorb both cations and anions.

In recent years, biochar has attracted attention as a strategy for mitigating soil pollution and enhancing soil restoration [28,29,30]. Reviews of current studies indicate that biochar contributes to the immobilization of heavy metals and persistent organic pollutants in soils, thereby lowering their bioavailability. This remediation effect is largely attributed to processes such as precipitation, electrostatic attraction, surface adsorption, structural entrapment, and promoted degradation. However, the efficiency of contaminant removal is influenced by factors including the type of biochar, the application rate, the characteristics of the soil, and the nature of the pollutants [31,32,33]. By acting as a nutrient reservoir and altering soil microbial composition, biochar contributes to both contaminant removal from soil and alterations in soil nutrient content. However, few reviews focus specifically on the latter.

Dai et al. [34] reviewed how biochar properties influence N and P cycles in soil and the abundance of soil microbial communities, but provided limited explanation on how biochar applications affect the levels and transformation of various nutrients in soil and the microbial communities for cycling of nutrients beyond N and P. Tsolis and Barouchas [35] reviewed the influence of adding biochar on soil properties and its biological interactions in soil, but did not discuss the influence of biochar on the retention, alteration, and leaching of various soil nutrients in detail. Similarly to the review of Dai et al. [34], the review of Anyebe et al. [36] highlights how biochar properties, shaped by feedstock and pyrolysis temperature, affect soil ecosystem services, nutrient cycling, and crop productivity, with the emphasis placed on biochar synthesis and properties. Singh et al. [37] reviewed biochar production, key properties, and its role in restoring degraded soils, with emphasis on its effectiveness in remediating saline and acidic soils, its interactions with soil properties, and its ecological functions influencing enzymes and crop performance. Their review did not focus on the underlying molecular and microbial mechanisms that alter soil nutrient levels.

It is important to review the effects of biochar on nutrient dynamics and microbial activity in soil because both are central to soil fertility, ecosystem functioning, and sustainable crop production. Biochar can alter nutrient availability by influencing soil pH, cation exchange capacity, and sorption processes, which in turn affect the efficiency with which plants and microbes access essential elements. The two are closely connected: microbial processes (e.g., mineralization, nitrogen fixation, and decomposition) drive nutrient cycling, while nutrient availability regulates microbial growth and metabolism. Thus, understanding how biochar simultaneously influences nutrient dynamics and microbial activity is critical for predicting its long-term impacts on soil health and agricultural productivity. Therefore, this review aims to examine how biochar influences soil nutrient levels, particularly nitrogen, phosphorus, carbon, and cations. Specifically, it aims to elucidate the physicochemical mechanisms by which biochar alters nutrient retention, transformation, and leaching. Additionally, it presents recent progress in understanding how biochar influences soil microbial communities (bacteria and fungi). It seeks to elucidate the mechanisms by which biochar–microbe interactions enhance nutrient availability. In doing so, it provides deeper mechanistic insights into how it regulates soil health.

2. Review Methodology

This narrative review was conducted to ensure a comprehensive and balanced synthesis of current knowledge on biochar amendments in soil, with a particular focus on nutrient dynamics and microbial activity. A broad search was undertaken in major scientific databases, including Web of Science, Scopus, and Google Scholar, to identify peer-reviewed journal articles, reviews, and relevant book chapters published between 2015 and 2025. Search terms were combined using Boolean operators and included variations of the following keywords: (1) “biochar” AND “soil restoration”, (2) “biochar” AND “nutrient dynamics” OR “nutrient cycling” OR “fertility”, (3) “biochar” AND “microbial activity” OR “microbial community” OR “soil microbiota”, and (4) “biochar” AND “soil health” OR “ecosystem functioning”.

Studies were included if they: (1) reported experimental, field, or modeling data on the impacts of biochar on nutrient availability, nutrient cycling, or microbial processes in soil; (2) provided mechanistic insights into biochar–soil interactions relevant to soil fertility, ecosystem functioning, or crop productivity focusing on the nutrient and microbial aspects; and (3) were published in English and in peer-reviewed outlets.

Studies were excluded if they: (1) focused solely on biochar production techniques without discussing impacts on nutrient dynamics or microbial activity; (2) addressed only physical soil properties (e.g., bulk density, porosity) without linking them to nutrient or microbial processes; (3) mixed biochar with other soil amendments or treatments except fertilization as the simplest form of soil improvement; (4) involved functionalized biochar due to the myriad of functionalizing materials and techniques; and (5) were non-peer-reviewed sources such as conference abstracts, opinion articles, or policy briefs.

Key information from each study was extracted, including biochar characteristics, soil type and experimental setting (laboratory, greenhouse, or field), reported impacts on nutrient dynamics (e.g., N, P, K availability, pH changes, cation exchange capacity), as well as reported impacts on microbial activity (e.g., microbial biomass, community composition, enzymatic activity, nutrient mineralization).

Extracted data were synthesized thematically to highlight the mechanisms by which biochar influences nutrient dynamics, the variations in microbial activity and community structure due to biochar addition, and the interlinkages between nutrient dynamics and microbial processes under biochar amendment. Thematic synthesis combined both a priori and emergent coding. Priori themes, namely nutrient dynamics and microbial community composition, were identified based on the review’s objectives. Additional subthemes, including variations by soil type, biochar feedstock, and environmental conditions, emerged inductively during literature analysis. No formal effect size calculations were performed, as this review aimed for qualitative integration rather than meta-analysis; however, directional trends were noted (e.g., the proportion of studies reporting increased nutrient availability or microbial biomass). Contradictory results were reconciled by considering contextual factors, including biochar properties, soil characteristics, and experimental duration. Study robustness was evaluated qualitatively, with greater weight given to peer-reviewed experimental data with clear methodological detail.

3. Impacts of Biochar on Nutrient Dynamics

Biochar amendments are known to significantly influence soil nutrient dynamics by altering chemical properties, nutrient availability, and nutrient cycling processes, thereby shaping soil fertility and plant productivity.

3.1. Phosphorus and Nitrogen Dynamics

Biochar plays a pivotal role in regulating soil P and N dynamics. A field study in western Montana examined the effects of wood biochar (0, 20, and 40 tons per ha), with or without short-term cattle trampling, on soil properties, nutrient dynamics, and grass nutrient uptake [38]. Trampling alone compacted surface soils and reduced nitrogen availability, whereas biochar application improved infiltration, enhanced net nitrification, and promoted biochar incorporation into soil. Biochar also increased organic P while lowering inorganic forms, with stronger effects under trampling, and these changes were mirrored in grass P concentrations (Figure 1). Cattle trampling drove biochar deeper into the soil, increasing its contact with soil matrices and enhancing P sorption, which lowered CaCl₂-P and citrate-P levels. These P shifts may also reflect higher phosphatase activity from plants or microbes responding to greater P demand under adequate N and better moisture conditions [38].

Long-term trials (7 years) confirm biochar’s ability to limit nutrient losses. On vegetated roofs in Finland, biochar consistently reduced P leaching in both seeded plantings and pre-grown mats, while N leaching decreased only in plantings, coinciding with improved plant cover [39]. Nutrient losses declined over time as roofs aged; however, fertilization six years post-establishment caused elevated P leaching that biochar did not mitigate. Runoff from plantings contained more total P than mats, despite mats having 1.4 times higher P in the substrate. P dynamics were governed mainly by substrate texture, chemistry, and pH, whereas N dynamics were driven by biological processes. Substrate pH declined from 7.2–7.9 (2016) to 6.3–7.0 (2020) over the study duration, a range that keeps P plant-available rather than particle-bound.

Dubrovina [40] examined how wood biochar affects phosphate forms in two Karelian soils with different phosphorus availability, namely a sandy Umbric Podzol and a heavy loamy Umbric Retisol. Biochar fractions (≤2 mm and 3–5 mm) were applied at 2% and 5% rates. Biochar, especially the ≤2 mm fraction at 5%, raised soil pH, available phosphorus (by 20–40%), and the proportions of Ca-, Al-, and loosely bound P, while enhancing phosphatase activity in the Umbric Podzol. In pot tests with spring barley, biochar further increased P₂O₅ levels. In the Umbric Retisol, biochar had a smaller effect, slightly increasing available and Ca-bound P but not phosphatase activity; significant P₂O₅ gains occurred only with the combined biochar–fertilizer treatment. Overall, fine biochar (≤2 mm) at 5% showed the strongest positive impact across parameters.

Soil incubation experiments provide further insight into N cycling. Gupta et al. [41] tested rice straw (RSB) (pH = 9.1) and acacia wood biochar (ACB) (pH = 7.3) (at 0–1% in clay loam (pH = 7.6) and loamy sand soils (pH = 7.0). Ammonium-N declined faster in biochar-treated soils, while RSB consistently raised nitrate-N more than ACB. In clay loam, nitrate levels increased with higher biochar application rates up to 15 days. However, in loamy sand, the 1% amendment reduced nitrate levels after 30–60 days compared to the 0.5% amendment. With ACB, nitrate remained higher at 1% than 0.5% for longer periods in both soils. Phosphorus stayed elevated across all biochar treatments throughout the experiment. Biochar application markedly reduced NH₄⁺-N in both soils compared to controls throughout the 60-day incubation. This effect is linked to carboxyl and ketone groups in biochar, which enhance NH₄⁺ retention via hydrogen bonding and electrostatic forces (Figure 1). By improving soil cation exchange capacity, biochar also suppressed ammonia volatilization, thereby lowering cumulative NH₄⁺-N losses in a dose-dependent manner [42].

In an acidic citrus orchard soil, pH was adjusted to five levels (4.0–7.2) with CaO. The soil was subsequently amended with 0 or 1% biochar [43]. Using 15N-tracing, the study showed that biochar generally increased gross mineralization (TM) and NH₄⁺ immobilization (TI), except for TI at pH 4.0. It also promoted heterotrophic nitrification across a pH range of 4.0–7.4, while autotrophic nitrification occurred above a pH of 6.4. Overall, biochar improved nitrogen retention within pH 4.5–6.4, with stronger effects on TI than TM, and reduced autotrophic nitrification, suggesting enhanced N use efficiency and lower NO₃⁻-N losses in subtropical pomelo orchards (Figure 1). Soil pH strongly influenced how biochar affected nitrogen cycling in subtropical citrus soils. Biochar enhanced N retention mainly at pH 4.5–6.4, with greater effects on NH₄⁺ immobilization than on mineralization, while suppressing autotrophic nitrification as a key mechanism for improved retention. This aligns with the findings of [41].

Ammonia nitrogen in farmland soils is easily lost due to environmental and human factors. Lu et al. [44] assessed how Gracilaria lemaneiformis biochar influences ammonium behavior in soils. Results showed that adding 10% biochar increased ammonium in the 0–10 cm layer by 1.57–2.36 times while reducing leaching losses by 44.83–72.27%. Adsorption and retention were mainly governed by electrostatic attraction and ion exchange, with additional effects from molecular interactions and system energy. Low-energy sites near C₆H₁₂O₆ favored ammonium binding, while structures like C₅H₁₀O₅, C-S-H, C-SO₃, and C₄H₇NO₄ contributed via physical or chemical adsorption mechanisms. The negative adsorption free energy (−394,590.84 kcal/mol) confirmed stable binding of ammonium to biochar surfaces.

Additionally, a study using pot experiments (farmland soil) reported that biochar improved soil N availability when applying corn straw biochar at rates of 0%, 0.5%, 1.0%, 2.0%, and 4.0%. It also evaluated soil nutrients and bacterial diversity [45]. Total and ammonium nitrogen rose by 4.7–32.3% and 8.3–101.5%, respectively, while microbial biomass nitrogen increased with higher biochar levels. This is primarily attributed to biochar’s highly porous structure and large surface area, which enable it to adsorb and retain nitrogen, minimize leaching losses, and enhance soil nitrogen levels [46].

Overall, biochar significantly regulates soil P and N dynamics across diverse systems by enhancing nutrient retention, limiting leaching, and improving plant uptake. Biochar increases organic P while reducing inorganic and leachable forms. It also promotes nitrification and enhances ammonium retention through electrostatic and chemical interactions. Soil type, pH, and biochar feedstock strongly mediate these effects, with optimal N retention often occurring under moderately acidic conditions. Trampling and substrate properties can further influence nutrient shifts, while crop- and system-specific responses highlight both benefits and potential constraints at high application rates.

3.2. Carbon Dynamics

Biochar influences soil carbon dynamics by stabilizing organic matter, enhancing carbon sequestration, and moderating microbial activity. Biochar stabilizes organic matter through several mechanisms. Its highly aromatic and condensed carbon structure resists microbial decomposition, thereby slowing down organic carbon mineralization. Additionally, biochar provides microsites that physically protect labile organic matter within its porous matrix and through organo-mineral associations. The surface functional groups (e.g., carboxyl, hydroxyl) can also adsorb soluble organic compounds, reducing their accessibility to decomposers. These physicochemical interactions contribute to long-term carbon stabilization and enhanced soil organic carbon persistence. The details are presented in the following reviews.

Wu et al. [47] conducted an experiment that included a no-amendment control and a treatment with peanut shell biochar. After 70 days of incubation, peanut shell biochar application increased soil organic carbon by 44.8% and particulate organic carbon by 27.4% compared to the control. Mantel analysis and structural equation modeling revealed strong links among cbbL, cbbM, Bacteroidota, total organic carbon, and particulate organic carbon. The biochar treatment enriched soil carbon fractions, enhanced Bacteroidetes abundance, and reduced Acidobacteria richness, influencing key carbon cycle genes (cbbL and cbbM) (Figure 1). Overall, peanut shell biochar markedly improved soil carbon content and fertility.

Furthermore, Abdul-Aziz et al. [48] assessed how biochar type and application rate affect soil fertility and maize yield over two field seasons (2022–2023). Biochar materials from rice husk, groundnut husk, and sawdust were applied at 0–8 t ha⁻¹. Groundnut husk biochar proved to be the most effective, with 8 t ha⁻¹ increasing grain yield by up to 218% in 2022 and 106% in 2023. Soil organic matter rose by 90–343%, and nitrogen availability by up to 220% (2022) and 70% (2023). Biochar’s large surface area and reactive groups (carboxyl, hydroxyl) enable it to hold NH₄⁺ and limit NO₃⁻ leaching. Maize yield strongly correlated with soil fertility indicators, except harvest index. Regarding the increase in soil organic matter, two main mechanisms may be involved: biochar directly adds stable carbon to the soil, and it suppresses the mineralization of soil organic matter by immobilizing microbes and adsorbing easily degradable organic substances.

Liu et al. [49] compared how biochar (1.0% w/w) with and without N addition influences the fate of photosynthetically fixed carbon in rice plant–soil systems using ¹³CO₂ pulse labeling at key growth stages. Biochar did not alter the distribution of ¹³C among plant–soil components but increased its accumulation in shoots, roots, and soil, particularly when combined with N addition. During the growth period, biochar raised total ¹³C in shoots, roots, and soil by 23%, 14%, and 20%, respectively, and improved nitrogen use efficiency by about 23% at the heading and ripening stages. This was likely due to two factors, namely its porous structure adsorbs labile compounds and enhances microbial carbon use efficiency, thereby stabilizing more photo-assimilated C [50], and greater root biomass under biochar plus N addition contributed additional ¹³C inputs to the soil, even though allocation patterns of ¹³C within the plant–soil system remained unchanged [51]. Overall, biochar promotes higher plant productivity and soil carbon sequestration through enhanced N use efficiency.

A field trial applied biochar at 0% (control), 1% (LB), 2% (MB), and 4% (HB) of topsoil dry weight before planting winter wheat [52]. Soil organic carbon and particulate organic carbon rose with higher biochar rates. After 8 months, readily oxidizable carbon was reduced but later returned to control levels. Biochar increased dissolved organic carbon without affecting water-soluble organic carbon, though both dissolved organic carbon and water-soluble organic carbon declined over time. Soil organic carbon rose with higher biochar rates due to its high stable carbon content (60–80%) [53]. While biochar did not affect water-soluble organic carbon, it was positively linked to dissolved organic carbon, likely from labile carbon release, leaching of dissolved organic matter, and selective adsorption of small dissolved organic molecules. Both water-soluble organic carbon and dissolved organic carbon declined over time, reflecting microbial carbon utilization [53].

Mesocosm experiments with Festuca arundinacea were conducted to investigate the impact of cornstalk biochar (100 g/kg soil) on soil organic carbon under simulated grazing conditions in two different soil types [54]. Biochar boosted soil organic carbon by 595% and mineral-associated organic carbon by 39%, mainly by stimulating microbes that promoted stable organo-metal complexes with Fe/Al in red soils and Ca in calcareous soils. Effects were stronger in red soils due to pH and mineral differences, while in calcareous soils, biochar also offered long-term protection against acidification. Biochar enriches soil carbon pools by directly adding carbon and stabilizing mineral-associated organic carbon through its high carbon content, aromatic structure, and resistance to decomposition [55]. It also raises soil pH, causing Fe and Al to precipitate, which allows high-molecular-weight organics to bind to these oxides and form stable Fe- or Ca-associated complexes. This process enhances the incorporation of plant inputs into stable soil organic carbon, particularly in acidic to neutral soils like red soils [56].

In Russia, an incubation study was conducted on three Podzol Antric soils differing in initial total organic carbon content for 90 days, using birch- and aspen-derived biochar (produced by fast pyrolysis at 550 °C) at application rates of 0.1% and 1.0%. The 0.1% dose showed no effect on soil organic matter mineralization, while 1.0% biochar increased it by 15–18%, accompanied by shifts in humic substance composition [57]. The higher proportion of aromatic humic acid fragments suggests greater stability. In humus-rich soils with abundant insoluble material, mineralization and humic acid formation occurred concurrently, likely due to enhanced transformation of resistant organic fractions and partial humification of the biochar itself.

Collectively, biochar enhances soil carbon dynamics by stabilizing organic matter, improving carbon sequestration, and boosting nutrient use efficiency. It increases soil organic and particulate carbon, stimulates carbon-fixing microbes, and reduces nitrogen losses. Higher application rates promote stable mineral-associated carbon and long-term C retention, especially in acidic and red soils. Even at moderate levels, biochar improves humic composition and aromatic carbon stability. Empirical evidence indicates that biochar carbon may persist for 100–1000 years depending on feedstock, production temperature, and environmental conditions [58], underscoring its potential as a long-term carbon sink and soil fertility enhancer. Overall, biochar enhances soil fertility and carbon storage through the activation of microbes and the chemical stabilization of soil.

3.3. Nutrient Dynamics in General

Numerous studies have examined the effects of biochar on various soil nutrients, including C, N, P, and minerals. For instance, peanut shell biochar (0–10%) was tested in acidified soils using experiments and simulations. The highest adsorption was for K⁺ (820.38 mg/g), followed by PO₄³⁻ (270.51 mg/g), NO₃⁻ (235.65 mg/g), and NH₄⁺ (130.93 mg/g) [59]. At 10% biochar, outlet volume decreased by 65.32%, nutrient leaching dropped by 48.40–68.28%, and surface soil nutrient retention increased by 437.80–913.87%. Nutrient concentrations declined with depth, confirming biochar’s potential for improving nutrient efficiency in acidic soils.

Similarly, Lewoyehu et al. [60] conducted an incubation study in Ethiopia, testing water hyacinth biochar (WHB) against lime and fertilizer in acidic Nitisol. WHB slowed nitrification, lowered H⁺ release, and improved N and P availability. Soil pH rose by 0.30–0.35 (1% WHB) and 0.72–0.86 (2% WHB), while exchangeable acidity dropped by 26.5–28.8% and 58.4–63%, respectively. The 2% WHB + fertilizer treatment further raised the pH by 0.71–0.90 and cut acidity by 49.8–64.7% compared to lime + fertilizer or fertilizer alone, showing WHB outperformed lime in resisting acidification. Biochar reduced soil acidity by retaining NH₄⁺ and neutralizing Al³⁺, H⁺, and Fe³⁺ via its surface functional groups. The more porous, phenolic- and carboxyl-rich pyrolyzed WHB raised the soil pH more than the ground WHB [60]. Overall, WHB mitigated acidification by slowing nitrification and enhancing soil pH buffering capacity [61]. The improvement in soil P availability was attributed to higher soil pH dissolving Al³⁺/Fe³⁺-bound phosphates, functional groups on biochar releasing P bound to metal cations, and the direct release of soluble P from biochar.

Long-term effects were explored by Burgeon et al. [62], who studied century-old kiln charcoal (~200 years) as a proxy for historical biochar in three land covers. They compared soils with old biochar (CoBC), fresh oak biochar (YBC, 80 t ha⁻¹), and no biochar. Results showed that the age of biochar strongly influenced nutrient dynamics. YBC reduced NO₃⁻ and K⁺ leaching but increased PO₄³⁻ in topsoil pore water, while CoBC raised subsoil K⁺ and Mg²⁺, enhanced total N, K⁺, and Ca²⁺, but decreased available P. YBC reduced NO₃⁻ leaching via ion diffusion into its porous structure and temporarily raised topsoil PO₄³⁻, though alkaline pH limited its availability. In contrast, CoBC immobilized P through cationic bridging on oxidized surfaces but improved Ca²⁺ availability through functional groups. While yields were unaffected, biochar reduced N, K, and Ca uptake but increased Mg uptake, showing that YBC affects short-term pore water dynamics, whereas CoBC drives long-term nutrient cycling [62].

Long-term field experiments have further highlighted biochar’s role in crop systems. In a seven-year rice study in northwest China, annual biochar application (9 t ha⁻¹ yr⁻¹) improved shoot and root growth, enhanced root traits, and delayed root senescence during grain filling [63]. Biochar alone increased root biomass and length, while its combination with N fertilizer altered root morphology and growth patterns. Beyond its effects on plants, biochar reduced soil bulk density by 10–12%, increased total organic carbon, and raised available N, P, and K, underscoring its contributions to soil quality and fertility.

Additionally, dos Santos et al. [64] examined how biochar materials from sugarcane bagasse (SB), orange bagasse (OB), and corncobs (CB) affect ion leaching in clay soil, with untreated soil (CT) and gypsum-amended soil (CTG) as controls. Biochar raised water infiltration and flow by 80% and 71%, respectively. SB and CB enhanced ion removal by 48%, while OB achieved 19%, likely due to its lower surface area and water retention. Biochar reduced exchangeable Ca²⁺ and Mg²⁺ by 44% and 76% but increased K⁺ by 233%. Notably, SB and CB cut exchangeable Na⁺ by 90% and OB by 50%, indicating strong potential for improving soil quality and reclaiming salt-affected soils [64]. Biochar improved anion removal from soil, but the effect differed by type, likely due to variations in elemental composition, high electronegativity that repels similar ions, and enhanced soil water distribution and retention.

Nutrient release from animal waste biochar has also been studied. Hadroug et al. [65] tested poultry manure biochar (RPM-B, 600 °C) under static and dynamic conditions. In a 10-day leaching test, P and K were gradually but substantially released (93.6 and 17.1 mg g⁻¹, about 95% and 43% of initial contents), outperforming many other biochar materials. Column leaching showed RPM-B (5–8% w/w) retained NO₃⁻-N and NH₄⁺-N in alkaline sandy soil but promoted leaching of PO₄³⁻-P, K⁺, Mg²⁺, and Ca²⁺ at low, sustained rates (<10 mg kg⁻¹ d⁻¹ over 40 days) [65]. This indicates that RPM-B could be a sustainable amendment for nutrient-poor semi-arid soils compared with synthetic fertilizers.

A six-week pot experiment assessed how wood biochar influences boreal soil chemistry and spring barley growth in sandy loam Umbric Podzol and clay loam Umbric Retisol [66]. Using two biochar fractions (3–5 mm and ≤2 mm) at 5% (w/w), soil pH, available P and K, and mineral N were measured, along with barley growth and protein content. In the Umbric Podzol, biochar raised pH by 0.5 units and increased K (17 mg/kg), P (7 mg/kg), and nitrate (4 mg/kg) levels, promoting plant growth, biomass yield, and protein accumulation, especially with fine biochar (3–5 mm) plus NPK fertilizer. In contrast, the same treatments in the Umbric Retisol slowed barley growth and lowered nitrate content by 4–7 mg/kg, likely due to enhanced denitrification.

In summary, biochar significantly improves soil nutrient dynamics by enhancing nutrient retention, reducing leaching, and alleviating acidity. Its porous structure and functional groups adsorb essential ions, buffer soil pH, and increase the availability of N, P, and K across diverse soil types. Long-term applications further stabilize nutrient cycling and improve root growth, soil structure, and fertility. Performance varies with feedstock and soil conditions, with materials like sugarcane bagasse and poultry manure biochar showing strong potential for nutrient recovery and soil reclamation. Overall, biochar enhances soil productivity by improving nutrient efficiency and providing long-term chemical stabilization. A summary of the effects of biochar on P, N, C, and nutrient dynamics, in general, is presented in

Table 1. Impacts of biochar on P, N, C, and general nutrient dynamics.

Reference	Location/System	Biochar Treatment	Main Findings	Implications
Effects on P and N dynamics
[38]	Field study on grassland, Montana	Wood biochar (0, 20, 40 t ha−1) with/without trampling	Trampling compacted soils, reduced N availability Biochar improved infiltration (no trampling: 70.5% at 20 t ha−1 and 26.5% at 40 t ha−1), nitrification, and nutrient incorporation Increased organic P, reduced inorganic P. Trampling enhanced P sorption (lower CaCl2-P, citrate-P)	Biochar offsets negative trampling effects, enhances P availability in plants, and stabilizes nutrients through deeper incorporation
[39]	Long-term (7 yr) green roof trial, Finland (plantings & mats)	Biochar in substrates	Reduced P leaching consistently Reduced N leaching only in plantings Fertilization after 6 years caused high P leaching not mitigated by biochar P dynamics governed by substrate chemistry/pH N regulated by biological processes	Biochar enhances long-term P retention, but effectiveness depends on fertilization and substrate properties
[40]	Karelia, Russia–Sandy Umbric Podzol & heavy loamy Umbric Retisol	Wood biochar (≤2 mm and 3–5 mm) applied at 2% and 5% (w/w); with and without NPK fertilizer	Fine biochar (≤2 mm, 5%) increased soil pH and available P (by 20–40%), and enhanced Ca-, Al-, and loosely bound P and phosphatase activity in Umbric Podzol Minor effects in Retisol; notable P2O5 gains only with biochar–fertilizer combination	Fine biochar at higher application rates improves P availability and soil enzymatic activity in low-P sandy soils but is less effective in clayey soils
[41]	Incubation in clay loam & loamy sand	Rice straw (RSB) & acacia wood biochar (ACB), 0–1%	NH4+ declined faster with biochar RSB raised NO3− more than ACB Effects varied by soil type and rate Biochar reduced NH4+ losses via retention (H-bonding, electrostatic)	Feedstock and soil type strongly affect N cycling; biochar improves NH4+ retention and reduces volatilization
[43]	Subtropical pomelo orchards with acidic soil	0–1% biochar at different pH (4.0–7.2)	Biochar increased mineralization & NH4+ immobilization (except TI at pH 4.0) Enhanced heterotrophic nitrification (pH 4–7.4) Suppressed autotrophic nitrification (pH > 6.4)	Biochar improves N retention mainly at pH 4.5–6.4, enhancing N use efficiency and lowering NO3− losses
[44]	Farmland soil	10% amendment with Gracilaria lemaneiformis biochar	Increased NH4+ by 1.57–2.36× in 0–10 cm layer Reduced leaching losses by 45–72% Retention via electrostatic attraction, ion exchange, stable molecular binding	Strong potential to reduce N losses and stabilize ammonium in soils
[45]	Farmland soil (pot experiment)	Corn straw biochar (0–4% peat)	Total and ammonium nitrogen rose by 4.7–32.3% and 8.3–101.5% Microbial biomass nitrogen increased with higher biochar levels	Biochar enhanced soil nitrogen availability and crop uptake by increasing ammonium and nitrate levels. Low doses suppressed ammonia oxidation, while higher application rates improved N use efficiency by stimulating nitrification and denitrification
Effects on C dynamics
[47]	Peanut shell biochar (treatment vs. no-amendment control)	70-day incubation experiment	Soil organic carbon ↑ 44.8% and particulate organic carbon ↑ 27.4% compared to control Enhanced Bacteroidetes abundance and reduced Acidobacteria richness Strong associations among cbbL, cbbM, Bacteroidota, total organic carbon, and particulate organic carbon	Biochar enriched soil carbon fractions, stimulated beneficial microbes, and improved soil fertility
[48]	Field maize trials, two seasons (2022–2023)	Biochar from different feedstocks (rice husk, groundnut husk, sawdust) at rates up to 8 t/ha	Groundnut husk biochar highest effect Soil organic matter increased by ~90–343% Nitrogen availability also increased Maize yield strongly correlated with increased soil fertility indicators Biochar’s reactive groups helped hold NH4+ and limit NO3− leaching	Biochar can significantly enhance soil C pools and fertility in real field settings; feedstock matters for maximizing effect
[49]	Rice plant–soil systems; 13CO2 pulse labeling	1.0% biochar with/without N fertilization	Biochar + N enhanced accumulation of newly fixed C in shoots (↑23%), roots (↑14%), and soil (↑20%) Improved nitrogen use efficiency (~23%) at heading and ripening stages Porous structure adsorbs labile compounds; more root biomass leads to more carbon inputs	Biochar + N fertilization can increase carbon retention from recent photosynthesis, boosting both plant growth and soil C storage
[52]	Field winter wheat trial, topsoil amendment	Biochar at control, low (LB-~1%), medium (MB-~2%), high (HB-~4%) rates (dry weight basis)	Soil organic carbon & particulate organic carbon increased with higher biochar rate Readily oxidizable carbon initially reduced, but later returned to control level Dissolved organic carbon increased; water-soluble organic carbon not strongly changed Over time, both dissolved organic carbon and water-soluble organic carbon declined (microbial use)	Higher rates of biochar build more stable carbon pools; labile C dynamics shift over time (initial flush then decline)
[54]	Mesocosm experiments with Festuca arundinacea planted on red soils and calcareous soils; simulated grazing	Cornstalk biochar at 100 g biochar per kg soil	Huge boost in soil organic carbon (~+595%) and mineral-associated organic carbon (~+39%) in red soils Stronger effects in red (acidic) soils than in calcareous soils Biochar helped stabilize organic matter via organo-metal complexes (with Fe, Al in red soils; with Ca in calcareous soils) Also mitigated soil acidification in calcareous soils	Biochar is especially effective in acidic soils for stabilizing carbon; mineral interactions are key (Fe/Al/Ca binding)
[57]	Northwestern Russia–Three Podzol Antric soils with varying organic C	Birch and aspen biochar (fast pyrolysis, 550 °C) at 0.1% and 1.0% (w/w); 90-day incubation	1.0% biochar increased soil organic matter mineralization by 15–18% and altered humic substances Higher aromatic humic acid fraction indicated greater stability In humus-rich soils, mineralization and humic acid formation occurred simultaneously	Moderate biochar addition stimulates organic matter turnover and humic substance formation, enhancing soil carbon dynamics
Nutrient dynamics in general
[59]	Acidified soils (experiments + simulations)	Peanut shell biochar (0–10%)	High adsorption: K+ (820.38 mg/g) > PO43− (270.51 mg/g) > NO3− (235.65 mg/g) > NH4+ (130.93 mg/g) At 10% biochar: outlet volume ↓65.32%, nutrient leaching ↓48.40–68.28%, retention ↑437.80–913.87%	Improves nutrient efficiency and reduces leaching in acid soils
[60]	Ethiopia, acidic Nitisol (incubation study)	WHB (1–2%), with lime/fertilizer comparisons	Slowed nitrification, lowered H+ release, improved N and P availability pH ↑0.30–0.35 (1% WHB) and ↑0.72–0.86 (2% WHB) Exchangeable acidity ↓26.5–28.8% (1%) and ↓58.4–63% (2%) WHB + fertilizer outperformed lime + fertilizer Functional groups neutralized Al3+/H+/Fe3+	WHB is more effective than lime in resisting acidification, enhancing soil buffering and nutrient availability
[62]	Historical kiln sites, various land covers	Century-old charcoal (CoBC), fresh oak biochar (YBC, 80 t/ha), no biochar	YBC: reduced NO3− and K+ leaching, increased PO43− in topsoil pore water CoBC: raised subsoil K+, Mg2+, total N, Ca2+, but reduced P YBC improved short-term nutrient retention; CoBC influenced long-term cycling	Biochar age shapes nutrient dynamics: YBC affects short-term processes, CoBC sustains long-term soil fertility
[63]	7-year rice field	Annual biochar (9 t/ha/yr) with/without N fertilizer	Improved root biomass, morphology, delayed senescence, boosted shoot/root growth Soil bulk density ↓10–12%, total organic carbon and available N, P, K ↑	Enhances soil fertility, crop growth, and long-term soil quality
[64]	Clay soil (Brazil), with gypsum control	Biochar from sugarcane bagasse (SB), orange bagasse (OB), corncobs (CB)	Biochar ↑ infiltration (80%) and flow (71%) SB & CB enhanced ion removal (48%), OB less effective (19%) Reduced exchangeable Ca2+, Mg2+; ↑K+ (233%) Strong Na+ reduction: SB & CB (90%), OB (50%)	Effective for salt-affected soil remediation and improving soil–water distribution; performance varies by biochar type
[65]	Alkaline sandy soil	Poultry manure biochar (RPM-B, 600 °C, 5–8% w/w)	Released P (93.6 mg/g) and K (17.1 mg/g) gradually (10-day test) Retained NO3−-N, NH4+-N, but promoted slow leaching of PO43−-P, K+, Mg2+, Ca2+ at <10 mg/kg/day (40 days)	Sustainable alternative to synthetic fertilizers for nutrient-poor semi-arid soils
[66]	Karelia, Russia–Boreal soils (Sandy loam Umbric Podzol; clay loam Umbric Retisol)	Wood biochar (≤2 mm and 3–5 mm) at 5% (w/w); 6-week pot study with spring barley	In Umbric Podzol, biochar increased pH (+0.5), K (+17 mg/kg), P (+7 mg/kg), NO3− (+4 mg/kg), and barley growth and protein yield In Retisol, growth slowed and NO3− decreased by 4–7 mg/kg, likely from denitrification	Biochar improves fertility and crop performance in acidic sandy soils but may hinder N dynamics in dense, clayey systems.

Note: The reported changes are statistically significant (p < 0.05) in the studies reviewed, unless otherwise specified with words like ‘slight’ and ‘not strongly’; ↑ represents increase and ↓ represents decrease.

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3.4. Nutrient Dynamics in Fertilized Soils

A two-year field experiment evaluated how rice-straw biochar influences the nutrient supply of organic fertilizer (OF) under rice–rapeseed rotation [67]. Results showed that using OF alone reduced rice yields by 2% and rapeseed yields by 6–10% compared with chemical fertilizer (CF). However, applying biochar at 15 t ha⁻¹ with OF (OF + B15) boosted rice yields by 10–17% and rapeseed yields by 20–25% relative to OF, and still outperformed CF with yield increases of 7–14% for rice and 12–13% for rapeseed. The OF + B15 treatment also improved soil quality: during rice and rapeseed seasons, it raised soil organic carbon by 57–90% and enhanced enzyme activities, i.e., catalase (14–19%), invertase (14–20%), urease (17–24%), and phosphatase (13–17%) [67]. Biochar improved the nutrient-supplying ability of organic manure by raising soil pH and organic carbon. This pH increase was linked to the high ash content (19.43%) and alkalinity (pH 9.42) of the biochar, which released cations that neutralized soil acidity [68]. Overall, biochar addition enhanced soil properties, enzymatic activity, and nutrient availability from OF.

Lu et al. [69] tested how biochar type (fine pine wood powder, 0.1 mm, vs. coarse fescue grass, 0.5–2 mm) and application method (surface vs. incorporated at 1% w/w) influenced N and P leaching from soils with inorganic fertilizer or biosolids. Soil columns were leached for six months, yielding ~16 pore volumes of drainage. Biochar showed limited retention of ammonium (NH₄⁺) and no sorption of nitrate (NO₃⁻). However, incorporating powder biochar reduced cumulative leaching of NH₄-N, NO₃-N, and total N by about 25% compared with controls. Although biochar’s negatively charged surface was expected to strongly adsorb NH₄⁺-N [70], powder biochar reduced NH₄⁺–N in leachate during only 6 of 16 events, mainly in biosolids and control soils, with effects declining after day 50. Still, incorporating powder biochar lowered cumulative NH₄⁺-N, NO₃⁻-N, and total N losses, especially in the early leaching phase (Days 1–36) when N release was greatest. Incorporation proved more effective than surface application, likely due to greater contact between biochar and soluble N [69].

In a greenhouse pot trial, Llovet et al. [71] compared three commercial organo-mineral fertilizers with their biochar-amended versions (NPK + B, NP + B, K + B) using barley as the test crop. Results showed that NPK + B slowed nutrient leaching, particularly nitrate (−28%) and potassium (−22%) in the first two weeks, though this effect reversed after week three, likely due to microbial nutrient immobilization supported by biochar. NPK + B also boosted barley straw biomass by 23.4% compared to NPK, while all biochar-based fertilizers improved plant nutrient content and export (K, S, Ca, Mn), suggesting biochar contributed as an additional nutrient source.

Furthermore, a lab incubation study tested biochar from spent mushroom substrate (2% w/w) and inorganic P addition (62 kg ha⁻¹) on soil P availability, pH, acid phosphatase activity, and bacterial abundance in acidic paddy and red soils from southern China at 15 °C and 25 °C [72]. Both amendments significantly raised soil pH by up to ~39–41% in paddy soil and ~43–45% in red soil. Inorganic P addition produced the highest available P, reaching 111.47 mg/kg at 15 °C and 100.17 mg/kg at 25 °C in paddy soil. The biochar contained labile P that directly released phosphate into the soil. Along with inorganic P, it reduced adsorption and complex formation, mobilizing native soil P beyond critical levels. Changes in soil pH, porosity, and microbial activity further enhanced available P.

Together, these studies demonstrate that biochar can significantly enhance fertilizer efficiency by improving soil fertility, reducing nutrient losses, and increasing crop yields. Its effects vary with feedstock, application method, and soil type, but overall, biochar strengthens nutrient retention, mobilizes otherwise unavailable pools, and supports long-term soil health, making it a promising amendment for sustainable crop production.

3.5. Effects of Nutrient-Enriched Biochar on Nutrient Dynamics

Sewage sludge biochar generally contains little K because much of it is lost with water during treatment. Supplementing sewage sludge biochar with mineral K can produce organo-mineral fertilizers that release K slowly, but their release patterns in different soils are not well understood. Ndoung et al. [73] examined K release from biochar-based fertilizers in pellet and granule forms in clayey soil, sandy soil, and silica over 60 days at an application rate of 4 Mg ha⁻¹. Compared to KCl, biochar-based fertilizers released K more slowly, with pellets showing the slowest release. K availability was greatest in clay soil, followed by sandy soil and silica. K release patterns from biochar-based fertilizers and KCl were similar across soils and silica, and particle size had little effect. However, release rates differed with soil moisture at 10 kPa tension: clay soil showed the fastest K release, followed by sandy soil and silica. The higher water content in clay likely enhanced fertilizer dissolution by maintaining closer contact with soil water [73]. Overall, enriched biochar-based fertilizers act as slow-release fertilizers, suggesting potential for improved plant K uptake during growth.

Yin et al. [74] assessed the impacts of N-enriched biochar (4 and 8 t ha⁻¹ y⁻¹) on C emissions, storage, rice yield, and Fe dynamics in subtropical paddy fields. Both application rates raised C emissions, but 4 t ha⁻¹ significantly improved yields. Field and culture experiments showed increased soil Fe²⁺ with biochar, which correlated positively with CO₂, CH₄, total C, and dissolved organic carbon, but negatively with Fe³⁺. In culture experiments without plants, biochar reduced cumulative CO₂ and CH₄ emissions. Both N-enriched biochar rates increased soil Fe²⁺, likely due to higher pH, lower redox potential, and enhanced Fe³⁺ reduction. Biochar’s porous structure and surface area also favor Fe-reducing microbes [75,76]. Additionally, biochar may serve as an electron donor or shuttle for Fe³⁺ reduction, roles considered more important than its potential function as an electron acceptor for Fe²⁺ oxidation [77]. Overall, moderate application (4 t ha⁻¹) enhanced yield, biomass, and dissolved organic carbon while helping regulate C emissions through effects on Fe dynamics.

Javeed et al. [78] tested the application of date palm biochar (at 2%) enriched with buffalo slurry (SEB) on soil C–N dynamics, nutrient leaching, and wheat growth. Enrichment raised biochar C and N contents by up to 310% and 286%, respectively. Both standard and enriched biochar reduced nitrification (81–94%), ammonification (48–74%), and CO₂ emissions (50–92%) compared to the control. SEB was especially effective, cutting leaching losses of C, N, P, K, Na, Ca, and Mg by 9–125%. Dissolved organic carbon supplied electrons for denitrification, releasing CO₂ as oxygen was consumed. In SEB treatments, higher water-filled pore space and organic carbon created anaerobic conditions that accelerated denitrification while reducing microbial respiration [78].

Additionally, a study developed two enriched biochar (EB) fertilizers with a 6–6–4 N-P₂O₅-K₂O grade: EB-1 (NPK intercalated into biochar) and EB-2 (EB-1 supplemented with humic acid and seaweed extract) [79]. Nutrient release tests showed EB fertilizers slowed the release of NH₄⁺, P, and K⁺ compared to conventional fertilizers, while NO₃⁻ release remained similar. Although total N release was comparable, EB fertilizers released considerably less P and K within 36 h. In field trials with direct-seeded rice, EB fertilizers improved apparent nutrient use efficiency, achieving 11.7–29.5% higher N, 32.9–64.0% higher P, and 31.4–38.0% higher K efficiency than conventional fertilizers at full application rates. These results highlight the potential of EB fertilizers for sustainable rice production by reducing nutrient losses and enhancing nutrient use efficiency.

Enriched biochar materials demonstrate strong potential as sustainable slow-release fertilizers that enhance nutrient efficiency and soil health. Sewage sludge- and organo-mineral-based biochar materials released K gradually, particularly in clay soils, improving nutrient availability over time. N-enriched biochar optimized rice yield and carbon regulation by promoting Fe reduction and moderating greenhouse gas emissions. Similarly, slurry- and nutrient-enriched biochar materials improved C–N cycling, reduced nutrient leaching, and increased fertilizer use efficiency in cereals. Overall, moderate application of enriched biochar supports higher productivity while minimizing nutrient losses and environmental impacts.

3.6. Implications

Biochar plays a multifaceted role in soil nutrient and carbon dynamics by combining physical, chemical, and biological mechanisms. Its porous structure and stable carbon backbone enhance soil organic carbon storage while also influencing CO₂ and CH₄ fluxes through changes in redox potential and microbial respiration [54,78,80]. Acting as an electron shuttle or donor, biochar supports Fe-reducing microbes, which further regulate carbon turnover and greenhouse gas emissions [74,75].

Nutrient cycling is similarly affected, as biochar improves N and P retention while reducing losses through leaching, runoff, and volatilization [38,39,41]. Functional groups such as carboxyl and ketone bond with NH₄⁺, while its high cation exchange capacity stabilizes mineral N [42]. Phosphorus is retained in more stable pools but is also made more plant-available through enhanced phosphatase activity and P-solubilizing microbes [38,72]. These processes increase nutrient-use efficiency and promote sustained plant uptake.

The magnitude and direction of these effects depend on soil type, feedstock, and environmental context. Acidic soils show greater ammonium immobilization and suppress nitrification, while clay soils enhance K and P availability through better water–fertilizer interactions [41,73]. Feedstocks such as rice straw enriched with Mg increase P solubility, whereas sewage sludge biochar requires mineral supplementation to supply K [73,81]. Moisture, pH buffering, and redox conditions further shape how biochar interacts with soil nutrients and microbial communities.

Mechanistically, biochar mediates nutrient dynamics not only through sorption and ion exchange but also by reshaping microbial networks. Its porous surface provides habitats for microbes, promoting groups such as Proteobacteria and Actinobacteria that drive organic matter decomposition, nitrification, and P solubilization [81]. In this way, biochar integrates physical stabilization with microbial mediation, supporting both soil fertility and environmental sustainability. Overall, its potential as a tool for nutrient management and climate mitigation is clear, though context-specific application strategies remain essential.

A zonal comparison of soils reveals that biochar’s influence on nutrient dynamics varies strongly with soil type, texture, and pH. In sandy Umbric Podzols, biochar (especially fine fractions ≤ 2 mm at 5%) significantly elevated soil pH, available P, and phosphatase activity, enhancing plant P uptake and growth [40]. In contrast, in heavy loamy Umbric Retisols, biochar produced smaller improvements in P availability and negligible effects on enzyme activity, with major benefits only when combined with fertilizer [40]. Similarly, studies on clay loam and loamy sand soils showed that biochar improved N retention and reduced NH₄⁺ losses more effectively in fine-textured soils, while in coarse soils, the benefits declined over time due to lower cation exchange capacity [66]. In acidic soils, biochar buffered pH and enhanced N and P availability, whereas in neutral to calcareous soils, its main role was improving C stabilization and reducing nutrient leaching [43,54,72,82]. Collectively, these zonal variations demonstrate that biochar’s efficiency depends on the inherent physicochemical properties of soils, where fine-textured and moderately acidic soils show the most consistent gains in nutrient retention, biological activity, and fertility improvement.

4. Impacts of Biochar on Microbial Activity

Biochar has gained significant attention as a soil amendment due to its potential to enhance soil fertility, sequester carbon, and mitigate environmental pollution. Beyond its physical and chemical contributions to soil systems, biochar exerts profound effects on soil microbial communities, which are central to nutrient cycling, organic matter decomposition, and overall ecosystem functioning. By modifying soil properties such as pH, porosity, moisture retention, and nutrient availability, biochar can create favorable or, in some cases, inhibitory conditions for microbial growth and activity. These shifts in microbial dynamics not only influence soil health and productivity but also determine the efficacy of biochar in delivering long-term agronomic and environmental benefits.

4.1. Recent Findings on the Influence of Biochar on Microbial Activity

Zhang et al. [83] examined the impacts of straw-based carbon substrate (100 kg), a new form of biochar, on soil fertility, microbial communities, and maize yield compared with traditional straw amendments. Both crop stage and treatment significantly shaped bacterial and fungal diversity. Key taxa, including Rhizobiales, Saccharimonadales, and Eurotiales, responded positively to the biochar substrate by enhancing nutrient cycling. Rhizobiales abundance was linked to higher soil nitrogen during early maize growth, while Saccharimonadales and Eurotiales correlated with elevated carbon and phosphorus at later stages (Figure 2). These findings indicate that straw-based carbon substrate fosters stage-specific recruitment of beneficial microbes, thereby improving nutrient supply and boosting crop productivity.

A two-year pot experiment (2020–2021) investigated how biochar (0–60 t ha⁻¹) and nitrogen fertilizer (0–450 kg ha⁻¹) influence soil properties, enzyme activity, nitrogen use efficiency, and rapeseed yield [84]. Compared with the control, biochar with high nitrogen markedly enhanced soil enzymes: B30 + N450 increased urease by 73–75%, B60 + N450 raised catalase by 16–17%, and B60 + N225 improved alkaline phosphatase by 17–19% (Figure 2). Although excessive nitrogen alone reduced plant nitrogen metabolism enzymes, adding 30 t ha⁻¹ biochar alleviated this stress and stimulated nitrate reductase, nitrite reductase, glutamate synthase, and glutamine synthetase at both seedling and flowering stages. The B30 + N450 treatment also increased plant height (11–18%), pods per plant (32–39%), and grain yield (54–64%), while B15 + N225 achieved the highest nitrogen use efficiency (29%) [84]. These results demonstrate that biochar enhances enzyme activity, offsets nitrogen stress, and improves nutrient efficiency and rapeseed productivity. Biochar addition enhanced key soil enzymes by improving nutrient cycling and microbial activity. Urease activity rose due to greater nitrogen mineralization and retention, while higher phosphatase activity was linked to biochar’s porous structure, ash content, and organic carbon, which increased soil pH and moisture [85]. Catalase activity improved as biochar boosted microbial oxidative metabolism through better soil aeration and moisture, and invertase activity increased with higher soil organic carbon and nutrient availability. However, excessive biochar (60 t ha⁻¹) reduced urease and invertase, likely due to elevated pH and adsorption effects [86].

Application of an enriched rice straw biochar at 4.24 g per treatment altered soil microbial communities by promoting beneficial groups like Proteobacteria and Actinobacteria, which support phosphorus solubilization and organic matter decomposition, while reducing taxa such as Methylomirabilota and Desulfobacterota (Figure 2) [81]. Biochar treatments increased the number of unique microbial operational taxonomic units, indicating community shifts that support nutrient cycling, plant–microbe interactions, and overall soil health. Biochar-based phosphate fertilizers further enriched rhizosphere diversity, enhancing nutrient solubilization, organic matter breakdown, and pathogen suppression [17].

In a seven-year maize-field trial testing biochar applications (0–126 Mg ha⁻¹) and their effects on root traits and soil microbes, biochar was found to significantly enhance root length, surface area, volume, and biomass. It also increased bacterial diversity and altered community structure, though fungal communities showed little change [87]. Root development was strongly associated with bacterial, rather than fungal, communities, with microbial network analysis linking key bacterial taxa to root growth parameters. Generally, fungi can utilize the stable carbon in biochar more efficiently than bacteria, colonizing its pores and exploiting available resources [88]. However, in this study, the limited fungal response may be due to the long interval since application (7 years), the low proportion of stable carbon, and the gradual collapse of biochar pores over time. Consequently, biochar had a stronger influence on bacterial communities than on fungal ones. Biochar application (15.75–126 Mg ha⁻¹) was observed to increase Firmicutes while reducing Actinobacteria and Acidobacteria in rhizosphere soils. Biochar can reshape rhizosphere microbial communities by stimulating root exudation and supplying nutrients and energy that support microbial activity, potentially favoring Firmicutes [89].

A study separating corn-straw biochar into aqueous extractable (AE), organic extractable (OE), and residual solid fractions (EBC) and incubating it with purple soil for 30 days revealed that biochar increased available P, K, and organic C but lowered pH and available N [90]. Sequencing showed fungal richness and diversity increased, while bacterial richness remained unchanged. Community shifts differed by group: AE mainly shaped bacterial composition through nutrient inputs, whereas EBC influenced fungal communities via its structure and stable compounds. The limited impact of biochar on fungi observed by Yan et al. [87] may be due to the seven-year gap since application, the low proportion of stable carbon in the biochar, and the gradual collapse of its pore structure over time.

In apple root zone soils, apple wood biochar (1% w/w) was found to reshape bacterial, fungal, and archaeal communities, raising bacterial α-diversity in the rhizoplane but lowering fungal and archaeal diversity in the rhizosphere [91]. It boosted nitrogen-fixing microbes and N₂O-reducing activity in root-associated soils, cutting N₂O emissions by up to 35%, while showing little effect on nifH and amoA genes in non-rhizosphere soils. Network analysis further linked nitrogen-cycling genes to diverse microbial groups.

Shang et al. [92] examined enzyme activity, microbial growth, and bacterial community dynamics in maize rhizospheres following straw and straw-derived biochar addition. Straw returned to soil boosted microbial biomass and specific growth rates (1.2–1.6 and 1.7–2.0 times higher, respectively), favoring fast-growing Firmicutes, which carried more β-1,4-glucosidase (BG) and β-N-acetylglucosaminidase (NAG) genes. Correspondingly, BG activity and affinity rose 2.2- and 1.8-fold, while NAG rose 4.0- and 2.0-fold. By contrast, biochar favored slower-growing Actinobacteria (K-strategists), resulting in weaker enzymatic responses but enhanced root biomass (+31%) and expanded rhizosphere hotspots for BG (+26%) and NAG (+47%). Biochar also increased network stability, whereas straw mainly accelerated nutrient cycling by promoting rapid microbial proliferation and enzyme production. Biochar enhanced nutrient mobilization in the rhizosphere by enlarging hotspot areas, allowing nutrient release from a broader soil volume [92].

Akumuntu et al. [93] assessed the impact of rice husk biochar (0–1.5%) on soil properties, lettuce growth, and earthworm toxicity. Rice husk biochar did not significantly alter soil pH but enhanced enzyme activities (dehydrogenase, alkaline phosphatase, and beta-glucosidase) after 28 days and increased beneficial growth-promoting microbes, such as Massilia, Bacillus, and Trichocladium (Figure 2). Lettuce root weight and leaf number rose with higher rice husk biochar levels, with growth improvements linked to enhanced nutrient availability driven by microbial and enzymatic shifts. Liu et al. [94] reported that the porous nature of biochar supports microbial diversity and protects microbes against stresses such as freeze–thaw events. Consequently, biochar contributes to creating favorable habitats for soil microorganisms and promotes the stability of microbial networks.

A six-year field study (2013–2018) examined how different biochar application rates (0, 15.75, 31.5, and 47.25 t ha⁻¹) with fertilizers influence soil fertility, organic N fractions, and microbial communities [95]. Biochar raised soil organic matter, C/N ratio, and pH, and significantly altered acid-hydrolyzable N and its fractions. Amino acid N was associated with enriched Verrucomicrobia, Cyanobacteria, Glucosylbacteria, and Nitrospira; ammonium N with Verrucomicrobia, Bacteroidetes, Gemmatimonadetes, Acidobacteria, and Proteobacteria; amino sugar N with Acidobacteria; and unknown N with Gemmatimonadetes (Figure 2). These shifts, along with changes in soil enzyme activities, indicate that biochar drives N cycling through specific microbial–enzyme interactions.

Moreover, Yin et al. [96] investigated the impact of rice and corn straw biochar materials on microbial communities in the rhizosphere of rice grown in albic soil. In a pot experiment, the application of biochar increased soil nutrient levels and enhanced bacterial and fungal abundance and diversity. Compared with the control, rice straw biochar increased the bacterial abundance-based coverage estimate (ACE) by 3.1%, and the fungal ACE and Chao indices by 7.9% and 14.2%, respectively. Both biochar materials increased the operational taxonomic units. Biochar also strengthened bacteria–fungi interactions, with shifts in community structure closely tied to changes in soil nutrients. Overall, rice straw biochar had the strongest positive effect on microbial diversity and soil quality. Biochar boosts microbial abundance by supplying labile C and minor nutrients, while in nutrient-poor, compact albic soil, it also alters community composition indirectly through improvements in pH, structure, and related properties. Biochar raised the relative abundance of Proteobacteria, Verrucomicrobia, and Bacteroidetes. Proteobacteria, the dominant phylum, increased by 13.95% with rice straw biochar and 9.94% with corn straw biochar compared to the control. Their enrichment reflects their eutrophic nature, as biochar improves nutrient conditions in albic soils, consistent with findings of Di et al. [95].

A three-month field trial tested peanut shell biochar (0, 5, and 10% v/v) in landfill cover soil to assess its impacts on plants and microbes [97]. Biochar raised soil pH, nutrients, and organic matter up to two times, boosting plant growth. It also increased bacterial richness, diversity, and community heterogeneity. However, plant diversity declined, and dominant species shifted. Specifically, it promoted Cyperus rotundus L. dominance, likely through its allelopathic traits in nutrient-rich conditions. Biochar significantly increased bacterial richness and diversity at all levels, while only 5% biochar boosted fungal richness and diversity. Functional genes linked to amino acid metabolism were abundant, likely from root growth, and biochar also raised genes for cofactor and vitamin metabolism. Improved soil chemistry from biochar also reinforced interactions between fungi and Cyperus rotundus L. [97].

In addition, a field experiment tested three treatments: no biochar, 10 t/m³, and 20 t/m³, with soil microbial diversity analyzed using high-throughput sequencing [98]. Biochar improved soil properties, and alpha diversity was highest at 20 t/m³. The dominant phyla included Proteobacteria, Cyanobacteria, and Actinobacteria, with Skermanella, Nostoc, Frankia, and unclassified Proteobacteria as key genera (Figure 2). Actinobacteria varied significantly across treatments. Microbial community structure was mainly shaped by soil porosity, moisture, and nitrogen and potassium fertilizers, rather than phosphate or organic matter.

Zhang et al. [99] tested four wheat straw biochar levels (0, 5, 10, 15 kg/plant) across two soil depths (0–20 cm, 20–40 cm). Biochar improved citrus fruit quality and soil properties (pH, organic matter, nutrients). It enhanced bacterial richness, evenness, and diversity but reduced fungal evenness, while enriching beneficial bacteria and saprophytic (Basidiomycota) fungi involved in nutrient cycling. Furthermore, Yang et al. [100] examined Panax quinquefolium L. under different rice straw biochar levels (0–2.4%) to assess plant performance, active compounds, soil enzymes, and microbial communities. Biochar enhanced plant growth, photosynthesis, chlorophyll b, and ginsenoside content, while also increasing sucrase, urease, and acid phosphatase activities (Figure 2). In contrast, catalase activity and microbial diversity declined. Notably, bacterial phyla (e.g., Proteobacteria) and fungal genera (e.g., Chalara, Sistotrema) varied between the control and 1.8% biochar. Linear discriminant analysis further indicated significant shifts in rhizosphere microbial taxa under 1.8% and 2.4% treatments. Biochar can enhance microbial growth by supplying organic carbon and nutrients [100]. In this study, the rhizosphere fungal community of P. quinquefolium was more affected by biochar, differing from Zhang et al. [98], who reported increased bacterial diversity but reduced fungal evenness.

In a separate study, Ren et al. [101] assessed biochar (20 kg/plant) effects on bayberry decline by examining tree growth, fruit quality, soil properties, microbial communities, and metabolites. Biochar improved tree vigor, fruit quality, and rhizosphere microbial diversity. It increased Mycobacterium, Crossiella, Geminibasidium, and Fusarium, while reducing Acidothermus, Bryobacter, Acidibacter, Cladophialophora, Mycena, and Rickenella. Redundancy analysis showed that soil pH, organic matter, nutrients, and exchangeable cations strongly shaped microbial composition, with a greater influence on fungi than bacteria at the genus level, in line with the findings of Yang et al. [100].

A pot experiment with cotton seedlings was conducted to test rice straw biochar at three levels (0, 1, and 2%, w/w) for saline–alkaline soil treatment [102]. At 2%, biochar lowered soil pH, EC, and soluble salts, while increasing organic matter, nutrients, and enzyme activity. It reduced salinity in both soil and plant tissues and altered microbial community composition, enhancing salt-tolerant phyla like Proteobacteria, Bacteroidota, Acidobacteriota, and Actinobacteriota. Biochar markedly enhanced extracellular enzyme activity, with nitrogen- and phosphorus-cycling enzymes rising sharply by 32.79% and 34.25%, respectively. Biochar also strengthened microbe–metabolite interactions, mitigating salt stress on microorganisms [102].

A field study in a tropical ecosystem tested biochar from corn cobs at 15 and 30 t/ha, with or without phosphate fertilizer, on soil microbes and enzymes [103]. Biochar boosted microbial biomass C (4.5–8.2×), N (1.4–2.7×), and mineralized C (1.2–1.7×), while 30 t/ha amendments reduced specific maintenance respiration rates by 66–73%, indicating higher efficiency. It also increased dehydrogenase and urease activity, as well as the abundance of arbuscular mycorrhiza and soil fungi. Only 30 t/ha and 30 t/ha + P raised Gram-positive (1.7–1.9×) and Gram-negative (1.5–1.6×) bacteria, and total phospholipid fatty acid was highest under 30 t/ha treatments [103]. Specific maintenance respiration reflects microbial oxygen use and stress levels. Biochar improves soil porosity and gas diffusion [104], raising O₂ availability and stimulating microbial activity. This explains the sharp decline in specific maintenance respiration with biochar, while higher specific maintenance respiration in controls indicates carbon stress. The lower stress in biochar-amended soils also favors greater fungal abundance, as fungi typically thrive under stable conditions [105].

In a double rice system, the effects of wheat straw biochar on microbes and soil fertility were assessed 3–4 years after a single application (0, 24, 48 t/ha) [106]. Biochar increased bacterial and fungal abundances by up to 102% and 178%, linked to higher total organic carbon, nitrogen, and rice biomass, while archaea rose only slightly. The bacteria/fungi ratio fell by up to 61.4%, as fungi dominated in decomposing recalcitrant carbon. Biochar also promoted Acidobacteria, Mortierella, and Westerdykella, which support carbon cycling and plant growth, while reducing phytopathogens like Athelia and Penicillium. A summary of these recent findings is shown in

Table 2. Biochar effects on soil microbial communities.

Ref.	Biochar Treatment	Crop/System	Duration	Key Findings	Microbial & Enzymatic Effects
[83]	Straw-based carbon substrate (100 kg)	Maize/Field	5 months	Enhanced soil fertility & yield; stage-specific nutrient effects (N early, C & P later)	↑ Rhizobiales, Saccharimonadales, Eurotiales; improved N, C, P cycling
[84]	Wood biochar (0–60 t/ha) + N (0–450 kg/ha)	Rapeseed	2 years	↑ Soil enzymes, plant height, pods, yield; alleviated N stress	↑ Urease, catalase, phosphatase, nitrate reductase, etc.; excess biochar (60 t/ha) inhibited urease/invertase
[81]	Enriched rice straw biochar (4.24 g/treatment)	General soils	60 days	Altered microbial structure; ↑ nutrient solubilization	↑ Proteobacteria, Actinobacteria; ↓ Methylomirabilota, Desulfobacterota; ↑ rhizosphere diversity
[87]	Biochar (0–126 Mg/ha)	Maize/Field	7 years	↑ Root traits, bacterial diversity (5–9%); minor fungal response	↑ Firmicutes; ↓ Actinobacteria, Acidobacteria; bacterial communities linked to root traits
[90]	Corn-straw biochar fractions (AE, OE, EBC)	Purple soil	30 days	↑ Available P, K, organic C; ↓ pH & N	AE affected bacteria via nutrients; EBC affected fungi via structure; ↑ fungal diversity
[91]	Apple wood biochar (1% w/w)	Apple orchard	1 year	↓ N2O emissions (−35%); altered rhizosphere diversity	↑ N-fixing microbes; altered bacterial/fungal/archaeal structure
[92]	Straw vs. straw-biochar	Maize rhizosphere	21 days	Straw ↑ microbial biomass; biochar ↑ root biomass & stability	Straw favored Firmicutes (r-strategists, ↑ enzyme genes); biochar favored Actinobacteria (K-strategists)
[93]	Rice husk biochar (0–1.5%)	Lettuce	28 days	↑ Enzyme activity, root & leaf growth	↑ Massilia, Bacillus, Trichocladium; biochar stabilized microbial networks
[95]	Biochar (0–47.25 t/ha) + fertilizer	Field	6 years	↑ Soil organic matter, pH, C/N; altered N fractions	N fractions correlated with Verrucomicrobia, Acidobacteria, Proteobacteria; ↑ N-cycling enzymes
[96]	Rice & corn straw biochar	Rice (albic soil)/Pot	Till rice maturity	↑ Nutrients, diversity, ACE & Chao indices (2–3% for bacteria; 7–14% for fungi)	↑ Proteobacteria, Verrucomicrobia, Bacteroidetes; stronger bacteria–fungi interactions
[97]	Peanut shell biochar (0–10%)	Landfill soil	3 months	↑ pH, nutrients, organic matter; ↑ plant growth, ↓ plant diversity	↑ Bacterial richness; 5% biochar ↑ fungal diversity; enriched amino acid metabolism genes
[98]	Biochar (0, 10, 20 t/m3)	Field soil	Not specified	↑ α-diversity (20 t/m3); improved soil properties	↑ Proteobacteria, Cyanobacteria, Actinobacteria; shaped by porosity & moisture
[99]	Wheat straw biochar (0–15 kg/plant)	Citrus	7 months	↑ Soil fertility & fruit quality	↑ Bacterial richness & diversity; ↑ Basidiomycota; ↓ fungal evenness
[100]	Rice straw biochar (0–2.4%)	Panax quinquefolium/Pot	7 months	↑ Growth, photosynthesis, ginsenosides (23–101%); ↓ catalase	↑ Proteobacteria, Chalara, Sistotrema; altered rhizosphere fungi strongly
[101]	Biochar (20 kg/plant)	Bayberry	11 months	↑ Growth (27–57% for various parameters), fruit quality, soil nutrients (14–58% for organic matter, alkali hydrolyzed N, exchangeable Ca, and Mg)	↑ Mycobacterium, Crossiella, Geminibasidium, Fusarium; ↓ Acidothermus, Mycena; fungi > bacteria
[102]	Rice straw biochar (0–2%)	Cotton, saline–alkaline soil/Pot	6 months	↓ pH (1%), electrical conductivity (19%), salts (25–28%); ↑ nutrients, enzyme activity	↑ Proteobacteria, Bacteroidota, Actinobacteriota; ↑ N, P cycling enzymes (32–34%)
[103]	Corn cob biochar (15–30 t/ha) ± P	Tropical soil/Field	2015 to Jan 2017	↑ Microbial biomass C (4.5–8.2 times), N (1.4–2.7 times), and enzymes; ↓ stress respiration	↑ Gram+ (1.7–1.9 times) & Gram– (1.5–1.6 times) bacteria; ↑ fungi & arbuscular mycorrhizal fungi; improved porosity, O2 diffusion
[106]	Wheat straw biochar (0–48 t/ha)	Double rice system	3–4 yrs post-application	↑ Bacteria (+102%), fungi (+178%); ↑ total organic carbon & N	↑ Acidobacteria, Mortierella, Westerdykella; ↓ Athelia, Penicillium (pathogens)

Note: The reported changes are statistically significant (p < 0.05) in the studies reviewed, unless otherwise specified with words like ‘slight’ and ‘not strongly’; ↑ represents increase and ↓ represents decrease.

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4.2. Variations in Bacterial and Fungal Communities

Biochar generally exerts positive effects on bacterial communities. It increases bacterial richness, diversity, and network stability by improving soil conditions, such as pH, porosity, nutrient availability, and aeration. Beneficial bacterial phyla, such as Proteobacteria, Actinobacteria, Rhizobiales, and Firmicutes, as well as genera like Massilia, often proliferate under biochar amendment, thereby supporting nutrient cycling, nitrogen fixation, and root development. Biochar’s porous structure provides microhabitats and labile carbon that stimulate bacterial activity, while its ability to retain nutrients enhances bacterial-mediated processes such as phosphorus solubilization, nitrogen mineralization, and organic matter decomposition [81,92,93,95]. However, at very high application rates, biochar can reduce bacterial enzyme activity or shift community composition in ways that are less favorable, often due to excessive pH increases or adsorption of available nutrients [86].

The effects of biochar on fungi are more variable and context-dependent. In many cases, biochar increases fungal abundance and richness, particularly of beneficial saprophytic fungi (e.g., Basidiomycota, Mortierella, Westerdykella) that drive carbon cycling and plant–soil symbioses [99,106]. Fungal communities often show stronger responses to biochar’s structural fractions (e.g., stable carbon compounds and pore architecture) than bacteria do [96,100]. Positive impacts include enhanced phosphorus mobilization, pathogen suppression, and increased resilience under stressors such as salinity.

However, biochar can also reduce fungal evenness or suppress certain groups, especially under high application rates, long-term use, or when biochar significantly alters soil pH [98]. Some studies reported little change in fungi compared to bacteria, while others observed declines in diversity or abundance of specific fungal taxa (e.g., Mycena, Rickenella) [100].

Biochar generally plays a suppressive role against soil-borne pathogens by modifying the soil environment in ways that reduce their competitiveness. Many studies show that biochar lowers the abundance of harmful fungi such as Athelia and Penicillium in rice systems, as well as Cladophialophora, Mycena, and Rickenella in bayberry soils [100,106]. These reductions are linked to improvements in soil fertility, organic matter, pH, and redox conditions, which create less favorable niches for pathogens to thrive. By improving overall soil quality, biochar makes conditions more conducive to beneficial microbes while simultaneously weakening pathogen survival.

An important mechanism for pathogen suppression is competitive exclusion by beneficial microbial taxa. Biochar stimulates the proliferation of groups like Proteobacteria, Actinobacteria, Mortierella, and Westerdykella, which are known for their roles in nutrient cycling, organic matter decomposition, and plant growth promotion [106]. These beneficial organisms occupy ecological niches and utilize resources that might otherwise support pathogens, thereby reducing their abundance and influence.

Enhanced soil enzyme activity also contributes to pathogen suppression. Biochar addition increases activities of urease, phosphatase, catalase, invertase, and other enzymes that strengthen nutrient cycling and microbial oxidative metabolism [67,84,103]. This enzymatic activity supports antagonistic microbial populations and biochemical processes that hinder pathogen growth. In particular, biochar’s porous structure and high surface area provide habitats for beneficial microbes that can outcompete or inhibit pathogens.

However, biochar does not always act purely suppressively. Some evidence suggests that, under certain conditions, biochar may shift microbial communities in ways that favor particular taxa with pathogenic potential. For example, increases in Fusarium in bayberry soils following biochar application suggest that some pathogenic groups may benefit from the altered nutrient and pH environment [101].

4.3. Connections Between Microbial Activity and Nutrient Dynamics

Biochar alters nutrient dynamics largely through its influence on microbial activity. By improving soil conditions, such as pH, aeration, and organic matter content, biochar creates favorable habitats that stimulate microbial growth and activity. Beneficial groups such as Proteobacteria and Actinobacteria become more abundant, alongside fungi like Mortierella and Westerdykella, which contribute to organic matter decomposition and nutrient release [92,98,106]. These shifts in community composition strengthen soil biochemical cycling.

Nitrogen cycling is particularly sensitive to microbial responses under biochar amendment. Proteobacteria, many of which include nitrogen-fixing (Rhizobium) and denitrifying (Pseudomonas) genera, are stimulated by improved soil conditions [83,107]. Increased urease activity, often linked to bacterial and fungal groups, accelerates N mineralization, raising the pool of plant-available nitrogen. Biochar also promotes Fe-reducing microbes, such as members of the Geobacter genus within Deltaproteobacteria, which drive Fe³⁺ reduction [74,75]. This process lowers redox potential and indirectly regulates denitrification pathways, thereby affecting nitrogen losses and greenhouse gas emissions.

Phosphorus dynamics are also strongly microbially mediated. Enrichment of Proteobacteria and Actinobacteria enhances the activity of phosphatase-producing taxa, which liberate inorganic phosphate from organic matter [81]. Some groups within Actinobacteria are known for secreting organic acids that solubilize phosphate minerals, complementing biochar’s direct effects on P sorption–desorption equilibria [108]. In this way, microbial turnover ensures greater and more sustained phosphorus availability.

For potassium, microbial activity plays a supporting role through mineral weathering and organic matter decomposition. Certain bacterial genera within Acidobacteria and Bacteroidetes contribute to the breakdown of plant residues, gradually releasing K⁺. This complements the cation exchange capacity of biochar, which stabilizes K in the soil and prolongs its availability [65,109].

Overall, microbial activities under biochar amendment drive nutrient dynamics by accelerating organic matter decomposition, stimulating enzyme-mediated processes, and regulating redox-sensitive pathways. The enhanced abundance of phyla such as Proteobacteria, Actinobacteria, Bacteroidetes, and Acidobacteria, along with fungal genera like Mortierella and Westerdykella, underscores the biological mechanisms linking biochar’s physicochemical properties with nutrient cycling. These microbial shifts underpin improvements in N, P, and K availability, thereby supporting soil fertility and plant nutrition.

5. Conclusions

Across a zonal series of soils, from acidic Podzols and Retisols to neutral loams and calcareous soils, biochar exerts soil-specific effects on nutrient dynamics, particularly phosphorus (P) and nitrogen (N) transformations. In coarse-textured, acidic soils such as Umbric Podzols, biochar markedly enhances nutrient availability by raising pH, stimulating phosphatase activity, and increasing exchangeable and Ca-bound P. Conversely, in fine-textured or neutral soils like Umbric Retisols and loamy sands, biochar’s influence is more moderate, often improving nutrient retention but showing limited stimulation of microbial processes. High surface area and functional groups in biochar promote cation exchange and ammonium adsorption, thereby reducing volatilization and leaching losses. However, its efficacy depends strongly on soil reaction and mineralogy: optimal N retention typically occurs under mildly acidic to neutral conditions (pH 4.5–6.4), where biochar enhances NH₄⁺ immobilization and suppresses nitrification. These zonal contrasts demonstrate that while biochar universally contributes to nutrient conservation and soil fertility enhancement, its mechanisms and magnitudes vary with soil type, texture, and pH. Hence, region-specific optimization of biochar formulation and application rate is essential for maximizing nutrient efficiency and sustaining soil productivity across diverse agroecosystems. For instance, higher application rates may be required for acidic soil to raise pH, increase available P, and improve nutrient retention, while lower rates are beneficial for calcareous or neutral to alkaline soils to avoid further alkalinization, which can immobilize P and hinder nutrient uptake.

Biochar has demonstrated consistent potential to improve soil fertility by enriching microbial diversity, stimulating enzyme activity, and supporting nutrient cycling, thereby boosting crop productivity. Its porous structure and nutrient content provide favorable habitats for beneficial bacterial and fungal taxa, which drive nitrogen fixation, phosphorus solubilization, and organic matter decomposition, while also suppressing certain pathogens. However, outcomes remain context-dependent, influenced by soil type, biochar feedstock, application rate, and crop species, with excessive use sometimes reducing enzyme activity, shifting microbial communities unfavorably, raising pH, and reducing nutrient availability.

Future research should prioritize long-term, multi-site field trials across varying climates, soils, and cropping systems to better understand biochar’s role in microbial succession and nutrient-use efficiency. Combining molecular, isotopic, and metabolomic tools will help clarify the mechanisms underlying biochar–microbe interactions. In addition, integrating biochar with nanotechnology, organic amendments, and precision fertilization strategies may reveal synergies that optimize soil health, crop productivity, and climate resilience in sustainable agricultural systems.