Biochar

Abstract Various studies have established that feedstock choice, pyrolysis temperature, and pyrolysis type influence final biochar physicochemical characteristics. However, overarching analyses of pre-biochar creation choices and correlations to biochar characteristics are severely lacking. Thus, the objective of this work was to help researchers, biochar-stakeholders, and practitioners make more well-informed choices in terms of how these three major parameters influence the final biochar product. Utilizing approximately 5400 peer-reviewed journal articles and over 50,800 individual data points, herein we elucidate the selections that influence final biochar physical and chemical properties, total nutrient content, and perhaps more importantly tools one can use to predict biochar’s nutrient availability. Based on the large dataset collected, it appears that pyrolysis type (fast or slow) plays a minor role in biochar physico- (inorganic) chemical characteristics; few differences were evident between production styles. Pyrolysis temperature, however, affects biochar’s longevity, with pyrolysis temperatures > 500 °C generally leading to longer-term (i.e., > 1000 years) half-lives. Greater pyrolysis temperatures also led to biochars containing greater overall C and specific surface area (SSA), which could promote soil physico-chemical improvements. However, based on the collected data, it appears that feedstock selection has the largest influence on biochar properties. Specific surface area is greatest in wood-based biochars, which in combination with pyrolysis temperature could likely promote greater changes in soil physical characteristics over other feedstock-based biochars. Crop- and other grass-based biochars appear to have cation exchange capacities greater than other biochars, which in combination with pyrolysis temperature could potentially lead to longer-term changes in soil nutrient retention. The collected data also suggest that one can reasonably predict the availability of various biochar nutrients (e.g., N, P, K, Ca, Mg, Fe, and Cu) based on feedstock choice and total nutrient content. Results can be used to create designer biochars to help solve environmental issues and supply a variety of plant-available nutrients for crop growth.

Abstract Biochar shows significant potential to serve as a globally applicable material to remediate water and soil owing to the extensive availability of feedstocks and conducive physio-chemical surface characteristics. This review aims to highlight biochar production technologies, characteristics of biochar, and the latest advancements in immobilizing and eliminating heavy metal ions and organic pollutants in soil and water. Pyrolysis temperature, heat transfer rate, residence time, and type of feedstock are critical influential parameters. Biochar’s efficacy in managing contaminants relies on the pore size distribution, surface groups, and ion-exchange capacity. The molecular composition and physical architecture of biochar may be crucial when practically applied to water and soil. In general, biochar produced at relatively high pyrolysis temperatures can effectively manage organic pollutants via increasing surface area, hydrophobicity and microporosity. Biochar generated at lower temperatures is deemed to be more suitable for removing polar organic and inorganic pollutants through oxygen-containing functional groups, precipitation and electrostatic attraction. This review also presents the existing obstacles and future research direction related to biochar-based materials in immobilizing organic contaminants and heavy metal ions in effluents and soil. Graphical Abstract

Abstract Biochar (BC) has exhibited a great potential to remove water contaminants due to its wide availability of raw materials, high surface area, developed pore structure, and low cost. However, the application of BC for water remediation has many limitations. Driven by the intense desire of overcoming unfavorable factors, a growing number of researchers have carried out to produce BC-based composite materials, which not only improved the physicochemical properties of BC, but also obtained a new composite material which combined the advantages of BC and other materials. This article reviewed previous researches on BC and BC-based composite materials, and discussed in terms of the preparation methods, the physicochemical properties, the performance of contaminant removal, and underlying adsorption mechanisms. Then the recent research progress in the removal of inorganic and organic contaminants by BC and BC-based materials was also systematically reviewed. Although BC-based composite materials have shown high performance in inorganic or organic pollutants removal, the potential risks (such as stability and biological toxicity) still need to be noticed and further study. At the end of this review, future prospects for the synthesis and application of BC and BC-based materials were proposed. This review will help the new researchers systematically understand the research progress of BC and BC-based composite materials in environmental remediation.

Abstract Biochar is a widely known soil amendment. Here we synthesize the available information on influence of biochar application on different soil properties and crop productivity using meta-analysis. Global data on influence of biochar applications on different soil physical, chemical, microbial properties, and crop productivity were extracted from literature and statistically analyzed. Based on selection criteria, 59 studies from the literature published between 2012 and 2021 were selected for the meta-analysis. Correlations were developed between effect size of biochar application on different soil properties and crop productivity. Application of biochar increased soil pH, cation exchange capacity, and organic carbon by 46%, 20%, and 27%, respectively, with greater effects in coarse and fine-textured soils. Effects on chemical properties were variable among biochar prepared from different feedstocks. Among physical properties, biochar application reduced bulk densities by 29% and increased porosity by 59%. Biochar prepared at higher pyrolytic temperatures (> 500 ℃) improved bulk density and porosity to greater extents (31% and 66%, respectively). Biochar prepared at lower pyrolytic temperatures (< 500 ℃) had a greater effect on microbial diversity (both bacterial and fungal), with more diverse bacterial populations in medium and coarse textured soils, while fungal diversity increased in fine textured soils. Biochar applications increased crop productivity only in fine and coarse textured soil. The effect size of biochar application on crop productivity was correlated with responses to physical properties of soils. The meta-analysis highlighted the need to conduct long-term field experiments to provide better explanations for changes in biochar properties as it undergoes aging, its longer-term effects on soil properties, and timing of re-application of different biochars.

The global prevalence of geogenically derived cadmium (Cd) enrichment poses significant risks to agricultural sustainability and food security. Micronutrients such as selenium (Se) usually co-occur with Cd in such regions but exhibit low bioavailability in soils, further causing a “hidden hunger” of nutrient depletion in the food supply chain. Through controlled field experiments, we tested strategically designed biochar-based amendments (clay-, manure-, and iron-enriched variants) at 5–20 t ha −1 on limestone-derived paddy soils to synchronously mitigate Cd mobility while enhancing grain Se accumulation. Biochar amendments decreased grain Cd bioaccumulation by 42.3–85.4% and increased grain Se bioaccumulation by 91.6–357.5% across both growing seasons, with a better effect observed in synergy with yield increase in the second growing season, reflecting longer-term soil conditioning and progressive weathering of the amendments. Among the three amendments, BC-ZVI at 10–20 t ha −1 achieved the fastest and most consistent Cd immobilization, lowering grain Cd below the national safety threshold (0.2 mg kg −1 ) in both growing seasons while increasing grain Se by 91.6–357.5%. BC-Clay and BC-Manure required the second season to reach comparable Cd performance across all application rates. Elevations in soil pH, CEC, and available phosphorus may have accounted for the (im)mobilization effect. Field and literature data further indicated a decoupling between soil total Cd-Se co-enrichment and their mobile or plant-accumulated pools. Findings from this study demonstrate a practical and effective strategy for producing Se-enriched, low-Cd rice in regions with elevated geogenic Cd-Se background.

The anisotropic shrinkage of wood during carbonization critically affects biochar microstructure, but its underlying mechanisms remain unclear. We examined shrinkage behavior in the radial (R) and longitudinal (L) directions in relation to polysaccharide thermal degradation, exchangeable cation species, and cell structure, using ten wood species (five softwoods and five hardwoods) and their cation-exchanged derivatives (H + , Na + , K + , Mg 2+ , Ca 2+ ). Thermogravimetric and continuous dimensional analyses revealed that the onset of shrinkage in the R-direction is primarily associated with hemicellulose degradation, whereas shrinkage in the L-direction is linked to cellulose degradation and occurs rapidly at temperatures 10–20 °C higher than the derivative thermogravimetric peak, when most cellulose microfibrils have already decomposed. Na + and K + lowered shrinkage onset temperatures across most species, whereas in hardwoods, R-direction shrinkage magnitude increased with H + and Mg 2+ substitution. Anatomical differences, particularly in ray parenchyma morphology, explained why hardwoods generally shrank more in the R direction. These results demonstrate that anisotropic shrinkage during wood carbonization is governed by polysaccharide-specific thermal degradation, modulated by exchangeable cation species and anatomical features. Furthermore, directional shrinkage can be predictably tuned through cation control and species selection to optimize biochar microstructure.

Enhancing dissimilatory nitrate reduction to NH 4 + –N (DNRA) in agricultural soils provides environmental and agronomic benefits by improving nitrogen retention and reducing nitrogen losses. However, DNRA requires a higher electron input (8 e − per NO 3 − ) than denitrification (5 e − per NO 3 − ), which often results in denitrification dominating nitrate reduction in soils. Although biochar can function as an electron shuttle to facilitate DNRA, it remains unclear whether enhancing its electron-shuttling capacity can further stimulate this nitrogen-conserving pathway. Here, we prepared a biochar with enhanced electron-shuttling capacity by loading redox-active Mn phases onto its surface and evaluated its effects on DNRA in soil. Mn-loaded biochar markedly enhanced electron-shuttling capacity and increased soil DNRA rates by 2.9-fold compared with the control. The electron-shuttling capacity of biochar was positively correlated with surface redox-active Mn content, and both DNRA rates and their contribution to total nitrate reduction increased accordingly. In addition, Mn-loaded biochar increased the relative abundance of nrfA -type bacterial taxa (e.g., Bacteroidota, Ignavibacteriota, and Bacillota) and significantly upregulated nrfA gene expression. These results demonstrate that loading redox-active Mn onto biochar effectively enhances its electron-shuttling function. Reversible Mn redox cycling may facilitate electron transfer, thereby increasing electron availability for nitrate reduction to NH 4 + –N. Meanwhile, Mn-mediated mobilization of native soil dissolved organic carbon (DOC), without increasing carbon loss, may provide supplementary bioavailable electron donors, collectively promoting DNRA and enhancing soil nitrogen conservation, which was accompanied by lower N 2 O emissions.

Abstract Biomass-derived carbon nanomaterials offer potential renewable alternatives to fossil-based carbon materials. It is, however, difficult to predict their performance since biomass composition, inorganic constituents, moisture, particle characteristics, and processing conditions all have strong influences on carbonization and structural evolution. The purpose of this review is to critically examine how precursor properties, carbonization/activation strategies, pore architecture, graphitic domains, defects, surface functionalities, and heteroatom configurations collectively affect the performance of materials. The paper discusses microwave-assisted pyrolysis as a rapid conversion route with distinct energy transfer and structural regulation characteristics, while also highlighting the challenges associated with energy absorption, temperature distribution, reactor design, and dielectric property reporting. The present study establishes an integrated precursor-process-structure-performance framework for biomass-derived carbon nanomaterials, in contrast to previous reviews that were primarily concerned with synthesis or applications. A standardized reporting approach and data-driven approaches are further recommended to improve reproducibility, enable reliable comparisons, and support scalable and application-oriented biochar research. Highlights Biomass can be converted into useful carbon materials for cleaner technologies There is a strong relationship between the quality and performance of materials and the raw materials used. Materials such as these are used for energy storage, carbon capture, and environmental cleanup applications.

Humic substance formation is essential for soil health, yet natural humification is slow, and synthetic production is often inefficient. White-rot fungi (WRF)-mediated lignocellulose humification shows potential but is limited by slow substrate degradation. Given the high biocatalytic potential of Mn(II) and its compatibility with the enzyme system of WRF, manganese-modified biochar (MBC) was utilized to enhance the degradation and conversion efficiency of lignocellulose. A 35-day solid-state fermentation experiment showed that 5% MBC addition increased lignin degradation by 93.4%, humic acid content by 47.7%, and polymerization degree by 112.4% compared with CK. Mechanistically, first, MBC enhanced extracellular lignocellulose-degrading enzymes and Mn-associated Fenton-like oxidation, thereby improving lignocellulose depolymerization. Second, interpretable machine learning and mass difference-based association analysis identified key molecular reactivity traits, suggesting that MBC facilitated humification by promoting the selective generation and conversion of reactive precursor molecules. In particular, enhanced DOM reactivity and increased precursor formation through deamination, dealkylation, and delignification were associated with the generation of more aromatic, oxygen-functionalized, and unsaturated humic-like substances, likely through polyphenol-dominated pathways. Finally, transcriptomic and metabolomic analyses indicated that MBC upregulated the expression of CAZyme-related genes and metabolic pathways associated with precursor generation, including phenolic/quinone biosynthesis, indicating that enhanced substrate depolymerization and subsequent precursor transformation jointly contributed to lignocellulose humification. Overall, this study provides mechanistic insights into how MBC enhances WRF-mediated lignocellulose humification and offers a theoretical basis for optimizing the bioconversion of lignocellulose-rich organic wastes.

The capacity of soil organic carbon (SOC) storage in agroecosystems can be effectively increased through the application of organic amendments. However, information on the pathways through which different organic amendments regulate SOC sequestration in degraded coastal saline land is limited. Thus, a two-year field experiment was established to assess SOC, microbial necromass C (MNC) contents, enzyme kinetics, pore structure, and microbial co-occurrence networks under different amendment applications (compound fertilizer, CK; compound fertilizer plus manure, OM; compound fertilizer plus manure plus straw, OM+ST; and compound fertilizer plus manure plus biochar, OM+BC). Compared with CK and OM, OM+ST and OM+BC increased SOC content and reduced soil salinity. However, their functional divergence clearly emerged. OM+ST primarily increased the activity of C-cycling enzymes, labile C pools (i.e., dissolved organic C and easily oxidizable organic C), and the accumulation of microbial necromass C. In contrast, OM+BC promoted macroaggregate formation and aggregate stability by transforming particles in the size range of 0.053–2 mm and optimizing macropores, ultimately increasing SOC storage capacity. Microbial network reconstruction revealed that the OM+ST treatment was associated with reduced abundance of Actinobacteria-dominated network modules, whereas OM+BC treatment increased the relative abundance of Ascomycota-dominated modules associated with SOC storage. Overall, our findings provide valuable insights into the promotion of SOC accumulation through integrated organic amendment strategies in coastal saline-alkali agroecosystems.

Biochar doped with heteroatoms showed superior performance in activating peroxymonosulfate (PMS) for contaminant degradation. The incorporation of nitrogen (N) enhanced surface functional groups and defects, while boron (B) improved pore structure; however, uncertainties remain regarding the identification of key active sites and reaction mechanisms under different N:B ratios. To address this issue, a series of co-doped biochar derived from waste sawdust was investigated in this work. Decreasing the N:B ratio (from 4 to 0.25) led to a significant enhancement of sulfamethoxazole (SMX) degradation (from 0.20 to 0.47 min −1 ) in the PMS system. Specifically, the contribution of catalysis was limited compared to that of adsorption at higher N:B ratios (> 1). Linear fitting analysis indicated that catalytic efficiency was primarily regulated by specific surface area. Additionally, C–OH groups acted as electron donors for generating radicals, while C = O groups were active sites for singlet oxygen ( 1 O 2 ). In contrast, the catalytic rate increased substantially at lower N:B ratios (0.25–1), suggesting that catalytic sites prevalent under boron-rich conditions became the dominant active centers. Correlation analysis revealed that decreasing N:B ratio (from 2 to 0.25) enhanced SMX degradation by elevating defect density (I D /I G from 1.40 to 1.46) and graphitic N content, thereby enhancing electron transfer and 1 O 2 formation for faster SMX degradation. The promotion of nonradical 1 O 2 pathway was further validated by theoretical calculations. Collectively, this study provides new insights into the mechanisms of PMS activation by biochar with varying N:B ratios, underscoring the potential of tailored biochar for efficient pollutant degradation.

Activated carbon (AC) is widely used in water and wastewater treatment, while biochar (BC) is emerging as a cost-effective and environmentally sustainable alternative. However, the release of superfine AC/BC particles may impair treated-water quality and facilitate contaminant transport. This study investigated residual superfine particles and particle-associated contaminant transport in fixed-bed filters and stirred-tank suspensions followed by sequential coarse-sand, fine-sand, 0.45 μm membrane, and 0.02 μm membrane filtration. Particle characterization revealed apparent size-distribution peaks at 5 μm, 0.5–1 μm, and 100–200 nm. Lead (Pb), used as a tracer contaminant, was detected on residual AC/BC particles. Quantitative fractionation combined with acid digestion and ICP-OES showed that particle-bound Pb accounted for 84.5–90.2% of total Pb in fine-sand filtrates. Pb-bearing particles in the 0.02–0.45 μm fraction also remained after 0.45 μm membrane filtration. The apparent 5 μm fraction is more plausibly attributed to secondary aggregation of nanoscale and submicrometer particles during filtration, sample handling, or measurement than to direct passage of intact 5 μm particles through the membrane. These findings identify particle-facilitated transport as a potentially overlooked pathway in post-filtration systems, and demonstrate the need to evaluate both dissolved and particle-associated contaminants when assessing treatment performance. Graphical Abstract

Dissimilatory nitrate reduction to ammonium (DNRA) conserves soil nitrogen (N), whereas denitrification and anaerobic ammonium oxidation (anammox) can result in soil N loss. Despite biochar’s potential to modulate N cycling pathways, its effects on dissimilatory nitrate reduction in alkaline paddy soils remain poorly characterized. Using 15 N tracer pairing techniques in soil slurries, we quantified denitrification, anammox, and DNRA rates in soils to 80 cm depth that had received surface annual application of 0.05 t/ha of rice straw biochar. We then partitioned nitrate reduction pathways and identified associated microbial communities. Denitrification showed the greatest variability in rates, proportional contribution and community composition, indicating its dominant role in partitioning nitrate reduction pathways in both surface and deep layers. Random forest and structural equation modeling identified the edaphic factors of soil pH, SOC/NO 3 ⁻ ratio, and Fe 2+ concentration as key drivers. In surface layers, biochar amendment optimized these parameters, stimulating both denitrification and DNRA rates. In deep layers, however, biochar progressively elevated pH above optimal thresholds and lowered SOC/NO 3 ⁻ ratio, further suppressing napA gene abundance and denitrification rates while increasing DNRA rates and proportions. Our findings demonstrate that long-term biochar amendment promotes DNRA and suppresses denitrification and anammox in deep alkaline paddy soil. These results provide mechanistic evidence for biochar’s role in conserving soil N through depth-dependent modulation of microbial N cycling pathways in alkaline paddy systems.

Abstract Biochar acts as a rhizosphere interface engineer, reshaping physical, chemical, and biological gradients across the root–soil–microorganism continuum. Physically, it enhances aggregation by 13.9–18.9% and increases porosity by 8.2–41.6%, improving root penetration and creating microbial refuges. Chemically, its functional groups buffer rhizosphere pH, adsorb root-derived organic acids, and mediate redox reactions that govern nutrient speciation and contaminant immobilization. Biologically, it provides protected microhabitats, enriches functional taxa, and stimulates enzyme activities. On average, urease activity rises by 23.1%, alkaline phosphatase by 25.4%, and nitrogen-cycling genes amoA and nosZ increase by 25.3% and 17.0%, respectively. These functional pathways are mechanistically coupled and shift with soil conditions. Enhanced pore connectivity promotes oxygen diffusion toward roots, driving iron redox cycling and iron plaque formation, which modulates phosphorus solubility and selects for microbial consortia coupling nitrogen transformation to iron reduction. The relative importance of these mechanisms varies by context: physical water retention dominates in coarse-textured soils under drought, pH buffering and sorption prevail in acidic soils, and microbial habitat effects are most significant in nutrient-limited systems driven by rhizodeposits. Collectively, these alterations decrease the mineralization of indigenous soil organic carbon by an average of over 5.5%, elevate the retention of root-derived carbon in subsoil by 20% on average, and lower nitrogen leaching by 10.9%. Meanwhile, plant stress resistance is improved via optimized rhizosphere microbial communities and enhanced rhizosphere signal transduction. Translating these benefits into practice requires matching biochar properties to specific soil conditions. For alkaline sandy loam soils, wood- or crop residue-derived biochar produced above 500 °C, applied at 20–40 t ha −1 with 0.5–2 mm particles, is optimal. Acidic soils, however, require lower application rates of 5–25 t ha −1 . Particles smaller than 0.5 mm, as well as sludge- or manure-derived biochar produced below 500 °C, should be avoided without prior ecotoxicological screening mainly due to the risk of its transport or cotransport with contaminants. Long-term efficacy depends on repeated moderate-rate applications to manage aging-related decline and nutrient imbalances. Future research should resolve context-dependent contradictions through systematic dose–response experiments, define safe application windows by tracking aging trajectories, and reconcile conflicting microbial responses using field-based metatranscriptomics paired with enzyme assays. Graphical Abstract Highlights Biochar acts as a rhizosphere interface engineer, regulating root–soil–microbe systems through combined multi-pathways. These changes cut soil carbon and nitrogen losses, boost nutrient retention, and enhance plant stress resilience. Matching biochar to local soil conditions maximizes agronomic benefits and mitigates environmental risks.