Beneficial microbes in soil biota are known to enhance plant growth by stimulating the nutrient supply and by devising certain mechanisms to cope up with the biotic (diseases) or abiotic (salinity, drought, and pollution) stresses. Owing to their effectiveness and sustainability concerns, the application of microbes in the agricultural sector has seen a positive surge recently. Biochar has been commended as an exemplary carrier material for beneficial microbes in the soil ecosystem. Biochar is generally produced from the waste biomasses, which not only resolve the management crisis of agricultural wastes but also render many benefits such as enhancement of soil properties, alteration of nutritional dynamics, removal of pollutants, and in the stimulation of beneficial microbial diversity in soil. The strategic application of biochar in agricultural land could help provide agronomic, economic, and environmental benefits. Since certain risks are associated with the application of biochar, attention needs to be paid while preferring for soil amendments. This present review focused on highlighting the role of microbes in plant growth. The influence of biochar on soil biota along with its detailed mechanisms was discussed further to delineate the scope of biochar in soil amendments. Further, the risks associated with the biochar amendments and the future perspectives in this research arena were highlighted.

Plant uptake of silver nanoparticles (Ag NPs) and ions (Ag⁺) largely depends on their exchangeable and soil-bound fractions in soils, which may be influenced by biochar amendment. This study investigated the effects of biochar amendment (0.1% and 1.0%) on soil sorption of Ag NPs and Ag⁺, their soil-bound fractions, and their uptake and translocation by radish grown in a loamy sand soil spiked with 1 mg/kg Ag NPs or Ag⁺. Sorption of Ag⁺ to the soils was much greater than that of Ag NPs, mainly because negatively charged soil particles would attract Ag⁺, but repel negatively charged Ag NPs. Biochar amendment at 1.0% (by weight) significantly decreased the reducible fraction of Ag⁺ in the soils with and without radish plants and increased the oxidisable fraction of Ag NPs in the soils with radish plants. Biochar amendment had no significant effect on Ag uptake by radish plants (p > 0.05), probably due to low exchangeable Ag fractions in all experimental treatments. In this short-term experiment (35 days), the addition of 1 mg/kg Ag NPs or Ag⁺ did not substantially elevate the level of Ag in radish roots (0.05 ± 0.02–1.06 ± 0.98 mg/kg) and shoots (0.01 ± 0.00–0.03 ± 0.01 mg/kg), compared to the blank control (p > 0.05). Radish uptake of Ag NPs and Ag⁺ at the environmentally-relevant concentration was low with root concentration factors between 0.03 ± 0.03 to 0.29 ± 0.21 and root-to-shoot translocation factors between 0.08 ± 0.10 to 0.89 ± 1.21, which may partially explain the non-significant effect of biochar amendment on Ag uptake.

This study focuses on the synthesis of metal-based biochar catalysts and their catalytic activation of peroxymonosulfate (PMS, HSO₅⁻) for the degradation of three different wastewater model pollutants employing advanced oxidation processes (AOP). Iron, copper, and two different cobalt-based catalysts were prepared and evaluated. The catalysts were supported on a biochar obtained from the pyrolysis of woody pruning wastes. They were characterized by C, H, and N elemental analysis, X-Ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and scanning electron microscope (SEM). The metal content in each catalyst was determined by means of atomic absorption spectroscopy (AAS). The degradation reactions of benzoic acid (BA), catechol (C), and cinnamic acid (CA) were carried out in a lab scale batch glass reactor and were followed by UV–Visible spectroscopy (UV–Vis). A colorimetric technique was employed to verify the presence of oxidant during the reaction progress. The catalyst/oxidant optimal ratio was determined for the cobalt catalysts. The mineralization degree of the pollutants after the degradations was verified by means of total organic carbon (TOC) content in the residual liquids. After 4 h of reaction, the maximum mineralization was reached when C was treated with a cobalt-based catalyst (> 80%), and its stability was evaluated through successive cycles of use.

Application of biochar technology in the remediation of organic contaminated soils has drawn growing interest in recent years. In this study, sorption and degradation of two typical neonicotinoid insecticides, imidacloprid (IMI) and clothianidin (CLO) in Chinese typical paddy soil and red soil amended with six kinds of biochars were investigated. The results showed that surface area (SA), pH, total organic carbon and dissolved organic carbon (DOC) of the two soils all increased after biochar amendment, while H/C decreased. With biochar pyrolyzing temperature (PT) increasing from 300 °C to 700 °C, the sorption of the two insecticides on biochar–soil mixtures increased by more than 4.3-fold, due to the increasing SA and decreasing H/C. The acidic pH of the two tested soils also favored the enhanced sorption of the insecticides by removing the ash on biochar. The amendment of low-PT (300 °C) biochar promoted the biodegradation of IMI and CLO by 11.3–41.9% via providing more DOC and available N for microorganisms, while inhibiting the chemical degradation. Oppositely, the high-PT (500–700 °C) biochars inhibited the biodegradation of the insecticides by decreasing their bioavailability and promoted the chemical degradation by providing mineral active groups, and generating ·OH and other free radicals. In addition, soil type also affected the effects of biochar remediation. The highest 60-day degradation extent was achieved for CLO (90.5%) and IMI (81.4%) in paddy soil by adding biochar derived from pig manure at 700 °C PT. In summary, the effect of biochar on the fate of organic contaminants in soil is a comprehensive result involving several processes and a systematic study considering the type and property of biochar and soil is needed to optimize biochar technology.

The application of designer biochar has the potential to impact soil enzyme activity and soil nitrogen dynamics. However, very little is known about the mechanisms responsible for biochar-enzyme-nitrogen interaction in highly weathered soils. The objective of our 3-year (2016–2018) field experiment was to evaluate the effectiveness of designer biochars (DB) in enhancing urease activity (UA), total nitrogen (TN), total inorganic nitrogen (TIN), and nitrogen uptake (NU) at different growth stages (GS) of corn in a highly weathered soil of southeastern Coastal Plain region, USA. Experimental treatments have consisted of the control, 100% pine chips (100PC), 100% poultry litter (100PL), 2:1 blend of PC and PL (PCPL), 100% raw switchgrass (Panicum vaginatum, L; 100RSG), and 2:1 blend of PC and RSG (PCRSG). All the designer biochar treatments were applied at the rate of 30,000 kg ha⁻¹ to a Goldsboro loamy sand in 2016. Urease activity, TN, TIN, and NU varied remarkably with DB (p ≤ 0.0001) at different GS (p ≤ 0.0001) of corn. Soils treated with 100PL had the greatest UA (28.18 µg N g⁻¹ h⁻¹), TN (0.087%), and TIN (14.53 mg kg⁻¹) while the least UA, TN, and TIN of 20.55 µg N g⁻¹ h⁻¹, 0.063%, and 5.42 mg kg⁻¹, respectively, were observed from the control. The three-year TN average increase over the control was in the order: 100PL (36.8%) > 100RSG (25.8%) > PCRSG (25.3%) > PCPL (23.9%) > 100PC (7.1%). The greatest NU of corn of 140.4 kg N ha⁻¹ was from soils treated with 100PL while the least amount of NU was from 100PC. Overall, our results showed promising significance for the treatment of highly weathered soils since the application of DB did enhance UA and improve TN and TIN in the soils.

Biochar has the potential to affect the cycle of phosphorus (P), but the underlying mechanisms of its effects remain poorly understood in calcareous soils. Our understanding of the effects of biochar is limited in calcareous soils during incubation. Therefore, this study was conducted to investigate how the availability and mineral fractions of P change after the addition of combined biochar and P fertilizer during incubation in calcareous soil. Sugarcane residue (raw SR) and biochar (400 °C for 2 h) were added to soils treated with 50 mg kg⁻¹ of P, in the form of Ca(H₂PO₄)₂·H₂O, at 0.5 and 1% (w/w). The soils were incubated at 25 ± 1 °C for 120 days. Available P (Olsen P) contents and mineral P fractions were measured after various incubation times (7, 30, 90, and 120 days). Biochar addition remarkably increased the amount of available P when compared with the raw SR treatment and the control condition (P < 0.05). After 30 days of incubation, the amount of available P in the soils decreased and remained unchanged thereafter. The results indicated that the addition of 50 mg kg⁻¹ of P as fertilizer significantly augmented the labile P and P associated with Fe and Al in all the treatments at all incubation times (P < 0.05). In comparison with P treated with raw SR, P associated with Fe and Al was significantly enhanced after biochar addition (P < 0.05). Significant correlations were found for available P with labile P and P associated with Fe and Al. We found that biochar addition could increase available pools, thus improving available P concentrations at various incubation times. Therefore, we conclude that sugarcane residue biochar can enhance the available P in calcareous soils.

Biochar is traditionally used as solid fuel and for soil amendment where its electrical conductivity is largely irrelevant and unexplored. However, electrical conductivity is critical to biochar’s performance in new applications such as supercapacitor energy storage and capacitive deionization of water. In this study, sugar maple and white pine were carbonized via a slow pyrolysis process at 600, 800 and 1000 °C and conductivities of monolithic biochar samples along the radial direction were measured using the 4-probe method. Biochars were characterized using an elemental analyzer, scanning electron microscopy, X-ray diffraction and Raman spectroscopy. The solid carbon in biochar samples was found to consist primarily of disordered carbon atoms with small graphitic nanocrystallites that grow with increasing temperature. The bulk conductivity of biochar was found to increase with pyrolysis temperature—1 to ~ 1000 S/m for maple and 1 to ~ 350 S/m for pine, which was accompanied by an increase in carbon content—91 to 97 wt% and 90 to 96 wt% for maple and pine, respectively. The skeletal conductivity of biochar samples carbonized at 1000 °C is about 3300 S/m and 2300 S/m for maple and pine, respectively (assuming solid carbon is amorphous); both values are above that of amorphous carbon (1250–2000 S/m). This work demonstrated the importance of carbonization and graphitization to electrical conductivity and suggested electron hopping as a likely mechanism for electric conduction in biochar—an amorphous carbon matrix embedded with graphitic nanocrystallites.