Soil microorganisms play crucial roles in soil nutrient cycling, carbon sequestration, fertility maintenance and crop health and production. To date, the responses of microorganisms, such as microbial activity, diversity, community structure and nutrient cycling processes, to biochar addition have been widely reported. However, the relationships between soil microbial groups (bacteria, fungi and microscopic fauna) and biochar physicochemical properties have not been summarized. In this review, we conclude that biochar affects soil microbial growth, diversity and community compositions by directly providing growth promoters for soil biota or indirectly changing soil basic properties. The porous structure, labile C, high pH and electrochemical properties of biochar play an important role in determining soil microbial abundance and communities, and their mediated N and P cycling processes, while the effects and underlying mechanisms vary with biochar types that are affected by pyrolysis temperature and feedstock type. Finally, we highlight some issues related to research methodology and subjects that are still poorly understood or controversial, and the perspectives for further research in microbial responses to biochar addition.
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.
Biochar is the carbon-rich product obtained from the thermochemical conversion of biomass under oxygen-limited conditions. Preparation methods contribute to biochar properties, which are widely concerned in the agronomic and environmental benefits of agro-ecosystems. This research aims to use bibliometrics methods to comprehensively and objectively analyze the research trends in the global biochar production field from 2006 to 2019 based on the Web of Science Core Collection database. The results showed that 1434 papers related to biochar preparation were published, which gradually increased each year. The research topics were diversified, which were mainly divided into “Environmental Sciences and Ecology,” “Engineering” and “Agriculture”. Moreover, Bioresource Technology was the most published journal contained biochar preparation. Authors from China had the most publications, followed by the US, Australia and the UK. Meanwhile, Yong Sik Ok from Korea University contributed most of the publications and had the highest H-index. Keywords analysis indicated that biochar preparation by pyrolysis was a current hotspot, of which microwave and hydrothermal were a future research trend. Besides, gasification would be used to develop new biomass energy. Biochar has the characteristics of high pH, high specific surface area, high carbon content, and rich functional groups. It is mainly used in soil improvement, water pollutant adsorption, carbon sequestration and emission reduction, and energy storage materials, etc. Furthermore, raw materials, pyrolysis temperature and pyrolysis time will also affect its characteristics and applications. It is expected that this study may provide insight into the future research interests regarding biochar preparation.
The Oronogo-Duenweg mining belt is a designated United States Environmental Protection Agency Superfund site due to lead-contaminated soil and groundwater by former mining and smelting operations. Sites that have undergone remediation—in which the O, A, and B horizons have been removed alongside the lead contamination—have an exposed C horizon and are incalcitrant to revegetation efforts. Soils also continue to contain quantifiable Cd and Zn concentrations. To improve soil conditions and encourage successful site revegetation, our study employed three biochars, sourced from different feedstocks (poultry litter, beef cattle manure, and lodgepole pine), at two rates of application (2.5%, and 5%), coupled with compost (0%, 2.5% and 5% application rates). Two plant species—switchgrass (Panicum virgatum) and buffalograss (Bouteloua dactyloides)—were grown in the amended soils. Amendment of soils with poultry litter biochar applied at 5% resulted in the greatest reduction of soil bioavailable Cd and Zn. Above-ground biomass yields were greatest with beef cattle manure biochar applied at 2.5% with 5% compost, or with 5% biochar at 2.5% and 5% compost rates. Maximal microbial biomass was achieved with 5% poultry litter biochar and 5% compost, and microbial communities in soils amended with poultry litter biochar distinctly clustered away from all other soil treatments. Additionally, poultry litter biochar amended soils had the highest enzyme activity rates for β-glucosidase, N-acetyl-β-D-glucosaminidase, and esterase. These results suggest that soil reclamation using biochar and compost can improve mine-impacted soil biogeophysical characteristics, and potentially improve future remediation efforts.
Little information is available on thallium (Tl) adsorption onto biochar amended soil for a relatively long term. In this study, bamboo-derived biochar (BDB), soil in pomelo orchard (SP), and biochar amended soil in pomelo orchard (BSP) were thus used to evaluate the potential remediation of thallium (Tl) using batch-adsorption techniques. Furthermore, we characterized the above-mentioned sorbents’ properties related to Tl adsorption to understand Tl’s adsorption mechanisms. The results showed that BDB, SP, and BSP achieved equilibrium adsorption capacity of 96.9, 95.43, and 96.76%, respectively, within the initial 15 min. This means that compared to other sorbents, BSP exhibited an efficient sorbent for Tl remediation even when applied in the agricultural field for one year. Multi-layer adsorption played a dominant role in the adsorption of Tl, which was supported by the suitability of Freundlich model for describing the adsorption behavior of Tl onto the selected sorbents. In addition, the pseudo-second kinetic order models strongly fitted Tl’s adsorption onto BDB, SP, and BSP, indicating that the process was accompanied by chemical adsorption. Observed on the surface of BDB by a Fourier Transform Infrared Spectrometer (FTIR) and an X-ray photoelectron spectroscopy (XPS), the presence of O–H groups and PO43− might promote chemical adsorption of Tl onto BDB. Overall, these findings can provide insights into comprehensively developed bamboo-derived biochar technology to remediate Tl contamination in agricultural soils.
The effects of uncoated and Fe-coated biochars (BC) on the removal of bacteria, microspheres, and dissolved reactive phosphorus (DRP) in sand filters were compared. Filters were packed with 1.2 or 2.0-mm sand mixed with 30% (vol/vol) uncoated BC, Fe-coated BC, or a control without BC. Removal of E. coli, Salmonella, and Enterococci increased from 23, 42, and 25% in the unamended 1.2-mm sand to 48, 80, and 75% in the uncoated BC treatment, though only the increase for Enterococci was significant (p < 0.05). For the Fe-coated BC, removal efficiencies were 89, 93, and 94%, respectively, which were all significantly (p < 0.05) greater than the unamended sand but only the removal of E. coli was significantly greater than the uncoated BC sand filter. For the 2.0-mm sands, the only significant increase in removal following BC addition was observed for Salmonella. Trends in microsphere removal were generally consistent with bacteria. Removal of DRP in the unamended and uncoated BC filters was 33 and 13% (p > 0.05), respectively, whereas removal in the Fe-coated BC filters was 98% (p < 0.05). Results from batch sorption experiments indicate that both BCs similarly increased bacterial sorption to sand. In contrast, DRP sorption to the unamended and uncoated BC-amended sands were similar (p > 0.05) with DRP sorption to the Fe-coated BC-amended sand being significantly greater (p < 0.05). Results indicate that Fe-coated BC is more effective at retaining DRP than bacteria and microspheres in sand filters.
Soil aggregation is one of the crucial processes that facilitate carbon sequestration and maintain soil fertility. So far, the effect of biochar amendment on soil aggregation remains inconclusive. Here, we tested the hypothesis that the response of soil aggregation to biochar addition varied with incubation duration and biochar chemistry. A one year microcosm experiment of soil with biochar was conducted that included biochar produced at three different temperatures (300, 450, and 600 °C), and three biochar application rates, i.e., 0, 1, and 3 wt%. It was observed that after one and three months, biochar mainly (> 90%) distributed in the micro-aggregates, and slightly reduced aggregate stability and increased proportion of micro-aggregates, which was demonstrated to result from the mechanical mixture of amended biochar with soil. Contrastingly, when the duration was prolonged to six months and one year, a significant increase in macro-aggregates (6.6–38.5%) and aggregate stability (7.3–29.4%) was detected, with the increasing extent being apparently higher for low-temperature biochar. This was related to the comparatively strong interaction of biochar particles with soil minerals or microbes after long-time incubation. The strong interaction was directly supported by the significant increase in H/C, O/C ratios of isolated biochar from treated soils, the detection of typical soil mineral elements on the surface of isolated biochar, and the increase in microbial biomass carbon of treated soils. The findings of this study highlighted the role of biochar type and amendment duration in mediating the effect of biochar application on soil aggregation.
The hydrothermal carbonization (HTC) of biogas digestate alters the raw materials inherent characteristics to produce a carbon (C)-rich hydrochar (HC), with an improved suitability for soil amelioration. Numerous studies report conflicting impacts of various HC application rates on soil properties and plant growth. In this study, the influence of HC application rate on soil improvement and plant growth aspects was investigated in three diverse soils (Chernozem, Podzol, and Gleysol). Pot trials were conducted in which all soils were amended with 5, 10, 20 and 30% (w/w) HC in quintuplicate, with two controls of pure soil (with and without plants, respectively) also included. Prior to potting, soil samples were collected from all HC-amended soils and controls and analyzed for soil pH, plant available nutrients (PO4-P and K), and microbial activity using standard laboratory and statistical methods. Immediately after potting, a 6-week seed germination experiment using Chinese cabbage was conducted to determine germination success, followed by a plant growth experiment of equal duration and plant species to determine biomass success. At the end of the study (after a total plant growth period of 12 weeks), each pot was sampled and comparatively analyzed for the same soil properties as at the beginning of the study. Soil pH shifted toward the pH of the HC (6.6) in all soils over the course of the study, but was most expressed in the 20% and 30% application rates, confirming the well-documented liming effect of HC. The addition of HC increased the PO4-P and K contents, particularly with 20% and 30% HC amendments. These results are proposedly due to the large labile C fraction of the HC, which is easily degradable by microorganisms. The rapid decomposition of this C fraction prompted the quick release of the HCs inherently high PO4-P and K content into the soil, and in turn, further stimulated microbial activity, until this fraction was essentially depleted. HC addition did not inhibit seed germination at any rate, presumably due to a lack of phytotoxic compounds in the HC from aging and microbial processes, and furthermore, showed no significant impact (positive or negative) on plant growth in any soil, despite improved soil conditions. In conclusion, although less pronounced, soil improvements were still achievable and maintainable at lower application rates (5% and 10%), whereas higher rates did not ensure greater benefits for plant growth. While the addition of high rates of HC did not detrimentally effect soil quality or plant growth, it could lead to leaching if the nutrient supply exceeds plant requirements and the soil’s nutrient retention capacity. Therefore, this study validates the previous study in the effectiveness of the biogas digestate HC for soil amelioration and suggests that smaller regularly repeated HC applications may be recommendable for soil improvement.
Combined application of biochar with fertilizers has been used to increase soil fertility and crop yield. However, the coupling mechanisms through which biochar improves crop yield at field scale and the time span over which biochar affects carbon and nitrogen transformation and crop yield are still little known. In this study, a long-term field trial (2013–2019) was performed in brown soil planting maize. Six treatments were designed: CK—control; NPK—application of chemical fertilizers; C1PK—low biochar without nitrogen fertilizer; C1NPK, C2NPK and C3NPK—biochar at 1.5, 3 and 6 t ha−1, respectively, combined with chemical fertilizers. Results showed that the δ15N value in the topsoil of 0–20 cm layer in the C3NPK treatment reached a peak of 291 ‰ at the third year (2018), and demonstrated a peak of 402 ‰ in the NPK treatment in the initial isotope trial in 2016. Synchronously, SOC was not affected until the third to fourth year after biochar addition, and resulted in a significant increase in total N of 2.4 kg N ha−1 in 2019 in C3NPK treatment. During the entire experiment, the 15N recovery rates of 74–80% were observed highest in the C2NPK and C3NPK treatments, resulting in an annual increase in yields significantly. The lowest subsoil δ15N values ranged from 66‰ to 107‰, and the 15N residual rate would take 70 years for a complete decay to 0.001% in the C3NPK. Our findings suggest that biochar compound fertilizers can increase C stability and N retention in soil and improve N uptake by maize, while the loss of N was minimized. Biochars, therefore, may have an important potential for improving the agroecosystem and ecological balance.
Water washing is a meaningful method to improve the surface’ characteristic of hydrochar produced using hydrothermal carbonization and minimize the negative effect on crop growth. However, the greenhouse effect resulting from water-washed hydrochar application was unclear in agricultural ecosystems. Hence, the effect of water-washed hydrochar on methane and nitrous oxide emissions was analyzed in an infertile paddy soil based on a soil-column experiment. Sawdust-derived hydrochar (WSH) and wheat straw-derived hydrochar (WWH) after water washing were selected and applied with low (5‰, w/w; 8.5 t ha−1) or high addition rate (15‰, w/w; 25.5 t ha−1). The study indicated that water-washed hydrochar could increase the grain yield; the difference between WWH with 5‰ application rate and CKU treatments was significant. WSH significantly decreased CH4 and N2O emissions in comparison with WWH addition treatments. For the same material, there were trends in reducing greenhouse gas (GHG) emissions at low application rate, although the differences were not significant. Compared with all treatments, WSH with 5‰ application rate achieved the lowest seasonal emissions for both GHGs. The mcrA gene was the critical factor affecting CH4 emission; soil NO3−–N concentration and the copy numbers of nirK, nirS, and nosZ jointly affected N2O emissions. Benefits from the high yield and low global warming potential, GHG emission intensity (GHGI) at low application rate was lower than at high application rate for WSH. Overall, the response of GHG emissions to water-washed hydrochar varies with the derived feedstock; WSH is a good additive for the mitigation of GHGI.
Biomass and pig manure have distinct compositions and the co-pyrolysis of them has gained much attention. However, the influence of volatiles interaction on the properties of the char was still unclear. In this study, lignin was selected as the model component of biomass with pig manure for co-pyrolysis at 600 °C. The results indicate that volatiles from co-pyrolysis promoted re-condensation reaction, resulting in the higher char yield (48.0% in co-pyrolysis versus 31.0% in pyrolysis of single manure) and the formation of more aromatics in bio-oil. The co-pyrolysis also facilitated the dehydrogenation and dehydration reactions, which accounted for the elimination of oxygen and nitrogen contents and thus a higher carbon content (64.7% in the co-pyrolysis versus the averaged value of 46.4% from the pyrolysis of single feedstock), higher crystallinity and thermal stability of the char. The in-situ diffuse reflection infrared Fourier transform spectroscopy (DRIFTS) characterization results demonstrated that the functionalities abundances of char with temperature was influenced by volatiles interaction via accelerating the carbonization reaction. In addition, the high heating value (HHV) of char was obviously improved by cross-interaction of volatiles during co-pyrolysis (24.4 MJ/Kg in co-pyrolysis versus averaged value of 15.1 MJ/Kg from single pyrolysis), implying that the co-pyrolysis enhanced the energy density of the resulting char.